Trop. Anita. Hlth Prod. (1991) 23, 29-38

DIFFERENCES IN S E N S m V I T Y OF K E N Y A N TRYPANOSOMA VIVAX POPULATIONS TO THE

PROPHYLACTIC A N D THERAPEUTIC ACTIONS OF ISOMETAMIDIUM CHLORIDE IN B O R A N CATrLE

A. S. PEREGRINE, S. K. MOLOO and D. D. WHITELAW t

International Laboratory for Research on Animal Diaeases (ILRAD), PO Box 30709, Nairobi, Kenya

SUMMARY

Isometamidium chloride was administered as a single prophylactic dose of 0.5mg kg -I body weight to each of 10 Boran (Bos indicus) steers. A t monthly interoals following drug administration, groups o f five cattle each were challenged with one of two different Trypanosoma vivax populations trans- mitted by infected Glossina morsitans centralis; one with a stock (IL 2982) from Galana, Kenya and the other with a stock (IL 2986)from Likoni, Kenya. Prophylaxis was afforded for less than one month against the Galana T. vivax and for one month against the Likoni T. vivax.

In a therapeutic study a further 10 Boran steers were similarly infected with either of the T. vivax populations; five steers per population. Eleven days after infection all animals were treated with 0.5mg kg- I isometamidium chloride and all were cured.

These findings demonstrate that, as defined in the field, the two Kenyan T. vivax populations express a high level of resistance to the prophylactic action of isometamidium yet a low level of resistance to the therapeutic action of the drug. The results also indicate that differences in drug resistance between different isolates play a major role in determining the apparent period of prophylaxis afforded by isometamidium chloride.

INTRODUCTION

Isometamidium chloride (Samorin; RMB Animal Health Ltd) (Berg, 1960) has been marketed since 1961 as a prophylactic and therapeutic drug against trypanosomiasis of domestic livestock. Although the drug has been used in the field for over 25 years, it is only recently that an extensive survey has been carried out to determine the economic benefit of using a regular isometamidium- treatment programme. Cattle maintained under isometamidium prophylaxis on the Mkwaja ranch, Tanzania, where cattle cannot survive without the use of trypanocides, were 80% as productive as Boran cattle maintained on trypanosomiasis-free ranches in Kenya (Trail, Murray, Sones, Jibbo, Durkin and Light, 1985).

The prophylactic period that isometamidium confers to cattle appears to vary widely; a dosage of 1 mgkg -~ body weight isometamidium chloride has been shown to confer protection for two to 22 weeks (Kirkby, 1964; Pinder and Authie, 1984; Whitelaw, Bell, Holmes, Moloo, Hirumi, Urquhart and Murray, 1986). This variation in prophylaxis could be due to a number of factors: vector density or level of trypanosomiasis risk (Davey, 1957; Whiteside, 1962), relapses

I Present address: Department of Veterinary Parasitology, University of Glasgow Veterinary School, Glasgow G61 1QH, UK. ILRAD Publication No. 784.

29

30 PEREORINE, MOLOO AND WHITELAW

from tissue sites inaccessible to the drug (Jennings, Whitelaw and Urquhart, 1977), acquisition of immunity following the use of trypanocides (Fiennes, 1953; Wilson, Le Roux, Paris, Davidson and Gray, 1975; Wilson, Paris, Luckins, Dar and Gray, 1976; Bourn and Scott, 1978), dosage of drug (Boyt, Lovemore, Pilson and Smith, 1962), infection at the time of treatment (Davey, 1957) and differing levels of drug sensitivity between trypanosome populations.

Experiments with cloned populations of Trypanosoma congolense in cattle have shown that the duration of chemoprophylaxis is a function of the dosage of drug used and not of the intensity of metacyclic challenge or the presence of an existing parasitaemia. Furthermore, acquisition of immunity to metacyclic variable antigenic types of T. congolense, whilst under chemoprophylactic cover, appeared unlikely to play any significant protective role in cattle maintained by an isometamidium prophylactic regime (Peregrine, Ogunyemi,Whitelaw, Holmes, Moloo, Hirumi, Urquhart and Murray, 1988).

