Congenital infections with nonarbo togaviruses

Veterinary Microbiology, 8 (1983) 321--361 321 Elsevier Science Publishers B.V., Amsterdam - -Pr in ted in The Netherlands

R e v i e w

C O N G E N I T A L I N F E C T I O N S W I T H N O N A R B O T O G A V I R U S E S

J.T. VAN OIRSCHOT

Central Veterinary Institute, Department of Virology, Houtribweg 39, 8221 RA Lelystad (The Netherlands)

(Accepted 31 January 1983)

ABSTRACT

Van Oirschot, J.T., 1983. Congenital infections with nonarbo togaviruses. Vet. Micro- biol., 8: 321--361.

The present review deals with the similarities and differences of selected aspects of prenatal pestivirus infections of domestic animals and congenital rubella of man.

Hog cholera virus, bovine virus diarrhoea virus and border disease virus are antigenically closely related, but unrelated to rubella virus. The nonarbo togaviruses are capable of producing congenital infections resulting in a wide spectrum of abnormalities. The infected foetus can die in utero, in the neonatal period, or it may be born with teratogenic defects. In addition, apparently healthy progeny can be delivered that develop a late onset disease, months, or years after birth, or remain clinically normal for life. The ult imate outcome of a congenital infection is mainly determined by the stage of foetal development, at which infection occurs. Foetuses exposed to rubella virus raise an ant ibody response to the virus, whereas domestic animals frequently fail to respond immunologically to a congenital pestivirus infection.

In congenital rubella the virus usually disappears from the host 's body 1--2 years after birth. However, congenital pestivirus infections may be characterized by a lifelong and widespread persistence of virus in clinically healthy animals. Such animals are of significance in the epizootiology of bovine virus diarrhoea, border disease or hog cholera.

INTRODUCTION

T h e v i ruses t h a t c a u s e h o g c h o l e r a (HC, s y n o n y m : s w i n e f eve r ) b o v i n e v i ra l d i a r r h o e a ( B V D , s y n o n y m : m u c o s a l d i s e a s e ) o r b o r d e r d i s e a s e (BD, s y n o n y m : h a i r y s h a k e r d i s e a s e ) b e l o n g t o t h e g e n u s Pestivirus. R u b e l l a v i ru s ( R U V ) is t h e so le m e m b e r o f t h e g e n u s Rubiv irus . T h e p e s t i v i r u s e s a r e se ro - l o g i c a l l y r e l a t e d , b u t u n r e l a t e d t o R U V ( P o r t e r f i e l d e t al . , 1 9 7 8 ) . T h e t w o g e n e r a a re , t o g e t h e r w i t h t h e g e n e r a Alphav irus a n d Flavivirus, i n c o r p o r a t e d i n t o t h e f a m i l y o f T o g a v i r i d a e . A l p h a - a n d m o s t f l av iv i ruses a r e t r a n s m i t t e d b y a r t h r o p o d s . S i n c e t h e v i ru ses b e l o n g i n g t o t h e g e n e r a Pestivirus o r Rubi - virus d o n o t d e p e n d o n b i o l o g i c a l t r a n s m i s s i o n b y a r t h r o p o d s f o r t h e i r c o n - t i n u a n c e in n a t u r e , t h e c o l l e c t i v e t e r m " n o n - a r t h r o p o d - b o r n e " o r n o n a r b o t o g a v i r u s e s has b e e n i n t r o d u c e d f o r t h e s e v i ruses ( H o r z i n e k , 1 9 7 3 ) . L a c t i c d e h y d r o g e n a s e v i ru s ( L D V ) a n d e q u i n e a r t e r i t i s v i rus ( E A V ) a re a d d i t i o n a l

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nonarbo togaviruses, outside the existing genera (Porterfield et al., 1978). After infecting a pregnant animal, the nonarbo togaviruses are capable of

crossing the placental barrier and invading the foetus. The congenital infection can result in a wide spectrum of abnormalities varying from death of the foetus to a persistent, lifelong infection without clinical signs.

HC is a very contagious disease of pigs, that can be present as an acute, chronic or clinically inapparent infection. The mortali ty rate ranges from nil to one hundred percent depending largely on the virulence of the virus. In the early decades of this century, it was recognized that hog cholera virus (HCV) could cause congenital infections. Benner (1928) and Michal- ka (1931) described the occurrence of abortion, stillbirth and neonatal death after vaccination of pregnant sows by the simultaneous method. A teratogenic activity of HCV was strongly suggested by the birth of malformed piglets after vaccination of pregnant dams with modified live viruses (Sautter et al., 1953). The definite evidence that HCV was able to cross the placenta came from in vivo transmission experiments (Young et al., 1955). Congenitally-infected piglets usually are stillborn, or succumb shortly after farrowing, but Van Oirschot and Terpstra (1977) reported that prenatally infected piglets can survive for months after birth.

Two clinically-distinct diseases of cattle, BVD (Olafson et al., 1946) and mucosal-disease (Ramsey and Chivers, 1953} were eventually shown to be caused by bovine viral diarrhoea virus (BVDV). Most BVDV infections are inapparent, though acute and chronic forms of the disease exist. BVDV infections have been associated with abortion and neonatal death since the initial description of the disease. It was not until 1967 that the virus was isolated from aborted foetuses (Gillespie et al., 1967). Experimental infections of pregnant cattle established that the virus can act as a teratogenic agent (Ward et al., 1969). Congenitally-infected calves do not always succumb shortly after birth, they may survive for years in a clinically healthy state, while remaining persistently infected (Coria and McClurkin, 1978; Strayer et al., 1983).

Infection of sheep with border disease virus (BDV) invariably remains subclinical. BD only becomes clinically manifest when a pregnant ewe encoun- ters the virus and consequently delivers a dead or clinically affected lamb. The existence of lambs with an abnormal hairy birthcoat, body tremors and a slow rate of growth in flocks of the Welsh border counties was first described by Hughes et al. (1959). The viral aetiology of the disease was established by in vivo transmission studies (Gardiner and Barlow, 1972). The teratogenic potential of BDV has already been indicated by the first description of the disease. Recent reports describe in utero infected sheep which had been persistently infected for years, while remaining apparently healthy (Barlow et al., 1980b; Terpstra, 1981).

Rubella is generally a mild disease in humans, but once the foetus becomes infected, serious damage may occur. Gregg (1941} initially recognized that maternal rubella in early pregnancy can lead to severe foetal malforma-

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tions, of which the principal ones were those affecting the eye, heart and ear. It was not until 1963, that Selzer (1963) demonstrated the presence of RUV in a foetus, 10 days after the onset of rubella in the mother. In the wake of the great epidemic that swept the U.S.A. in 1964, a tremendous amount of new information on clinical manifestations, pathogenesis and epi- demiology of congenital rubella has been reported. These studies were facili- tated by the successful isolation of RUV in vitro in 1962 (Parkman et al., 1962; Weller and Neva, 1962). Persistent infections are common after birth in congenital rubella.

Most EAV infections take an inapparent course (McCollum and Bryans, 1973; MoraiUon and Moraillon, 1978). However, the virus may occasionally cause severe disease in horses (Bryans et al., 1957}. When a susceptible pregnant mare is exposed to the virus, the foetus may become infected and abortion can occur. The virus was first isolated from an aborted foetus in 1957 (Doll et al., 1957). The significance of subclinical EAV infection with regard to abortion is not known.

LDV causes an asymptomatic persistent infection in mice. In the acute phase of the infection in pregnant mice LDV can cross the placental barrier and infect the offspring, without any serious effect (Georgii et al., 1964; Crispens, 1965).

There appear to be no serological relationships between the pestiviruses and RUV, EAV and LDV (Horzinek, 1981).

However, the viruses constituting the genus Pestivirus are antigenically and structurally closely related. HCV and BVDV cross-react in immunodiffu- sion (Darbyshire, 1960), immunofluorescence (Mengeling et al., 1963) and neutralization assays (Kumagal et al., 1962; Corthier et al., 1974}. Pigs exposed to certain strains of HCV or BVDV can develop neutralizing antibody to both HCV and BVDV (Fernelius et al., 1973b; Carbrey et al., 1976; Liess et al., 1976}. Immunization of pigs with BVDV can confer some protection against challenge with virulent HCV (Sheffy et al., 1961; Snowdon and French, 1976). BDV-immunized pigs also appear to be cross-protected against HCV infection (Laude and Gelfi, 1979). Sheep infected naturally or experimentally with BDV produce neutralizing antibody to all 3 pestiviruses (Acland et al., 1972; Hamilton and Timoney, 1972; Osburn et al., 1973) and BDV antigen can be demonstrated in cell culture or tissue sections by immunofluorescence with a BVDV or HCV antiserum (Harkness et al., 1977, Terpstra, 1978). Sheep inoculated with a virus isolated from a lamb showing signs of BD produced higher neutralizing antibody titres to BVDV than to BDV (Barlow et al., 1979). In addition, BVDV and BDV may cross- infect the ovine and bovine species and induce comparable congenital defects (Ward, 1971; Gibbons et al., 1974; Terlecki et al., 1980). Laude (1979) has reported that the pestiviruses, when grown in the same cell system, have a similar buoyant density and sedimentation coefficient.

In view of the observations mentioned above, one may question whether it is justified to consider the pestiviruses, and BVDV and BDV in particular,

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as distinct viral entities. However, for the sake of clarity the 3 pestiviruses are described here in relation to their "natura l" host, which means that HCV is related to swine, BVDV to cattle and that viruses isolated from sheep with signs of border disease or hairy shaker disease are termed BDV. A separate section will deal with the various cross-infections.

This paper is concerned mainly with the similarities and differences between selected aspects of prenatal infection of domestic animals with pestiviruses and additionally with congenital rubella infection of man.

