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

•Haemonchus contortus was the predominant nematode identified in the studied flock.•Moxidectin/albendazole/levamisole treatment decreased nematode egg-shed by 99.99 %.•Trichostrongyle egg output the following spring was 1.81 % of pretreated values.•Next spring egg output increased by 16.88 fold to 30.58 % of pretreated mean.•Cold winters delay Haemonchus population recovery after spring deworming.