Post on 08-Mar-2023
www.elsevier.com/locate/vetpar
Available online at www.sciencedirect.com
Veterinary Parasitology 153 (2008) 100–107
The ecology of horse cyathostomin infective larvae
(Nematoda-Cyathostominae) in tropical southeast Brazil
Simone Quinelato a, Melissa C.M. Couto a, Bruno C. Ribeiro b,Claudia N. Santos b, Luciene S. de Souza b, Debora H.S. dos Anjos c,
Ivan B.M. Sampaio d, Lurdes M.A. Rodrigues e,*a UFRRJ, Postgraduate Program in Veterinary Sciences, Dept. of Animal Parasitology, BR 465, Km 7, 23890-000, Seropedica, RJ, Brazil
b UFRRJ, Scientific Initiation CNPq/PIBIC, Dept. of Animal Parasitology, BR 465, Km 7, 23890-000, Seropedica, RJ, Brazilc UFRJ, Carlos Chagas Filho Biophysics Institute, CCS-BI, 21949-900, Rio de Janeiro, RJ, Brazil
d UFMG, Veterinary School, Zootechny Dept., Av. Antonio Carlos, 6627, 31270-901 Pampulha, MG, Brazile UFRRJ, IV, Dept. of Animal Parasitology, CPGCV, BR 465, Km 7, 23890-000 Seropedica, RJ, Brazil
Received 24 August 2007; received in revised form 9 January 2008; accepted 11 January 2008
Abstract
Experimental studies about the recovery, survival and migration to pasture of cyathostomin infective larvae (L3) from fresh feces
depositions were conducted from February 2005 to March 2007 in a tropical region of southeast Brazil. Grass and feces were
collected weekly at 8 a.m., 1 and 5 p.m. and processed by the Baermann technique. Multivariate analysis (principal components
method) showed the influence of time and environmental variables on the number of infective larvae recovered from the feces and
pasture. In the rainy period (October–March), more infective larvae were recovered on the feces and grass apex. In contrast, in the
dry period (April–September), the recovery was higher only on the grass base, as well as the L3 survival on feces and grass. More
larvae were recovered at 8 a.m., except from the grass apex, where the highest recovery was at 1 p.m. Few studies investigating the
seasonal transmission of equine cyathostomin have been conducted in South American tropical climates. These results demonstrate
that in tropical conditions L3 are available on feces and pasture throughout the year. Knowledge of climatic influences on the
development and survival of L3 is crucial to designing integrated parasite control programs that provide effective protection while
slowing the development of anthelmintic resistance.
# 2008 Elsevier B.V. All rights reserved.
Keywords: Cyathostomin; Horses; Tifton 85; Infective larvae
1. Introduction
Gastrointestinal nematodosis is an important disease
among grazing horses in all regions of the world (Love
and Duncan, 1992). Control programs are based on the
intensive use of drugs, promoting the development of
* Corresponding author.
E-mail addresses: squinelato@ufrrj.br (S. Quinelato),
lurdesar@ufrrj.br (L.M.A. Rodrigues).
0304-4017/$ – see front matter # 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.vetpar.2008.01.027
nematode resistance to most anthelminthics groups.
Alternative methods to control cyathostomin infection
should take into account the infective larvae dynamics
on pasture (Ramsey et al., 2004; Nielsen et al., 2007).
It has been proposed that the ‘‘parasite refugium’’ is
the most important factor in the development of
nematode resistance (van Wyk, 2001; Kaplan, 2004).
This refugium is constituted by free-living stages in
pasture and untreated animals and some encysted
parasites, such as the histotrophyc phase of the
cyathostomin (Nielsen et al., 2007).
S. Quinelato et al. / Veterinary Parasitology 153 (2008) 100–107 101
The development, survival and migration of infective
larvae are influenced by environmental conditions, such
as temperature and rainfall. Larvae migration is
regulated by available moisture and survival is
influenced strongly by temperature (Ogbourne, 1973;
Craig et al., 1983; Eysker et al., 1986; Quinelato et al.,
2007).