CLINICO-PATHOLOGICAL FEATURES

Congenital infection with HCV, BVDV, BDV or RUV ranges from ex- tremely severe to clinically inapparent. The results are mainly dependent on the stage of foetal development at which infection takes place. In general, the risk for the foetus is highest during early pregnancy.

An in utero infection can result in prenatal death, foetal malformations and lesions, or death of the neonate or later on in life. In addition, apparently healthy offspring can be delivered that develop a late onset disease months or years after birth, or remain clinically normal for life.

As a rule, clinical evidence of maternal disease is minimal or absent.

Prenatal death

Early embryonic death with subsequent complete resorption may occur after infection in the very early stages of pregnancy, before the skeleton has developed fully. This is suggested by the increase of barrenness of sows infected with HCV (Dunne and Clark, 1968; Stewart, 1969) and of ewes exposed to BDV (Manktelow et al., 1969; Porter et al., 1972). Several investi- gations have suggested that BVDV causes early embryonic death. Cows arti- ficially inseminated and subsequently given an intrauterine infusion with BVDV around breeding showed a significantly decreased pregnancy rate (Archbald and Zemjanis, 1977; Whitmore et al., 1981) and an abnormal development of preimplantation embryos (Archbald et al., 1979). On the other hand, cows infected oronasally 2 h after breeding have normal pregnancy rates (Whitmore et al., 1981) and BVDV does not appear to cross the placenta before the 58th day of gestation (Kendrick, 1976). Thus, in view of the latter findings and because BVDV is rarely excreted in semen it is unlikely that, under natural conditions, BVDV is a major cause of early embryonic death. Whether RUV causes early embryonic death is not known.

Abortion is a common sequel to congenital infections with nonarbo togaviruses, including EAV. Unfortunately, in many studies, virus isolation from the aborted foetus is not at tempted, or remained unsuccessful due to the ad- vanced stages of autolysis. In the field, abortion of animals may often go un- noticed. Thus, the real incidence of abortion is difficult to determine, but it seems to be more common in sheep and cattle than in man and swine. Abor-

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tion is usually the consequence of infection during the first third of pregnancy, but in sheep it can also be the result of maternal BVD infection around mid-pregnancy (Barlow and Gardiner, 1969). The period that elapses between virus exposure and abortion is variable and seems to be different for the various species. Obviously, it is dependent upon the mode of virus administration, but the minimum period required is usually 3 weeks. Spon- taneously-aborted foetuses are often autolyzed, which makes the presence of lesions difficult or impossible to observe. Occasionally, oedema of the subcutaneous tissues and excessive fluid in the pleural and peritoneal cavities have been noted in aborted foals {Doll et al., 1957), piglets (Meyer, 1978) and calves (Gillespie et al., 1967; Casaro et al., 1971). In addition, the latter reports describe hyperplasia of reticulo-endothelium in spleen and lymph nodes in aborted calf foetuses.

Foetuses, which die in utero in early pregnancy, may become mummi- fied due to the resorption of fluid. In this case, the uterus is probably not properly stimulated to cause regression of the corpus luteum (Kendrick, 1971) and the conceptus consequently remains in utero. Mummies have been born from sows, cows and ewes after infection with pestiviruses.

Stillbirth is common after pestivirus infections, but seems to be an infre- quent complication of congenital rubella (Dudgeon, 1975). Pregnant sows infected with HCV seem to be more prone to delivering stillborn progeny than to producing aborted foetuses, whereas the reverse holds true for the ovine and bovine species after BDV and BVDV infections, respectively. Stillbirth is usually the consequence of foetal death in mid- and in the last third of pregnancy. In pigs, stillbirth occurs after maternal infection during the last third of gestation (Cowart and Morehouse, 1967; Meyer, 1978). At post-mortem examination, subcutaneous oedema, hydrops ascites and hydro- thorax are the most pronounced lesions in stillborn pigs. These excessive ac- cumulations of fluids are often associated with mottling or pitting of the liver or kidney (Fig. 1) (Cowart and Morehouse, 1967; Johnson et al., 1974; van Oirschot, 1979a). The mechanisms responsible for these lesions still remain unexplained.

As a rule, prenatal death due to congenital nonarbo togavirus infections is more common when the maternal infection takes place earlier during gestation.

Teratogenic effects

RUV, BVDV, BDV and HCV can act as teratogenic agents and when the foetus survives the initial infection a whole spectrum of abnormalities can develop. Manifestations of such deviant foetal development are malformations, generalized or localized growth retardation, pathological lesions, which are not always clinically obvious, and more subtle biochemical or functional defects. In this context the term "malformat ion" is confined to cases where there are gross anatomical defects of one or more body components.

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Fig. 1. A stillborn piglet, born to a sow experimentally-infected with HCV during pregnancy, showing oedema of the subcutis, excessive fluid in body cavities and a mottled liver.

Ma l fo rma t ions and g rowth r e t a rda t ion m a y arise when virus in fec t ion occurs dur ing organogenes is and thus in terferes wi th g rowth , d i f f e ren t i a t ion and m a t u r a t i o n of foe ta l t issue, whereas lesions m a y be the resul t o f virus infec t ions o f a l ready m a t u r e d tissue. This implies t ha t the vulnerabi l i ty o f foeta l t issue to viral t e ra togens is d i rec t ly re la ted to the stage o f develop- m e n t a t which in fec t ion occurs . In congeni ta l rubel la the re is a m u c h higher incidence o f p e r m a n e n t d a m a g e in chi ldren in fec ted in the first 8 weeks t h a n in the second 8 weeks o f p regnancy . Af t e r 20 weeks the risk to the foe tus is vir tual ly negligible (Dudgeon , 1975) . Bovine foetuses exposed to B V D V between 3 and 6 m o n t h s of ges ta t ion o f ten show m a l f o r m a t i o n s o f the bra in and eyes at b i r th (Sco t t e t al., 1973; Brown et al., 1974, 1975) , whereas BVDV infec t ions a f te r the 6 th ges ta t ional m o n t h usual ly do no t resul t in

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foe t a l a b n o r m a l i t i e s (Casaro e t al. , 1 9 7 1 ; K e n d r i c k , 1 9 7 1 ) . By e x p e r i m e n t a l - ly m a n i p u l a t i n g t h e t i m i n g o f B D V i n f e c t i o n d u r i n g p r e g n a n c y , n e u r o l o g i c a l a n d c u t a n e o u s l es ions c o u l d be s u p p r e s s e d a n d a r te r ia l l e s ions i n t r o d u c e d ( Z a k a r i a n e t al. , 1 9 7 5 ; D e r b y s h i r e a n d Ba r low , 1 9 7 6 ) . This m a y be i n d i c a t i v e o f t he c h a n g i n g ava i l ab i l i t y or s u s c e p t i b i l i t y o f t a r g e t cells as foe t a l t i s sue d i f f e r e n t i a t e s a n d m a t u r e s .

T h e n o n a r b o togav i ruses i n d u c e a va r iab le p a t t e r n o f foe ta l d i sorders . H o w e v e r , t h e de fec t s p r o d u c e d in t he r e spec t ive hos t s have several charac- te r i s t i cs in c o m m o n ( T a b l e I).

TABLE I

Unity and diversity in clinico-pathological features of congenital nonarbo togavirus infections

Hog Bovine Border Rubella cholera virus disease

diarrhoea

Abortion + ++ ++ + Foetal mummification + + + - Stillbirth ++ + + +

Intrauterine growth retardation - + ++ ++ Selective organ stunting + + + + Cerebral malformations + + + + Cerebellar hypoplasia + + + - Dysmyelination + + ++ + Chorioretinopathy - + - ++ Cataracts - + - + Skeletal defects + + + + Arthrogryposis + - + - Skin abnormalities + + ++ +

Neonatal morbidity and mortality ++ ++ ++ ++

Clinically inapparent persistently- + + + + infected individuals

++ = frequently occurring; + = recorded; - = not (yet) recorded.

I n t r a u t e r i n e g r o w t h r e t a r d a t i o n ( I U G R ) is a c o m m o n f e a t u r e o f c o n g e n i - ta l B V D V , BDV a n d R U V i n f e c t i o n s . I t m a y be d e f i n e d as h a v i n g t o o low a b i r t h w e i g h t for t h e g e s t a t i o n a l age. Se lec t ive o r ga n s t u n t i n g c a n also occu r . B D - a f f e c t e d l a m b s f r e q u e n t l y have a b o d y w e i g h t a n d c r o w n m a n u s l e n g t h b e l o w t h e n o r m a l r ange , t h e y possess f i ne r l o n g b o n e s (Te r l eck i e t al., 1 9 7 3 ) a n d have s i g n i f i c a n t l y r e d u c e d visceral o rgan a n d t h y m u s w e i gh t s ( R i c h a r d - s o n , 1 9 7 7 ) . L o w b i r t h we igh t is also very c o m m o n in c o n g e n i t a l r u b e l l a ( F o r r e s t a n d Mense r , 1975}. A s i g n i f i c a n t I U G R , d e f i n e d as h a v i n g a b i r t h w e i g h t b e l o w the 1 0 t h p e r c e n t i l e , was n o t e d in 59% of 92 i n f a n t s s t u d i e d ( D e s m o n d e t al. , 1 9 6 7 ) . In genera l , severe I U G R is o b s e r v e d in c h i l d r e n w i t h m u l t i p l e de fec t s , b u t i t is s e l d o m the sole m a n i f e s t a t i o n o f c o n g e n i t a l r u b e l l a

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(Lejarraga and Peckham, 1974). In-utero infected children can have significantly lower organ weights than healthy children (Naeye and Blanc, 1965). Newborn calves, whose dams were exposed to BVDV at 100 days of pregnancy showed evidence of IUGR (Done et al., 1980). The thymus weights were also significantly decreased. Kendrick (1975 ) r epo r t ed a reduction in foetal lung size after in utero BVDV infection. In congenital HC general IUGR has not been recorded, but a selective growth retardation of the lung has been observed (Young et al., 1955; Emerson and Delez, 1965).

