- Email: [email protected]
The prevalence and intensity of internal parasites of horses in the U.S.A.
Veterinary Parasitology, 15 (1984) 75--83 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
75
THE PREVALENCE A N D INTENSITY OF INTERNAL PARASITES OF HORSES IN THE U.S.A.
C.R. R E I N E M E Y E R * , S.A. SMITH, A.A. GABEL 1 and R.P. HERD
Department of Veterinary Pathobiology and ~Department of Veterinary Clinical Sciences, The Ohio State University, 1925 Coffey Road, Columbus, OH 43210 (U.S.A.) (Accepted for publication 4 October 1983)
ABSTRACT Reinemeyer, C.R., Smith, S.A., Gabel, A.A. and Herd, R.P., 1984. The prevalence and intensity of internal parasites of horses in the U.S.A. Vet. Parasitol., 15: 75--83. Fifty-five adult horses were necropsied over a 15-month period, and their worm burdens counted and speciated. Twenty-one species of Cyathostominae were recovered. Ten species: Cyathostomum catinatum, Cylicocyclus nassatus, Cylicostephanus longibursatus,
Cyathostomum coronatum, Cylicostephanus goldi, C. calicatus, C. minutus, Cylicocyclus leptostomus, C. insigne and Cyathostomum pateraturn, comprised 98.9% of the total cyathostome burdens. These same 10 species also demonstrated high prevalences in 4 previous surveys. Eight o f these 10 species have been shown to be resistant to benzimidazole anthelmintics. Anthelmintic resistance apparently had little effect on the prevalence of a species, but appeared to increase its relative abundance within a mixed population. The prevalence of other internal parasites was similar to previous reports, but Strongylus vulgaris adults and arterial lesions were less common.
INTRODUCTION
There have been few comprehensive surveys of internal parasites of the horse in the U.S.A., and no attempts have been made to speciate the Cyathostominae as done by Ogbourne (1976) in Britain. The m o s t complete survey of equine internal parasites in the U.S.A. is a compilation of data accumulated over several years from 779 horses from 12 different regions (Hass, 1979), b u t the cyathostomes were not differentiated. A survey of 49 thoroughbreds in central K e n t u c k y was undertaken to evaluate the effectiveness of existent control measures against Strongylus vulgaris and Parascaris equorum, b u t other parasites were disregarded (Lyons et al., 1981). Some investigators have speciated the cyathostomes in the course of anthelmintic trials and found ten species to be resistant to benzimidazole drugs (Drudge et al., 1977, 1983; Wescott et al., 1982), however, most of these studies were done with small numbers of horses killed over a short period of time. * Author to w h o m correspondence should be addressed.
76
It was therefore decided to survey 4 horses per m o n t h over a 15-month period in Ohio, with special emphasis on the speciation, prevalence and relative abundance of the Cyathostominae. MATERIALS AND METHODS
Subjects were selected on the basis of a positive fecal egg count, as determined by the modified McMaster technique (Whitlock, 1948), from horses presented for necropsy at the Ohio State University College of Veterinary Medicine from May, 1981 to July, 1982. Age, sex, breed and relevant clinical data were recorded, and an a t t e m p t was made to determine the anthelmintic history of each animal. The aorta and major branches of the cranial mesenteric artery were examined for endarteritic lesions associated with larval Strongylus vulgaris. The stomach, small intestine, cecum and colon were ligated and separated. The stomach was opened longitudinally and examined for the presence of Draschia megastoma lesions. Gasterophilus larvae were detached from the mucosa, enumerated and speciated. Gastric contents were processed in the same manner as the contents of the large bowel, as described below. The small intestine was opened and examined grossly for parasites. The cecum and colon were opened separately, the contents removed and the mucosa washed with water. Contents and washings of each organ were made up to a convenient volume with water and a 10% aliquot removed. Each aliquot was washed over stacked 35 and 100 mesh screens (0.5 and 0.15 mm apertures, respectively) and preserved in 10% formalin. The entire aliquot, or in some cases a 10% subaliquot if the volume was excessive, was examined under an illuminated 3× lens and all parasites removed and counted. One-hundred worms were randomly selected from each aliquot and identified to developmental stage, sex, genus and species using the key of Lichtenfels (1975). The entire mucosa was scraped from the stomach and digested in pepsinhydrochloric acid at 37°C for 2--6 h. Digestion was halted by the addition of 37% formaldehyde. Ten percent aliquots of the stomach digests were examined with a dissecting microscope, and all worms were counted and speciated. Fecal samples from each horse were cultured at 27°C for 10 days to determine the relative contribution of cyathostomes to the strongyle egg counts. RESULTS
A total of 55 horses were examined from May, 1981 to July, 1982. An att e m p t was made to examine 4 horses per month, and this was accomplished with the exceptions of July, 1981 and June, 1982, when 1 and 2 horses were surveyed, respectively. Median age of the horses was 6 years and there was a nearly equal ratio of males to females. Breed distribution was divided among standardbreds, thoroughbreds and quarterhorses. Fecal eggs counts ranged from 24 to 4325 eggs per gram (e.p.g.), with a median of 328 e.p.g. Small
77
strongyles comprised 94.7% of the larvae recovered from fecal cultures. Anthelmintic histories proved t o o incomplete to be of any value in assessing the effects of prior treatment. Cyathostomes were recovered from all horses surveyed. Twenty-one small strongyle species were found. Species distribution ranged from 2 to 11 species per horse, with a median of 7 spp. per horse. Total c y a t h o s t o m e burdens ranged from 680 to 663 100 with a mean of 75 566 worms per horse. The colon contained 94.2% of the total lumen population of small strongyles. The prevalence and mean intensity of individual species of cyathostomes are represented in Fig. 1. Those species which were most prevalent also occurred in the highest average numbers. The 5 most prevalent species: Cya-
thostomum catinatum, Cylicocyclus nassatus, Cy licostephanus longibursatus, Cyathostomum coronatum and Cylicostephanus goldi, comprised 84% of the total lumen small strongyle population. The 10 most prevalent species, consisting of the previous 5 plus Cylicostephanus calicatus, C. minutus, Cylicocyclus leptostomus, C. insigne and Cyathostomum pateratum, comprised 98.9% of the total adult population. The remaining 11 species made up only 1.1% of the total population. Data on the non-cyathostome internal parasites of the horse were also recorded (Table I). The prevalence of various species of Strongylinae was low, averaging 27% for Strongylus vulgaris, and 10.9% and 1.8% for S. edentatus and S. equinus, respectively. Arterial lesions associated with larval S. vulgaris were present in 41.8% of horses. The prevalence of cestodes, MEAN INTENSITY (THOUSANDS)
PREVALENCE (%) 100 9 0 8 0 7 0 I
I
I
I
60 I
50 4 0 I
I
30 20 I
I
I0
0
I
I
0
L
i
5
I0
15
20
25
30
I
I
I
I
i
I
~
2
3 ~ 411 5 ~ 6i 711 8 ~ 9i ~omm 1 2 3 4 5 {. 7 8. 9 I0
C/athostomum catlrlatum C}izcocyclus nassatus C/llcostephanus longlbursa~us C/athostomum coronatum C~[icostephanus goldl C?llco~tephanus callcatus C~llcostephanus mlnutus C/llcocyclus leptostomus Cyllcoc/clus inslgne CIathostomum paterattur
II •
1i
C/athostomum
•
I~' •
12
• • •
13 11 I4 i I 5 II
Cyathostomum labratum 3~ alocephalus capltatu~ Poterlostomum Fatzll CyllcostephanJs pocdlatu ~ Cyllcocyclus brevlcapsulatus Cyllcodontophoru% blcoronatu~ Cyll _odontophorus euproctus C]il-~ocyclus ultra]ectlnus C}llcodontophoru~ mettaml Pote rlostomum impar identa ttml
11 11
I6 i I7 •
i
18 ~
|Mgi I 2011 B2MI
13 14 15 16 17 18 19 20 21
lablatum
Fig. 1. Prevalence and mean intensity of cyathostome species from Ohio horses at necropsy, 1981--1982.
