Equine Parasite Resistance

Gastrointestinal parasite resistance is an emerging problem in horses. For decades we have seen the development of resistance to dewormers in small strongyles (cyanthostomes). Results of a multistate study showed 97.7% of farms had small strongyles with resistance to fenbendazole and 40.5% had resistance to pyrantel (Kaplan 2004). Resistance to other types of dewormers has also been documented. This has forced veterinarians to critically evaluate the use of dewormers currently available to horse owners, with no new products available in the near future!

Not every horse needs to be dewormed every two months. This approach has been the reason why we have seen resistance to the common dewormers. Resistance is a farm situation, not an individual horse situation. The goal of new parasite control programs are to maintain a population of susceptible parasites called “refugia” on the farm. These parasites can breed with resistant parasites and produce more susceptible parasites that can be killed with dewormers. If a farm develops a large number of resistant parasites, the horses that live there risk serious health problems such as colic!

Resistance is identified by evaluating the number of eggs a horse sheds in his feces, called fecal egg count (FEC) both before and after deworming. In order to start this program, a fecal sample should be collected at least 10 weeks after horses have been dewormed. The horses are then dewormed and a second fecal sample is collected two weeks later. The difference (ideally a decrease in FEC) is a means of quantifying the effectiveness of the dewormer. This is termed fecal egg count reduction (FECR). If the post-deworming (fecal performed 2 weeks after deworming) FECR is <90-95%, this indicates resistance of the parasite to that dewormer. FECR enables identification of horses that carry a higher worm burden (>400 egg/gram of feces [EPG]) versus those that carry a low worm burden (<400 EPG). While it is not possible or necessary to eliminate all worm burden, animals with low FECR do not need to be dewormed, while those with high FECR should be targeted. By not deworming those horses that have a low load, we can preserve the effectiveness of our dewormers by minimizing overuse.

Fecal samples should be obtained periodically from your horses to determine which horses have high parasite burdens and need to be dewormed. If your horse consistently has a FEC >400 EPG, it is considered a “high shedder” and will need to be dewormed at least four times per year. If your horse has a FEC <400 EPG, then it is considered a low shedder and may only need to be dewormed twice each year. Tapeworms are a medical threat to all horses, but there is no reliable fecal diagnostic test for them. All horses should be dewormed for tapeworms in the fall, regardless of their fecal egg count (praziquantel or triple dose of pyrantel). Weigh your horse or use a weight tape to ensure accurate dosage. Under-dosing also promotes parasite resistance!

While this may seem labor intensive and costly, by identifying some horses that actually do not need to be dewormed, the horse owner will actually save money over time. Futhermore, by minimizing the overuse of dewormers we aim to preserve their effectiveness.

In addition, environmental contamination with feces can serve as an important reservoir for parasite re-introduction. It is prudent to minimize pasture contamination. This includes removing feces before eggs become infective and avoiding spreading manure on pastures which will be grazed. Environmental management will become increasingly important as anthelmintics are less capable of eradicating the gastrointestinal parasites from our horse population.

Kaplan RM, et al. Prevalence of anthelmintic-resistant cyathostomes on horse farms. JAVMA 2004;225(6):903-910.