Parasite Resistance in US Cattle

Abstract

Parasite resistance to the macrocyclic lactones (ivermectin, doramectin, eprinomectin, and moxidectin) is receiving considerable attention in the US cattle industry at a time when the economics of parasitism constitute one of the most important factors involved in beef production. Knowing whether a dewormer is effective is extremely important to an operation. If parasites become resistant to a particular product or product formulation, a serious problem can develop unknowingly unless producers have an easy way to determine product efficacy. The fecal egg count reduction test (FECRT) is a simple test recommended by the American Association of Veterinary Parasitologists (AAVP) as the best way for the practitioner to help producers verify that the dewormer(s) they are using is effective. The FECRT involves conducting a fecal check at the time of treatment and again 14 days following treatment. In the fall of 2007 continuing through the summer of 2008, free lab support was offered to bovine practitioners throughout the US to conduct FECRTs with their clients. This was done according to a standard protocol involving a minimum of 20 samples per treatment group at each collection time. The results are being recorded in a national data base supported by Intervet/Schering-Plough Animal Health and the University of Nevada-Reno. Over 58 veterinary clinics in 19 states have already participated in this program, with over 119 separate tests involving 4,765 samples using a wide range of products and formulations. These data confirm that macrocyclic lactone resistance is widespread and that continued vigilance is required by the veterinary profession, since the problem now appears to be at a critical stage with millions of dollars in production losses at stake.