Anthelmintic resistance in gastrointestinal nematodes and associated management factors in Intermountain West sheep flocks

Authors: STEWART, WHIT - University Of Wyoming; SCOTT, DAVE - National Center For Appropriate Technology; HOWELL, SUE - University Of Georgia; KAPLAN, RAY - University Of Georgia; ROEDER, BRENT - Montana State University; Murphy, Thomas - Tom

Submitted to: Sheep and Goat Research Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/26/2020
Publication Date: 11/20/2020
Citation: Stewart, W.C., Scott, D.M., Howell, S.B., Kaplan, R.M., Roeder, B.L., Murphy, T.W. 2020. Anthelmintic resistance in gastrointestinal nematodes and associated management factors in Intermountain West sheep flocks. Sheep and Goat Research Journal. 35:30-37.

Interpretive Summary: Blood sucking, gastrointestinal nematodes (GIN) present a significant challenge to forage based, small ruminant production systems worldwide. Chemical-based GIN control was inexpensive and extremely effective for many decades. However, over-reliance and, oftentimes, misuse of these chemical dewormers has led to widespread resistance in GIN populations. Most economically important sheep GIN flourish in tropical, sub-tropical, and temperate geographic regions with above average precipitation. Therefore, the semi-arid Intermountain West is not typically thought of as a region where GIN should be a concern to sheep producers. However, grazing on irrigated pastures is becoming a more common production model for farm flock producers in this region but reports of predominant GIN species and dewormer resistance are lacking. A total of 25 sheep operations that grazed irrigated pastures in Montana (n = 15), Wyoming (n = 9), and Utah (n = 1) were selected to participate in the study. Overall, Haemonchus contortus was the most commonly identified GIN from fecal samples across operations (average = 68.5%, minimum = 2%, and maximum = 100%) followed by Trichostrongylus (12.4%, 0%, and 98%, respectively), Oesphagostumum (8.9%, 0%, and 54%, respectively), Teladorsagia (8.4%, 0%, and 48%, respectively), and Cooperia (1.8%, 0%, and 33%, respectively). Results indicated that resistance to H. contortus was highly prevalent with benzimidazoles (91.7%), followed by ivermectin (50%) and moxidectin (8.3%). Effective husbandry and parasite control strategies to mitigate GIN burdens in the Intermountain West should be an area of further research.

Technical Abstract: The objectives of this study were to provide baseline estimates of gastrointestinal nematode (GIN) prevalence and species composition on sheep operations grazing irrigated or subirrigated pastures, quantify anthelmintic resistance utilizing a commercially available larval development assay (LDA), and identify management risk factors from producer responses to survey data. Sampling occurred during the summers of 2017 to 2019 on 25 sheep operations in Montana (n = 15), Wyoming (n = 9), and Utah (n = 1). Operations were selected for the study based on word-of-mouth solicitation and limited to those with a history of GIN challenges. Fecal samples collected at each operation were composited into a single sample for coproculture and LDA analysis. Overall, H. contortus was the most commonly identified GIN across operations (68.5%) followed by Trichostrongylus (12.4%), Oesphagostumum (8.9%), Teladorsagia (8.4%), and Cooperia (1.8%). Twelve operations were evaluated for resistance to H. contortus using the LDA for benzimidazoles, ivermectin, and moxidectin. Results indicated that resistance to H. contortus was highly prevalent with benzimidazoles (91.7%), followed by ivermectin (50%) and moxidectin (8.3%). Grazing system and prior use of the corresponding anthelmintic class did not significantly impact H. contortus susceptibility to ivermectin. Questionnaire responses indicated that 56% of producers attributing production losses to GIN in 0 to 10% of their flock but only 25% utilized targeted treatment methods to guide anthelmintic administration. Results from the present study indicate that anthelmintic resistance to multiple drug classes is a concern in Intermountain West flocks that routinely utilize irrigated pastures as a forage base.