Since T. vivax is also a major pathogen of domestic livestock, we have determined if variation in sensitivity to isometamidium, between different populations of T. vivax, is a major factor influencing the duration of prophylaxis conferred by this drug. Preliminary drug-sensitivity studies on a clone Of T. vivax from Nigeria and a T. vivax clone from Kenya have been described by Peregrine, Moloo and Whitelaw (1987). These observations have been extended in the present study to two other Kenyan T. vivax populations. In addition we have determined if antigenic priming of the immune system occurs in cattle repeatedly bitten by T. vivax-infected tsetse whilst under isometamidium prophylaxis.

MATERIALS AND METHODS

Yearling Boran steers were obtained from the Athi River basin of Kenya, an area not inhabited by tsetse. Prior to experimentation all cattle were treated with 5 mgkg -1 fenbendazole (Panacur, Hoechst). Antibodies to T. congolense, T. vivax or 7". brucei were not detected in any of the steers using an enzyme-linked immunosorbent assay (Voller, Bidwell and Bartlett, 1975) or by an im- munofluorescent antibody test (Katende, Musoke, Nantulya and Goddeeris, 1987). Every four months all steers were inoculated with an inactivated quadrivalent (O/A/SAT1/SAT2) foot-and-mouth disease vaccine (Wellcome Kenya Ltd, Cooper). The housing and management of steers has been previously described (Whitelaw et al., 1986). At the beginning of each experiment all steers weighed approximately 200 kg.

Adult male castrated goats (crossbreeds between East African Masai and GaUa) were obtained from the Naivasha/Nakuru re#on of the Kenyan Rift Valley, an area known to be free from trypanosomiasis. On arrival at ILRAD goats were treated with 7-5 mgkg -1 rafoxanide and 44mg kg -1 thiabendazole (Ranizole Drench, MSD AGVET) for helminths and nasal bots, and 50mg powder of amprolium (Amprol 20%, MSD AGVET) daily for three days for coccidiosis. Goats were maintained as described by Whitelaw et aL (1986).

Guinea-pigs were multicoloured, English type, and were obtained from the ILRAD breeding colony.

T. vivax IL 2968 was a trebly-cloned derivative (Peregrine et aL, 1987) of an isolate made from a naturally infected cow in Zaria, Nigeria and was infective for rodents (Leeflang, Buys and Blotkamp, 1976).

SENSITIVITY OF T. VIVAX TO ISOMETAMIDIUM CHLORIDE 31

T. vivax IL 2969 was derived from T. vivax IL 2710 by one passage in a cow. T. vivax IL 2710 is an in vitro-derived clone (Hirumi and Hirumi, 1986) of a stock originally isolated from a cow in Kilifi, situated 70 to 80 km north of Mombasa, Coast Province, Kenya. It is non-infective for rodents.

T. vivax IL 2982 is a population cryopreserved after one bovine passage of stock IL 2337. IL 2337 is an uncloned derivative of an isolate made from a cow on a ranch in Galana, Coast Province (Nantulya, Musoke and Moloo, 1986), and is non-infective for rodents. The ranch on which the isolate was made is situated about 100 km north of Mombasa and 60 km west of Malindi.

T. vivax IL 2986 is a stock derived by three bovine passages of an isolate (KETRI 2375) from a naturally infected cow in Likoni, Coast Province (Wellde, Chumo, Adoyo, Kovatch, Mwongela and Opiyo, 1983). It is uncloned and is non-infective for rodents. Likoni is situated approximately 12km south of Mombasa, in the Coast Province, Kenya.

Glossina morsitans centralis obtained from the ILRAD-breeding colony were infected with one of the four trypanosome populations and used to challenge cattle. Flies were infected by allowing them to feed on infected goats or Boran calves; goats were used for the Nigerian T. vivax (IL 2968) whilst Boran calves were used for the three Kenyan T. vivax populations (IL 2969, IL 2982 and IL 2986) as these three populations give rise to only transient parasitaemia in goats. Tsetse flies were maintained by feeding them daily (except weekends) on infected animals. Identification of infected tsetse by examination of salivary probes (Burtt, 1946) proved efficient for flies infected with the Nigerian T. vivax but not for those infected with the Kenyan T. vivax populations; only a small number of flies infected with these three populations extruded metacyclic trypanosomes in their salivary probes. Thus for challenge of cattle with each of the three Kenyan T. vivax populations each fly that fed on an animal was dissected. Each uninfected fly was replaced with another until the required number of infected flies had fed on each animal. Each steer was challenged by feeding 10 infected tsetse on the caudal flank of each animal.

