Repetitive Cattle Deworming May Cause Drug Resistant Worms

From: CattleNetwork.com
April 21, 2010

Dewormers have provided effective parasite control, which has resulted in returns to farmers between $20 to $200/hd. The cost of these products is reasonable when compared to potential productions gains provided. Sheep and goat farmers have long battled with drug resistant worms, however until recently there has not been evidence of this is occurring in beef cattle. In fact, some of the first evidence of worm resistance in cattle was found in Wisconsin in 2002, when a backgrounder, who acquired calves from the Southeast, experience lower than expected weight gain during the fall.

According to Dr. Shulaw, Extension Veternarian at Ohio State University, "Unlike sheep and goats, cattle tend to develop a much stronger immune response to gastronintestinal parasites after a season of grazing exposure." However, research from USDA Agriculture Research Service (ARS) and North Carolina State University (NCSU) have reported evidence of increasing resistant worm populations and decreasing efficacy of deworming drugs like avermectin pour-ons.

Dr. Gasbarre, recently retired ARS parasitologist, conducted research at the Wisconsin backgrounding operation, which used intensive grazing management and strategically timed deworming for more than 17 years. The research confirmed the decreased performance in calves was due to internal parasites still present after deworming. Research at NCSU, compared efficacy of various in anthelmintics (generic label ivermectin pour-on, brand-name ivermectin pour-on, injectable ivermectin, or drench of fenbendazole) in two different research herds. In one herd, where previously worm resistance had been identified, the only dewormer to have greater than 90% reduction in fecal egg counts, was fenbendazole. In contrast, all the anthelmintic treatments in the other herd reduced fecal counts by more than 90%. This indicates worm resistance to anthelmintics may be specific to herds and locations, therefore internal parasite control may need to be developed specifically for a farm.

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