Extended effects of an aggressive spring-time anthelmintic treatment on Haemonchus contortus in sheep from a North America cold winter climate

Abstract

Anthelmintic-resistant haemonchosis is a growing problem even among sheep located in winter climates lethal to the free-living third-stage larvae (L3) of Haemonchus contortus. As survival in cold climates is entirely dependent on tissue-dwelling fourth-stage larvae (L4s), an effective winter or spring-time anthelmintic treatment can significantly collapse H. contortus populations in cold winter regions. This extended collapse would be useful for maximizing population-replacement strategies designed to reverse anthelmintic resistance if the imported susceptible worms become well established before the collapsed resistant population rebounds. This study evaluated the extended effects of an aggressive anthelmintic treatment with moxidectin, albendazole, and levamisole given during the first spring lambing season (2014) and a levamisole treatment given the following spring on adult ewes from the USA Northern Plains naturally infected with a population of H. contortus showing moderate levels of anthelmintic resistance. The size of the trichostrongyle populations was measured via fecal egg counts (FECs) performed pre- and post-treatment, then bimonthly during the first grazing season, and then yearly during the next three years. Prior to treatment, H. contortus was identified in 97.8 % of the PCR-tested fecal samples; Teladorsagia circumcincta and Trichostrongylus spp. were found in 26.7 % and 50.0 % of the tested samples respectively. Pre- and 2-week post-treatment FECs showed a 99.99 % reduction to 0.16 eggs per gram (EPG). Ewes showing less than 1.0 EPG (N = 248) were grazed 2–3 times through eight paddocks from June 30 to November 4, 2014. Bi-monthly FECs showed very low egg output until ten weeks on paddocks which increased steadily and peaked on the sixteenth week at 42.1 EPG (1.15 % of the pretreatment mean). During the following year, mean FEC was 66.1 and 5.1 EPG for the spring and fall, respectively which increased by 16.88-fold to 1116.0 EPG by spring 2016 and remained high in 2017. PCR analysis showed that H. contortus remained the predominant species throughout the study, but T. circumcincta and Trichostrongylus spp. prevalence increased by the study’s end. This extended population collapse would greatly maximize population-replacement strategies designed to reverse anthelmintic resistance.