Jugular blood samples were collected three times weekly in 10 ml ethylenedi- amine tetra-acetic acid (EDTA)-treated vacutainer tubes for determination of the packed red cell volume percent (PCV) and the level of parasitaemia. The level of parasitaemia was determined using the blood buffy-coat phase-contrast technique described by Murray, Murray and Mclntyre (1977). A scoring system described by ParAs, Murray and McOdimba (1982) was used for correlating the number of trypanosomes counted to the approximate level of parasitaemia in the blood sample.

Sera were collected weekly and stored at -70~ until required for serological investigations.

Isometamidium chloride (Lot No. DM 4532) was used throughout the study. The drug was freshly prepared in sterile distilled water. For cattle a 2% w/v solution was used for dosage of 0-5 mg kg - t as recommended by the manufac- turer, and administered as a deep-intramuscular injection in the middle third of the side of the neck using an 18 gauge 1�89 inch needle. Prior to administration, the injection site area was washed thoroughly with 70% ethanol. Care was taken that none of the drug was deposited subcutaneously. The body weight of each steer was determined accurately prior to administration of the drug.

Serum was heat-inactivated at 56~ for 30 min and assayed for the presence of anti-trypanosomal antibodies using an immunofluorescent antibody test

32 PEREGRINE, MOLO0 AND WHITELAW

(IFAT) and a trypanolysis test. Sera were examined for the presence of antibodies to trypanosomes collected from the first parasitaemic peak of cattle originally bitten by tsetse infected with homologous trypanosomes. Preinfection sera were used as negative controls.

T. vivax bloodstream forms for use as antigen in the IFAT were separated from bovine blood as follows: 40 ml of RPMI 1640 medium containing 20 mM HEPES buffer, 10ram NaHCO3 and 1% (v/v) L-Glutamine (29-2mgml -~ medium RPMI 1640) were placed in a 50 ml plastic centrifuge tube. To this was added 10 ml of blood from an infected steer, collected in a heparinised vacutainer tube. After centrifugation at 330 x g for 15 min, 45 ml of medium was removed and centrifuged at 1,300 x g for 15 minutes. The resultant pellet was resuspended in 1 to 2 ml of phosphate saline glucose (PSG), pH 8.0, and the trypanosomes separated from other blood elements on a diethylaminoethyl (DEAE) cellulose- 52 column according to the method described by Lanham and Godfrey (1970). The eluate was collected and centrifuged at 380 x g for 10 min and the resultant pellet resuspended in PSG, pH 8-0. Trypanosomes were fixed in formaldehyde as described by Nantulya and Doyle (1977). The suspension was then centrifuged at 380 x g for 15 rain and the pellet resuspended in phosphate buffered saline containing 1% v/v rabbit serum so as to produce a trypanosome suspension of approximately 5 x 106m1-1. The IFAT was then conducted as described by Katende et al. (1987) except that serum dilutions of 1:10, 1:50, 1:250 and 1 : 1,250 were used.

The trypanolysis test was a modification of the technique described by Van Meirvenne, Janssens and Magnus (1975). Trypanosomes for trypanolysis tests were collected from control steers when the level of first peak parasitaemia reached approximately 5 x 106 ml-k Samples of blood collected into heparinised vacutainer tubes were mixed with 10 times excess of fresh guinea-pig serum. Fifty microlitres of trypanosome suspension was added to each 10/~1 aliquot of test serum in the well of a round-bottomed microtitre plate. After incubation for 30min at room temperature, the contents of each well were placed on a microscope slide, examined under x 400 magnification and the percentage of lysed trypanosomes determined.