Some types of grass maybe favor L3 survival and
migration. Studies of the development and survival of
cyathostomin eggs and larvae in pastures have been
carried in temperate climate areas (Ogbourne, 1972;
Duncan, 1974), however few such studies have been
conducted in tropical areas (Mfitilodze and Hutchinson,
1988; Hutchinson et al., 1989; Quinelato et al., 2007)
and subtropical ones (English, 1979a,b; Craig et al.,
1983; Courtney and Asquith, 1985; Baudena et al.,
2000a,b).
This effort is essential to South American tropical
region, since there are only a few studies related to these
conditions. The objective of this study is to evaluate the
recovery, survival and migration of cyathostomin
infective larvae on Tifton 85 grass (Cynodon spp. cv.
Tifton 85) at different times of the day during 24
months. Such knowledge will contribute to control
horse infection while reducing the use of anthelmintics.
2. Material and methods
2.1. Experimental design
From February 2005 to March 2007 the potential
transmission of infective larvae (L3) of equine
cyathostomin was studied, employing the methodology
described in Quinelato et al. (2007). The samples of
feces and grass were collected at three times of the day
(8 a.m., 1 p.m. and 5 p.m.). The grass was divided into
base (0–20 cm) and apex (20–40 cm). For data analysis,
two different periods were considered: rainy (October–
March) and dry (April–September).
2.2. Location and meteorological data
The study was carried out at the Helminthology
Laboratory of W. O. Neitz Parasitology Station,
UFRRJ, Rio de Janeiro, Brazil. The climate is AW
type (tropical humid), according to the Kopen
classification.
The weather data were supplied by the Seropedica
Agricultural Weather Station, INMET/PESAGRO, RJ.
The soil temperature was measured at the experi-
mental area with a soil thermometer at each time of
collection.
2.3. Animals and pasture
Fecal pats were obtained from horses naturally
infected with cyathostomin. These animals did not
receive any anthelminthic treatment during experi-
mental period. ATifton 85 plot (Cynodon spp. cv.Tifton
85) was set off measuring 11.50 m � 1.00 m, fenced to
prevent the animals from entering and grazing.
2.4. Fecal pats
A fecal pat of approximately 1 kg was collected
directly from the horse, the number of eggs per gram
(epg) was processed by McMaster technique (Gordon
and Whitlock, 1939) and the larvae were cultured
according to Roberts and O’Sullivan (1950). After these
procedures, the feces were placed in the pasture plot in
the first week of each month, separated by �50 cm.
2.5. Procedures
Seven days after the deposit, one sample of feces
(�2 g) and another of grass were collected weekly from
three different field sites. Collection of feces and grass
were done at three different times a day (8 a.m., 1 p.m.
and 5p.m.) from the feces deposited and the surrounding
grass plot. Grass sample was divided into base (0–
20 cm) and apex (20–40 cm). The samples were
weighed weekly and processed by the Baermann
technique (Ueno and Goncalves, 1998). After 24 h,
the L3 were recovered and observed under an optical
microscope. Infective larvae were counted and identi-
fied according to Bevilaqua et al. (1993). To obtain the
number of L3 kg�1, the feces and grass samples were
dried at 75 8C for 48 h.
The fecal plots were abandoned when no more L3
could be recovered from the feces and grass samples.
The data on larvae number in feces and grass,
survival period, average air and soil temperature and
total rainfall were recorded weekly in an Excel file.
2.6. Statistical analysis
To evaluate the effect of the three collection times,
the average values were compared by the Kruskal–
Wallis (P < 0.05) (Zar, 1999) nonparametric test
through the BioEstat program (Ayres et al., 2005).
Mann–Whitney nonparametric test (P < 0.05) (Zar,
1999) was used to analyze means values of temperature
(air and soil), rainfall, epg, infective larvae recovery and
survival in the rainy and dry periods, again through the
BioEstat program (Ayres et al., 2005).
