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 he

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