By George Lager, PhD
Anthelmintic resistance is recognized globally as a serious problem in the control of equine internal parasites. Fecal egg count reduction tests indicate that on many farms, certain drug classes are no longer effective, or egg reappearance periods have shortened significantly. Drug-resistant parasite emergence is related primarily to anthelmintic overuse and the lack of “evidence-based” deworming strategies.
When we started our small farm in south-central Indiana, we decided to base our parasite control program on a rigorous management plan, which involved frequently removing all fecal matter from pastures (about 5 acres), corrals, and stalls. We do not deworm until fecal egg counts (FECs) reach a predetermined target value. This is a labor-intensive approach, but it’s more sustainable than traditional protocols because it limits anthelmintic use and delays drug resistance. In addition, horses carry a small worm burden that helps build immunity to more serious parasite infections.
Beginning with the adoption of our first BLM mustang, we collected all feces manually from pastures three times/week and from corrals and from stalls twice daily. We tracked FECs of two mustangs at one- to three-month intervals over a period of about two years (2-year-old colt at adoption on Aug. 3, 2007, named Nevada) and about one year (1-year-old colt at adoption on March 11, 2008, named Mesteñ).
We dewormed both mustangs with ivermectin-praziquantel at the time of adoption and quarantined them for one month before release to pasture. Within one to two months of adoption, we used either pyrantel/piperazine or piper