78 TABLE I P r e v a l e n c e a n d m e a n i n t e n s i t y o f n o n - c y a t h o s t o m e internal parasites o f Ohio horses at necropsy, 1981--1982 Parasite
Prevalence
Mean intensity
Strongylus vulgaris Strongylus edentatus Strongylus equinus Craterostomum acuticaudatum Triodontophorus serratus Triodontophorus minor Ha bronema muscae Draschia megastoma Anoplocephala perfohata Gasterophilus in testinahs Gasterophilu s n asalis Oxyuris equi Parascaris e q uo rum Draschia lesions Strongylus vulgarls lesions
27 10.9 1.8 1.8 3.6 1.8 71 47 18
71 10. 9 10.9
18 40 41.8
91 5 600 1744 20 1888 504 155 147
119 14.5 4708
167 ---
ascarids, and pinworms was low, averaging 18% or less. Gastric parasites, including bots, spirurids, and associated lesions were c o m m o n . Digestion of gastric mucosa revealed Habronema muscae and Draschia megastoma, and in every case, digests recovered the same species found in the contents. Setaria equina was f o u n d in small numbers in the peritoneal cavity of one animal. A single adult male Haemonchus contortus was f o u n d in the colon cont ent s of one horse. Strongyloides westeri, Habronema majus, Probstmayria vivipara, Anoplocephala magna and Paranoplocephala mamillana were n o t found. DISCUSSION
The results o f this survey are similar t o those o f Ogbourne (1976). Twent y - o n e species o f C y a t h o s t o m i n a e were observed in each study, although horses in the British survey had larger w orm burdens and were infected with m o r e species on the average. Great variability in w o r m numbers was obvious, because at least a h u n d r e d fold difference existed between the lowest and highest burdens in each study. Ranges of 6 8 0 - - 6 6 3 1 0 0 and 1 2 0 0 0 - 1 239 000 worms per horse were seen in the present and British studies, respectively. These differences m ay be attributable to varying systems of management or to t he degree o f anthelmintic exposure. The cecum was shown to be a m i n o r source o f cyat host om es, containing only 5.8% o f the total adult population. This is in agreement with the results o f Ogbourne (1976} w h o recovered only 5% of the total small strongyles fr o m th e cecum. In his review o f the Cyat host om i nae, Ogbourne (1978)
79 states that less than 10% of cyathostomes normally reside in the cecum. Cyathostomes comprised 94.7% of strongyle larvae derived by fecal culture. This is in agreement with other publications that credit 95% of the equine strongyle egg output to the Cyathostominae (Drudge, 1978). Thus, small strongyles are the major objective of control programs based on reduction of fecal egg counts. The results show that a small number of species comprise the majority of the total cyathostome population. This finding is consistent with the conclusions of Ogbourne (1978) in his comprehensive review of the Cyathostominae, and with survey results from Panama (Foster, 1936), Czechoslovakia (Barus, 1962), and Britain (Mathieson, 1964; Ogbourue, 1976). This was a conclusive finding in spite of the fact that the 55 horses examined originated from 54 different farms with various systems of housing, management and anthelmintic exposure. These factors could be expected to give marked variability to the parasite populations examined. The species comprising the 10 most prevalent cyathostomes in this survey are in close agreement with previous reports. Although the relative ranking varied among surveys, the composition of the 10 major species was quite stable. Accordingly, Ogbourne (1976) concurred 100% with the parasites listed in the present report. Foster (1936), Barus (1962) and Mathieson (1964) each included 8 of the same species. The disparate 2 species were a non-cyathostome, Strongylus vulgaris, and Cyathostomum labiatum (Foster, 1936), C. labiatum and Gyalocephalus capitatus {Mathieson, 1964), and G. capitatus and Cylicodontophorus bicononatus (Barus, 1962). The relative prevalence of the 10 most common cyathostome species was compiled from the present report and the papers cited above. As determined by the median prevalence ranking in these 5 studies, Cylicostephanus longibursatus, Cyathostomum catinatum, Cylicocyclus nassatus, Cylicostephanus goldi and C. calicatus are the 5 most prevalent species, and Cyathostomum coronatum, Cylieostephanus minutus, Cylicocyclus insigne, C. leptostomus and Cyathostomum pateratum complete the top 10. The strong similarities in these surveys demonstrate the stability of cyathostome populations in different areas of the world over the past 45 years, and suggest that the prevalence of most species has been altered minimally by the advent and utilization of modern equine anthelmintics. Populations of strongyles resistant to phenothiazine were reported in the United States in 1961 (Drudge and Elam), and resistance of small strongyles to thiabendazole was documented in 1965 (Drudge and Lyons). Since that time, resistance to benzimidazoles has been documented in horses in Britain (Round et al., 1974), Canada (Slocombe and Cote, 1977) and Australia (Arundel, 1978; Barger and Lisle, 1979; Kelly et al., 1981). Ten species of small strongyles have been reported to be resistant: Cylieocyclus nassatus, C. insigne, Cylicostephanus goldi, C. longibursatus, Cyathostomum catinarum, C. coronatum, and C. labiatum (Drudge et al., 1977), Cylicocyclus leptostomus and C. brevicapsulatus (Wescott et al., 1982) and Cylico-
80