Congenital nonarbo togavirus infections frequently interfere with the development of the central nervous system (CNS).

Intracranial malformations have been reported in association with prenatal infection with pestiviruses. Hydranencephaly, which is defined as complete or near~omplete absence of cerebral hemispheres, with the space filled with cerebrospinal fluid, surrounded by thin, membraneous cerebral tissue (Cho and Leipold, 1977), was observed in lambs with apparently normal heads, whose dams were exposed to BDV at 54 days of gestation (Barlow, 1980). Hydranencephalic calves were born during field outbreaks of BVD (Markson et al., 1976; Axthelm et al., 1981; Badman et al., 1981) and after experimental BVDV infection (Done et al., 1980). Porencephalic cavitations of cerebrum and cerebellum can develop in foetal lambs infected with BDV (Barlow, 1980) and calves exposed to BVDV in utero (Markson et al., 1976; Axthelm et al., 1981).

Brain malformations are relatively uncommon in congenital rubella and HC, and are far overshadowed by retardation in growth and maturation of the brain, that may eventually lead to microencephaly (Rorke, 1973; John- son et al., 1974). Also in BD and BVD a decreased brain weight is a consistent morphological defect (Scott et al., 1973; Terlecki et al., 1973; Sweasey et al., 1979; Done et al., 1980).

Cerebellar hypoplasia is the most common brain defect in prenatal HCV and BVDV infections. It is often associated with congenital t remor in the newborn. Varying degrees of cerebellar hypoplasia can be induced by inocu- lating pregnant sows with field (Done and Harding, 1969; Vannier et al., 1981) or vaccine strains of HCV (Emerson and Delez, 1965; Johnson et al., 1974). The defect appears more severe the earlier sows are infected during gestation. After experimental infection of cattle with BVDV between 100 and 150 days of gestation cerebellar hypoplasia of the foetus can occur (Brown et al., 1973, 1974; Done et al., 1980). This cerebellar defect has also been noted in field outbreaks (Markson et al., 1976; Badman et al., 1981). Cerebellar hypoplasia is also part of the BD syndrome (Barlow, 1980). Hypoplasia refers to a lack of development, wi thout indicating how this arises. In BVD it appears to be the result of destruction of tissue formed previously rather than impaired cerebellar development. Severe folial oedema in conjunction with necrosis of cells in the external germinal layer appears to cause the cerebellar hypoplasia (Brown et al., 1974).

Dysmyelination Or hypomyelinogenesis is a disorder frequently seen after congenital nonarbo togavirus infections. In BDV infected lambs dysmyelination of the CNS is a characteristic feature, and it may lead to congenital

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tremor in the newborn lamb. The congenital defect is present in varying degrees in the brain and spinal cord, the latter being constantly the most severely affected (Hughes et al., 1959). Histopathologically, the anomaly is characterized by a reduced affinity for myelin-specific stains, aggregation of interfascicular lipid droplets and the presence of numerous neuroglial cells, many of them abnormal (Patterson, 1975). Biochemically, there is evidence of a disturbed lipid metabolism and an enhanced macrophage enzyme activity, which suggests a process of myelin degeneration (Storey and Barlow, 1972). Other studies suggest that BDV affects the precursors of myelin- forming oligodendrocytes and thus results in reduced myelin formation (Barlow and Storey, 1977). The above data support the concept that the dysmyelination is due to impaired myelination associated with concurrent destruction of newly formed myelin (Patterson, 1975; Terlecki, 1977). Re- cently, defective myelination of the spinal cord was demonstrated neuro- chemically in 8 calves prenatally-infected with BVDV (Done et al., 1980). Congenital t remor of pigs resulting from transplacental infection with HCV is, among other things, characterized by dysmyelination of the CNS. Again, the spinal cord shows the most severe disturbance of myelination, whereas the peripheral nerves are not significantly affected (Emerson and Delez, 1965; Done and Harding, 1969). Retardation of myelination of the CNS, in some case with a decrease in oligodendroglia, has also been described in congenital rubella (Naeye and Blanc, 1965; Rorke and Spiro, 1967).

The developing eye appears to be very susceptible to the teratogenic ac- tion of RUV, and BVDV also can induce eye anomalies. Eye defects do not seem to have been described in congenital BDV and HCV infections. The most common ocular defect in congenital rubella is the typical pigmentary chorioret inopathy, which rarely impairs vision (Forrest and Menser, 1975). Infection of pregnant cows with BVDV at 150 days, but not between 65 and 115 days of pregnancy, can also lead to focal or total ret inopathy in the foetus. In the acute stage inflammation and necrosis of the retina are seen (Brown et al., 1975). Cataracts are frequently encountered in congenital rubella, usually involving both eyes (Dudgeon, 1976), and are often associated with microphthalmos. Calves congenitally-infected with BVDV can also develop cataracts and microphthalmos (Bistner et al., 1970; Scott et al., 1973). However, the cataracts induced by the 2 infections seem to be histologically distinct (Bistner et al., 1970). Other ocular defects are less common than ret inopathy and cataracts. Severe eye defects may result in blindness.

In BD, BVD and rubella, abnormalities of the skeleton have been reported. These are skeletal growth arrest lines in BD, indicative of disturbed osteogenesis (Terlecki et al., 1973). Calves that died in utero after maternal BVDV infection showed growth arrest lines in long bones {Done et al., 1980). BD lambs possess finer limb bones and exhibit skull deformities. The bone of the skull is radiographically less dense (Terlecki et al., 1973}. De- formed heads have also been noted in congenital HC (Sautter et al., 1953) and BVD (Scott et al., 1973). Bone lesions in congenital rubella consisted of a large anterior fontanelle, long bone changes and demineralization of

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the skull bones. There is a defect in bone formation, involving principally the laying down and calcification of osteoid (Rudolph et al., 1965). It is not clear whether the bone changes are secondary to a metabolic or nutritional disturbance or are a direct effect of viral infection of the bone, as suggested by Heggie (1977),

Defects in distal front legs, suggestive of arthrogryposis, have been observed in piglets exposed in utero to HCV (Emerson and Delez, 1965; Johnson et al., 1974). Arthrogryposis also appears as a rare sequel of a foetal BDV infection (Vantsis et al., 1980).

Skin abnormalities occur following congenital nonarbo togavirus infections. A characteristic feature of BD is an abnormally hairy birthcoat, which is the result of an enlargement of primary follicles (Derbyshire and Barlow, 1976}. Alopecia may be the consequence of transplacental BVDV and HCV infections (Kendrick, 1971; Stewart et al., 1972). Achs et al. (1966} found dermatoglyphic abnormalities in children with severe congenital rubella. Some infants infected in utero with RUV show a rash at birth (Lindquist et al., 1965; Plotkin et al., 1965b).

The nonarbo togaviruses can cause additional foetal abnormalities. A consistent feature of congenital rubella is its multiplicity of defects, the only defects that commonly occur alone being deafness and retinopathy. Deafness is the most common defect in infants with congenital rubella, and it is usually of the sensoneurinal type (Forrest and Menser, 1975). Deafness is often not detected at birth, and in some cases it may deteriorate after birth (Peckham, 1972). Other major manifestations of the rubella syndrome are the structural cardiovascular defects, which mainly include persistent ductus arteriosus, pulmonary artery stenosis and ventricular septum defects (Dudgeon, 1976). The teratogenicity of RUV can cause numerous other anomalies and these are listed in several review articles (Dudgeon, 1976; Charles, 1980).

In addition to malformations nonarbo togaviruses can produce lesions in the foetus. Lesions are the result of infection of already fully-formed tissue, and the foetal immune response may contribute to their development. Various lesions have been observed after congenital infections with nonarbo togaviruses, among which are hepatitis and thrombocytopenia purpura in rubella (Dudgeon, 1975), hepatitis and petechiation in HC (Von Benten et al., 1980), periarteritis in BD (Gardiner et al., 1980) and mouth lesions in BVD (Scott et al., 1973).

Neonatal disease

After in utero infection with nonarbo togaviruses the clinical picture covers a broad spectrum of illness ranging from an apparently normal newborn to moribund individuals with extensive lesions or malformations. The affected newborn may die or recover from the disease. Neonatal mortal i ty is relatively high. Most of the prenatally-infected piglets succumb in the first days or weeks after birth, and BVD- or BD-affected offspring also have a

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low viability. The mortali ty figures of children with congenital rubella vary considerably. When severe disease occurs in combinat ion with malformations it can lead to a high rate of death in early infancy (Cooper et al., 1965; Forrest and Menser, 1975).

