Effectiveness of current anthelmintic treatment programs on reducing fecal egg counts in US cow-calf operation

Authors: Gasbarre, Louis; BALLWEBER, LORA - Colorado State University; STROMBERG, BERT - University Of Minnesota; DARGATZ, DAVID - Animal And Plant Health Inspection Service (APHIS); RODRIGUEZ, JUDY - Animal And Plant Health Inspection Service (APHIS); KOPRAL, CHRISTINE - Animal And Plant Health Inspection Service (APHIS); Zarlenga, Dante

Submitted to: Canadian Journal of Veterinary Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/10/2013
Publication Date: 10/1/2015
Citation: Gasbarre, L.C., Ballweber, L., Stromberg, B.E., Dargatz, D.A., Rodriguez, J.M., Kopral, C.A., Zarlenga, D.S. 2015. Effectiveness of current anthelmintic treatment programs on reducing fecal egg counts in US cow-calf operation. Canadian Journal of Veterinary Research. 79:296-302.

Interpretive Summary: Development of the currently used anthelmintics has resulted in significant changes in how cattle are raised in the US. The combination of high efficacy combined with large margins of safety changed the timing, manner, and frequency of drug use. The goal has been to attain optimal animal productivity through the reduction of parasite transmission by reducing the number of infectious larvae on pastures. As a result, producers have been able to employ higher stocking rates and reduced or eliminated non-chemical adjuncts in their parasite control programs. Recently there has been an increasing perception that resistance to the most commonly used cattle anthelmintics is on the rise in the US. In an attempt to gain insights into the effectiveness of producer applied drug treatments and as part of the NAHMS Beef 2007-08 study of cow-calf operations, producers were given the opportunity to assess the effect of their current anthelmintic program on reducing fecal egg counts in weaned calves on pasture. The results of these studies clearly showed that in nearly one-third of the operations tested, drug intervention resulted in a less than an 85% reduction in fecal egg counts 2 weeks after treatment which is in agreement with drug resistance. The results reported here should alert American cattle producers to a number of important points. First, they need to check the efficacy of their anthelmintic program and not assume that treatment equates with successful control. Second, producers may need to treat with multiple classes of drugs to achieve adequate parasite control. Third, pour-on formulations though easy to apply provide the least effective level of nematode control. Finally, successful long-term and sustainable GI nematode control cannot be obtained by reliance only of anthelmintic treatment; sustainable nematode control must include good pasture management and animal husbandry. These results can be used by both scientists and producers to help identify nematode problems with pastured animals and interject new control measures before drug resistance becomes unmanageable in a herd of animals.

Technical Abstract: During the USDA National Animal Health Monitoring System’s (NAHMS) Beef 2007-08 study producers from 24 states collected fecal samples from weaned calves for evaluation of internal parasite burdens and response to treatment with an anthelmintic product. Participating producers were provided instructions and materials to collect fecal samples at the time of treatment and again 2 weeks after treatment. Samples were only accepted where at least 45 days lapsed between initial sampling and any previous anthelmintic treatment. The choice of anthelmintic product was entirely at the discretion of the producer. Fresh fecal samples were collected from either 20 different animals, or if less than 20 animals, the number in the group. Samples were randomly assigned for submission to one of three participating laboratories. Analyses consisted of double centrifugation floatation followed by enumeration of strongyle, Nematodirus, and Trichuris eggs, and simple notation of the presence or absence of coccidian oocysts and tapeworm eggs. In submissions where strongyle eggs per gram exceeded 30, aliquots from 2-6 animals were pooled for extraction of egg DNA. Extracted DNA was subjected to PCR for the presence of Ostertagia, Cooperia, Haemonchus, Oesophagostomum, and Trichostrongylus. A total of 72 producers from 19 States participated in this portion of the survey. Treatment options included oral benzimidazoles, and both injectable and pour-on endectocides. Results indicated that in approximately one-third of the operations participating, fecal eggs counts reductions were less than 85%. All operations that showed less than an 85% reduction had used either a brand name or generic pour-on macrocyclic lactone as the anthelmintic treatment. While some of these less than expected reductions could have been the result of improper application of the drugs, PCR analyses of the parasite populations surviving treatment, coupled with follow-up studies at a limited number of sites, indicated that in the majority of cases, the less than expected reductions were due to anthelmintic resistance in Cooperia sp. and possibly Haemonchus sp.