Experimental design To determine the duration of prophylaxis that isometamidium conferrred

against T. vivax IL 2982 and T. vivax IL 2986, 0.5 mgkg -~ isometamidium chloride was administered to 10 Boran steers. Thereafter, cattle were challenged at monthly intervals using G. m. centralis infected with one of the two T. v ivax populations (five cattle per population; 10 infected flies per animal). Similar studies with T. vivax IL 2968 and T. v ivax IL 2969 have been described (Peregrine et al., 1987).

At each challenge one control for each group, to which isometamidium chloride had not been administered, was challenged in a similar manner.

Five cattle were infected with T. vivax IL 2982 and five with T. v ivax IL 2986 using 10 infected G. m. centralis per animal. Eleven days later when all cattle had a parasitaemia of approximately 5 x 104mi -~ blood, all were treated with 0.5mgkg -x isometamidium chloride. For a period of 200 days following treatment jugular blood was collected from each animal three times weekly and examined for the presence of trypanosomes.

SENSITIVITY OF T. VIVAX TO tSOMETAM1DIUM CHLORIDE 33

RESULTS

Duration of prophylaxis The duration of prophylactic activity of isometamidium against tsetse-

transmitted challenge with the two T. vivax populations is shown in Table I. A short period of prophylaxis was observed with T. vivax IL 2982 from Galana; all five drug-treated animals became infected following the first challenge. In contrast, all five drug-treated animals exposed to the Likoni T. vivax (IL 2986) were refractory to the first challenge but became parasitaemic following the second.

The initial decline in PCV in all cattle infected with the Galana and Likoni T. vivax coincided with the first detection of parasites.

A haemorrhagic syndrome (Hudson, 1944; Wellde et al., 1983) [produced by the parental population of T. vivax IL 2982 (P. R. Gardiner, pers. comm.)] was not observed in any group A animal. By approximately 30 days post-infection PCV values in all group A animals had declined to approximately 20%. Thereafter PCV values in all animals gradually increased.

TABLE I

Duration of prophylactic activity of 0-5 mg kg-i body weight isometamidium chloride in cattle against challenge with tsetse-transmitted T. vivax

Challenge: months after administration of isometamidium chloride

Trypanosoma Animal vivax 1 2

A1 A2 A3 A4 A5 Control A

B1 B2 B3 B4 B5 Control Bt

IL 2982, Galana IL 2982, Galana IL 2982, Galana IL 2982, Galana IL 2982, Galana IL 2982, Galana

IL 2986 Likoni IL 2986 Likoni IL 2986 Likoni IL 2986 Likoni IL 2986 Likoni IL 2986 Likoni

9* 9 9

14 9 9

11

12 12 12 12 14 12

* Numbers correspond to the day following challenge when parasites were first detected. --Refractory to challenge. t Different animals used for each challenge.

Immunological data Immunological studies were conducted on sera collected from cattle chall-

enged with the Zaria or Kilifi T. vivax (Peregrine et al., 1987). These animals (five per group) were treated and challenged in an identical manner to those challenged with the Galana or Likoni T. vivax and the development of infections in these animals is described elsewhere (Peregrine et aL, 1987). Whilst all five drug-treated animals were refractory to both the one month and two month

34 PEREGRINE, MOLO0 AND WHFrELAW

TASLE II

Indirect immunofluorescent antibody (IFAT) test with sera from cattle treated with 0.5 rag kg -1 body weight isometamidium chloride and challenged monthly with T. vivax IL 2968 or T. vivax

IL 29699

Monthly IFA titre in serum samples: days chal lenge following infection

tha t resulted in Ant igen used

Animal Chal lenge infection in IFA test* day 0 day 7 day 14 day 21 day 28

1 IL 2968 None IL 2968 No antibody detected in any s ample 2 IL 2968 None IL 2968 No antibody detected in any s ample 3 IL 2968 3 months IL 2968 - - - - - - 1 : 250 1 : 250 4 IL 2968 4 months IL 2968 - - - - 1 : 10 1 : 50 1 : 50 5 IL 2968 3 months IL 2968 - - - - - - 1 : 50 1 : 250