stephanus minutus (Drudge et al., 1983). Eight of these resistant species are among the 10 most c o m m o n , as listed above. It appears that the relative prevalence has not been changed by anthelmintic resistance in view of the continued stability of the dominant species for 45 years. The question has been raised as to whether certain species are so prevalent because anthelmintic resistance has made them predominant, or whether the size, genetic potential and plasticity of a population enable it to respond successfully to selection pressure from certain anthelmintics. The evidence supports the latter conclusion. Anthelmintic resistance appears to have a greater influence on the mean intensity of c y a t h o s t o m e species. Cylicocyclus leptostomus, which was recently shown to be resistant to fenbendazole (Wescott et al., 1982) is a case in point. It was ranked 16th in prevalence by Foster (1936), 10th by Barus (1962), and was not found at all by Mathieson (1964). Ogbourne (1976) listed C. leptostomus as the 8th most prevalent small strongyle and as having the 7th highest mean intensity. In the present survey, C. leptostomus was still 8th in prevalence, b u t was elevated to the third highest mean intensity. This pattern suggests the evolution of C. leptostomus from relative obscurity in the pre-benzimidazole era to a position of significance after several years of benzimidazole availability. The remaining species of small strongyles that fall outside the top 10 comprised only 1.1% of the total adult population and are therefore of little practical importance. Their total numbers within a host m a y be limited by factors such as specific nutritional requirements (Crompton, 1973) or low fecundity. Anthelmintic resistance has been demonstrated in Cyathostomum labiatum (Drudge et al., 1977) and Cylicocyclus brevicapsulatus (Wescott et al., 1982), b u t it is dubious whether these species would ever establish themselves in significant numbers in a mixed population of small strongyles. Cyathostome larvae were also recovered from the gut lumen, and their mean intensities determined. Larval cyathostomes currently cannot be differentiated to species. Further investigations are required in order to identify them and determine their population dynamics. The prevalence of non-cyathostome parasites recorded in Table I is similar to most previous reports, but adult Strongylus vulgaris were present in only 27% of horses, compared to 100% (Foster and Ortiz, 1937; Mathieson, 1964), 84% (Whitlock and Leasure, 1939), 71.4% (Barus, 1962) and 85.4% (Slocombe and McGraw, 1973). However, a report from Kentucky (Lyons et al., 1981) showed only 39% of horses to be mfected with S. vulgaris. Associated arterial lesions, 41.8%, were similar to 40% (Lyons et al., 1981), b u t less c o m m o n compared to 79% (Foster and Clark, 1937), 95.4% (Ottaway and Bingham, 1946), 93% (Poynter, 1960) and 64.4% (Ogbourne, 1975). The decreased prevalence in recent North American reports may be due in part to the frequent, repeated use of benzimidazole anthelmintics in the United States. Resistance has drastically reduced the efficacy of these drugs against cyathostomes, b u t benzimidazole resistance has n o t y e t been
81 reported in large strongyles. These drugs appear to have been relatively successful in controlling S. vulgaris in the United States. Habronema muscae was the most c o m m o n gastric nematode found. It was present in 71% of horses examined and averaged over 500 worms per infection. Habronema rnuscae was also shown to be the dominant gastric spirurid {74%) by Foster and Ortiz (1937), but burdens averaged only 21 worms per horse. Habronema muscae was also present in 95.8% of equine stomachs examined in Morocco (Pandey et al., 1981). Draschia megastoma, 47%, was relatively c o m m o n and frequently associated with fibrous nodules near the margo plicatus. In contrast to this survey, D. megastoma was found in very low numbers in only 8% of horses examined in Panama (Foster and Ortiz, 1937), and was not seen in any of 94 horses examined in Morocco (Pandey et al., 1981). Hass (1979) reported the prevalence o f H . muscae and D. megastoma under the combined designation "stomach w o r m s " as 59% in adult horses from various regions of the United States. Habronema majus was n o t observed in the present survey, nor by Hass (1979), although it had a prevalence o f 75.6% and 62% in Morocco (Pandey et al., 1981) and Panama (Foster and Ortiz, 1937), respectively. Trichostrongylus axei was n o t observed in this study although Herd et al. (1981) reported finding T. axei larvae in 14% of 188 positive fecal cultures from two horse farms in Ohio, and T. axei accounted for 100% of the larvae in 6% of the cultures. Trichostrongylus axei has a reported prevalence of 80.9% in Morocco (Pandey et al., 1981). Gasterophilus intestinalis was the most c o m m o n bot, being present in 71% of the stomachs examined. Other North American surveys have also demonstrated a high prevalence: 100% (Schooley et al., 1971), 98.7% (Drudge et al., 1975), and 99% (Hass, 1979). Gasterophilus nasalis was found in only 10% of the horses. This figure is lower than previously reported values of 65% (Schooley et al., 1971), 80.7% (Drudge et al., 1975), and 62% (Hass, 1979). Two larvae of Gasterophilus hemorrhoidalis were found in one horse. This parasite has been reported to be u n c o m m o n in the United States (Hass, 1979). The low prevalences of Anoplocephala perfoliata, Parascaris equorum and Oxyuris equi were probably related to the mature age of the horses examined. Cestodes (Dunn, 1978; BeUo, 1979) and P. equorum and O. equi (Drudge and Lyons, 1977) have been reported more frequently from y o u n g animals, and acquired resistance to P. equorum usumly develops before the second year of life (Drudge, 1972). The prevalence of ascarids in this survey is lower than the 39% reported from K e n t u c k y thoroughbreds (Lyons et al., 1981), b u t the K e n t u c k y horses had a lower mean age and were less likely to have developed immunity to P. equorum. Strongyloides westeri was n o t observed, but no attempt was made to examine the contents or mucosa of the small intestine microscopically. In addition, foals become resistant to S. westeri after 6 months of age (Drudge, 1972), and the youngest horse in this study was a yearling. Probstmayria
82