Since nonarbo togavirus infections are widely disseminated throughout the foetus, the symptoms in the neonate may involve virtually every organ, either transiently, progressively or permanently. Low birth weight is a fairly constant feature in congenital rubella, BVD and BD and it is frequently associated with a failure to thrive postnatally. Epidemiological studies show an abnormal incidence of long-term growth retardation in infants and adults, who have had congenital rubella (Menser et al., 1967a; Hardy, 1976). Lambs with BD are usually recognized in infected flocks by being small and s tocky, with a poor growth rate (Terlecki, 1977). Clinical manifestations of CNS disorders are common in the neonatal period. In BD, congenital t remor is one of the main characteristics, and is associated with dysmyelination. Af- fected lambs show a slight muscular t remor to rhythmic shaking movements; symptoms which may gradually subside in the course of the first months (Westbury et al., 1979; Terpstra, 1981). Congenital t remor has also frequently been noted after in utero HCV infection (Emerson and Delez, 1965; Done and Harding, 1969; Vannier et al., 1981). Calves with prenatally acquired BVD also can suffer from body tremor (Done et al., 1980; Binkhorst et al., 1983). Other nervous signs in pestivirus-infected animals include inability to rise and suck, ataxia and abnormal stance (Emerson and Delez, 1965; Terlecki, 1977; Done et al., 1980). The spectrum of neurological and neurosensory involvement in infants with congenital rubella is wide. These include lethargy, motor paresis, restlessness and developmental delay. Patho- logical changes in the CNS are vasculitis, leptomeningitis and encephalitis (Desmond et al., 1967; Rorke, !973). In addition to nervous signs and malformations numerous other symptoms of congenital rubella may be present in the newborn, notably thrombocytopenia purpura and hepatitis, frequently with jaundice. These neonatal lesions often are transient. Other neonatal manifestations of congenital rubella are listed in several articles (Dudgeon, 1975; Forrest and Menser, 1975; Charles, 1980).

In addition to the signs referred to above, BD-affected lambs frequently show an abnormally hairy birthcoat. Many lambs with BD die shortly after birth, but they can survive for more than 100 days (Gard et al., 1976). At necropsy there may be evidence of intercurrent disease, particularly pneumonia (Terlecki, 1977). Haemorrhages of skin and internal organs are the most pronounced lesions at necropsy in piglets which died in the neonatal period (Wrathall, 1975; Van Oirschot, 1979a; Meyer et al., 1981).

It is also possible that after prenatal nonarbo togavirus infections apparently healthy individuals may be born. Later such individuals either develop clinical signs or remain normal.

Late onset disease and long-term sequelae

An in utero infection may follow an asymptomatic course in the neo-

332

natal period, and clinical signs may not appear until the progeny are several months old. In the case of rubella, foetal infection after the first trimester of pregnancy is more subtle in its manifestations than infection during early pregnancy. Such children often appear normal at birth, but lesions continue to evolve and can give rise to disease months or years after delivery. Thus, rubella acquired in utero during the latter half of pregnancy produced active postnatal disease, including pneumonia and encephalitis, from 3 months of age (Marshall, 1973). Deafness, mental retardation, heart and blood vessel defects and other dysfunctions may not be recognized before the child enters school. Peckham (1972) described children with congenital rubella, who were apparently normal at 2 years of age, but were profoundly deaf by the age of 7 years. Another long-term effect is a poor intellectual performance. Children experience problems in perception, memory, sequencing and recall, which interferes with normal functioning. Such long,term sequelae may not be uncommon (Hardy, 1976). Congenital rubella has been im- plicated in the evolvement of endocrinopathies like diabetes mellitus (Menser

1976; Floret et al., 1980) and growth hormone deficiency (Preece et al., 1977). A, probably uncommon, late effect has been recognized as "progressive rubella panencephalitis". It is a slowly progressive and fatal CNS disorder of children in their second decade o f life that is characterized byh igh rubella ant ibody titres in the cerebrospinal fluid and the presence of virus in brain tissue (Townsend et al., 1975; Weil et al., 1975).

Swine exposed to HCV in utero, under natural conditions, looked healthy during the first months of life, but they invariably developed a runting-like syndrome, characterized by conjunctivitis, dermatitis, leucopenia and loco- mot ion disturbances and eventually died (Van Oirschot and Terpstra, 1977). This "late onset disease" could be reproduced experimentally, however, the infected piglets commenced to show symptoms 3 to 7 weeks after birth (Van Oirschot, 1979a; Meyer et al., 1981). These congenital, persistent HCV infections induced severe depletion of lymphocytes in the thymus and peripheral lymphoid organs and proliferation and reticuloendothelial tissue (Hermanns et al., 1981; Van der Molen ar, d VarrOirsc,hot, t981a) . I r rad- dition, vascular lesions, especially hydropic degeneration of endothelium, were a consistent finding in these pigs (Van der Molen and Van Oirschot, 1981b).

In contrast with congenital HC, prenatally-infected sheep and cattle that are born apparently healthy, may remain normal for years or even during their whole lifetime, in spite of the lifelong virus persistence in these animals (Coria and McClurkin, 1978; Westbury et al., 1979; Terpstra, 1981, Strayer e ta l : ,~983) : -

Conceptuses congenitally-infected with BDV (Terpstra, 1981), BVDV (Braun et al., 1973), RUV (Phillips et al., 1965; Hardy et al., 1969) or HCV (Van Oirschot, 1979a) can eliminate the virus infection during the prenatal or newborn period and consequently grow up healthy.

3 3 3

Infections of other host species

Besides their "natural" host the nonarbo togaviruses can also infect and subsequently cross the placenta of other animal species (Table II).

Some strains of BVDV are highly pathogenic for the sheep foetus. After experimental infection of ewes by the intramuscular or intravenous route BVDV is capable of causing early embryonic death, abortion, foetal mummification, stillbirth, or foetal malformations such as IUGR, dysmyelination, hydranencephaly and cerebellar hypoplasia. Lambs with signs typical for BD, or healthy lambs with neutralizing ant ibody in their pre-colostral serum, can occasionally be born (Ward, 1971; Snowdon et al., 1975; Parsonson et al., 1979; Barlow et al., 1980a; Terlecki et al., 1980). These BVDV infections caused a high rate of foetal mortal i ty in sheep.

T A B L E I I

C o n g e n i t a l n o n a r b o t o g a v i r u s i n f e c t i o n s o f s e ve r a l h o s t s p e c i e s

H o g B o v i n e v i ru s B o r d e r

c h o l e r a d i a r r h o e a d i s e a se

R u b e l l a

S w i n e + + ±b _

C a t t l e _a + ± _ S h e e p - + + - M a n - - - +

a N o t ( y e t ) r e c o r d e d . b F o e t a l d e f e c t s r e c o r d e d , b u t f o e t a l i n f e c t i o n n o t c o n f i r m e d .

There is circumstantial evidence that in utero infection of pigs with BVDV occurs naturally (Stewart et al., 1971; Fernelius et al., 1973a). After experimental infection by the oronasal route of 20 pregnant gilts, BVDV could be isolated from the foetuses of only one gilt. The foetuses showed nonsuppu- rative meningitis and chorioditis (Stewart et al., 1980). Thus, BVDV can cause congenital infections in swine, although it is probably not common.

Some observations indicate that BDV may infect calf, pig and goat foetuses. After intramuscular inoculation of 10 pregnant heifers at 50--51 days of gestation, 6 of the cows aborted and 3 showed evidence of foetal death. All foetuses were severely retarded in growth and 3 had cerebellar malformations (Gibbons et al., 1974). Nine out of 19 piglets born to 3 sows inoculated intramuscularly with BDV at 34 days of pregnancy exhibited reduced cerebellum weights in the terminal stages of gestation (Wrathall et al., 1978). After inoculation of pregnant goats with BDV a placentitis developed, foetal death and abort ion occurred, and some newborn kids showed neurological signs (Huck, 1973; Barlow et al., 1975). Regrettably, in these studies there is no conclusive evidence that BDV caused the congenital abnormalities in calves, pigs and goats, because no mention is made of at- tempts to confirm foetal infection.

334

The author is unaware of any reports regarding the inoculation of pregnant cattle or sheep with HCV, although HCV appears to propagate in calves, sheep and goats (Loan and Storm, 1968).

Thus under experimental conditions BVDV and BDV may cause congenital infections in "non-natural" hosts. It may be assumed that such infections occasionally occur in the field, since BVDV can be transmitted to sheep by contact with infected cattle (French et al., 1974) and BD-affected lambs can be sources of infection for cattle (Barlow et al., 1980b) and pigs (Terpstra, 1981).

Unfortunately, there is no experimental animal model suitable for study- ing congenital rubella. Experimental vertical transmission of RUV has been achieved in monkeys (Parkman et al., 1965), rats (Cotlier et al., 1968), ferrets (Rorke et al., 1968), rabbits (Kono et al., 1969) and mice (Matsuo, 1975). Occasionally, foetal defects and pathological changes, similar to those observed in humans, have been found in animal foetuses (Cotlier et al., 1968; Kono et al., 1969). However, none of these positive teratogenic animal studies has been confirmed by other investigators.

PATHOGENESIS

Route o f infection

When a pregnant animal becomes infected, the virus can reach the foetus in a number of ways: (1) it can spread haematogenously and cross the placenta, (2) the virus can enter via the genital mucosa and pass the amnion, (3) the gamete in the ovary can be infected.

In nonarbo togavirus infections the transplacental route is the most important. Cows bred by a persistently-infected bull, whose semen contained BVDV gave birth to uninfected normal calves (McClurkin et al., 1979). Con- versely, it was possible to produce BD-affected lambs by artificial insemination of ewes with infected semen and by natural mating with BD rams (Gardiner and Barlow, 1981}. In addition, BDV antigen has been demonstrated in the germinal cells of the ovaries (Gardiner, 1980), which provides opportunities for transovarian passage to the foetus. However, there is no evidence that these modes of infection do occur under natural conditions. After a nonarbo togavirus infection of a susceptible pregnant mammal the virus disseminates by way of the bloodstream to the placenta and subsequently to the foetus. This maternal infection usually runs a subclinical course. Immuni ty of the dam gained by prior infection Or vaccination pro- tects the foetus from becoming infected, but exceptions have been documented in HC (Stewart et al., 1972), BD (Barlow et al., 1980a) and rubella (Eilard and Strannegard, 1974).

Placental infection

Little is known on the course of infection in the placenta, which presents a physical barrier to the transmission of viruses.