Control IL 2968 4 months IL 2968 - - - - - - 1 : 50 1 : 250

6 IL 2969 2 months IL 2969 - - - - 1 : 50 1 : 50 1 : 50 7 IL 2969 2 months IL 2969 . . . . 1 : 10 8 IL 2969 1 month IL 2969 . . . . 1 : 10 9 IL 2969 1 month IL 2969 . . . . 1 : 50 10 IL 2969 : 1 month IL 2969 . . . . 1 : 10 1 : 10

Control IL 2969 1 month IL 2969 - - - - - - 1 : 50 1 : 50

* First peak bloodstream forms from a s teer infected with the homologous clone by tsetse transmission.

challenge with T. vivax IL 2968 (from Zaria), animals 3 and 5 (see Table II) became infected following challenge at three months and animal 4 following the fourth monthly challenge. Animals 1 and 2 remained aparasitaemic for the entire observation period of 200 days. In contrast to the Zaria T. vivax, three of the drug-treated animals challenged with the Kilifi T. vivax (IL 2969) (8, 9 and 10) became parasitaemic following the first challenge. The remaining two animals (6 and 7) became parasitaemic following the second challenge. Table II shows the results of IFATs conducted with sera collected from the 10 cattle in these two groups, zero to 28 days following the day on which challenge resulted in infection. In nine animals antibody against first peak bloodstream forms was not detected until after the onset of parasitaemia (for each T. vivax population, trypanosomes for the IFAT were collected from one challenge control steer infected by tsetse transmission). In contrast in animal 10, whilst antibody was detected at a titre of 1:10 in sera collected on day 21 following the first challenge, parasites were not detected until day 24 following the same challenge.

Whereas the 10 challenge controls for animals 1 to 5 and the four controls for animals 6 to 10 were all detected parasitaemic by day 11 following challenge (Peregrine et aL, 1987), antibody was detected by indirect immunofluorescence in day 14 sera from only two out of the 14 animals (data not shown). However, antibody was detected in day 21 sera from all 14 animals (data not shown).

Only two animals were refractory to all five challenges with the Zaria T. vivax; animals 1 and 2. Examination of sera collected from both animals during the entire course of the experiment failed to reveal the presence of antibody in any sample.

Data from trypanolysis tests were similar to that from IFATs. In all instances, greater than 20% lysis only occurred in sera collected subsequent to the detection

SENSITIVITY OF T. VIVAX TO ISOMETAMIDIUM CHLORIDE 35

of parasites in an animal (Peregrine, 1987). As before, analyses of all sera from animal numbers 1 and 2 (animals challenged with the Zaria T. vivax [IL 2968] and refractory to all five challenges) failed to reveal the presence of trypanolytic antibody in any sample.

Therapeutic activity Relapses were not detected in any of the animals infected with either the

Galana or Likoni T. vivax during the 200-day observation period following treatment. Thus infections in all 10 steers appeared to have been sensitive to a dose of 0.5 mg kg -1 isometamidium chloride.

DISCUSSION

In the studies described here with two T. vivax populations, isometamidium chloride at a dosage of 0-5 mg kg -I body weight has been shown to confer complete prophylaxis to cattle for one month against T. vivax IL 2986 (Likoni, Kenya) and less than one month against T. vivax IL 2982 (Galana, Kenya). The same dosage of drug has previously been shown to confer cattle complete prophylaxis for two months against T. vivax IL 2968 (Zaria, Nigeria) and for less than one month against T. vivax IL 2969 (Kilifi, Kenya) (Peregrine et al., 1987). Further studies both in vivo (Peregrine et al., 1987)and in vitro (Peregrine, 1987) have shown that the short period of prophylaxis conferred against the Kilifi T. vivax was associated with the expression of a high level of resistance by that population. Similar experiments were not undertaken with either the Likoni or Galana T. vivax.

Previous reports describing the expression of resistance to isometamidium by field isolates from East Africa have been associated with both T. congolense (Gitatha, 1979) and T. vivax (Schonefeld, Rottcher and Moloo, 1987). These reports concern a limited number of isolates and no conclusions concerning the prevalence of drug resistance in any one locality could be made.