vivipara w a s l i k e w i s e n o t seen, b u t t h i s n e m a t o d e is v e r y small ( D u n n , 1 9 7 8 ) a n d m a y h a v e p a s s e d t h r o u g h t h e 0 . 1 5 m m a p e r t u r e screen. A n a d u l t m a l e Haemonchus contortus was f o u n d in t h e c o l o n c o n t e n t s o f o n e a n i m a l . This p a r a s i t e m a y h a v e b e e n i n g e s t e d a c c i d e n t a l l y a n d was passing o u t of the digestive tract when recovered. However, the w o r m was m o r p h o l o g i c a l l y i n t a c t , a n d t h e h o r s e h a d b e e n c o n f i n e d t o a stall a n d segreg a t e d f r o m r u m i n a n t s f o r several d a y s p r i o r t o e u t h a n a s i a . T h i s raises t h e p o s s i b i l i t y t h a t H. contortus c a n d e v e l o p in t h e g a s t r o i n t e s t i n a l t r a c t o f t h e horse. REFERENCES Arundel, J.H., 1978. Parasitic diseases of the horse. Veterinary Review No. 18. The PostGraduate Foundation in Veterinary Science, Sydney, N.S.W., 83 pp. Barger, I.A. and Lisle, K.A., 1979. Benzirnidazole resistance in small strongyles of horses. Aust. Vet. J., 55: 594--595. Barus, V., 1962. Helmintofauna koni v Ceskoslovensku. Cesk. Parazitol., 9: 15--94. Bcllo, T.R., 1979. Perspectives on current equine anthelmintic therapy misunderstandings and clarification. In: Proc. 25th Annu. Meeting Am. Assoc. Equine Pract., Miami Beach, FL, pp. 261--265. Crompton, D.W.T., 1973. The sites occupied by some parasitic helminths in the alimentary tract of vertebrates. Biol. Rev., 48: 27--83. Drudge, J.H., 1972. Endoparasitism. In: E.J. Catcott and J.F. Smithcors (Editors), Equine Medicine and Surgery, Vol. 2. American Veterinary Publications, Wheaton, IL, 960 pp. Drudge, J.H., 1978. Parasites of Horses. American Hoechst Corp., Somerville, NJ, 17 pp. Drudge, J.H. and Elam, G., 1961. Preliminary observations on the resistance of horse strongyles to phenothiazine. J. Parasitol.,47 (Suppl.): 38--39. Drudge, J.H. and Lyons, E.T., 1965. Newer developments in helminth control and Strongylus vulgaris research. In: Proc. 11th Annu. Meeting A m . Assoc. Equine Pract., Miami Beach, FL, p. 381. Drudge, J.H. and Lyons, E.T., 1977. Methods in the evaluatlon of antiparasitic drugs in the horse. Am. J. Vet. Res., 38: 1581-1586. Drudge, J.H., Lyons, E.T., Wyant, Z.N. and Tolliver, S.C., 1975. Occurrence of second and third instars of Gasterophilus intestinalis and Gasterophilus nasahs in stomachs of horses in Kentucky. Am. J. Vet. Res., 36: 1585--1588. Drudge, J.H., Lyons, E.T. and Tolliver, S.C., 1977. Resistance of equine strongyles to thiabendazole: critical tests of two strains. Vet. Med. Small Anita. Clin., 72: 433--438. Drudge, J.H., Lyons, E.T., Swerczek, T.W. and Tolliver, S.C., 1983. Cambendazole for strongyle control in a pony band: selection of a drug-resistant population of small stongyles and teratologic implications. Am. J. Vet. Res., 44: 110--114. Dunn, A.M., 1978. Veterinary Helminthology. Butler and Turner, Frome and London, 323 pp. Foster, A.O., 1936. A quantitative study of the nematodes from a selected group of equines in Panama. J. Parasitol., 22: 479--510. Foster, A.O. and Clark, H.C., 1937. Verminous aneurysms in the equines of Panama. Am. J. Trop. Med., 17: 85--93. Foster, A.O. and Ortiz, O.P., 1937. A further report on the parasites of a selected group of equines in Panama. J. Parasitol., 23: 360-364. Hass, D.K., 1979. Equine parasitism. Vet. Med. Small Anita. Clin., 74. 980--988. Herd, R_P., Miller, T.B. and Gabel, A.A., 1981. A field evaluation of pro-benzimidazole, benzimidazole and non-benzimidazole anthelmintics in the horse. J. Am. Vet. Med.
83 Assoc., 179: 686--691. Kelly, J.D., Webster, J.H., Griffin, D.L., Whitlock, H.V., Martin, I.C.A. and Gunawan, M., 1981. Resistance to benzimidazole anthelmintics in equine strongyles. Aust. Vet. J., 57: 163--171. Lichtenfels, J.R., 1975. Helminths of domestic equids. Illustrated keys to genera and species with emphasis on North American forms. Proc. Helminthol. Soc. Wash., 42: (special issue) 92 pp. Lyons, E.T., Drudge, J.H., Swerczek, T.W., Crowe, M.M. and Tolliver, S.C., 1981. Prevalence of Strongylus vulgaris and Parascaris equorum in Kentucky thoroughbreds at necropsy. J. Am. Vet. Med. Assoc., 179: 818--819. Mathieson, A.O., 1964. A study into the distribution and tissue responses associated with some internal parasites o f the horse. Master's Thesis, University of Edinburgh, 160 pp. Ogbourne, C.P., 1975. Studies on the epidemiology of Strongylus vulgaris infection of the horse. Int. J. Parasitol., 5: 423--426. Ogbourne, C.P., 1976. The prevalence, relative abundance and site distribution of nematodes of the subfamily Cyathostominae in horses killed in Britain. J. Helminthol., 50: 203--214.
Ogbourne, C.P., 1978. Pathogenesis of cyathostome (Trichonema) infections of the horse. A review. Commonwealth Institute of Helminthology. Commonwealth Agricultural Bureau of the United Kingdom. Miscellaneous Publication No. 4, 25 pp. Ottaway, C. and Bingham, M.L., 1946. Further observations on the incidence of parasitic aneurysms in the horse. Vet. Rec., 58: 155--159. Pandey, V.S., OuheUi, H. and Elkhalfane, A., 1981. Epidemiological observations on stomach worms o f horses in Morocco. J. Helminthol., 55: 155--160. Poynter, D., 1960. The arterial lesions produced by Strongylus vulgaris and their relationshiv to the migratory route of the parasite in its host. Res. Vet. Sci., 1: 205--217. Round, M.C., Simpson, D.J., Haselden, C.S., Glendenning, E.S.A. and BaskerviUe, R.E., 1974. Horse strongyles' resistance to anthelmintics. Vet. Rec., 95: 517--518. Schooley, M.S., Marsland, W.P. and Fogg, T.J., 1971. Monthly distribution of Gasterophdus species in horses in the United States: Implications on treatment schedules. Vet. Med. Small Anita. Clin., 66: 592--593. Slocombe, J.O.D. and Cote, J.F., 1977. Small strongyles of horses with cross-resistance to benzimidazole anthelmintics and susceptibility to unrelated compounds. Can. Vet. J., 18: 212--216. Slocombe, J.O.D. and McGraw, B.M., 1973. Gastrointestinal nematodes in horses in Ontario. Can. Vet. J., 14: 101--105. Wescott, R.B., Jen, L.W., Hellier, L.E., Stenslie, J.L. and Torbeck, R.L., 1982. Efficacy of combinations of piperazine and fenbendazole against benzimidazole-resistant small strongyles in horses. Vet. Med. Small Anita. Clin., 77: 247--249. Whitlock, H.V., 1948. Some modifications of the McMaster helminth egg-counting technique and apparatus. J. Counc. Sci. Ind. Res., 21: 177--180. Whitlock, J.H. and Leasure, E.E., 1939. Studies u p o n Strongylus vulgaris I. The incidence of Strongylus vulgaris in mid-continental North America and the reaction of the infested caeca. Am. J. Hyg. Sect. D, 29: 83- 87.
