Journal of NACAA

Anthelmintic Resistance on Commercial Sheep Farms in the Southeastern US

ISSN 2158-9429

Volume 12, Issue 2 - December 2019

Editor: Bindu Poudel-Ward

Abstract

In 2016, the Let’s Grow Committee of the American Sheep Industry Association funded a study to determine anthelmintic resistance on 30 commercial sheep farms in the southeastern US: Maryland (MD), Virginia (VA), and Georgia (GA). Pooled fecal samples were collected from 10 farms in each state and submitted to the University of Georgia for fecal egg count, larvae identification, and DrenchRite® larval development assay (LDA). Percentage and fecal egg count data were log transformed for analysis/inference, but LSMEANS are reported. Fecal egg count averaged 5080, 3015, and 3695 epg (SEM=487; MD=GA>VA, P<0.05), respectively for MD, VA, and GA farms. Coprocultures identified Haemonchus contortus as the predominant worm species: 89.4, 73.3, and 83.9% (SEM=3.1%; MD=GA>VA, P<0.02), respectively for MD, VA, and GA farms. All farms had high resistance to fenbendazole (SEM=9.4%). Resistance to ivermectin was similar among states, averaging 80, 100, and 100%, respectively for MD, VA, and GA. Resistance to moxidectin and levamisole was lower in MD as compared to the more southern states (P<0.04). All farms in VA and GA had resistance to three of the four anthelmintics tested. Multiple drug resistance in MD was lower, with only 80% of farms having resistance to two drugs and 60% having resistance to three drugs. The percentage of farms with resistance to all tested drugs was 0, 60, and 40%, respectively for MD, VA, and GA farms. While anthelmintic resistance varied by state and farm, all farms had resistance to at least one anthelmintic, which underscores the importance of anthelmintic resistance testing.