S. Quinelato et al. / Veterinary Parasitology 153 (2008) 100–107102
The selected collections (n = 139) were submitted to
multivariate analysis (principal components method)
(Judez, 1989) using the INFOSTAT software. Larvae
counts within observation periods were set in percen-
tages in relation to total count of each period, in order to
properly observe migration dynamics. Graphical repre-
sentation of the variables is shown in a three-
dimensional system, allowing visualization of associa-
tions among them.
3. Results
3.1. Meteorological data
The average air and soil temperature and total
rainfall were high in the rainy period (P < 0.05)
(Fig. 1). In this period the average temperatures were
25.5 8C for air and 27.9 8C for soil, while in the dry
season these were 22.8 8C for air and 24.3 8C for soil.
The total rainfall was 141.93 mm and 56.55 mm in
rainy and dry period respectively.
Fig. 1. Monthly average air temperature (T), soil temperature (
Fig. 2. Number of eggs per gram (epg) fr
3.2. epg
epg levels varied during the experimental period,
with values between 850 and 4500 (Fig. 2). Peaks were
observed in December 2006 (rainy) and April and
August 2005 (dry). The average in the rainy period was
1823 (�269.49) and during the dry period the average
was 1925 (�346.92) (P > 0.05).
3.3. Feces
L3 recovery was observed in the first week, with larger
numbers in November 2006, (107,482 L3 kg�1 dm) and
April 2005 (106,695 L3 kg�1 dm). In the rainy period,
the average number of larvae recovered was
26,961 L3 kg�1 dm, and during the dry period it was
30,566 L3 dm (P > 0.05) (Fig. 3).
Infective larvae were recovered at three times of day
(8 a.m., 1 p.m. and 5 p.m.), with the largest recovery at 8
a.m. (P < 0.05), and with peaks in April 2005 and
November 2006 (Fig. 4).
Ts) and total rainfall from February 2005 to March 2007.
om February 2005 to January 2007.
S. Quinelato et al. / Veterinary Parasitology 153 (2008) 100–107 103
Fig. 3. Percentage of cyathostomin infective larvae recovered from fecal and grass samples from February 2005 to January 2007.
Larval survival varied from 2 to 9 weeks in the
rainy period and from three to 14 weeks in the dry
(Fig. 5), with highest L3 survival in the dry season
(P < 0.05).
3.4. Grass base
L3 migration in the rainy and dry seasons occurred
from the second week. The highest larval recoveries
were in August 2005 (3933 L3 kg�1 dm) and October
2005 (1728 L3 kg�1 dm). The average number of larvae
recovered was 555 L3 kg�1 dm in the rainy season and
762 L3 kg�1 dm in the dry (P < 0.05) (Fig. 3).
Fig. 4. Cyathostomin infective larvae recovery in feces at d
The L3 larval were collected at the three times
(P > 0.05), with the largest recovery at 8 a.m. and peaks
in August and October 2005 (Fig. 6).
The L3 survival varied from 1 to 9 weeks in the rainy
period and from 2 to 11 weeks in the dry (Fig. 5). There
was a difference in L3 survival between the periods
(P < 0.05), with the highest survival in the dry season.
3.5. Grass apex
The L3 migration occurred from the first week in the
rainy period and second week in the dry. The largest
number of L3 was recovered in December 2006
ifferent collection times (8 a.m., 1 p.m. and 5 p.m.).
S. Quinelato et al. / Veterinary Parasitology 153 (2008) 100–107104
Fig. 5. Cyathostomin infective larvae survival (weeks) in feces and grass samples from February 2005 to January 2007.
(919 L3 kg�1 dm) and June 2006 (335 L3 kg�1 dm).
The average number of larvae recovered were
263 L3 kg�1 dm (rainy) and 185 L3 kg�1 dm (dry)
(P > 0.05) (Fig. 3).
Just as for the other samples, the L3 cyathostomin
were recovered at three times of day, with the highest
recovery at 1 p.m. (P > 0.05) and a peak in October
2006 (Fig. 7).
