Horse Illustrated Magazine

Q: I recently had a fecal egg count done for my horse, and after we got the results, it was recommended that I deworm him with fenbendazole for five consecutive days, then wait ten days to deworm him again. Is this safe? Are there side effects I should watch out for?

A: Fecal egg counts (FECs) are an extremely important tool in today’s equine parasite control programs. For decades, horse owners and veterinarians alike merely administered dewormers because it was understood this was part of the routine management of horses, whether the animals needed the medication or not. This, among other practices, has caused the rapid development of antiparasitic resistance whereby a certain drug may not work anymore on a particular population of parasites. Antiparasitic resistance is already a huge problem of international concern in the small ruminant (sheep and goats) industry and it is a matter of time before this problem reaches the horse world if proper scientific and pharmacological practices are not implemented on a routine basis. FECs are one such practice.

FECs are performed on a fairly fresh fecal sample that your veterinarian examines under the microscope. Parasite eggs are then counted and used to infer the approximate parasite load in your horse. FECs provide your vet an objective measurement of the parasite burden in your horse at a particular point in time. If the FEC reveals a certain level of eggs beyond which is acceptable, then it is appropriate to treat your horse with a dewormer. FECs prevent treating horses who are not infected at all or who only have a low acceptable number of parasites, keeping in mind that up to a certain level, some parasites are tolerable in your horse’s gut.

FECs can also identify, to an extent, what types of parasites are infecting your horse. Knowing both the amount of infection and what worms are involved greatly helps your vet decide on the appropriate dewormer to use since not all dewormers are created equal. Take a peek in the equine section of your local farm supply store or catalog and you may likely get overwhelmed by the myriad of different brands of dewormers. The worms that are sensitive to each drug are listed on the label, making it a little easier to narrow down the choices once you know what your horse is infected with.

The recommendation to treat your horse with fenbendazole in such a way has been marketed as a “PowerPac”. This is indicated for cases of small strongyles, a very common type of equine internal parasite, and is a common way to deworm horses.

Fenbendazole is an extremely safe antiparasitic and in fact one of the safest drugs you can give your horse. It has been demonstrated that fenbendazole is safe even when overdosed at one hundred times the appropriate dose. Aside from the safety of the drug, the level of parasite load being targeted may have an impact on a horse’s health. Occasionally, severely parasitized horses can suffer hypersensitivity reactions to the sudden kill of a large amount of parasites in their gut. A large parasite kill can also occasionally result in intestinal impaction, as the massive amount of dead parasites travels through the gut en route to be expelled. Each of these scenarios, however, is reserved in rare cases of extreme parasitism.

— Anna O’Brien, DVM

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