75
THE PREVALENCE A N D INTENSITY OF INTERNAL PARASITES OF HORSES IN THE U.S.A.
C.R. R E I N E M E Y E R * , S.A. SMITH, A.A. GABEL 1 and R.P. HERD
Department of Veterinary Pathobiology and ~Department of Veterinary Clinical Sciences, The Ohio State University, 1925 Coffey Road, Columbus, OH 43210 (U.S.A.) (Accepted for publication 4 October 1983)
ABSTRACT Reinemeyer, C.R., Smith, S.A., Gabel, A.A. and Herd, R.P., 1984. The prevalence and intensity of internal parasites of horses in the U.S.A. Vet. Parasitol., 15: 75--83. Fifty-five adult horses were necropsied over a 15-month period, and their worm burdens counted and speciated. Twenty-one species of Cyathostominae were recovered. Ten species: Cyathostomum catinatum, Cylicocyclus nassatus, Cylicostephanus longibursatus,
Cyathostomum coronatum, Cylicostephanus goldi, C. calicatus, C. minutus, Cylicocyclus leptostomus, C. insigne and Cyathostomum pateraturn, comprised 98.9% of the total cyathostome burdens. These same 10 species also demonstrated high prevalences in 4 previous surveys. Eight o f these 10 species have been shown to be resistant to benzimidazole anthelmintics. Anthelmintic resistance apparently had little effect on the prevalence of a species, but appeared to increase its relative abundance within a mixed population. The prevalence of other internal parasites was similar to previous reports, but Strongylus vulgaris adults and arterial lesions were less common.
INTRODUCTION
There have been few comprehensive surveys of internal parasites of the horse in the U.S.A., and no attempts have been made to speciate the Cyathostominae as done by Ogbourne (1976) in Britain. The m o s t complete survey of equine internal parasites in the U.S.A. is a compilation of data accumulated over several years from 779 horses from 12 different regions (Hass, 1979), b u t the cyathostomes were not differentiated. A survey of 49 thoroughbreds in central K e n t u c k y was undertaken to evaluate the effectiveness of existent control measures against Strongylus vulgaris and Parascaris equorum, b u t other parasites were disregarded (Lyons et al., 1981). Some investigators have speciated the cyathostomes in the course of anthelmintic trials and found ten species to be resistant to benzimidazole drugs (Drudge et al., 1977, 1983; Wescott et al., 1982), however, most of these studies were done with small numbers of horses killed over a short period of time. * Author to w h o m correspondence should be addressed.
76
It was therefore decided to survey 4 horses per m o n t h over a 15-month period in Ohio, with special emphasis on the speciation, prevalence and relative abundance of the Cyathostominae. MATERIALS AND METHODS
Subjects were selected on the basis of a positive fecal egg count, as determined by the modified McMaster technique (Whitlock, 1948), from horses presented for necropsy at the Ohio State University College of Veterinary Medicine from May, 1981 to July, 1982. Age, sex, breed and relevant clinical data were recorded, and an a t t e m p t was made to determine the anthelmintic history of each animal. The aorta and major branches of the cranial mesenteric artery were examined for endarteritic lesions associated with larval Strongylus vulgaris. The stomach, small intestine, cecum and colon were ligated and separated. The stomach was opened longitudinally and examined for the presence of Draschia megastoma lesions. Gasterophilus larvae were detached from the mucosa, enumerated and speciated. Gastric contents were processed in the same manner as the contents of the large bowel, as described below. The small intestine was opened and examined grossly for parasites. The cecum and colon were opened separately, the contents removed and the mucosa washed with water. Contents and washings of each organ were made up to a convenient volume with water and a 10% aliquot removed. Each aliquot was washed over stacked 35 and 100 mesh screens (0.5 and 0.15 mm apertures, respectively) and preserved in 10% formalin. The entire aliquot, or in some cases a 10% subaliquot if the volume was excessive, was examined under an illuminated 3× lens and all parasites removed and counted. One-hundred worms were randomly selected from each aliquot and identified to developmental stage, sex, genus and species using the key of Lichtenfels (1975). The entire mucosa was scraped from the stomach and digested in pepsinhydrochloric acid at 37°C for 2--6 h. Digestion was halted by the addition of 37% formaldehyde. Ten percent aliquots of the stomach digests were examined with a dissecting microscope, and all worms were counted and speciated. Fecal samples from each horse were cultured at 27°C for 10 days to determine the relative contribution of cyathostomes to the strongyle egg counts. RESULTS
A total of 55 horses were examined from May, 1981 to July, 1982. An att e m p t was made to examine 4 horses per month, and this was accomplished with the exceptions of July, 1981 and June, 1982, when 1 and 2 horses were surveyed, respectively. Median age of the horses was 6 years and there was a nearly equal ratio of males to females. Breed distribution was divided among standardbreds, thoroughbreds and quarterhorses. Fecal eggs counts ranged from 24 to 4325 eggs per gram (e.p.g.), with a median of 328 e.p.g. Small
77
strongyles comprised 94.7% of the larvae recovered from fecal cultures. Anthelmintic histories proved t o o incomplete to be of any value in assessing the effects of prior treatment. Cyathostomes were recovered from all horses surveyed. Twenty-one small strongyle species were found. Species distribution ranged from 2 to 11 species per horse, with a median of 7 spp. per horse. Total c y a t h o s t o m e burdens ranged from 680 to 663 100 with a mean of 75 566 worms per horse. The colon contained 94.2% of the total lumen population of small strongyles. The prevalence and mean intensity of individual species of cyathostomes are represented in Fig. 1. Those species which were most prevalent also occurred in the highest average numbers. The 5 most prevalent species: Cya-
thostomum catinatum, Cylicocyclus nassatus, Cy licostephanus longibursatus, Cyathostomum coronatum and Cylicostephanus goldi, comprised 84% of the total lumen small strongyle population. The 10 most prevalent species, consisting of the previous 5 plus Cylicostephanus calicatus, C. minutus, Cylicocyclus leptostomus, C. insigne and Cyathostomum pateratum, comprised 98.9% of the total adult population. The remaining 11 species made up only 1.1% of the total population. Data on the non-cyathostome internal parasites of the horse were also recorded (Table I). The prevalence of various species of Strongylinae was low, averaging 27% for Strongylus vulgaris, and 10.9% and 1.8% for S. edentatus and S. equinus, respectively. Arterial lesions associated with larval S. vulgaris were present in 41.8% of horses. The prevalence of cestodes, MEAN INTENSITY (THOUSANDS)
PREVALENCE (%) 100 9 0 8 0 7 0 I
I
I
I
60 I
50 4 0 I
I
30 20 I
I
I0
0
I
I
0
L
i
5
I0
15
20
25
30
I
I
I
I
i
I
~
2
3 ~ 411 5 ~ 6i 711 8 ~ 9i ~omm 1 2 3 4 5 {. 7 8. 9 I0
C/athostomum catlrlatum C}izcocyclus nassatus C/llcostephanus longlbursa~us C/athostomum coronatum C~[icostephanus goldl C?llco~tephanus callcatus C~llcostephanus mlnutus C/llcocyclus leptostomus Cyllcoc/clus inslgne CIathostomum paterattur
II •
1i
C/athostomum
•
I~' •
12
• • •
13 11 I4 i I 5 II
Cyathostomum labratum 3~ alocephalus capltatu~ Poterlostomum Fatzll CyllcostephanJs pocdlatu ~ Cyllcocyclus brevlcapsulatus Cyllcodontophoru% blcoronatu~ Cyll _odontophorus euproctus C]il-~ocyclus ultra]ectlnus C}llcodontophoru~ mettaml Pote rlostomum impar identa ttml
11 11
I6 i I7 •
i
18 ~
|Mgi I 2011 B2MI
13 14 15 16 17 18 19 20 21
lablatum
Fig. 1. Prevalence and mean intensity of cyathostome species from Ohio horses at necropsy, 1981--1982.