The L3 survived up to 7 weeks in the rainy period and
10 weeks in the dry (P < 0.05) (Fig. 5).
Fig. 6. L3 recovery from grass base at different c
The factors identified in the multivariate analysis as
being associated with the number of L3 recovered from
the pasture samples included the time since the feces
were deposited, the average temperature (air and soil)
and the rainfall during the previous week. There was a
decrease in the percentage of L3 in feces (% lgf) and
grass (% lgg) as a function of time, with a great
difference observed in feces. Increasing rainfall
promoted an increase of the larval percentage in grass
(% lgg). Air temperature had a negative effect on L3
ollection times (8 a.m., 1 p.m. and 5 p.m.).
S. Quinelato et al. / Veterinary Parasitology 153 (2008) 100–107 105
Fig. 7. L3 recovery from grass apex at different collection times (8 a.m., 1 p.m. and 5 p.m.).
percentage in feces (% lgf), while soil temperature (tsoil)
negatively influenced negatively the L3 grass percen-
tage (% lgg) (Fig. 8).
4. Discussion
4.1. Feces
In tropical climates, horses can be infected at all
times of the year because L3 are present in the pasture
Fig. 8. Graphical representation of variables acting on infective larvae
pasture dynamics, according to axis 2 and 3. The coordinate and
direction of the first axis are represented by the arrow and its value.
System inertia: 72%. %lf, larvae in feces; %lg, larvae in grass; day,
days; rf, rainfall; tair, average of air temperature; tsoil, average soil
temperature.
throughout the year. The animals are more exposed to
infection in dry periods than in periods with intense
rains (Quinelato et al., 2007; Couto et al., personal
comment, 2007). If the quantify of forage is not enough,
animals can graze close to fecal pats, seeking more
palatable and new growth grass, with greater infection
risk (Craig, 1999).
In the present study, the larger L3 recovery in the dry
period occurred due to ideal environmental conditions
for larval development and migration to the pasture,
showing the importance of the fecal pats as an ideal
microclimate for egg and larval development and as
larvae reservoirs, as observed by Mfitilodze and
Hutchinson (1987), Uhlinger (1991), Quinelato et al.
(2007), Couto et al. personal comment (2007).
The higher number of L3 recovered at 8 a.m. was
probably associated to the mild air temperature and
fecal moisture in the morning (Hasslinger and Bittner,
1984).
According to the literature, factors that favor larval
development do not favor survival. High temperature
and moisture, typical of the rainy period, lead to fast
larvae development but limit their survival. In contrast,
in the dry period these climatic factors are milder and
larvae develop more slowly but survive longer
(Courtney, 1999; Nielsen et al., 2007).
The larger L3 survival observed in this study during
dry period can be explained also by the fact that pats
stay intact during this period, protecting the eggs and
larvae (Rupasinghe and Ogbourne, 1978). In the rainy
season, intense rains can destroy the fecal pats, reducing
the survival period. These results are in agreement with
S. Quinelato et al. / Veterinary Parasitology 153 (2008) 100–107106
English (1979a), who in studies in the Australian
subtropical area reported that development of eggs to L3
was completed in 2 weeks during the summer (rainy
period), but only 1–10% of the larvae survived. During
the winter (dry period), L3 development was completed
in 5 weeks, with 80% survival.
4.2. Grass
The larger L3 recovery in the grass base in the dry
period (total rainfall 56.55 mm), in contrast with the grass
apex in the rainy period (total rainfall 141.93 mm), can be
explained by the lower rainfall in the dry period, because
rainfall acts as limiting factor for larval migration to the
pasture (English, 1979b; Ogbourne, 1972, 1973; Hutch-
inson et al., 1989; Langrova et al., 2003). No correlation
was found by Hutchinson et al. (1989) between L3
recovered from pasture and the amount of rain. They
considered that rain just supplies the minimum moisture
necessary for migration and that great amounts of rain
would disperse L3. Most of the studies have analyzed
only L3 horizontal migration, preventing comparison
with our results. The larger recovery at 1 p.m. here does
not agree with the findings of Langrova et al. (2003),
where the larger values at 8 a.m. were explained by the
presence of morning dew.
