Prevalence of anthelmintic resistance on sheep and goat farms in the mid-Atlantic region and comparison of in vivo and in vitro detection methods

Authors: CROOK, ELIZABETH - DELAWARE STATE UNIVERSITY; O'BRIEN, DALIA - DELAWARE STATE UNIVERSITY; HOWELL, SUSAN - UNIVERSITY OF GEORGIA; STOREY, BOB - UNIVERSITY OF GEORGIA; WHITLEY, NIKI - FORT VALLEY STATE UNIVERSITY; Burke, Joan; KAPLAN, RAY - UNIVERSITY OF GEORGIA

Submitted to: Small Ruminant Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/10/2016
Publication Date: 11/1/2016
Citation: Crook, E.K., O'Brien, D.J., Howell, S.B., Storey, B.E., Whitley, N.C., Burke, J.M., Kaplan, R.M. 2016. Prevalence of anthelmintic resistance on sheep and goat farms in the mid-Atlantic region and comparison of in vivo and in vitro detection methods. Small Ruminant Research. 143:89-96.

Interpretive Summary: One of the greatest health concerns for small ruminants is gastrointestinal parasite infection, particularly Haemonchus contortus or barberpole worm, a blood sucking nematode. Dewormers have been used for several years to control these parasites, but recently, dewormer resistance has become prevalent. Scientists at Delaware State University, University of Georgia, Fort Valley State University, and USDA, Agricultural Research Service determined that the degree of dewormer resistance in the mid-Atlantic U.S. was similar to the southern U.S. or highly prevalent, indicating a serious problem for small ruminant farmers, who will need to find alternative control methods for this life threatening disease. These results are important to U.S. farmers, extension specialists, veterinarians, and scientists who work with sheep and goats.

Technical Abstract: Despite strong economic opportunities and incentives for small ruminant production, their health and productivity are often severely affected by parasitic disease. To combat these effects, most farms administer anthelmintics to their animals at frequent intervals, and without consideration to principles of sustainable integrated parasite management (SIPM). This has led to growing problems caused by the development of drug-resistant populations of gastrointestinal nematodes (GIN) in much of the world, particularly in Haemonchus contortus. The objectives of this research were to characterize levels of anthelmintic resistance on small ruminant farms located in the mid-Atlantic US and to compare the fecal egg count reduction test (FECRT) and larval development assay (LDA) for detecting resistance. To achieve these objectives, the DrenchRite® LDA was used to evaluate resistance status to benzimidazoles, ivermectin, moxidectin, and levamisole on 20 goat and 14 sheep farms in the Mid-Atlantic US over a 2-year period. A FECRT was also conducted on 14 of the same farms and on 2 additional farms in which the LDA was not completed. For the LDA and coprocultures, fecal samples were collected rectally from a minimum of 10 individual animals, pooled, and express-mailed to the University of Georgia for analysis. For the FECRT, albendazole, ivermectin, moxidectin, and/or levamisole were tested on each farm. Animals were allocated randomly based on FAMACHA© scores to 2-5 treatment groups, which included an untreated control group. The number of treatment groups on a farm depended on the number of qualified animals present. Haemonchus contortus was the most common parasite recovered from fecal cultures; the mean level across all farms was 79%. Results of the LDA indicated resistance to benzimidazoles, ivermectin, moxidectin, and levamisole on 100%, 82%, 47%, and 24% of farms, respectively. Multi-drug resistance to all 3 drug classes was detected for H. contortus on 18% of farms (1 sheep and 5 goat farms). Of the 16 farms tested by FECRT, resistance to albendazole was present on 8/10 farms, to ivermectin on 4/4 farms, to moxidectin on 7/9 farms and to levamisole on 2/5 farms tested. Results obtained from the FECRT and the LDA (p = 0.51) were similar. The prevalence of resistance found in this study in the mid-Atlantic region of the US is very similar to that reported in an earlier survey of resistance performed in the Southern US, demonstrating that anthelmintic resistance in GIN is a serious problem on small ruminant farms throughout the Eastern US.