Horse Care - Parasite Management

All equines have a parasite burden, and therefore treatment is periodically needed throughout life. Some steps to reduce parasite infection include regularly removing droppings from the animal's stall, shed or field; breaking up droppings in fields by harrowing or disking; minimizing crowding in fields; periodically leaving a field empty for several weeks; or placing animals other than equines on the field for a period of time, particularly ruminants, which do not host the same species of parasites as equines. If botflies are active, frequent application of fly spray may repel insects. A small pumice stone or specialized bot egg knife can also scrape off any bot eggs that were laid on the hairs of the horse.

However, internal parasites cannot be completely eliminated. Therefore, most modern horse owners commonly give anthelmintic drugs (wormers) to their horses to manage parasite populations.
Methods of Deworming

There are 2 common methods of deworming. Purge dewormers that kill parasites with a single strong dose, are given periodically, depending on local conditions and veterinary recommendations. Continuous dewormers, also known as "daily" dewormers, are given in the horse's feed each day, in small doses, and kill worms as they infect the horse. Neither of these methods is perfect; purge dewormers are effective for rapidly killing parasites, but are gone from the horses' body in a few days, and then the horse may start to be re-infected. Continuous dewormers are a mild low dose and may be easier on the horse, but may not be effective in quickly killing worms in a heavily-infected horse and may contribute to drug resistance. If a treatment doesn't kill at least 95% of a worm species, that species is classed as 'resistant' to the drug. For adult horses, frequent rotation of several types of dewormers is no longer recommended, as it can often lead to overtreatment and subsequent drug resistance. Another way of combating drug resistance in adult horses is to deworm less frequently, by performing fecal egg counts on manure and deworming only horses with a high count. This strategy is now recommended by most veterinarians and parasitologists, as it reduces the probability of resistance. For horses that are consistently deemed "low shedders," it is still recommended to deworm at least 1-2 times per year with ivermectin + praziquantel or moxidectin + praziquantel to target tapeworms, bots, and small strongyles. This is typically done in the fall and spring.

Dewormers come in several forms, including pastes, gels, powders, and granules or pellets. Powders and granules normally come in single-dose packaging, and the dewormer is normally mixed in with the horse's feed. Pastes and gels normally come in a plastic syringe which is inserted in the side of the horse's mouth and used to administer the dewormer onto the back of the horse's tongue. A dewormer syringe has a plastic ring on the plunger that is turned to adjust the dosage for the horse's weight.
Risks of Deworming

Drug resistance is a growing concern for many horse owners. Resistance has been noted with ivermectin to ascarids, and with fenbendazole, oxibendazole, and pyrantel to small strongyles. Development of new drugs takes many years, leading to the concern that worms could out-evolve the drugs currently available to treat them. As a result, most veterinarians now recommend deworming for small strongyles based on fecal egg counts to minimize the development of resistant parasite populations.[13] Fecal egg count reduction tests can also be performed to identify which dewormers are effective on a particular farm.

If a horse is heavily infested with parasites, dewormers must be given carefully. Small strongyles can form cysts embedded in the intestinal epithelium. A decrease in the active population of worms, as in the case of deworming, can cause larvae to emerge from the cysts (larval cyathostomiasis).