Moisture influences larval migration, while tempera-
ture more directly influences survival, both in temperate
and tropical climates (Ogbourne, 1973; Craig et al., 1983;
Eysker et al., 1986). The larger L3 survival in the dry
period can be explained due to mild temperature and little
rain, allowing fecal pats to serve as an L3 reservoir. In
contrast, during the rainy season, the higher temperatures
raises larval metabolism, resulting in fast depletion of
their energy reserves, reducing the survival and
infectivity of the larvae (Cheah and Rajamanickam,
1997; Medica and Sukhdeo, 1997).
The type of grass can influence L3 recovery and
survival. The plants have different growth patterns and
wide morphologic contrasts, and these characteristics
alter the vegetation microclimate, directly affecting the
larval development and survival (Niezen et al., 1998).
Tifton 85 (Cynodon spp. cv. Tifton 85) is widely used
for equine grazing, it is a hybrid characterized by wide
leaves, few trichomes, big stems, fine stalks and well-
developed rhizomes (Burton et al., 1993). It grows low
to the ground, allowing wide solar light incidence on the
vegetation, which might have contributed to smaller
larval recovery in the hottest months (rainy period),
besides the drying of the fecal plot, hindering the
liberation of infective larvae. In the dry period, this
pasture type is more fibrous protecting the larvae from
unfavorable environmental conditions (Brady and Weil,
1999). In the present study, the highest number of
infective larvae was recovered from grass base, maybe
because the infective larvae find favorable conditions
for survival on this part of the grass and because the fine
stems provide smaller area for larval migration. Another
factor could be the presence of few trichomes, hindering
the formation of a moist layer that allows the access of
infective larvae to the upper parts of the grass.
The environmental conditions of the studied area are
favorable for the development of cyathostomin infective
larvae. The L3 are present in the pasture throughout of
the year, so methods of prevention and control are
necessary to help reduce the number of larvae,
especially those in refugia. Some methods of prevention
are simple and inexpensive, like monitoring feces epg
and pasture rotation. These can help to prevent infection
caused by larvae in refugia.
Another important point is the frequency of treatment
with anthelmintics. This frequency will depend on the
percentage of feces contamination and development of
anthelmintics resistance. It is important to know the time
between treatments and the expected reappearance of
eggs in feces. An optimum treatment interval must be
established considering some risk factors, such as pasture
type, grazing history, time of the year, stock density and
weather conditions (Jacobs et al., 1995).
5. Conclusion
At all collection times, the number of larvae
recovered was high with potential infection risk to
equines.
The L3 recovery was larger from the grass base
(0–20 cm) than apex (20–40 cm), with the highest L3
recovery from the base during the dry season. The largest
survival was observed in the dry period. The fecal pats
acted as reservoirs of the cyathostomin in both periods.
Knowledge of climatic influences on the develop-
ment and survival of free-living stages is essential for
the development of control programs that provide
efficient protection and still reduce the development of
anthelminthic resistance.
Acknowledgements
This study had financial support from CPGCV,
UFRRJ, CAPES and CNPq. We also so thank the
Seropedica Agricultural Weather Station (INMET/
PESAGRO) for providing the meteorological data used
in this study and Martin K. Nielsen for his research
contribution.
S. Quinelato et al. / Veterinary Parasitology 153 (2008) 100–107 107
References
Ayres Jr., M., Ayres, M., Ayres, D.L., Saints, A.S.D., 2005. BioEstat
4.0—Statistical Applications in the Areas of the Biological and
Medical Sciences, fourth ed. Belem IOEPA, p. 324.
Baudena, M.A., Chapman, M.R., French, D.D., Klei, T.R., 2000a.
Seasonal development of equine cyathostome larvae on pasture in
south Louisiana. Vet. Parasitol. 88, 51–60.
