Control of internal parasites is a cornerstone of equine management and a continual endeavor. Although anthelmintics are the primary method of parasite control, other factors such as pasture management and stable environment are also important. 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 and tapeworms.
Unfortunately, anthelmintic resistance has arisen in many GI parasites of horses. Ascarids and cyathostomes have developed resistance to standard doses of the macrocyclic lactones, benzimidazoles, and the tetrahydropyrimidines. Tapeworms now sometimes demonstrate multidrug resistance, and pinworms are globally resistant to macrocyclic lactones. Among intensively managed horse populations, treatment with macrocyclic lactones (eg, ivermectin, moxidectin) does not suppress strongyle egg counts for as long as previously expected.
A program to prevent 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.
The parasites of greatest concern in horses are large strongyles, small strongyles, roundworms, pinworms, 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.
Ascarids (P equorum) are the primary cause of pulmonary inflammation and intestinal impaction in foals and weanlings. The control of large strongyles with improved anthelmintics has allowed the small strongyles (cyathostomes) to become a greater problem. Cecocecal or cecocolic intussusceptions caused by tapeworms have been reported.
The two currently available methods of diagnosing parasites are fecal flotation to determine concentrations of worm eggs as well as culture of feces for infective larva. The flotation method for parasite egg concentration can determine the type and number of parasites. The culture method is more suitable for laboratory use and research purposes. PCR has been described for detection of Strongylus vulgaris but is not yet available for clinical use.
The effectiveness of any parasite control program can be evaluated by fecal egg count reduction. An initial fecal egg count should be done before administration of a particular dewormer and then again 14 days later. The amount of decrease 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 macrocyclic lactones have a broad range of activity, are safe and effective at low doses, and control adult and migrating larval nematodes. This drug class is not effective against tapeworms; however, the combination of ivermectin with praziquantel is effective against tapeworms as well as large and small strongyles, ascarids, and bots.
The benzimidazoles (oxibendazole, fenbendazole, and oxfendazole) are effective against most nematodes but have no efficacy against encysted small strongyles or tapeworms.
The pyrantels are effective against ascarids and pinworms. Pyrantels are available as a paste (single dose) and as a tartrate salt that can be administered as a daily dewormer mixed in the feed.
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 decrease 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 shedders) and maximizing refugia (those parasites not exposed to drug selection pressure) by decreasing 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.
Effective deworming programs should also include one or more of the following nonchemical methods of parasite control:
Avoid overstocking and overgrazing pastures.
Keep pasture roughs mowed to a height of 3–8 inches.
During hot, dry weather, harrow or rake pastures to disperse manure piles and expose larvae to sun.
Cross-graze pastures with other species. Cattle, sheep, and goats serve as biological vacuums for equine parasites.
Make at least one cutting of hay from some pastures, to help decrease the parasite burden.
Plant an annual crop such as winter wheat.
Feed hay and grain in raised containers and not directly on the ground.
Remove manure from stalls, paddocks, and pastures every 24–72 hours before strongyle eggs have a chance to hatch and develop into infective larvae (5–7 days during optimal conditions).
Quarantine new arrivals and perform fecal examinations.
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, and to evaluate new arrivals.
Compost manure. Properly composted manure will kill strongyle larvae and many ascarid eggs.
