Comparison of targeted selective and whole flock treatment of periparturient ewes for controlling Haemonchus sp. on sheep farms in Ontario, Canada

Abstract

Haemonchosis is commonly associated with late gestation and parturition in ewes during the spring in Ontario, Canada. Due to widespread anthelmintic resistance, targeted selective treatment (TST) of ewes with closantel, an anthelmintic not previously available in Canada, at lambing was investigated as a potential strategy to manage haemonchosis in both ewes and lambs, while also maintaining parasite refugia (i.e., a population of parasites not exposed to anthelmintic). In a longitudinal pilot study in 2013 and 2014, six client-owned farms were randomly assigned as either whole treatment (WT) farms (n=3), where all the ewes were treated with oral closantel (10mg/kg) at lambing, or TST farms (n=3), where ewes were treated with oral closantel (10mg/kg) at lambing only if they met at least one of four criteria: a) the last grazing season was their first grazing season; b) body condition score ≤2; c) FAMACHA© score ≥4; and/or d) three or more nursing lambs. Pasture samples to assess larval burdens and fecal samples from a minimum of 20 ewes and 20 lambs on every farm were collected monthly during the grazing season (May-October). Additional anthelmintic treatments given by producers (either fenbendazole or ivermectin) and deaths attributable to parasitism were also recorded. Pre-treatment arithmetic mean Haemonchus sp. fecal egg counts (FECs) on each farm at the beginning of the study in May 2013, ranged from 506 to 2738 eggs per gram of feces. The number of animals treated on TST farms was reduced by as much as 47% compared to WT farms, and there was no significant difference between TST and WT farms in the proportion of additional treatments given by producers throughout the grazing season or the percentage of parasitism deaths reported. In the first year of the study, Haemonchus sp. larval burdens on pasture were generally higher on TST than WT farms at the beginning of the grazing season, but were not significantly different at the end of the grazing season. In the second year, there were no significant differences in Haemonchus sp. larval burdens on pasture between TST and WT farms throughout the grazing season. This study suggests that, under Ontario conditions, when TST indicators are able to accurately identify animals with high Haemonchus sp. FECs, TST of ewes at lambing can be employed to control Haemonchus sp. FEC in the flock, thereby maintaining refugia and delaying the onset of anthelmintic resistance.