Benzimidazole resistance in Nematodirus spathiger and Nematodirus filicollis in New Zealand

AIM: To determine the prevalence of benzimidazole (BZ) resistance in Nematodirus spathiger and Nematodirus filicollis from a sample of New Zealand farms. METHODS: The efficacy of albendazole against Nematodirus spp. was assessed by faecal egg count reduction tests (FECRT) undertaken on 21 sheep farms throughout New Zealand, with the majority of these farms being selected as a result of samples sent to the laboratory for routine testing. On each farm, one group of 12 animals was treated with albendazole (ABZ) (4.75mg/kg) while another group of 12 remained as an untreated control. Faecal samples were collected from all animals at the time of treatment and 7-10 days later. Faecal egg counts were performed using a modified McMaster method. Larvae were cultured from pooled post-treatment faecal samples from each group by incubation at 20°C for 6 weeks, 4° for 26 weeks then 13°C for 2 weeks. The resulting infective larvae were identified to species using PCR-based techniques with species specific primers for the ITS-2 region of ribosomal DNA. The proportion of the two species in culture was applied to the group mean faecal egg counts and used to calculate the efficacy for both species. RESULTS: ABZ resistance was found on 95% (20/21) of farms for N. spathiger which was a higher prevalence (p<0.05) than for N. filicollis with only 40% (4/10) of farms. CONCLUSION: The results confirm the high level of resistance in N. spathiger in New Zealand and that ABZ resistance was more common in N. spathiger than N. filicollis. While resistance to benzimidazole anthelmintics has been reported previously in New Zealand, this is the first report of N. filicollis being resistant to BZ anthelmintics.