SR-109: Strongyles in Horses

AGRICULTURAL EXPERIMENT STATION UNIVERSITY OF KENTUCKY COLLEGE OF AGRICULTURE, FOOD AND ENVIRONMENT, LEXINGTON, KY, 40546 SR-109 Strongyles in Horses Update 2015 E.T. Lyons and S.C. Tolliver, Veterinary Science Introduction Parasites live in a host from which they obtain food and pro- tection. They may harm but usually do not benefit the host. The Mature worms word “parasite” is derived from the Immature worms live in Latin and Greek languages mean- migrate in tissues intestinal tract ing, in general, “one who eats at the table of another.” It is said that Stages inside the horse a “good” parasite does not overtly harm or kill its host. It is theoreti- Stages outside the horse cally possible that a more benign parasite (e.g. Gasterophilus spp.) is Infective stages much “older in eons of time” and ingested in food and water Eggs passed it and its host have adjusted better in feces to each other than a conceivably “newer” parasite (e.g. Strongylus spp.) which may be more harmful Infective larvae to its host. develop Taxonomy Horses can harbor over 100 species of internal parasites. About Figure 1. Strongyle life cycle. one half of these species are nema- todes in the strongyle group (fam- ily Strongylidae Baird, 1853). They the genera Bidentostomum Tshoijo to the first stage larva (L1) which are separated taxonomically into in Popova, 1958, Craterostomum hatches and then develops to the two categories—large strongyles Boulenger, 1920, Oesophagodon- second stage larva (L2), and finally (subfamily Strongylinae Railliet, tus Railliet et Henry, 1902, and to the third stage larva (L3) which 1893) and small strongyles (cya- Triodontophorus Looss, 1902. The is the infective stage (Figure 3). The thostomes) (subfamily Cyathos- latter four genera are much less L1 and L2 feed on organic matter, tominae Nicoll, 1927). Historically, important because, unlike Stron- but the L3 does not feed but sub- large strongyles included only gylus spp., they do not migrate sists on internal nutrients. The L3 is those strongyles in the genus outside the intestinal tract. the infective stage which typically Strongylus Müller, 1780. In this crawls up on pasture vegetation, discourse, the latter designation Life Cycle especially under moist conditions will be used for large strongyles Outside the Horse which make movement easier than i.e. it includes only Strongylus spp. Strongyles live as adults in the dry situations, and is then acciden- More recently in revision of the large intestine (cecum, ventral tally ingested by the grazing horse taxonomy of the strongyles, based colon, and dorsal colon) of the (Figure 4). Inside the horse the L3 on morphology, large strongyles horse and lay eggs that are voided develops to the fourth (L4) and include, besides, Strongylus spp., in the feces (Figures 1 and 2). In the then fifth (L5) (adult) stage. environment, an egg embryonates University of Kentucky • Lexington, Kentucky 40546 RESEARCH Figure 2. Strongyle eggs (small and large Figure 3. Free-living third stage (L3) infective Figure 4. Free-living L3 in moisture strongyle similar). strongyle larvae. droplet on grass. Inside the Horse parasites usually are most evident terior part of the small intestine, Within the genus Strongylus are during migration of immature cecum, and ventral colon, and en- four species—S. asini, S. edentatus, stages in organs outside the gas- ter arterioles in the walls of these S. equinus, and S. vulgaris. These trointestinal tract. organs. About two weeks later they species are the most pathogenic Strongylus vulgaris will be begin arriving and accumulating of the strongyles because they highlighted because it is the most primarily in the cranial mesenteric can cause colic and even death of pathogenic parasitic nematode artery (CMA) (Figures 5 and 6). horses. Strongylus vulgaris is the species in horses. When L3 are There they undergo development most damaging of the four spe- ingested they penetrate the intes- to the L4 and L5. Their presence cies. Detrimental effects of these tinal mucosa, mainly of the pos- stimulates the immune system POSTERIOR AORTA renals (eyes) cranial mesenteric (CMA) (nose) coeliac (mouth) Figure 5. Posterior aorta showing orifices of some Figure 6. Verminous aneurysm in cranial mesenteric ar- branching arteries with their equine anatomic names tery with celluar debris and migrating Strongylus vulgaris (= some features of a human face): renals (eyes), cranial larvae (see arrow) evident. mesenteric (nose), coeliac (mouth). Note: The cranial mesenteric artery is the major location of migrating Strongylus vulgaris larvae causing development of an aneurysm. 