Submitted to: American Association of Veterinary Parasitologists Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: May 20, 2004
Publication Date: July 20, 2004
Citation: Gasbarre, L.C., Smith, L.L., Lichtenfels, J.R., Pilitt, P.A. 2004. The identification of cattle nematode parasites resistant to multiple classes of anthelmintics in a commercial cattle population in the U.S. [abstract]. American Association of Veterinary Parasitologists Proceedings.
As reported in an accompanying paper, a backgrounding producer noticed an apparent decrease in the effectiveness of his strategic anthelmintic regimen. Upon testing with a fecal egg reduction test, it was noted that treatment with either ivermectin injectable(I), moxidectin pour-on(M), doramectin injectable(D), eprinomectin pour-on(E), or albendazole oral(A) did not result in EPG reductions of at least 80%. To determine the species of parasites responsible for the eggs found in the feces, 3 animals from each of the treatment groups plus 3 untreated controls (U) were slaughtered 7 days after the appropriate treatment. Calves for slaughter were chosen by their EPG rank, i.e. 1 block of 6 calves encompassing all treatments near the median EPG value, 1 block near the high and 1 block near the low. Worms were recovered from washes of the abomasum and small intestine and from an 4 hour incubation in warm saline of the abomasum. Separate 20% aliquots from each sample were processed at 2 different locations and the mean of these counts was used as an indication of parasite numbers. The abomasum of all groups harbored significant numbers of Haemonchus placei and H. contortus. The results for each treatment group presented as total worms/ % H. placei/ % H. contortus were: U = 1582/29/71 ; I = 2616/28/72; M =1950/33/67 ; D = 2034/27/73; E = 2019/45/55; and A = 2291/0/100 . The small intestines from all avermectin treated groups contained significant numbers of Cooperia punctata, and smaller numbers of C. oncophora, and C. spatulata . The results for each treatment group presented as total worms/ % C. punctata/ C. oncophora/ % C. spatulata were U = 242/89/0/11 ; I = 11875/90/8/2; M = 5308/96/0/4; D = 6482/86/10/3; E = 3712/85/2/13; and A = 17/100/0/0. These results imply that the pastures studied contain substantial numbers of H. contortus resistant to both avermectins and benzimadoles, and H. placei and Cooperia sp. resistant to all the commonly used avermectin anthelmintics. While this is the first report of anthelmintic resistance in American cattle parasites, it is very important to ascertain the extent of the drug resistant phenotype in American cattle operations, and that caution be used in espousing treatment regimens that could hasten the spread of such resistance.