78 TABLE I P r e v a l e n c e a n d m e a n i n t e n s i t y o f n o n - c y a t h o s t o m e internal parasites o f Ohio horses at necropsy, 1981--1982 Parasite
Prevalence
Mean intensity
Strongylus vulgaris Strongylus edentatus Strongylus equinus Craterostomum acuticaudatum Triodontophorus serratus Triodontophorus minor Ha bronema muscae Draschia megastoma Anoplocephala perfohata Gasterophilus in testinahs Gasterophilu s n asalis Oxyuris equi Parascaris e q uo rum Draschia lesions Strongylus vulgarls lesions
27 10.9 1.8 1.8 3.6 1.8 71 47 18
71 10. 9 10.9
18 40 41.8
91 5 600 1744 20 1888 504 155 147
119 14.5 4708
167 ---
ascarids, and pinworms was low, averaging 18% or less. Gastric parasites, including bots, spirurids, and associated lesions were c o m m o n . Digestion of gastric mucosa revealed Habronema muscae and Draschia megastoma, and in every case, digests recovered the same species found in the contents. Setaria equina was f o u n d in small numbers in the peritoneal cavity of one animal. A single adult male Haemonchus contortus was f o u n d in the colon cont ent s of one horse. Strongyloides westeri, Habronema majus, Probstmayria vivipara, Anoplocephala magna and Paranoplocephala mamillana were n o t found. DISCUSSION
The results o f this survey are similar t o those o f Ogbourne (1976). Twent y - o n e species o f C y a t h o s t o m i n a e were observed in each study, although horses in the British survey had larger w orm burdens and were infected with m o r e species on the average. Great variability in w o r m numbers was obvious, because at least a h u n d r e d fold difference existed between the lowest and highest burdens in each study. Ranges of 6 8 0 - - 6 6 3 1 0 0 and 1 2 0 0 0 - 1 239 000 worms per horse were seen in the present and British studies, respectively. These differences m ay be attributable to varying systems of management or to t he degree o f anthelmintic exposure. The cecum was shown to be a m i n o r source o f cyat host om es, containing only 5.8% o f the total adult population. This is in agreement with the results o f Ogbourne (1976} w h o recovered only 5% of the total small strongyles fr o m th e cecum. In his review o f the Cyat host om i nae, Ogbourne (1978)
79 states that less than 10% of cyathostomes normally reside in the cecum. Cyathostomes comprised 94.7% of strongyle larvae derived by fecal culture. This is in agreement with other publications that credit 95% of the equine strongyle egg output to the Cyathostominae (Drudge, 1978). Thus, small strongyles are the major objective of control programs based on reduction of fecal egg counts. The results show that a small number of species comprise the majority of the total cyathostome population. This finding is consistent with the conclusions of Ogbourne (1978) in his comprehensive review of the Cyathostominae, and with survey results from Panama (Foster, 1936), Czechoslovakia (Barus, 1962), and Britain (Mathieson, 1964; Ogbourue, 1976). This was a conclusive finding in spite of the fact that the 55 horses examined originated from 54 different farms with various systems of housing, management and anthelmintic exposure. These factors could be expected to give marked variability to the parasite populations examined. The species comprising the 10 most prevalent cyathostomes in this survey are in close agreement with previous reports. Although the relative ranking varied among surveys, the composition of the 10 major species was quite stable. Accordingly, Ogbourne (1976) concurred 100% with the parasites listed in the present report. Foster (1936), Barus (1962) and Mathieson (1964) each included 8 of the same species. The disparate 2 species were a non-cyathostome, Strongylus vulgaris, and Cyathostomum labiatum (Foster, 1936), C. labiatum and Gyalocephalus capitatus {Mathieson, 1964), and G. capitatus and Cylicodontophorus bicononatus (Barus, 1962). The relative prevalence of the 10 most common cyathostome species was compiled from the present report and the papers cited above. As determined by the median prevalence ranking in these 5 studies, Cylicostephanus longibursatus, Cyathostomum catinatum, Cylicocyclus nassatus, Cylicostephanus goldi and C. calicatus are the 5 most prevalent species, and Cyathostomum coronatum, Cylieostephanus minutus, Cylicocyclus insigne, C. leptostomus and Cyathostomum pateratum complete the top 10. The strong similarities in these surveys demonstrate the stability of cyathostome populations in different areas of the world over the past 45 years, and suggest that the prevalence of most species has been altered minimally by the advent and utilization of modern equine anthelmintics. Populations of strongyles resistant to phenothiazine were reported in the United States in 1961 (Drudge and Elam), and resistance of small strongyles to thiabendazole was documented in 1965 (Drudge and Lyons). Since that time, resistance to benzimidazoles has been documented in horses in Britain (Round et al., 1974), Canada (Slocombe and Cote, 1977) and Australia (Arundel, 1978; Barger and Lisle, 1979; Kelly et al., 1981). Ten species of small strongyles have been reported to be resistant: Cylieocyclus nassatus, C. insigne, Cylicostephanus goldi, C. longibursatus, Cyathostomum catinarum, C. coronatum, and C. labiatum (Drudge et al., 1977), Cylicocyclus leptostomus and C. brevicapsulatus (Wescott et al., 1982) and Cylico-
80