Baudena, M.A., Chapman, M.R., Larsen, M., Klei, T.R., 2000b.
Efficacy of the namatophagus fungus Duddingtonia flagrans in
reducing equine cyathostome larvae on pasture in south Luisiana.
Vet. Parasitol. 89, 219–230.
Bevilaqua, C.M.L., Rodrigues, M.L.A., Concordet, D., 1993. Identi-
fication of infective larvae of some common nematode strongylids
of horses. Rev. Med. Vet. 144, 989–995.
Brady, N.C., Weil, R.R., 1999. The Nature and Properties of Soils,
12th ed. Prentice Hall Upper Saddle River, NJ, p. 881.
Burton, G.W., Gates, R.N., Hill, G.M., 1993. Registration of Tifton 85
bermuda grass. Crop Sci. 33, 644–645.
Cheah, T.S., Rajamanickam, C., 1997. Epidemiology of gastro-intest-
inal nematodes of sheep in wet tropical conditions in Malaysia.
Trop. Anim. Health Prod. 29, 165–173.
Courtney, C.H., 1999. Seasonal transmission of equine cyathostomin
in warm climates. Vet. Parasitol. 85, 173–180.
Courtney, C.H., Asquith, R.L., 1985. Seasonal changes in pasture
infectivity by equine cyathostomin in central north Florida. Eq.
Vet. J. 17, 240–242.
Couto, M.C.M., Quinelato, S., Souza, T.M., Bevilaqua, C.M.L.,
Anjos, D.H.S., Sampaio, I.B.M., Rodrigues, M.L.A., 2007. Desen-
volvimento e migracao de larvas infectantes de ciatostomıneos
(Nematoda-Cyathostominae) em gramınea coast-cross (Cynodon
dactylon) em clima tropical, na Baixada Fluminense, RJ, Brasil.
(personal communication).
Craig, T.M., Bowen, J.M., Ludwig, K.G., 1983. Transmission of
equine cyathostomin (Strongylidae) in central Texas. Am. J.
Vet. Res. 44, 1896–1897.
Craig, T.M., 1999. Considarations for the control of equine cyathos-
tomin in arid arias. Vet. Parasitol. 85, 181–188.
Duncan, J.L., 1974. Field studies on the epidemiology of mixed
strongyle infection in the horse. Vet. Res. 94, 337–345.
English, A.W., 1979a. The epidemiology of equine strongylosis in
southern Queensland. Part 1. The bionomics of the free-
living room stages in faeces and on pasture. Aust. Vet. J. 55,
299–305.
English, A.W., 1979b. The epidemiology of equine strongylosis in
southern Queensland. Part 2. The survival and migration of
infective larvae on herbage. Aust. Vet. J. 55, 306–309.
Eysker, M., Jansen, J., Kooman, F.N.J., Mirck, M.H., 1986. Compar-
ison of two control systems for cyathostome infections in the horse
and further aspects of the epidemiology of these infections. Vet.
Parasitol. 22, 105–112.
Gordon, H.M., Whitlock, H.V., 1939. A new technique for counting
nematode eggs in sheep faeces. J. Coun. Sci. Ind. Res. Aus. 12,
50–52.
Hasslinger, M.A., Bittner, G., 1984. Zur Saisondynamik gives Larven
von Pferdestrongyliden und deren Beziehung zoom Infektionsri-
siko auf gives Weide. Zentralblatt fur Veterinarmedizin 31, 25–31.
Hutchinson, G.W., Abba, S.A., Mfitilodze, M.W., 1989. Seasonal
translation of equine strongyle infective larvae to tropical herbage
in Australia. Vet. Parasitol. 33, 251–263.
Jacobs, D.E., Hutchinson, M.J., Parker, L., Gibbons, M., 1995. Equine
cyathostome infection: suppression of faecal egg output with
moxidectin. Vet. Rec. 137, 545.
Judez, A.L., 1989. Tecnica de Analisis de Datos Multidimensionales.
Ministerio da Agricultura, Pesca y Alimentacion, Madrid, p. 301.