335

The histological structure of the placenta varies in different mammals. In the human haemochorial placenta the maternal blood is in direct contact with the chorion epithelium and is separated from foetal blood by 3 tissue layers. In the epitheliochorial placenta of ruminants and swine 6 tissue layers separate maternal from foetal blood. In addition, there are differences in placental structure at various stages of gestation.

Theoretically, a virus can leak across, be carried across or grow across the placenta. BDV (French et al., 1974) and BVDV (Snowdon et al., 1975) replicate to high titres in placentomes of sheep and RUV (Alford et al., 1964) and HCV (Van Oirschot, 1980) are regularly isolated from placental tissue. Thus, these viruses appear to grow across the placental barrier. The transmission is of ten associated with the product ion of lesions in the placenta.

Histological work of TSndury (1975) on infected placentae and foetuses obtained by therapeutic interruptions of pregnancy after maternal rubella shows that necrosis occurs in the epithelial cells of the chorion and in the endothel ium of the chorionic vessels, the latter lesions being most promi- nent. Frequently, thrombi develop in the vascular areas, jDamaged endothelial cells are desquamated and consequently emboli are found in the branches of the umbilical vein. Emboli become fastened to a foetal vessel wall and induce foci of damage in the endothelium of arteries in various organs, resembling those in the chorionic vessels. These lesions are the first to be observed in the foetus. Thus, these observations suggest that RUV se- q u e n t i a t t y ~ f e c t s epithelium and endothelium of the chorion and spreads to the foe tus by way of infectecl emboii, consistihg o f necrotic vascular endothelial cells.

In cattle and sheep the mode of infection with pestiviruses is probably through the placentome, which comprises a foetal portion, the cotyledons, and a maternal portion, the caruncles. Pregnant ewes inoculated with BDV developed lesions, consisting of necrosis o f the placentomal septa, :within 10 days of exposure. The lesions either remained small and focal and showed a tendency to heal or extended to diffuse necrosis. The lesions apparently commenced in the maternal capillary walls with endothelial swelling and occlusion of the vessels; in a later stage the epithelium became involved. Ulti- mately, the cellular debris was discharged into the foeto--maternal space and ~ppare~tty ingested by the trophobl~st (Barlow, 1972). Af t e r experimental infection of sheep with strains of BVDV, thrombi in small b lood vessels or focal areas of haemorrhage and necrosis were seen at the junction of the foetal and maternal component of the placentome (Snowdon et al., 1975; Parsonson et al., 1979). These histological data suggest a passage of pestiviruses to foetal tissue within cellular debris. However, other studies demon- strate that_BDV and BVDV can replicate in the placentome of sheep and cattle, respectively, wi thout producing lesions and yet readily cross the placenta (Casaro et al., 1971; French et al., 1974). Thus, it seems that detritus-free virus also may pass the placental barrier and infect the foetus.

A congenital infection with HCV may result in the farrowing of a completely or a partially-infected litter. There is evidence that may indicate

336

that HCV passes the placenta at one or more sites and subsequently spreads from foetus to foetus (Van Oirschot, 1980) . Obviously, this does not ex- clude the possibility that all pig foetuses may also become infected simul- taneously.

A placental infection does not necessarily lead to a foetal infection, as indicated by a higher percentage of RUV (Alford et al., 1964), and BDV (French et al., 1974) isolations from placentae than from corresponding foetuses, The factors determining whether infection remains ccnfLucd to p ~ n ~ m l : ~ - 6 r - s p ~ d ~ t 6 ~ e f o ~ t ~ ar~ n~t well tmd~rsmod,(bu~ appear to be-related-to a certain.extent,-to gestationa! age~In B:~ mud BD thereis limited ~ c e t h a t : t h e : : ~ s = d o e s - n u t ~ v a ~ ; t h e ~ , , ~ , v . p ~ a ~ , ~ in the first month of pregnancy (Manktelow et a l , 1969; French et al., 1974; Whitmore et al., 1981). A possible explanation is that close contact between t rophoblast and well-developed maternal villi is required for vertical transmisaion_~ occux, and~ucha,~hridge£ ' ~eve!op.~ around . 3 O A a ~ ~ nancy (Barlow, 1972; Kendrick, 1976). Essentially, the mammalian placenta seems to be as p e ~ e a b I e to nonarbo t0gavirus~ during the firs~ :as during the last third of pregnancy (Kendrick, 1971; French et al., 1974; Van Oirschot, 1979a; Cradock-Watson et ~ . , !980) .

Foetal infection

Once the xdrus gainsaccess to the foetus-it,car~ spread sys temicat !y-byway of the bloodstream, or from cell to cell by continuity. The virus finds a favourab~ environment fo r replication in ~ e m etabol ica l~ ~_ctl.vefoet~_A ~!!s undergO~mg rap~d[mitoSiS ~nd [~ffe~f i t ia t iom ~ e [ u l t i ~ t ~ 5utcOni~ 'ofcon- g e n i * ~ ~ n s ...... -rml .... ~ ~.~ . . . . . . . . . . ~ . c l s - ~ . n u m b e ~ ~ , such ~ : ~ e d e v e I o p m e n ~ ~ e t ~ the genotype of the host: and' t he foetal ability tO repair damage, which is generally very high (Hicks, 1954; Willis, 1962). The age of the foetus deter-

tance for the extent of foetal damage. In general, when infection takes place before the onset of immune competence the virus is widely dissemmated in the foetus. RUV has been isolated from many organs (Monif et al., 1965; Rawts e t ate, t 965 ) and HGV ~ t i g e n (VanOi~sehot andT e rps t ~ a ,1977 )and BDV antigen (Terpstra, 1981) have been found to be widespread in foetuses. C0ngenjt~ virus infections ~ o u n d or a f a r t h e o n ~ t 0f immune c o m p e t ? n c e are more likely to be controlled or eliminated, due t o the development of a specific immune response by the foetus, In general, the risk of foetal damage is higher, the earlier infection occurs during pregnancy. However, a congenital infection, even in the first month, does not necessarily lead to foetal abnormalities.-Thi~ is-sho-wn, in the-ca~e-o~f ru~el la .by t~e~bi r th-of~pparent ly normal infants, who excrete virus, born to mothers with a history of rubella during early pregnancy (Rawls, 1968). In addition, Woods et al. (1966) detected foci of infected cells in tissues that were histologically normal. In the

337

case of pestiviruses, the incidence of foetal infection is clearly higher than the incidence of foetal malformations or lesions.

The specific properties of a virus strain have effects on the course of a foetal infection. Virus strains vary in their teratogenic potential. Pregnant sheep inoculated with BDV strain BP-77 gave birth to lambs with hypomyelinogenesis, whereas strain H-77 produced hydranencephaly and arthrogryposis in the same breed of sheep (Barlow et al., 1979). There are HCV strains that induce a high rate of congenital tremors in piglets infected in utero (Done and Harding, 1969; Vannier et al., 1981), whereas other strains do not (Van Oirschot, 1980). The NADL strain of BVDV did not produce severe abnormalities in foetal calves older than 100 days (Kendrlck, 1971; Braun et al,, 1973), as opposed to the Holmes strain, that gave rise to cerebellar hypoplasia and ocular lesions (Brown et al., 1973, 1974). Japanese strains of RUV seem to be less teratogenic than American strains (Kono et al., 1969). This diversity may perhaps be explained by differences in interferon production. Japanese strains induced high levels of interferon in human placental cultures, whereas strains derived from the U.S.A. induced low levels (Potter et al., 1973). The virulence of a virus strain may influence the outcome of a congenital infection. Piglets prenatally-infected with a moderate-virulent HCV strain were more severely affected and died earlier than piglets exposed in utero to a low-virulent HCV strain (Van Oir- schot, 1980). Attenuated RUV (Vaheri et al., 1972; Ebbins et al., 1973) and HCV vaccines (Bran et al., 1971; Tesmer et al., 1973) can also transplacentally infect the foetus, but the risk for the conceptuses is virtually negligible. That the dose of virus influences the outcome is illustrated by Richardson et al. (1976) in the case of BD. The progeny of ewes inoculated with a 100 X higher dose were more severely affected than the progeny of ewes that re- ceived the low dose.

There is evidence indicating that virus strain--host genotype interactions affect the character of disease in the offspring. While pregnant ewes of 2 breeds inoculated with the H-77 strain delivered typical BD lambs, ewes of 2 other breeds inoculated with the same virus, gave birth to lambs without signs characteristics of BD (Barlow et al., 1979). Congenital rubella has rarely been noted in Japan even after extensive epidemics (Kono et al., 1969). An explanation for this phenomenon may be that, genetically, the Japanese are less prone to congenital rubella. An increased incidence of HLA antigens A1 and A8 has been found in cases of congenital rubella, and these antigens are seldom encountered among Japanese (Honeyman et al., 1975). However, there is also suggestive evidence that Japanese strains are of low teratogenicity (Koni et al., 1969).

Teratogenesis

Nonarbo togaviruses can induce abortion. It may be the result of maternal disease or foetal damage, or a combination of both. The general impression is that it is principally the result of foetal infection, since maternal infections

338

are often asymptomatic. In addition, hyperthermia, as such, does not produce abortion in sows (Omtvedt et al., I 9 7 I ) . Moreover, virus ~ d lesions are found in aborted foetuses which are not autolyzed (Doll et al., I957; Gillespie et al., 1967; Meyer, 1978; Terpstra, 1981). The vascular lesions in the placenta, as has been reported for BDV (Barlow, 1972), BVDV (Snowdon et al., 1975) and rubella (TSndury and Smith, 1966; Ornoy et at., 1973) may contribute to foetal death and expulsion by, for instance, impairing the nutrit ion of the foetus. Also mummification and stillbirth are caused by foetal infection rather than by maternal illness.

