Repetitive Cattle Deworming May Cause Drug Resistant Worms

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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.

Internal parasite management is no longer as simple as deworming cattle, but should include some of the following strategies:

1. Fecal Egg Count Reduction Test (FECRT) protocol, used by USDA National Animal Health Monitoring System (NAHMS), is a valuable tool to determine when animals need to be dewormed. In addition, this can be used to detect worm resistance in a group of cattle. The test can cost between $10-15 per head, but the entire herd does not need to be tested. A farmer should test at least 20 of the most susceptible animals in the herd.
2. Calves are more susceptible, shed the most eggs, and require less product to deworm than cows in the herd. Farmers could reduce cost and overuse of product by only deworming calves in the herd at the appropriate times.
3. Farmers should avoid overusing dewormers and use these products in moderation. This can occur by deworming when the drugs are most effective instead of when convenient to administer. Furthermore, the dewormers should be given using the correct dose. This requires knowing the actual individual weight or average body weight of the animal when administering the drug.
4. If resistance is detected though testing, changing drug class is recommended. Another alternative could be rotating chemicals in the deworming program.
5. Good pasture management is also critical to reduce resistance. Rotate cattle through pastures to reduce re-infections, and it is recommended to let the pastures rest for 3 to 4 weeks before re-introducing cattle.

Source: Amy Radunz, Beef Cattle Extension Specialist, UW-Madison



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