First detection and evaluation of ivermectin resistance in Haemonchus contortus from sheep in Zhaosu, Xinjiang, China

Abstract Background: In 2017 in Zhaosu, Xinjiang, China, a series of sheep death suspected to have reduced the potency of ivermectin (IVM) have raised the vigilance of herdsmen. At the same time, veterinary practitioners warned of the potential risk of the lack of efficacy of IVM in the treatment of Haemonchus contortus. Methods: In this study, the efficacy of IVM against H. contortus was assessed under controlled experimental conditions by using larval developmental assay (LDA) and faecal egg count reduction test (FECRT) to determine whether the reduced efficacy in sheep in Zhaosu was caused by IVM resistance. Results: In total, 1560 adult worms were isolated from the abomasum of post mortem examination of dead sheep which received recent IVM (0.2 mg/kg) treatment before death and eggs were collected from female worms for usage in LDA. The LDA result revealed that significant difference in resistance ratio (RR) was detected between the IVM-susceptible and -resistant isolates (6.09 for Zhaosu strain) of H. contortus. The Zhaosu strain was maintained in laboratory for FECRT, which showed that the efficacy of IVM was 0% at 1, 2 and 3 times dose, but at 4.8 times dose, it reached 87.5% at 14 days post-treatment. Conclusion: The findings suggested that controlling H. contortus of small ruminants in Zhaosu requires an integrated parasite management strategy that should include monitoring of infection and anthelmintic resistance, and increasing the awareness of herdsmen on the importance of using the appropriate drug usage to improve nematode control in sheep.