Kaplan, R.M., 2004. Drug resistance in nematodes of veterinary
importance: a status report. Trends Parasitol. 20, 477–481.
Langrova, I., Jankovska, I., Borovskı, M., Fiala, T., 2003. Effect of
climatic influences on the migrations of infective larvae of
Cyathostomin. Vet. Med-Czech 48, 18–24.
Love, S., Duncan, J.L., 1992. The development of naturally acquired
cyathostome infection in ponies. Vet. Parasitol. 44, 127–142.
Medica, D.L., Sukhdeo, M.V.K., 1997. Role of lipids in the transmis-
sion of the infective stage (L3) of Strongylus vulgaris (Nematoda:
Strongylida). J. Parasitol. 83, 775–779.
Mfitilodze, M.W., Hutchinson, G.W., 1987. Development and survival
of free-living room stages of equine strongyles under laboratory
conditions. Vet. Parasitol. 23, 121–133.
Mfitilodze, M.W., Hutchinson, G.W., 1988. Development of free-
living room stages of equine strongyles in faeces on pasture in the
tropical environment. Vet. Parasitol. 26, 285–296.
Nielsen, M.K., Kaplan, R.M., Thamsborg, S.M., Monrad, J., Olsen,
S.N., 2007. Climatic influences on development and survival of
free-living room stages of equine strongyles: implications for
worm control strategies and managing anthelmintic resistance.
Vet. J. 174, 23–32.
Niezen, J.H., Charleston, W.A.G., Miller, C.M., Warghorn, T.S.,
Robertson, H.A., 1998. Effect of plant species on the larvae of
gastrointestinal nematodes wich parasites sheep. I.J. Parasitol. 28,
791–803.
Ogbourne, C.P., 1972. Observations on the free-living room stages of
strongylid nematodes of the horse. Parasitology 64, 461–771.
Ogbourne, C.P., 1973. Survival on herbage plots of infective larvae of
strongylid nematodes of the horse. J. Helminthol. 47, 9–16.
Quinelato, S., Couto, M.C.M., Souza, T.M., Bevilaqua, C.M.L.,
Anjos, D.H.S., Sampaio, I.B.M., Rodrigues, M.L.A., 2007. Cia-
tostomıneos (Strongylidae-Cyathostominae) parasitas de cavalos:
ecologia experimental dos estagios pre-parasıticos em gramınea
tifton 85 (Cynodon spp. cv. tifton 85) na Baixada Fluminense, RJ,
Brasil. Parasitol. Latinoam. 62, 27–34.
Ramsey, Y.H., Chistley, R.M., Matthews, J.B., Hodgkinson, J.E.,
McGoldrick, J., Love, S., 2004. Seasonal development of Cyathos-
tominae larvae on pasture in a northern temperate region of the
United Kingdom. Vet. Parasitol. 119, 307–318.
Roberts, H.S., O’Sullivan, P.S., 1950. Methods for egg counts and
larval cultures goes Strongyles infesting the gastrointestinal tract
of cattle. Aust. J. Agric. Res. 1, 99–102.
Rupasinghe, D., Ogbourne, C.P., 1978. Laboratory studies on the effect
of temperature on the development of the free-living room stages of
strongylid nematodes of the horse. Parasitol. Res. 55, 249–253.
Ueno, H., Goncalves, P.C., 1998. Manual Para Diagnostico das
Helmintoses em Ruminantes. Japan International Cooperation
Agency, Toquio, Japao, p. 143.
Uhlinger, C.A., 1991. Equine small strongyles: epidemiology pathol-
ogy and control. Comp. Cont. Edu. 13, 863–869.
van Wyk, J.A., 2001. Refugia—overlooked perhaps the most potent
factor concerning the development of anthelmintic resistance.
Onderstepoort J. Vet. Res. 68, 55–67.
Zar, J.H., 1999. Biostatistical Analysis, fourth ed. Prentice Hall,
Upper Saddle River, NJ, p. 663.