2 leaf crowns buccal teeth capsule tooth plug Figure 7. Strongylus vulgaris adult Figure 8. Cross-section of head/buccal capsule of Strongylus vulgaris adult at- male—anterior end. tached to plug of intestinal mucosa; plugs are bitten off/sucked in, dissolved and eaten. to “repel” these invaders. Upon and dorsal colon. After L5 excyst initial infection, this action is a they locate primarily in the ce- negative feature because the host cum, but also in the ventral colon, reaction produces cellular debris which are the usual locations for which may result in thrombi and mature adults that attach to and Adult male Strongylus vulgaris emboli that can block blood flow. feed on the mucosa of these organs Thus resulting in chronic or acute (Figures 7, 8, and 9). The prepatent colic and possible death of the period, the time since acquisition horse because of lessened or lack of L3 until females begin laying of blood supply to the intestine. In eggs, is about six months. Future Bottle Nose Dolphin addition, there is a condition called improved methodology may al- a “verminous aneurysm” which low more exact determination of Figure 9. Adult male Strongylus vulgaris (S.v.) showing is enlargement and thickening of the route of migrating larval S. typical “dolphin-type” posture in gross lateral view. Two the CMA and adjoining arteries. vulgaris. similarities of S.v. and the dolphin are body curvature and tails—bursa of S.v. and fluke of dolphin. One difference Acquired immunity to S. vulgaris Small Strongyle is that the base of the S.v. tail is slightly up curved (each is typical in varying degrees. The unit on scale = one millimeter representing length of S.v.) (Cyathostome) Group prepondence of literature states (size of dolphin not related to scale). The small strongyle group that the L4 and L5 come back to the large intestine via arteries. includes more than 50 species Some researchers wonder how worldwide. Only 10 or 12 species such large larvae can get back are the most common. Virtually, through the small arterioles. They 100 percent of horses are infected with at least some species of hypothesize that maybe the L5 that actually come back to the intestine small strongyles. Numbers of are ones that do not migrate to these worms are usually lower in older horses that have had time to the CMA. Possibly, L3 penetrate through the large intestinal wall develop some immunity to them. They are much less harmful than and undergo development to L4 Strongylus spp. because the infec- and L5 there on the serosa surface tive third stage (L3) penetrates before returning. In any event, L5 coming back (assuming they left) only into the lining of the large Figure 10. Small strongyle larvae encysted in mucosa— can be found encysted in the mu- intestine where it encysts (Fig- large intestine. cosa of the cecum, ventral colon ure 10). Here they develop to the fourth (L4) and sometimes young 3 Treatment/Drug Resistance compound. It should be men- Over several centuries various tioned that in the early 1950s the methods have been advocated barber pole nematode in sheep for control of internal parasites was documented in Kentucky of horses. It was not until the to be resistant to phenothiazine. early 1900s that scientific methods This was the first report on drug were begun to detect actual effi- resistance of any internal parasite cacy of antiparasitic compounds species in any animal. Finding against horse parasites. The first of small strongyles resistant to compound so-tested was carbon phenothiazine was an indicator disulfide which was proven to (in retrospect) that these parasites be efficacious against horse bots would potentially become resis- (Gasterophilus spp.) and asca- tant to other parasiticides used rids (Parascaris equorum). Since frequently. Thiabendazole, one of the early 1900s until presently numerous benzimidazoles, was (2015), more than 25 products commercially available for horses have become commercially avail- in the early 1960s, but very soon able for control of endoparasites small strongyles were observed to Figure 11. Small strongyle (cyathostome)—head in horses. These compounds are be resistant to this drug. Currently, (Cyathostomum catinatum). in only a few chemical classes both of the commercially available including benzimidazoles (BZs) benzimidazoles (fenbendazole (e.g. thiabendazole), phenylgua- and oxibendazole) and pyrantel fifth (L , adult) stages which excyst 5 nidines (Pro-BZ) (e.g. febantel); pamoate are ineffective on small usually by trickling out into the imidothiazoles (e.g. levamisole + strongyles (cyathostomes). The intestinal lumen and maturing piperazine), macrocyclic lactones macrocyclic lactones (ivermectin (Figure 11). Also they may remain (e.g. ivermectin and moxidectin), and moxidectin) are now less ef- encysted for long periods. In a organophosphates (e.g. dichlor- fective against small strongyles recent study, the earliest prepatent vos), pyrimidines (e.g. pyrantel) then initially. It was established period for a small strongyle species and others (carbon disfulfide, phe- that these compounds now have (Cylicostephanus longiburatus) nothiazine, and piperazine). No lower activity on luminal stages was about two months. There is new classes of equine parasiticides (especially fourth stages) of small great variation in the fecundity have been marketed in the past 25 strongyles so the life cycle is short- of species of strongyles.

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