Experiments conducted with T. vivax populations originating from Kilifi (IL 2969) (Peregrine et al., 1987), Galana (IL 2982) and Likoni (IL 2986), in Kenya, have shown them to be sensitive to the therapeutic activity of isometamidium but resistant to the prophylactic activity of the drug, as defined by "early" breakthroughs at dosages recommended for field usage. In contrast, studies with the Zaria T. vivax (IL 2968) showed it to be sensitive to isometamidium's therapeutic and prophylactic activities (Peregrine et al., 1987), whilst studies on a T. vivax population isolated in Bamburi, Kenya (15 km north of Mombasa), showed it to be resistant to the drug's therapeutic and prophylactic activities (Schonefeld et al., 1987). Differences between the prophylactic and therapeutic activity of a drug have also been reported for the 2,7-di-m-amidinophenyldiazo- amino derivative of homidium (Berg, Brown, Hill and Wragg, 1961), another member of the phenanthridine series o f compounds to which isometamidium belongs. Although the compound expressed lower therapeutic activity than metamidium (a mixture of isometamidium chloride and an isomer of isometami- dium) against a T. congolense population, it was equally active prophylactically. The findings described here suggest that the form(s) of isometamidium exerting the drug's therapeutic activity may be different from the form(s) exerting the prophylactic activity. Possibly the therapeutic activity is associated with isometa- midium per se whilst isometamidium metabolite(s) exert(s) the prophylactic

36 PEREGRINE, MOLOO AND WHITELAW

activity of the drug. Alternatively, different isomers of isometamidium (Kinabo and Bogan, 1988) may be responsible for the different activities.

Fiennes (1953) and Whiteside (1958) suggested that a decrease in the prophylactic activity of a trypanocide may be due to a rapid increase in expression of resistance when trypanosomes are inoculated into an animal under prophylaxis. In similar studies to those described here, but with T. congolense (Peregrine et al., 1988), trypanosome populations arising in drug-treated cattle did not appear to express a decreased level of sensitivity to isometamidium in mice as compared to populations from bovine controls.

In studies conducted on the prophylactic formulation of quinapyramine (a commonly used chemoprophylactic against trypanosomiasis of domestic livestock in the 1950s and 1960s), Whiteside (1962) suggested that the total duration of prophylaxis consisted of two additive components: one due to the trypanocidal activity of the drug, and the other due to an immune response by the host as a result of antigenic priming during part or all of the protective period. In the work described here with T. vivax populations originating from both East and West Africa, antibodies recognising first peak bloodstream variable antigenic types (VATs) were not detected in any animal repeatedly challenged whilst under isometamidium prophylaxis until after the onset of parasitaemia. Thus antigenic priming to surface antigens of early arising variants of T. vivax (Barry, 1986) does not occur in cattle on isometamidium prophylaxis and this does not appear to contribute towards the prophylactic activity of the drug.

Although humoral immune responses to metacyclic VATs of T. congolense (Peregrine et al., 1988) and first peak bloodstream VATs of T. vivax do not appear to be acquired whilst under chemoprophylactic cover, protective immune responses may be acquired under therapeutic regimes. Wilson et al. (1975, 1976) described a field-based experiment in which partial immunity, as determined by a lengthening of the intertreatment period, appeared to develop after two years in a group of cattle treated with diminazene aceturate. Animals were treated on an individual basis when they developed clinical disease or the PCV had decreased below 20 per cent. In contrast, immunity, as judged by the same criteria, did not appear to develop in cattle treated on a group basis with diminazene aceturate when one animal in the group was parasitaemic. Thus development of immunity in cattle was greatest in animals in which infections were well established at the time of treatment: T. congolense, T. vivax and T. brucei were detected in cattle either as single or mixed infections. In conclusion, whilst there may be an immune contribution to the prophylactic activity of isometamidium chloride when it is used as a therapeutic, the results described here indicate that the major factor determining the drug's apparent period of prophylaxis is differences in drug resistance between different isolates.

