|Smith, Larry - RES & DEVELOPMENT, INC|
Submitted to: American Association of Veterinary Parasitologists Proceedings
Publication Type: Proceedings
Publication Acceptance Date: May 2, 2005
Publication Date: July 15, 2005
Citation: Gasbarre, L.C., Smith, L.L., Pilitt, P.A. 2005. Identification of cattle nematode parasties resistant to multiple classes of anthelmintics in a commercial cattle population in the US [abstract]. American Association of Veterinary Parasitologists Proceedings. p. 57. Technical Abstract: As reported in an accompanying paper, we have monitored for the past 2 years pastures of a commercial backgrounding operation for the presence of drug resistant nematodes by assessing the effectiveness of drug treatment in reducing fecal egg counts (EPG). Treatments used over the 2 years consisted of : ivermectin injectable(I), moxidectin pour-on(M), doramectin injectable(D), eprinomectin pour-on(E), albendazole oral(A), fenbendazole (F), levamisole (L) and 2 combination treatments of eprinomectin plus levamisole (EL) and eprinomectin plus fenbendazole (EF). Over the 2 years the only treatments that resulted in at least 80% reduction in EPG included levamisole either singly or in combination with a second drug. To determine the species of parasites responsible for EPG in year 2, 6 animals from each group were slaughtered at 14 days post treatment to insure the drugs had sufficient time to kill the parasites. Additionally, each group receiving a pour-on treatment was segregated to preclude the possibility of cross transfer of drug. Total worms recovered from the groups were: U = 10,138, I = 15,785, D = 3,634, E = 2,721, M = 1,211, EF = 548, L = 409, F = 350, and EL = 35. The results for both years were very similar and show the following pattern. Haemonchus contortus was found in the abomasa of all groups tested over the 2 year period indicating resistance to both classes of drugs employed. In contrast H. placei with few exceptions was restricted to the groups receiving the avermectins or milbimycins indicating that this parasite was still sensitive to the benzimidoles. Few Ostertagi ostertagi were seen in the studies, and they were mostly restricted to the levamisole treated animals. In the small intestine the predominant species encountered was Cooperia punctata, with smaller numbers of C. oncophora, and C. spatulata. Similar to H. placei, results indicate that these parasites remained sensitive to the benzimidoles. Interestingly in both years the mean number of Cooperia recovered from the ivermectin treated group exceeded that found in the untreated control group. There was a slight increase in the numbers of Ostertagia recovered in year 2, although the total number of Ostertagia remained low.