For malformations to be produced, infection has to occur during periods of organogenesis, so that virus replication can interfere with growth, maturation and differentiation of foetal tissue. Foetal organs that are defined at the time of infection may escape gross anatomical damage. In addition, the virus must damage the foetus without killing it, thus it seems necessary that the virus is of sufficiently low virulence. The persistence of nonarbo togaviruses in the conceptuses is also an important factor in teratogenesis.

The most likely mechanisms for the teratogenic effects of nonarbo togaviruses are (1) an inhibition or distortion of normal cellular development, (2) cell destruction as a direct consequence of virus infection, and (3) the production of vascular lesions. It is surprising that the teratological mechanisms have been studied better in rubella than in the animal nonarbo togavirus infections. Most data on the pathogenesis of congenital rubella had to be based on studies of spontaneously or therapeutically-aborted foetuses.

Human foetal cells, infected in vitro with RUV, showed a marked depres- sion in mitotic activity (Plotkin et ah, 1965a). Carrier cultures, derived from tissues of infected foetuses, were found to have a reduced growth rate and appeared to have a shortened lifespan (Rawls and Melnick, 1966). In such carrier-cultures the virus does not cause obvious morphological changes. It is passed from parent to daughter cells during successive mitoses and this may give rise to clones of infected cells. In addition, RUV-infected cells appeared to produce a soluble growth-inhibitor, which might in part, account for the reduced rate of cell growth (Plotkin and Vaheri, 1967}. Children with congenital HC, BDV and BD. These viruses also persist in foetal tissues and basis for this is a subnormal number of cells in body organs (Naeye and Blanc, 1965). This defect is probably due to the inhibition of multiplication of RUV-infected cells. Since RUV is present in clones and cells in various organs (Woods et al., 1966) it seems probable that the slower rate of cell division of infected cells could also account for the development of specific organ defects. This is in keeping with changes of malformed organs that were interpreted as interrupted, retarded or disorganized maturation of tissue (Singer et al., 1967). Presumably, the above mechanism also plays a role in congenital HC, BDV and BD. These viruses also persist in foetal tissues and IUGR and selective organ stunting are frequently observed in these diseases. The pestiviruses are of low cytopathogenici ty in vitro and persistently-infected cultures are readily established (Mengeling et al., 1970; Terpstra,

339

1978). Such carrier cultures show no morphological alterations and infection is maintained by distribution of virus from mother to daughter cells (Mengeling et al., 1970). HCV-infected cells also have a slightly extended period of DNA synthesis (Pirtle and Mengeling, 1970). It is not known, whether the infection of thyroid and other internally-secreting organs, as shown in BD (Terpstra, 1978), contribute to the general growth retardation.

Another possible teratogenic mechanism is an alteration in the cell differentiation process. In congenital rubella the lens cataract may be partially due to a disturbed differentiation of lateral epithelial cells of the lens into lens fibre cells (TSndury and Smith, 1966). BDV seems to affect the differentiation of oligodendrocytes from their precursors, thus leading to reduced myelin formation (Barlow and Storey, 1977). The typical abnormally coarse birthcoat in BD lambs appears to be the result of abberant differentiation of primary follicles (Derbyshire and Barlow, 1976).

In the case of rubella, there is histological evidence that the cytolyt ic ac- tion of the virus is operative in the production of malformations. Necrosis and cellular damage, without inflammation and fibrosis, have been found in the heart, developing lens, inner ear, teeth and other tissues of foetuses obtained after therapeutic interruption of pregnancy (TSndury and Smith, 1966). It has been suggested that BVDV has an affinity for mitotically active cells and causes destruction of the external germinal layer and other layers of the cortex of the cerebellum of foetal calves (Brown et al., 1973). Casaro et al. {1971) considered necrosis in lung, brain and skin in calf foetuses less than 150 days of age as potential causes of teratological defects. The early phases of CNS abnormalities produced by BDV are also associated with necrosis of the germinal layers (Barlow, 1980). Hence, the cytolytic activity of nonarbo togaviruses for cells of particular susceptibility appears to play a role in teratogenesis.

Foetal defects may also be a consequence, in an indirect way, of vascular lesions. Vasculitis has been noted in congenital rubella (TSndury and Smith, 1966}, predominantly in the brain (Rorke and Spiro, 1967; Singer et al., 1967). In these studies, vasculitis was considered to be the initial lesion leading to necrosis of surrounding tissue. The congenital heart malformations may also be initiated by vascular lesions (Overall, 1972). Also in congenital HC (Emerson and Delez, 1965; Van der Molen and Van Oirschot, 1981b) and BVD (Brown et al., 1973) substantial vascular damage occurs. In this context , it is notewor thy that all nonarbo togaviruses of domestic animals and RUV show a predilection for the vascular endothelium. The vascular disturbance is probably initiated by degenerative changes of the endothelial cells. Damage of endothelial cells may lead to thrombus formation, that can detach and circulate as emboli, giving rise to generalized vasculitis. At the same time, the blood~lot t ing mechanism can be triggered, directly or in- directly, by the virus refection, which may result in disseminated intravascular coagulation. The brain is among the organs most frequently involved in the disseminated intravascular coagulation syndrome. This process, which

340

may become manifest by clinical signs such as haemorrhagic diathesis and thrombocytopenia p u r p u r a , occurs in congenital rubella (McKay and Margaretten, 1967). Haemorrhagic diathesis is also a common finding in intrauterine-infected piglets (Van Oirschot, 1979a; Hermanns et al., 1981). The thrombosis and vasculitis of small blood vessels can produce partial to complete occlusion of these vessels, which may result in organ hypoxia, in turn leading to organ hypoplasia or necrosis. Hypoxia has been shown to induce specific malformations in laboratory animals (Wilson, 1973). It has been suggested that the cerebellar hypoplasia in newborn pigs congenitally- infected with HCV results from hypoxia due to vasculitis (Emerson and Delez, 1965). The cerebellar hypoplasia in BVD appears to be due to loss of vascular integrity by vasculitis, leading to folial oedema that in turn causes the-severe cerebettardestruct ion (Brown et al~, t973~, tn ~xtbetta~ ";a~ cular lesions are associated with ischaemic damage and retardation in growth and maturation of the brain (Rorke, 1973). The brain malformations in BD appear not to originate from vascular lesions, though, occasionally capillary thromboses are seen within the lesions (Barlow, 1980).

Chromosomal abnormalities have been described in children with congenital rubella (Nusbacher et al., 1967; Ansari and Mason, 1977), and in cells infected with RUV in vitro (Plotkin et al., 1965a). Others have failed to find chromosomal damage (Mellman et al., 1965). Also in the case of HC there are contradictory findings on the induction of cytogenetic alterations in cells infected in vitro (Pirtle and Woods, 1968; Mengeling et al., 1970). Theoreti- cally, chromosomal damage in differentiating cells may cause malformations. However, there is no convincing evidence at hand for virus-induced chromosomal changes as a mechanism of teratogenesis.

It remains speculative whether maternal disease affects foetal development. The changes in the growth-zone of the long bones may reflect a reduced overall growth due to severe disturbances in placental function. It is unlikely, that maternal fever contributes to foetal malformations in the pig, since exposure of pregnant sows to high environmental temperatures appears to be without effect on foetuses (Omtvedt et al., 1971). Conversely, hyperthermia has teratogenic effects in other animals, e.g., sheep (Berry et al., 1977).

Lesions, instead of true malformations, are produced in already fully- formed foetal organs according to principles that also apply to animals after birth, except that repair and regeneration are more important and immunological and inflammatory responses are less important in foetal life.

Virus persistence

In general, the nonarbo togaviruses persist in the foetus after congenital infection, a condition which usually continues into postnatal life. In HC, BD and BVD, the virus persistence can be lifelong. In congenital rubella, however, the virus usually disappears within 1 to 2 years after birth. The

342

Fig. 2. Demonstration of BDV antigen by immunofluorescence in the cytoplasm of epi- theUal cells. Cryostat sections of thyroid ( top )and rectum (bo t tom)f rom a 5-year-old sheep with congenital persistent BD (courtesy Drs. C. Terpstra and J. Vreeswijk).

343

TABLE III

Factors o f (poss ib le ) importance in virus pers is tence in congeni ta l nonarho togav.iruses in fec t ions

Hog Bovine virus Border Rubella cholera diarrhoea disease

Foeta l age at t ime o f in fec t ion + + Low or no viral cytopathology + + Cell to cell spread + ? Little or no viral antigen in

cell membrane + ? Infec t ion of lymphoid tissue + + I m m u n e to lerance + + Defe c t s in i m m u n e system + ? Inadequate interferon response ? -

+ +

+ +

? +

?

+

? +

tively low or absent in foetuses exposed during the last tr imester (Casaro et al., 1971). In general, infants infected during the first third of pregnancy are more likely to shed virus for longer periods (Phillips et al., 1965; Cooper e t al., 1969) than children exposed during later stages (Hardy et al., 1969).

Several virus-specific factors have been proposed to be of importance in the initiation and maintenance of virus persistence. There is evidence that temperature-sensitive mutants (Williams et al., 1 9 8 1 ) a n d defective inter- fering particles are involved in the establishment of RUV persistence in cell culture (Norval, 1979). Cells infected with HCV appear to bear no, or very few, viral antigens on their surface, thereby allowing infected ceils to escape immunological attack (Van Oirschot, 1980). Most viruses that can produce persistent infections cause minimal cy topa thology (Mims, 1974) and this applies also to the nonarbo togaviruses. Many, if not all viruses that persist, infect lymphocytes or macrophages (Mims, 1974), and in this way elimina- t ion of virus by macrophages may be prevented. RUV (Simons and Jack, 1968) and HCV (Van Oirschot, 1979b) have been isolated from mononuclear cells of the peripheral blood from persistently-infected infants and pigs, respectively. Cultures of mononuclear cells support the growth of RUV (Van der Logt et al., 1980}, BVDV (Truit t and Shechmeister, 1973; Atluru et al., 1979) and HCV (Korn and Lorenz, 1976; Van Oirschot, 1980). BDV has been isolated from thymus, spleen and lymph nodes (Terpstra, 1981). It is obvious that viruses which infect cells of the immune system may be able to induce defects in immunological functions. In fact, persistent nonarbo togavirus infections are associated with specific and more generalized dysfunctions of the immune system. Such defects in immune response probably play an impor tant role in the establishment of nonarbo togavirus persistence.