stephanus minutus (Drudge et al., 1983). Eight of these resistant species are among the 10 most c o m m o n , as listed above. It appears that the relative prevalence has not been changed by anthelmintic resistance in view of the continued stability of the dominant species for 45 years. The question has been raised as to whether certain species are so prevalent because anthelmintic resistance has made them predominant, or whether the size, genetic potential and plasticity of a population enable it to respond successfully to selection pressure from certain anthelmintics. The evidence supports the latter conclusion. Anthelmintic resistance appears to have a greater influence on the mean intensity of c y a t h o s t o m e species. Cylicocyclus leptostomus, which was recently shown to be resistant to fenbendazole (Wescott et al., 1982) is a case in point. It was ranked 16th in prevalence by Foster (1936), 10th by Barus (1962), and was not found at all by Mathieson (1964). Ogbourne (1976) listed C. leptostomus as the 8th most prevalent small strongyle and as having the 7th highest mean intensity. In the present survey, C. leptostomus was still 8th in prevalence, b u t was elevated to the third highest mean intensity. This pattern suggests the evolution of C. leptostomus from relative obscurity in the pre-benzimidazole era to a position of significance after several years of benzimidazole availability. The remaining species of small strongyles that fall outside the top 10 comprised only 1.1% of the total adult population and are therefore of little practical importance. Their total numbers within a host m a y be limited by factors such as specific nutritional requirements (Crompton, 1973) or low fecundity. Anthelmintic resistance has been demonstrated in Cyathostomum labiatum (Drudge et al., 1977) and Cylicocyclus brevicapsulatus (Wescott et al., 1982), b u t it is dubious whether these species would ever establish themselves in significant numbers in a mixed population of small strongyles. Cyathostome larvae were also recovered from the gut lumen, and their mean intensities determined. Larval cyathostomes currently cannot be differentiated to species. Further investigations are required in order to identify them and determine their population dynamics. The prevalence of non-cyathostome parasites recorded in Table I is similar to most previous reports, but adult Strongylus vulgaris were present in only 27% of horses, compared to 100% (Foster and Ortiz, 1937; Mathieson, 1964), 84% (Whitlock and Leasure, 1939), 71.4% (Barus, 1962) and 85.4% (Slocombe and McGraw, 1973). However, a report from Kentucky (Lyons et al., 1981) showed only 39% of horses to be mfected with S. vulgaris. Associated arterial lesions, 41.8%, were similar to 40% (Lyons et al., 1981), b u t less c o m m o n compared to 79% (Foster and Clark, 1937), 95.4% (Ottaway and Bingham, 1946), 93% (Poynter, 1960) and 64.4% (Ogbourne, 1975). The decreased prevalence in recent North American reports may be due in part to the frequent, repeated use of benzimidazole anthelmintics in the United States. Resistance has drastically reduced the efficacy of these drugs against cyathostomes, b u t benzimidazole resistance has n o t y e t been
81 reported in large strongyles. These drugs appear to have been relatively successful in controlling S. vulgaris in the United States. Habronema muscae was the most c o m m o n gastric nematode found. It was present in 71% of horses examined and averaged over 500 worms per infection. Habronema rnuscae was also shown to be the dominant gastric spirurid {74%) by Foster and Ortiz (1937), but burdens averaged only 21 worms per horse. Habronema muscae was also present in 95.8% of equine stomachs examined in Morocco (Pandey et al., 1981). Draschia megastoma, 47%, was relatively c o m m o n and frequently associated with fibrous nodules near the margo plicatus. In contrast to this survey, D. megastoma was found in very low numbers in only 8% of horses examined in Panama (Foster and Ortiz, 1937), and was not seen in any of 94 horses examined in Morocco (Pandey et al., 1981). Hass (1979) reported the prevalence o f H . muscae and D. megastoma under the combined designation "stomach w o r m s " as 59% in adult horses from various regions of the United States. Habronema majus was n o t observed in the present survey, nor by Hass (1979), although it had a prevalence o f 75.6% and 62% in Morocco (Pandey et al., 1981) and Panama (Foster and Ortiz, 1937), respectively. Trichostrongylus axei was n o t observed in this study although Herd et al. (1981) reported finding T. axei larvae in 14% of 188 positive fecal cultures from two horse farms in Ohio, and T. axei accounted for 100% of the larvae in 6% of the cultures. Trichostrongylus axei has a reported prevalence of 80.9% in Morocco (Pandey et al., 1981). Gasterophilus intestinalis was the most c o m m o n bot, being present in 71% of the stomachs examined. Other North American surveys have also demonstrated a high prevalence: 100% (Schooley et al., 1971), 98.7% (Drudge et al., 1975), and 99% (Hass, 1979). Gasterophilus nasalis was found in only 10% of the horses. This figure is lower than previously reported values of 65% (Schooley et al., 1971), 80.7% (Drudge et al., 1975), and 62% (Hass, 1979). Two larvae of Gasterophilus hemorrhoidalis were found in one horse. This parasite has been reported to be u n c o m m o n in the United States (Hass, 1979). The low prevalences of Anoplocephala perfoliata, Parascaris equorum and Oxyuris equi were probably related to the mature age of the horses examined. Cestodes (Dunn, 1978; BeUo, 1979) and P. equorum and O. equi (Drudge and Lyons, 1977) have been reported more frequently from y o u n g animals, and acquired resistance to P. equorum usumly develops before the second year of life (Drudge, 1972). The prevalence of ascarids in this survey is lower than the 39% reported from K e n t u c k y thoroughbreds (Lyons et al., 1981), b u t the K e n t u c k y horses had a lower mean age and were less likely to have developed immunity to P. equorum. Strongyloides westeri was n o t observed, but no attempt was made to examine the contents or mucosa of the small intestine microscopically. In addition, foals become resistant to S. westeri after 6 months of age (Drudge, 1972), and the youngest horse in this study was a yearling. Probstmayria
82
