Merial: Don’t Jump To Conclusions About Parasite Resistance
10/05/2009 02:51PM
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With only two well-documented cases of nematode resistance in cattle, Dr. Michael Hildreth, parasitologist and professor, South Dakota State University, says subpar parasite control results are more likely due to improper timing or application.
“We shouldn’t be too quick to blame parasite resistance,” he says. “Improper product use is often mistaken for parasite resistance. Products have label instructions for a reason, and it is very important that producers follow these instructions properly for the products to work correctly and to their full potential.”
Another problem may be that producers are not treating their cattle often enough to achieve the best results, says Dr. Frank Hurtig, director, Merial Veterinary Services.
“Producers may not realize it, but parasite control treatments are only effective against worms for the day you treat in the case of some white dewormers, and 14 to 28 days for endectocides, depending on the product used and the parasite,”1 he explains.
Dr. Hildreth says producers in his area aren’t treating enough to properly control the parasites — let alone cause parasite resistance.
“Most producers only treat cattle once a year — if at all — and they may not treat all of their cattle each time,” he says. “In our area, cow/calf producers are treating for parasites at the most twice a year, once in the spring at turnout and again in the fall.”
Dr. Hildreth’s experience aligns with research released by Iowa State University. Researchers reported that 73 percent of cow/calf operations treat their cattle for parasites.2 This means that 27 percent of operations do not treat for parasites at all.2
This research does not report how many times producers treat their cattle through the year, but Dr. Hildreth believes that treating once or twice a year is not enough to create resistance problems.
“If we look at the two cases in this country where resistance has been scientifically documented, those animals were being treated multiple times throughout the year — not once or twice,” Dr. Hildreth says. “We have not seen any evidence globally that parasite resistance is emerging as a problem under normal conditions and management practices.”
Of the two well-documented cases of resistance in U.S. herds,3,4,5 one case involved stocker cattle that were managed with very high stocking rates in a rotational grazing program (75 head/acre or more) with multiple endectocide treatments per year (five to eight times or more).6 This operation has used this approach for more than two decades on the same pastures and only recently had a problem.3
Dr. Hildreth says these cases are the exception, not the rule.
“The current prevalence of endectocide-resistant worms in a population is very low at best,” he says. “We may be able to find a few with a very sensitive assay. However, currently, there is not any clinical evidence or detectable production differences that we can say definitively are a result of parasite resistance.”
Dr. Hurtig says that to avoid the losses associated with less-than-ideal parasite control, producers should work with their veterinarian to develop a parasite control plan that best fits their individual needs.
“Producers should consult with their veterinarian when it comes to forming a year-long parasite control plan to help ensure their program will be most effective based on their operation and area,” he says. “It also is important that they use products that come from a trusted manufacturer and are backed by a 100% product satisfaction guarantee. Then, if there is a problem, producers have somewhere to turn.”
1Based on data provided in FDA Freedom of Information summaries.
2Lawrence JD, Ibarburu MA. Economic analysis of pharmaceutical technologies in modern beef production. 2007 Iowa State University.
3Gasbarre LC, Smith LL. Identification of cattle nematode parasites resistant to multiple classes of anthelmintics in a commercial cattle population. Proceedings, AAVP, 2004;44:55.
4Smith LL, Gasbarre LC. Evaluation of individual or a combination of anthelmintics in a commercial cattle population where anthelmintic-resistant parasites had been observed the previous year. Proceedings, AAVP, 2005;55:63.
5Edmonds, et al. Efficacy of two pour-on products in two feedlot studies: Proceedings, the American Association of Veterinary Parasitologists 52nd annual meeting 2007;14:42.
6Smith L. Personal conversation. Nov. 27, 2007.
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