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IMMUNOLOGY

Effects on the immune system

Since the nonarbo togaviruses replicate in cells of the immune system, it is conceivable that they induce defects in the host defence mechanism, which in turn may favour virus persistence.

Congenital infections with RUV (Singer et al., 1969), BVDV (Braun et al., 1973) and HCV (Von Benten et al., 1980) may lead to precocious maturation of lymphoid tissue, i.e., germinal centre formation and plasma cell development, and this correlates well with elevations in serum levels of IgM and IgG. In infants with congenital rubella the IgM concentration may remain above normal for the first 6 months of life (Cradock-Watson et al., 1976). Lambs, prenatally-infected with BVD have augmented IgG levels (Osburn et al., 1973). In congenital HC (Richter-l~eichhelm et al., 1980) and BVD (Braun et al., 1973; Brown et al., 1979) the elevated Ig levels have no or low specificity for the causative virus. These immunoglobulins may be directed to altered host tissue antigens {Brown et al., 1979) or they may be incomplete molecules of smaller size and low avidity (Richter-Reichhelm et al., 1980). In congenital rubella not all the extra IgM seems to be specific for RUV (Cradock-Watson et at., 1976),

There is evidence of lymphoid tissue damage in congenital infections with nonarbo togaviruses. Singer et al. (1969) observed a slight thymic involution in children with congenital rubella, who died in the first months after parturition. Lambs with BD have a significantly smaller thymus at birth than normal lambs (Richardson et al., 1976; Sweasey et al., 1979). Necrosis of lymphoid tissue has been observed in congenitally-infected piglets, shortly after birth (Brack, 1971). In utero-infected pigs that survive for months after birth show severe morphological changes in lymphoid tissue. The most prom- inent lesions are an almost complete thymus atrophy, depletion and destruction of lymphocytes , histiocyte hyperplasia with phagocytosis of lymphocyt ic debris and proliferation of reticulo~endothelial tissue (Hermanns, 1978; Van der Molen and Van Oirschot, 1981a). It is of interest to note that such pigs show a marked adrenal hyperptasia, and this may indicate an enhanced production of glucocorticosteroids. The adronncor~ical hormones might~ at i e ~ t partially, be responsible for the severe lymphocyte destruction in these persistently-infected pigs (Van der Molen and Van Oirschot, 1981a). In children with congenital rubella histiocytosis has also been observed (Claman et al., 1970).

Various abnormalities in humoral immunity can occur in congenital rubella. The most common aberration observed is a dysgammaglobulinaemia, with high IgM, low IgG, and low or undetectable IgA levels (Soothill et al., 1966; Singer et al., 1969; Stern and Forbes, 1975). The abnormality may be corrected as the child grows older, or the IgA deficiency may persist (South et al., 1975; Stern and Forbes, 1975). Transplacentally-infected piglets that

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survive after birth appear to have a depressed IgG synthesis concomitant with an enhanced IgM production (Hermanns, 1978). In view of the thymic atrophy found in HC as well as in rubella, the abnormal IgG:IgM ratio may be caused by a disturbance in the switch from IgM to IgG, that appears to be under some degree of thymus or T cell control (Roitt , 1980).

The antibody response to antigens unrelated to the causative virus does not seem to be decreased in animals infected in utero with nonarbo togaviruses (Van Oirschot, 1977; Coria and McClurkin, 1978; Terpstra, 1981). except in the terminal stages of congenital HC (Van Oirschot, 1979b). In infants with congenital rubella the antibody response to unrelated antigens appears to depend on the state of illness. Children with active disease are deficient in antibody production when stimulated with specific antigens (Michaels, 1969, 1972), but children, who no longer excrete virus, respond normally (White et al., 1968; Michaels, 1972).

There are conflicting reports regarding responsiveness of peripheral blood lymphocytes (PBL) from infants, which acquired rubella in utero, to phytohaemagglutinin (PHA). Some authors found a hypo- or unresponsiveness of PBL to PHA (Montgomery et al., 1967; Olson et al., 1968), whereas others have described a normal lymphocyte proliferation to PHA (White et al., 1968; Marshall et al., 1970). In a more recent paper (Buimovici-Klein et al., 1979) the PBL response to PHA was correlated with the foetal age at the time of infection. Children, aged 1--13 years, who had been infected with RUV in the first 2 months of gestation, and having clinical manifestations, showed a more severely depressed PHA response than children exposed after the third month of gestation. In another study, the cutaneous response of RUV-infected children to vaccination with vaccinia virus did not appear to be impaired, but there was a diminished in vitro response of PBL to vaccinia virus (White et al., 1968). Thus, in a number of children with congenital rubeUa a hyporesponsiveness of the immune system can be demonstrated, the severity probably depending on the foetal age at the time of infection and on the state of illness of the infant. In fact, most children may pass through a state of immune deficiency, from which they may recover, others may manifest permanent dysfunctions. However, these immune deficiencies rarely become clinically apparent (Forrest and Menser, 1975). PBL from pigs with congenital HC respond normally to PHA and pokeweed mitogen, except a few days before death (Van Oirschot, 1977, 1979b). No information is at hand on the effect of congenital BVDV or BDV infections on cell-medi- ated immune (CMI) functions. There are too few data available to draw con- clusions on immune func t ion in congenital pestivirus infections, but the re- suits obtained suggest that, apart from the specific immune tolerance, there is a normal immune responsiveness.

The mechanism involved in the immune dysfunct ion in congenital rubella is poorly understood. The defect is often considered to be a direct teratogenic effect of RUV on the not fully differentiated immune apparatus, al-

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though the presence of virus in PBL does not appear to be required for impairment of the immune response (Buimovici-Klein et al., 1979). The immune dysfunction may also be the result of an inhibition of macrophage function by RUV (Van der Logt et al., 1980) or of antigenic competi t ion (Chandra and Soothill, 1971; Gershon et al., 1974).

Another factor of possible importance in viral persistence is the interferon response. Desmyter et al. (1967) reported a defect in the interferon-producing capacity of cells derived from a t~UV-infected human foetus. More- over, cell cultures persistently infected with RUV are not sensitive to interferon, whereas acutely infected cells are. In addition, no interferon could be detectectin- serum or urine -of infants with congenital ~-abella,~y~-en i n those who were actively excreting virus (Desmyter et al., 1967; Rawls, 1967). Thus, an inadequate interferon response might be involved in persistence of RUV. In contrast, foetal calves infected in utero with BVDV at Day 95 or Days 149--150 of pregnancy produce interferon to levels comparable to those in adult bovines (Rinaldo et al., 1976), indicating that a defective interferon response is not a factor involved in BVDV persistence.

Immune mechanisms, among which is macrophage function, that normally terminate infection in postnatal life are absent or not fully developed in foetal life. In addition, the developing immune system appears to be highly susceptible to virus-induced defects. It is likely that these are the most important factors involved in the establishment of virus persistence in congenital nonarbo togavirus infections.

I m m u n e response to congenital infection

In man, maternal antibody of the IgG class is transmitted through the haemochorial placenta after the first trimester of pregnancy. In contrast, the epitheliochorial placenta of domestic animals is not permeable for maternal immunoglobulins during the whole period of pregnancy (Brambell, 1970). Instead, maternal antibodies are transferred to the neonate in the colostrum of the dam. Thus, antibodies present in the newborn animal before sucking have been formed by the foetus itself.

Man and domestic animals gain immunocompetence during foetal life. The onset of immune reactivity in man, calf, lamb and pig is around mid-gestation (Solomon, 1971; Schultz, 1973; Bourne et al., 1974; Fahey and Morris, 1978), indicating that, basically, foetuses are capable of producing ant ibody to nonarbo togaviruses. The human foetus commonly responds to a RUV infection with the production of specific IgM antibody, irrespective of the time of prenatal infection (Cradock-Watson et al., 1980). This antibody synthesis may continue for 6 months after birth, a period which seems to correlate with the period of virus shedding, which suggests that IgM antibody production results from viral persistence. IgG ant ibody titres to RUV increase during the first 3 months of life, and this response may have started during the last weeks of pregnancy (Cradock-Watson et al., 1976).

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In BD, BVD and HC the age of the foetus at the time of infection appears decisive as to whether or not a specific ant ibody response will be elicited. A foetal infection before the onset of immune competence does not usually result in the production of specific antibody, whereas an infection during the late stages of gestation can lead to ant ibody formation. Although domestic animals can respond immunologically around mid-gestation BVD neutralizing ant ibody cannot be detected until about 200 days of gestation (Braun et al., 1973; Brown et al., 1979). Porcine foetuses may only produce neutralizing ant ibody to HCV, when infected after the 90th day of pregnancy (Van Oirschot, 1979b; Frey et al., 1980). It is not clear, why in BVDV and HCV, such a long period elapses between the onset of immune reactivity and appearance of viral specific antibody.

