Equine Parasitology
Holly Dorchak
Biology 4800: Symbiotic Interactions
March 18th 2011
Horse Parasites
•More than 150 species of parasites that attack horses:
•Endoparasites:
-trypanosomes
-nematodes (strongyles, ascarids, pinworms)
-cestodes
-arthropods (bot flies)
•Ectoparasites:
-biting flies (horse, stable…)
-ticks
-mites
-lice
•Clinical parasitism vs. subclinical parasitism
•Can look healthy but still be infected with half a million GI nematodes
•Left untreated parasite infections can be fatal
Bot Flies
•Very common
•Three species that parasitize horses:
common horse bot (Gasterophilus
intestinalis), throat bot and nose bot
•Lifecycle: lay eggs on horse (up to
900 in 3 hours), horse grooms itself
and ingests eggs, larva burrow in
mouth, month later they emerge, are
swallowed and burrow in intestine
and mature, attach themselves to
anal region and drop into soil and
pupate
•Pathology: digestive problems,
nutrient deprivation, larvae in mouth
causes irritation
Pinworms
•Oxyuris equi
•lifecycle: eggs ingested, hatch in
intestine, adults live in intestines
and migrate to anus to lay eggs,
causes frequent tail itching
•pathology: usually harmless-don’t
migrate through tissues, severe
itching can cause a secondary
infection
Strongyles
•most harmful horse parasite
•aka redworms, bloodworms
•Small (cyathastomes) and large
•S. vulgaris, S. edentates, S. equines
•lifecycle: larvae ingested on grass or water,
adults live in large intestine, eggs released
in feces, hatch, larvae climb grass
•pathology: the larvae of large strongyles
migrate through blood vessels of intestines
and damage them-leading to colic or vessel
rupturing
:small strongyles can encyst in the
intestines and can cause colic, diarrhea and
weight loss
Tapeworms
•3-8 cm long
•Anoplocephala perfoliata, A.
magna and Paranoplocephala
mamillana
•Lifecycle: soil mites eat tapeworm
eggs from feces, horse eats mites
containing cysticercoids, hook onto
illeo-cecal junction, mature into
adults, shed proglottids in feces
•Detection: hard to detect eggs in
feces microscopically, ELISA test
on blood
•Pathology: colic, diarrhea, poor
growth
Colic
•Abdominal pain
•Leading cause of death in horses (10% of horses die from colic)
•Not just due to parasites, also from gas build up, food impaction, sand
impaction, twisting of the gi tract…
•Symptoms: biting the stomach, pacing, rolling, loss of appetite, decreased fecal
output, high pulse
•Best way to prevent is by deworming
•Can have irreversible damage
•May need surgery
Parasite Avoidance
•Remove manure from pen daily
•Mowing and harrowing land can decrease amount of eggs and larvae present
•Keeping dewormed horses off pasture for 3 days post treatment
•Rotate pastures to reduce parasite build up
•Avoid overstocking
•Don’t feed on the ground or floor
•Provide clean water
•Give yearly veterinarian exams
Dewormers
•Not all dewormers can act on immature, encysted and migrating worms
•Three classes of dewormers:
1. Macrocyclic lactone (Ivermectin)
2. Benzimidazole
3. Pyrantel
•Two types of dewormers:
1. Daily-added to feed, continual presence of drugs
2. Purge-can be paste or on-feed treatments
Drug Resistance
•Ability of parasites in a population to survive a treatment that used to be
effective against the same population
•Each time a horse is dewormed, the resistant worms live to shed their eggs
onto the pasture
•Resistance seen to all 3 classes of dewormer drugs
•Small strongyles-97.7 percent resistant to some classes of drugs
•Not seeing resistance to ivermectin but are seeing some parasite eggs are
returning more quickly (2X) than they did previously
•Decrease of refugia of parasites can lead to resistance development
(Refugia=proportion of parasite population not affected by treatment)
Determining Resistance
•Fecal egg count reduction test (FECRT)-compares the number of parasite eggs
in the feces after treatment with the number that were there before treatment
•Egg reappearance period (ERP)- is the time interval between deworming and
the recurrence of parasite eggs in the feces (ivermectin usually has an ERP of 8
weeks)
Ways to Avoid Resistance
•Pasture management instead of heavy dewormer use
•Monitor fecal counts and treat only when necessary (100-500epg)
•Proper choice of anthelmintics
•Proper timing of treatment (infective larvae at their lowest numbers in the
winter, deworming during these unnecessary periods increases resistance)
•Monitor for resistance and act accordingly
•Try and determine which horses are more at risk or have weaker immune
systems
•Rotate between dewormers of different drug classes
•Give the correct dose of dewormers
•Control vs. eradication
References
Bliss, D. H. Equine parasitology the control of gastro-intestinal nematode parasites in horses with emphasis on
reducing environmental contamination: A new control strategy for an old problem.
Brady, H. A., Nichols, W. T. (2009). Drug resistance in equine parasites: an emerging global problem. J. Equine
Veterinary Science 29(5):285-295.
Herd, R. P. (1993). Control strategies for ruminant and equine parasites to counter resistance, encystment, and
ecotoxicity in the USA. Veterinary Parasitology, 48:327-336.
Kaufman PE, Koehler PG, Butler JF. (2006). Horse bots. EDIS. http://edis.ifas.ufl.edu/IG136 (13 December 2007).
McCraw, B. M., Slocombe, J. O. (1976). Strongylus vulgaris in the horse: a review. Can Vet J 17(6): 150-157.
Proudman, C. J. (2003). Diagnosis, treatment, and prevention of tapeworm-associated colic. Journal of Equine
Veterinary Science, 23:6-9.
Stoletenow, C. L., Purdy, C. H., (2003) Internal parasites of horses.
http://www.ag.ndsu.edu/pubs/ansci/horse/v543.pdf
Womack, S., Tarpley, H. L., Little, S. E., Latimer, K. S. (2006). Trypanosoma evansi in horses.
http://www.vet.uga.edu/vpp/clerk/womack/index.php
http://www.thehorse.com/Parasites/Parasites0904.pdf