Equine Parasitology

Holly DorchakBiology 4800: Symbiotic InteractionsMarch 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. Benzimidazole3. Pyrantel •Two types of dewormers:1. Daily-added to feed, continual presence of drugs2. 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

ReferencesBliss, 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