vivipara w a s l i k e w i s e n o t seen, b u t t h i s n e m a t o d e is v e r y small ( D u n n , 1 9 7 8 ) a n d m a y h a v e p a s s e d t h r o u g h t h e 0 . 1 5 m m a p e r t u r e screen. A n a d u l t m a l e Haemonchus contortus was f o u n d in t h e c o l o n c o n t e n t s o f o n e a n i m a l . This p a r a s i t e m a y h a v e b e e n i n g e s t e d a c c i d e n t a l l y a n d was passing o u t of the digestive tract when recovered. However, the w o r m was m o r p h o l o g i c a l l y i n t a c t , a n d t h e h o r s e h a d b e e n c o n f i n e d t o a stall a n d segreg a t e d f r o m r u m i n a n t s f o r several d a y s p r i o r t o e u t h a n a s i a . T h i s raises t h e p o s s i b i l i t y t h a t H. contortus c a n d e v e l o p in t h e g a s t r o i n t e s t i n a l t r a c t o f t h e horse. REFERENCES Arundel, J.H., 1978. Parasitic diseases of the horse. Veterinary Review No. 18. The PostGraduate Foundation in Veterinary Science, Sydney, N.S.W., 83 pp. Barger, I.A. and Lisle, K.A., 1979. Benzirnidazole resistance in small strongyles of horses. Aust. Vet. J., 55: 594--595. Barus, V., 1962. Helmintofauna koni v Ceskoslovensku. Cesk. Parazitol., 9: 15--94. Bcllo, T.R., 1979. Perspectives on current equine anthelmintic therapy misunderstandings and clarification. In: Proc. 25th Annu. Meeting Am. Assoc. Equine Pract., Miami Beach, FL, pp. 261--265. Crompton, D.W.T., 1973. The sites occupied by some parasitic helminths in the alimentary tract of vertebrates. Biol. Rev., 48: 27--83. Drudge, J.H., 1972. Endoparasitism. In: E.J. Catcott and J.F. Smithcors (Editors), Equine Medicine and Surgery, Vol. 2. American Veterinary Publications, Wheaton, IL, 960 pp. Drudge, J.H., 1978. Parasites of Horses. American Hoechst Corp., Somerville, NJ, 17 pp. Drudge, J.H. and Elam, G., 1961. Preliminary observations on the resistance of horse strongyles to phenothiazine. J. Parasitol.,47 (Suppl.): 38--39. Drudge, J.H. and Lyons, E.T., 1965. Newer developments in helminth control and Strongylus vulgaris research. In: Proc. 11th Annu. Meeting A m . Assoc. Equine Pract., Miami Beach, FL, p. 381. Drudge, J.H. and Lyons, E.T., 1977. Methods in the evaluatlon of antiparasitic drugs in the horse. Am. J. Vet. Res., 38: 1581-1586. Drudge, J.H., Lyons, E.T., Wyant, Z.N. and Tolliver, S.C., 1975. Occurrence of second and third instars of Gasterophilus intestinalis and Gasterophilus nasahs in stomachs of horses in Kentucky. Am. J. Vet. Res., 36: 1585--1588. Drudge, J.H., Lyons, E.T. and Tolliver, S.C., 1977. Resistance of equine strongyles to thiabendazole: critical tests of two strains. Vet. Med. Small Anita. Clin., 72: 433--438. Drudge, J.H., Lyons, E.T., Swerczek, T.W. and Tolliver, S.C., 1983. Cambendazole for strongyle control in a pony band: selection of a drug-resistant population of small stongyles and teratologic implications. Am. J. Vet. Res., 44: 110--114. Dunn, A.M., 1978. Veterinary Helminthology. Butler and Turner, Frome and London, 323 pp. Foster, A.O., 1936. A quantitative study of the nematodes from a selected group of equines in Panama. J. Parasitol., 22: 479--510. Foster, A.O. and Clark, H.C., 1937. Verminous aneurysms in the equines of Panama. Am. J. Trop. Med., 17: 85--93. Foster, A.O. and Ortiz, O.P., 1937. A further report on the parasites of a selected group of equines in Panama. J. Parasitol., 23: 360-364. Hass, D.K., 1979. Equine parasitism. Vet. Med. Small Anita. Clin., 74. 980--988. Herd, R_P., Miller, T.B. and Gabel, A.A., 1981. A field evaluation of pro-benzimidazole, benzimidazole and non-benzimidazole anthelmintics in the horse. J. Am. Vet. Med.
83 Assoc., 179: 686--691. Kelly, J.D., Webster, J.H., Griffin, D.L., Whitlock, H.V., Martin, I.C.A. and Gunawan, M., 1981. Resistance to benzimidazole anthelmintics in equine strongyles. Aust. Vet. J., 57: 163--171. Lichtenfels, J.R., 1975. Helminths of domestic equids. Illustrated keys to genera and species with emphasis on North American forms. Proc. Helminthol. Soc. Wash., 42: (special issue) 92 pp. Lyons, E.T., Drudge, J.H., Swerczek, T.W., Crowe, M.M. and Tolliver, S.C., 1981. Prevalence of Strongylus vulgaris and Parascaris equorum in Kentucky thoroughbreds at necropsy. J. Am. Vet. Med. Assoc., 179: 818--819. Mathieson, A.O., 1964. A study into the distribution and tissue responses associated with some internal parasites o f the horse. Master's Thesis, University of Edinburgh, 160 pp. Ogbourne, C.P., 1975. Studies on the epidemiology of Strongylus vulgaris infection of the horse. Int. J. Parasitol., 5: 423--426. Ogbourne, C.P., 1976. The prevalence, relative abundance and site distribution of nematodes of the subfamily Cyathostominae in horses killed in Britain. J. Helminthol., 50: 203--214.
Ogbourne, C.P., 1978. Pathogenesis of cyathostome (Trichonema) infections of the horse. A review. Commonwealth Institute of Helminthology. Commonwealth Agricultural Bureau of the United Kingdom. Miscellaneous Publication No. 4, 25 pp. Ottaway, C. and Bingham, M.L., 1946. Further observations on the incidence of parasitic aneurysms in the horse. Vet. Rec., 58: 155--159. Pandey, V.S., OuheUi, H. and Elkhalfane, A., 1981. Epidemiological observations on stomach worms o f horses in Morocco. J. Helminthol., 55: 155--160. Poynter, D., 1960. The arterial lesions produced by Strongylus vulgaris and their relationshiv to the migratory route of the parasite in its host. Res. Vet. Sci., 1: 205--217. Round, M.C., Simpson, D.J., Haselden, C.S., Glendenning, E.S.A. and BaskerviUe, R.E., 1974. Horse strongyles' resistance to anthelmintics. Vet. Rec., 95: 517--518. Schooley, M.S., Marsland, W.P. and Fogg, T.J., 1971. Monthly distribution of Gasterophdus species in horses in the United States: Implications on treatment schedules. Vet. Med. Small Anita. Clin., 66: 592--593. Slocombe, J.O.D. and Cote, J.F., 1977. Small strongyles of horses with cross-resistance to benzimidazole anthelmintics and susceptibility to unrelated compounds. Can. Vet. J., 18: 212--216. Slocombe, J.O.D. and McGraw, B.M., 1973. Gastrointestinal nematodes in horses in Ontario. Can. Vet. J., 14: 101--105. Wescott, R.B., Jen, L.W., Hellier, L.E., Stenslie, J.L. and Torbeck, R.L., 1982. Efficacy of combinations of piperazine and fenbendazole against benzimidazole-resistant small strongyles in horses. Vet. Med. Small Anita. Clin., 77: 247--249. Whitlock, H.V., 1948. Some modifications of the McMaster helminth egg-counting technique and apparatus. J. Counc. Sci. Ind. Res., 21: 177--180. Whitlock, J.H. and Leasure, E.E., 1939. Studies u p o n Strongylus vulgaris I. The incidence of Strongylus vulgaris in mid-continental North America and the reaction of the infested caeca. Am. J. Hyg. Sect. D, 29: 83- 87.