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item Gasbarre, Louis
item Hoberg, Eric
item Pilitt, Patricia

Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/12/2006
Publication Date: 12/23/2009
Citation: Gasbarre, L.C., Smith, L.L., Hoberg, E.P., Pilitt, P.A. 2009. Further characterization of a cattle nematode population with demonstrated resistance to current anthelmintics. Veterinary Parasitology. 166:275-280.

Interpretive Summary: The appearance of cattle nematode parasites resistant to all the commonly used dewormers was first reported in the US last year. In a follow-up study, cattle at the same location were again tested for the presence of drug-resistant parasites. Major differences between the two studies were that the number and types of drugs used was increased, and certain of the sample collection procedures were changed to lessen the chances of sampling errors. The results of this study strongly support those of the previous study, and conclusively demonstrate that heavy use of anthelmintic drugs in cattle can and does select for drug resistant parasites. In this study parasites resistant to all the commonly used anthelmintics were found to increase over the grazing season. An older drug that is now rarely used in cattle was effective in killing most parasites, but this drug is the least effective against the most pathogenic parasite, and as such could lead to additional problems in the future. This study underscores the danger of relying on repeated drug treatments to control parasite infections and demonstrates the need for new tools to control production losses resulting from nematode parasites of cattle.

Technical Abstract: We had previously documented the appearance of cattle nematode parasites resistant to commonly used avermectins, milbymycin, and a benzimidole at the end of a grazing season on a backgrounding operation in the upper Midwestern US. To further characterize the pattern of drug resistance we extended the study to: 1. monitor the animals over the course of the grazing season, 2. increase the number of animals slaughtered at the end of the season to minimize the effect of potential outlying observations, 3. increase the time interval between treatment and slaughter to insure sufficient time for drug action, 4. utilized repeated fecal sampling in the fecal egg reduction test to minimize procedural variation, and 5. increase the number of drugs tested . The results of the present study were in agreement with those of the previous study and demonstrated that during the course of the grazing season parasites were selected that were refractory to avermectins, milbimycin, and a benzimidole at the label recommendations. As seen previously, Haemonchus contortus resistant to all these compounds were selected over the course of the study period. In contrast, Cooperia sp., mainly punctata, and H. placei were resistant only to the ivermectin-like drugs. There was no apparent resistance against the older drug compound, levamisole, which had not been previously used in the cattle operation; but animals treated with this drug continued to harbor small but measurable numbers of Ostertagia.