Parasites and the damage they cause to the intestinal vasculature and mucosa contribute to GI disturbances, including impactions, motility disorders, diarrhea, and peritonitis. Migrating ascarids contribute to pulmonary inflammation in foals. The principal internal parasites of horses are nematodes. With the advent of more effective anthelmintics, the nematode species of importance have shifted from large strongyles to cyathostomes. Unfortunately, many species of adult cyathostomes have developed resistance to standard doses of the benzimidazoles and the tetrahydropyrimidines. Among intensively managed horse populations, treatment with macrocyclic lactones (eg, ivermectin and moxidectin) does not suppress strongyle egg counts for as long as previously expected. Among foals, weanlings, and even yearlings, there are increasing reports of ascarids becoming resistant to ivermectin and moxidectin, and more recently, to the tetrahydropyrimidines. The problem of anthelmintic resistance involves multiple parasites, multiple drug classes, and all age horses.

There is no single parasite control program that is ideal for all horses. Age of the horse, population density, region of the country, climate, and pasture size and quality can affect the choice of parasite control programs. As horses age, they develop resistance to reinfection with certain parasites, such as Strongyloides westeri and Parascaris equorum. Resistance to most strongyles is incomplete.

The 3 major classes of anthelmintics commonly used are macrocyclic lactones (avermectins and milbemycins), benzimidazoles, and tetrahydropyrimidines. Avermectins have a broad range of activity, are effective at low dosages, suppress fecal egg counts for moderate periods of time, and are active against adult and migrating larval nematodes and various ectoparasites including lice, mites, ticks, and bots (eg, Gasterophilus spp). Treatment of mares with ivermectin within 24–48 hr of foaling greatly reduces transmission of S westeri to foals via the dam's milk. Prompt deworming of the mare soon after delivery minimizes the newborn foal's potential exposure to parasite ova in the mare's manure as a result of coprophagy. Due to their efficacy against adult and migrating roundworms, avermectins should be included in the foal's anthelmintic schedule. Avermectins are not effective against tapeworms (Anaplocephala), Onchocerca adults, flukes, or encysted cyathostome larvae. Milbemycins (eg, moxidectin) have a longer tissue half-life and egg reappearance period and are effective against encysted cyathostome larvae. Milbemycins should not be used in foals <6 mo old or in horses that are underweight and debilitated.

Benzimidazoles are effective against ascarids, pinworms, and most nematodes; however, many species of adult cyathostomes have developed resistance to this class of anthelmintics. Increased dosages of this drug class are required to kill migrating immature large strongyles, encysted cyathostomes, and migrating ascarid larvae.

The tetrahydropyrimidines (eg, pyrantel) have reasonable efficacy against nematodes and ascarids and good activity against tapeworms when used at increased dosages.

A fourth class of anthelmintics, the isoquinolines, contains the drug praziquantel, which is a narrow-spectrum drug approved for horses as a cestocide. Praziquantel is marketed in combination with either ivermectin or moxidectin.

To slow the development of drug-resistant parasites, the focus of most parasite control programs is to reduce selection pressure for resistance by customizing deworming protocols for the farm and individual horses. This strategy includes identifying horses that are the most susceptible to parasites (ie, the high egg shedders) and maximizing refugia (those parasites that are not exposed to drug selection pressure) by reducing the overall number of anthelmintic treatments administered and by monitoring the efficacy of different classes of dewormers using the fecal egg count reduction test.

An effective deworming protocol for horses should incorporate a cestocide (eg, a praziquantel-containing product or a double dose of pyrantel pamoate) and a boticide (eg, ivermectin or moxidectin) once or twice a year to control tapeworms and bots, respectively, and a larvicidal dose of anthelmintic effective against encysted cyathostomes (eg, 10 mg/kg fenbendazole, PO, sid for 5 days, or a single dose of moxidectin) during early winter months in northern climates or during early summer in warmer climates. (Also see Gastrointestinal Parasites of Horses.) Foals should be maintained on a regular deworming schedule every 60 days that includes anthelmintics that are safe and effective against ascarids. Treatments effective against other nematodes (including encysted cyathostomes) and cestodes should be included in the deworming protocols for foals during the first year of life.

Effective deworming programs should also include one or more of the following nonchemical methods of parasite control: 1) Avoid overstocking and overgrazing pastures. 2) Keep pasture roughs mowed (3–8 in.). 3) During hot, dry weather, harrow or rake pastures to disperse manure piles and expose larvae to sun. Rest the pasture at least 3–4 wk after harrowing. 4). Cross-graze pastures with other species. Cattle, sheep, and goats serve as biological vacuums for equine parasites. 5) Make at least one cutting of hay off some pastures to help reduce the parasite burden. 6) Plant an annual crop such as winter wheat. 7) Feed hay and grain in raised containers and not directly on the ground. 8) Remove manure from stalls, paddocks, and pastures every 24–72 hr before strongyle eggs have a chance to hatch and develop into infective larvae (~5–7 days during optimal conditions). 9) Clean water sources regularly to prevent fecal contamination. 10) Quarantine new arrivals and check fecals. Use a larvicidal treatment before turning new arrivals out on pastures. 11) Use fecal egg counts, performed at the proper times, to identify and monitor high, medium, and low strongyle egg shedders, to monitor the efficacy of anthelmintics being used, and to evaluate new arrivals. 12) Compost manure. Properly composted manure will kill strongyle larvae and many ascarid eggs.

Last full review/revision July 2011 by Wendy E. Vaala, VMD, DACVIM