Intercepting Parasite Resistance

The evidence is there: The internal parasites present in all horses are evolving to be resistant to the very drugs designed to combat them. And the number of new drugs being developing to overcome this issue is a whopping zero. So over the past decade, veterinarians have begun encouraging horse owners to pay closer attention to their parasite control approaches and practice selective deworming.

Selective deworming entails administering anthelmintics (anti-parasite drugs) only as often as necessary to reduce fecal egg counts (FEC), a practice that flies in the face of the long-standing approach of deworming horses every two months regardless of their need. For selective deworming to be effective, owners must rely heavily on FECs, which calculate the number of parasite eggs in an individual horse’s feces.  

As we’ve established in related articles, “The majority of the parasites in any group of animals are concentrated in a minority of the animals,” says Craig Reinemeyer, DVM, of East Tennessee Clinical Research and co-author of The Handbook of Equine Parasite Control. Fecal egg counts help practitioners identify this minority by revealing differences in each horse’s immune response to worms and their likelihood of shedding eggs into the environment (whether the horse is a low, moderate, or high shedder), along with showing which deworming products might no longer be effective on certain farms.

While practitioners can make generalizations about how frequently parasite populations develop resistance to specific dewormer classes, such as the benzimidazoles (fenbendazole), pyrimidines (pyrantel), and macrocyclic lactones (including ivermectin and moxidectin), the American Association of Equine Practitioners (AAEP) emphasizes in its Guidelines for Parasite Control that “the occurrence of resistance is very variable and large differences can be found between individual farms, and resistance cannot be concluded on any given farm without proper testing.” Ensuring that all anthelmintics used are still effective has become more important than following any traditional rotational patterns the industry once kept.

Source: TheHorse