THE MERCK VETERINARY MANUAL
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Parasite Control in Horses

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Control of internal parasites is a cornerstone of equine management and a continual endeavor. Control programs should be tailored to each farm situation and require a cooperative program between the horse owner and the veterinarian. The principal internal parasites of horses are nematodes. 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, 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. (Also see Gastrointestinal Parasites of Horses and see Anthelmintics.)

A program of anthelmintic resistance involves multiple parasites and multiple drug classes, and must consider horses of all ages. No single parasite control program is ideal for all horses. Age of the horse, population density, region of the country, climate, method of confinement (eg, stall or pasture), 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. Although anthelmintics are the primary method of parasite control, other factors such as pasture management and stable environment are also important.

The parasites of greatest concern in horses are large strongyles, small strongyles, roundworms, pinworms, stomach bots, and tapeworms. GI parasites can cause acute medical problems as well as chronic debilitation. They may be responsible for diarrhea, intestinal impactions or irritation (colic), and poor performance. The result can be slow growth in young horses, poor performance, acute bouts of colic, and death in severe instances. The migrating ascarids (Parascaris equorum) are the primary cause of pulmonary inflammation and intestinal rupture in foals and weanlings. The control of large strongyles with improved anthelmintics has allowed the small strongyles (cyanthostomes) to become a greater problem. Reports of tapeworms have also increased.

The three methods of diagnosis of parasites are direct examination of feces, concentrations of worm eggs by flotation, and culture of feces for infective larva. The direct examination method is quick but not always accurate. The flotation method for parasite egg concentration is more accurate to determine the type and number of parasites. The culture method is more situated for laboratory use and research purposes. The effectiveness of any parasite control program can be evaluated by fecal examination. The fecal examination should be done before administration of a particular dewormer and then again 14 days later. The amount of decreases in egg population will determine the effectiveness of the dewormer. Parasite egg counts that remain high after use of a particular dewormer indicate the presence of resistance and a need to change the control program.

The three major classes of anthelmintics available for parasite control in horses are the avermectins, benzimidazoles, and pyrantels. The avermectins have a broad range of activity, are safe and effective at low doses, and control adult and migrating larval nematodes. Deworming of mares right after foaling lessens the foal's exposure to parasites in the mare's manure. Avermectins are not effective against tapeworms; however, the combination of ivermectin with praziquantel is effective against tapeworms as well as large and small strongyles, ascarids, pinworms, and bots. The benzimidazoles (oxibendazole, fenbendazole, and oxfendazole) are effective against most nematodes but not against encysted small strongyles or tapeworms. The pyrantels are effective against large and small strongyles, ascarids, and pinworms. Pyrantels kill nematode parasites slowly. Pyrantel is available as a paste (single dose), and as a tartrate salt that can be used as a daily dewormer mixed in the feed; it is effective against tapeworms but not bots. The avermectins are effective against bot larva and are recommended in the fall after the botfly season.

A fourth class of anthelmintics, the isoquinolines, contains praziquantel, a narrow-spectrum drug approved for horses as a cestocide. Praziquantel is available 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 most susceptible to parasites (ie, the high egg shredders) and maximizing refugia (those parasites 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 cyanthostomes (eg, fenbendazole 10 mg/kg/day, PO, for 5 days, or a single dose of moxidectin) during early winter months in northern climates or during early summer in warmer climates. Foals should be maintained on a regular deworming schedule every 60 days that includes anthelmintics 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 to a height of 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 perform fecal examinations. Use a larvicidal treatment before turning out new arrivals 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 November 2014 by Thomas J. Lane, BS, DVM

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