ACKNOWLEDGEMENTS

We thank Mr D. Muteti, Mr J. Katende and the staff of the I L R A D tsetse laboratory for technical assistance and Mrs D. M. Lewa for typing the manuscript. Thanks are also extended to Professors P. H. Holmes and G. M. Urquhart and Dr P. R. Gardiner for helpful discussions. RMB Animal Health Ltd kindly donated the Samorin used in these studies. The financial assistance of the Overseas Development Administration, UK, under research scheme R3910

SENSITIVITY OF T. VIVAX TO ISOMETAMIDIUM CHLORIDE 37

and the Commission of the European Communities Research and Development Programme (Tropical Agriculture) Contract number TSD-A-199 are gratefully acknowledged.

Accepted for publication January 1990

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38 PEREGRINE, MOLOO AND WHITELAW

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DIFFERENCES DE SENSIBILITE DES POPULATIONS KENYANNES DE T R Y P A N O S O M A VIVAX AUX ACTIONS THERAPEUTIQUES ET PROPHYLACTIQUES DU C H L O R U R E

D'ISOMETAMIDIUM SUR DU BETAIL BORAN

R~sam~---Du chlorure d'isom6tamidium a 6t~ administr6 a la dose individueUe unique de 0,5 mg par kg de poids vif a 10 bouvillons Boran ( 8os indicus) ~ titre prophylactique. A un mois d'intervalle, apr~s I'administration du produit, deux grouper de 5 animaux ont 6t6 soumis a Fune des deux populations diff6rentes de Trypanosorna vivax transmises par Giossina morsitans centralis; l 'une avec la souche IL 2982 provenant du Galana (Kenya) et l 'autre avec la souche IL 2986 de Likoni (Kenya). La protection a 6t6 inf6rieure a un mois contre T. vivax Galana et ~gale a un mois pour T. vivax Likoni. Lors d'une ~tude th~rapeutique, un autre lot de 10 bouviUons Boran a 6t6 infect6 clans les m~mes conditions avec I'une et l 'autre des populations de T. vivax, ~ raison de cinq anirnaux pour chacune d'elles. Onze jours apr~s l'infectiun, tOUR ont re~u une dose unique individueUe de 0,5 mg par kg de chlorure d'isom6tamidium et tOUR ont ~t6 gu6ris. Ces r6sultats d6montrent que, telleR qu'elles sont pr6sentes sur le terrain, ies deux souches kenyannes de T. vivax pr6sentent un haut degr6 de r~sistance it Faction prophylactique de l'isom~tamidium, en d6pit d'un niveau peu 61ev6 de Faction th~rapeutique du m~me produit. Les r6sultats montrent 6galement que la variation de la r6sistance au produit entre cliff, rents isolats joue un r61e majeur dans la d6termination de la p~riode apparente de protection assur6e par l'isom6tamidium.

DIFFERENCIA EN SENSIBILIDAD DE LAS POBLACIONES DE T R Y P A N O S O M A VIVAX EN KENYA" A LA ACCION PROFILACTICA Y TERAPEUTICA DEI C L O R U R O DE

ISOMETAMIDIO EN GANADO BORAN

Resumen---Se administr6 cloruro de isometamidio en doris profilacticas sencillas de 0-5 mg kg -1 de peso corporal a cada uno de 10 novilios Bor~in (Bos indicus). Con intervalos de un mes despu6s de la administraci6n de la droga, grupos de cinco animales cada uno recibieron una descarga con una de dos cepas diferentes de 1". viva.r, transmitidas por Giossina morsitans centralis infectadas; un grupo con la cepa (IL 2982) de Galana Kenya y el otro con la cepa (IL 2986) de Likoni, Kenya. La acci6n profil~ctica dur6 menos de un rues contra la cepa Galana y pot un rues contra la cepa Likoni.

En un estudio terap~utico, otros 10 novillos Bor~in fueron infectados con una de las dos cepas (cinco animales pot grupo). Once dias despu6s de la infecci6n, todos los animales se t ra taron con 0.5 mg kg -1 de cloruro de isometamidio, curandose to<los.

Estos hallazgos demuestran que como en el campo, las dos cepas de 7". vivax tienen u n alto nivel de resistencia a la acci6n profil~ictica del isometamidio y una baja resistencia a la acci6n terap6utica de la droga. Los resultados tambi6n demuestran que la diferencia en la resistencia a la droga de ias dos cepas, juega un papel importante y determinante en el efecto profil~ctico inducido por el cloruro de isometamidio.