A foetal immune response to pestiviruses is usually followed by virus elimination from the host~ Such non-persistent infections become evident by the birth of animals with specific virus-neutralizing ant ibody in the pre- colostral serum and from which no virus can be recovered (Braun et al., 1973; Frey et al., 1980; Terpstra, 1981). In addition, the offspring of sows infected with HCV may be born viraemic, but shortly after birth the virus is no longer detectable {Van Oirschot, 1979a). In this case, an ongoing immune response in conjunction with the ingested colostral ant ibody may effect the virus disappearance. The mechanism, by which the virus infection is termi- nated before parturition is not completely understood, but it is likely that the foetal immune response plays the most important role in curing the infection in domestic animals. In congenital rubella, however, the passively- acquired maternal virus-specific IgG ant ibody and the foetal IgM ant ibody often fail to eliminate the infection. This results in RUV persistence despite the presence of circulating ant ibody (Alford et al., 1965; Cooper et al., 1965). The virus is passed from parent to daughter cell thus enabling the virus to persist in the presence of neutralizing antibody. The ant ibody may only serve to limit further dissemination of the virus via the circulation. Yet, virtually all infants with congenital rubella, ultimately, eliminate the virus.

It is a well documented phenomenon that exposure to antigens during early foetal life can lead to immune tolerance to that particular antigen. It was therefore surprising to find that foetuses exposed to RUV in early pregnancy are able to mount a specific ant ibody response to the virus. In contrast, domestic animals often do not respond with ant ibody synthesis after infection with nonarbo togaviruses during early or mid-gestational life (Gard et al., 1976; Van Oirschot, 1977; Coria and McClurkin, 1978; Terpstra, 1981). One of several plausible explanations for this discrepancy might be the presence of maternal virus-specific IgG in the RUV-infected conceptuses, which limits the spread of virus and consequently facilitates the production of antibody by the foetus. Since there is no passage of maternal immunoglobulins in swine, cattle or sheep, virus can disseminate unlimitedly in foetal tissues, resulting in an excessive load of viral antigen, during the development of

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the immune system, which may lead to the induction of immunological tolerance. In congenital pestivirus infections immunological tolerance appears neither absolute nor permanent. A few persistently-infected pigs developed low levels of precipitating antibody or antibody detectable by the indirect immunofluorescence test (Van Oirschot, 1977; Van Oirschot, 1979b). Persistently-infected sheep can produce a transient or persistent level of neutralizing antibody, postnatally (Barlow et al., 1980a; Terpstra, 1981).

Immune tolerance, generally, can be the result of deletion, destruction, in- activation or dysfunct ion_of specific antigen-~eac32~e ~mphQcytes. It can re- side at the level of B or T cells or both, or it may be a disturbance of the im- munoregulation mechanism. With regard to nonarbo togaviruses, it has been s h ~ w ~ h a ~ lymph~cytes f rom caive~chronicaily-infec~ed wRh BVDV had a decreased ability to secrete immunogtobutins (Muscoptat et al., 1973). In addition, BVDV appears to depress in vitro plasma cell development and subsequent synthesis of IgG and IgM after pokeweed mitogen stimulation (Atluru et al., 1979). A possible explanation for the immunological tolerance is that pestiviruses may act by infecting pestivirus-specific lymphocytes and in that way interfere with the production of specific antibody. The obser- vation that immune tolerance is neither absolute nor permanent suggests that the unresponsiveness is not due to a complete deletion or destruction o l ives- reac t ive lympho cytes.

It is not known whether tolerance of the CMI response exists in congenital nonarbo togavirus infections. A few reports have dealt with the CMI response to RUV antigens. Simons and Fitzgerald (1968) found that lymphocytes from infants with congenital rubella showed blastogenesis in the presence of viral antigens. In another report (Buimovici-Klein et al., 1979) the lymphocyte response to RUV was found to be absent or relatively low in children with congenital rubella, when examined years after birth. Immune interferon production by stimulated lymphocytes showed a similar pattern. In addition, there was no detectable production of leucocyte migration inhibition factor by RUV-stimulated lymphocytes. These authors observed a more severe impairment of the CMI in children infected in the first 2 months than in the latter stages of pregnancy. Fuccfllo et al. (1974) described that lymphocytes of infants with congenital rubella failed to lyse RUV-infected non-autologous target cells. The above data suggest that a hypo- or unresponsiveness of CMI might exist in some children with congenital rubella. Although some work has been done with respect to HC (Van Oirschot, 1979b), it is not known whether animals congenitally-infected with pestiviruses mount a CMI response to the causative virus.

The specific unresponsiveness of the humoral immune system in swine, sheep and cattle after congenital pestivirus infection appears to be only partially responsible for the lifelong virus persistence. In vitro experiments have shown that HCV can spread from cell to cell through intercellular bridges (Pirtle, 1969b), and from mother to daughter cell in the presence of HC ira-

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mune serum (Pirtle, 1969a). Ingestion of high-titre colostral ant ibody only transiently suppressed viraemia in pigs persistently-infected with HCV (Van Oirschot, 1979a). A lamb persistently-infected with BDV developed neutralizing ant ibody at the age of 14 months, but the virus was still widely disseminated throughout the tissues when it was killed 7 months later (Terpstra, 1981). In congenital rubella the virus persists despite the presence of neutralizing ant ibody (Cooper et al., 1965) and RUV carrier-cultures could not be cured by rubella ant ibody (Rawls and Melnick, 1966). Thus, neutralizing ant ibody by itself appears to be incapable of curing the virus infection. This is in keeping with the concept of Notkins (1974) that CMI plays a major role in terminating virus infections that spread from cell to cell. Unlike the pestiviruses, RUV ultimately disappears in children with congenital rubella in spite of the inability of ant ibody to eliminate the infection and the possible existence of tolerance of the CMI system. A plausible explanation for the eventual cessation of RUV persistence is the clonal hypothesis (Rawls, 1974). In vitro, RUV-infected cells have a reduced growth rate and a limited doubling potential. If a similar virus--host relationship exists in vivo, clones of infected cells gradually die out, leaving the infant free of virus. Thus, according to this concept, the duration of virus persistence is largely determined by the lifespan of the infected cell.

The discrepancy between congenital RUV and HCV infection with regard to immune response and virus persistence may theoretically be explained by a difference in maturation of these viruses. HCV appears to mature at intra- cytoplasmatic membranes (Scherrer et al., 1970; Rutili and Titoli, 1977) and induces few or no viral antigens in the cell membrane (Van Oirschot, 1980). Thus, a cell infected with HCV may not elicit a specific immune response and in addition may not be recognized by the immune system, a condition which obviously favours virus persistence. On the other hand, RUV develops by budding from plasma membranes, which results in antigenic changes of the cell's surface (Oshiro et al., 1969). The expression of new antigens in the cell's surface triggers a predominant CMI response, which in turn may contribute to the termination of RUV infection.

CONCLUDING REMARKS

Viruses, which produce congenital infections, belong to many families. In this respect the nonarbo togaviruses may be considered to be a unique group, since they all are able to cross the placenta, notwithstanding the diversity in natural host species and the consequent variation in placental structure. It is likely that one unifying mechanism underlies this capability. One pronounced biological feature the nonarbo togaviruses have in common is their predilection for vascular endothelial cells. Viral replication in these cells results in vascular lesions. Apparently, the placental endothelium is highly susceptible for these viruses, and this characteristic might well be responsible for the common ability of the nonarbo togaviruses to grow through the placenta.

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Foeta l infec t ion with nona rbo togaviruses can resul t in a wide spec t rum o f congeni ta l abnormal i t ies , ranging f rom prenata l dea th to inapparen t infec t ion which persists for life. The nona rbo togaviruses, with the ex cep t i o n o f LDV, can all cause mild to severe deve lopmen ta l defects , which is remark- able since only a relat ively small p r o p o r t i o n o f viruses are proven tera togens. The te ra togenic nona rbo togaviruses usually are of low virulence and have low to zero cy topa thogen i c i t y . These features al low the foe tus to survive a n d consequen t ly the virus ~o persist and disseminate widely, which enables it to p roduce its de t r imenta l e f fec ts on the foetus . The mechanisms m o s t l ikely to be opera t ing in teratogenesis are an inhibi t ion, or d i s to r t ion , of normal cellular deve lopment , and the induc t ion o f vascular lesions, ini t ia ted by endothel ia l degenera t ion .

The age of the foe tus at the t ime o f exposure to the virus largely deter- mines the course and eventual o u t c o m e of infect ion. Virus pers is tence usually fol lows a foetal in fec t ion with nona rbo togaviruses be fore or a round the onset o f i m m u n o c o m p e t e n c e . In congeni ta l rubel la the virus persists in spite o f the p r o d u c t i o n o f specific an t i body by the h u m a n foetus . The in u te ro infec ted foetuses o f domes t i c animals f r equen t ly are i m m u n o t o l e r a n t to the causative pestivirus. Virus pers is tence in congeni ta l pestivirus infec- t ions can be lifelong, bu t in congeni ta l rubel la it eventual ly ceases. Despite the widespread and con t inued presence o f virus in congeni ta l ly in fec ted catt le , sheep and swine they may appear hea l thy at bir th and remain clinically normal for m o n t h s or years. These animals shed virus co n t i n u o u s ly and act epizoot ica l ly like " t h e Tro jan ho r se " tha t surrept i t ious ly in t roduces an a rmy of pestiviruses into animal popula t ions . Such a vi rus--host in te rac t ion is vir tual ly never encoun t e r ed in any o the r viral infect ions o f man or animals.

F u r the r studies on the cellular level as well as the level of the macroor - ganism should con t inue to yield in fo rma t ion useful for unders tand ing the, in some respects un ique , congeni ta l nona rbo togavirus infect ions .

ACKNOWLEDGEMENTS

The a u tho r thanks Drs. J.G. van Bekkum, A.C. Hekker , C. Terps t ra and D. van Zaane for careful ly reading the manuscr ip t .

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Congenital infections with nonarbo togaviruses

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