Submitted to: Journal of the American Veterinary Medical Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/7/2008
Publication Date: 12/15/2008
Citation: Howell, S., Burke, J.M., Miller, J., Thomas, T., Valencia, E., Williams, M., Zajac, A., Williamson, L., Kaplan, R. 2008. Anthelmintic resistance on sheep and goat farms in the southeastern United States. Journal of the American Veterinary Medical Association. 233(12):1913-1919.
Interpretive Summary: Recent studies on sheep and goat farms in the Georgia indicated that multiple-anthelmintic resistance in Haemonchus contortus was a severe problem. The current study examined the extent of multiple drug resistance in various southern states, Puerto Rico and the Virgin Islands. Drug resistance of Haemonchus contortus and Trichostrongylus colubriformis, which cause severe health problems, was highly prevalent. This information is important to veterinarians, scientists, extension agents, and small ruminant producers throughout the U.S., Puerto Rico and Virgin Islands to manage movement of livestock and treatment of internal parasites.
Technical Abstract: Anthelmintic resistance in gastrointestinal nematodes (GIN) is recognized as a severe problem for small ruminant production worldwide. In the United States, a high prevalence of drug resistance to multiple drug classes was reported in GIN of goats in Georgia, but no studies have examined the prevalence of resistance in other states of the region and virtually no recent data is published for sheep. In this study the DrenchRite® larval development assay was used to evaluate the prevalence of anthelmintic resistance on both sheep and goat farms in the southeastern states. Several farms in Puerto Rico and St. Croix, U.S. Virgin Islands were also evaluated. Twenty-six sheep and 20 goat farms in Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana, Maryland, Virginia and Puerto Rico and St. Croix, U.S. Virgin Islands were tested. Haemonchus contortus was the major parasite on 44 of 46 farms tested while Trichostrongylus colubriformis was the second most commonly identified parasite. For H. contortus, benzimidazole (BZ) resistance was detected on 98% of farms, levamisole (LEV) resistance was detected on 54% of farms and ivermectin (IVM) resistance was detected on 76% of farms tested. Multiple drug resistance to all three classes of anthelmintics was detected on 22 of 46 farms (48%). Using recently published guidelines for diagnosing moxidectin resistance to H. contortus with the DrenchRite® assay 24% of farms (4 sheep, 7 goats) had moxidectin resistant H. contortus. Resistance to all 3 drug classes plus moxidectin was detected on 17% of farms (4 sheep, 4 goats). Resistance status in T. colubriformis was evaluated only when this species made up = 20% of the L3 identified in the assay control wells: this criterion was met on 14 farms. Resistance of these farms was detected on 100 % to BZ, 57% to LEV and 50% to IVM. Four farms had T. colubriformis resistant to all three classes of anthelmintics. Moxidectin resistance status was not evaluated for T. colubriformis because there are no criteria for establishing a diagnosis of moxidectin resistance in this species using this assay. These data provide strong evidence that anthelmintic resistance is a serious problem on small ruminant farms throughout the southeastern United States. Due to the frequent movement of animals it is highly likely that a high prevalence of resistance exists in other regions as well. Consequently, examination of resistance status should be made a part of routine parasite management to ensure that efficacious anthelmintics are administered.