Stability of invermectin resistance in a field strain of Ostergagia spp

T.G. Watson, B.C. Hosking, P.F. McKee

AgResearch, Ruakura Agriculture Centre, Private Bag 3123, Hamilton, New Zealand

Proceedings of the New Zealand Society of Animal Production. 1993, 53: 239-242

Multiple anthelmintic resistance ivermectin (IVM), morantel (MOR), oxfendazole (OXF) was first demonstrated in 1988 from a goat property in New Zealand. Total spelling of all anthelmintics on the site has not been a management option. Since detection, a strict drench rotation has been adopted - levamisole (LEV) 1988-89, IVM 1990 and combination product 1991-92. IVM has been used very strategically as animal health demanded, 2 occasions in 1988, 6 in 1990 and twice in each of 1991 and 1992.

In 1992 surveillance monitoring of anthelmintic status using lambs indicated that LEV remained highly effective (99.3%) against the resistant Ostertagia spp. but there was a subtle shift downward for IVM (87.5%). The alternate milbemycin, moxidectin (MOX), removed 99.9% of the Ostertagia spp. population.

Stability of resistance is an important issue to address when designing and evaluating management options to delay or restrict resistance and minimise the impact on farming profitability. These results suggest that anthelmintic rotation and strategic use of IVM have not had a significant effect on the field isolate over the past 5 years. Currently in the field, there appears to be a significant difference in efficacy between IVM and MOX despite their close structural similarities. It is proposed that MOX will not be used on the site but efficacies will be monitored at regular intervals.

Keywords: NZSAPAB; Invermectin; Ostertagia; resistance; persistence; goats; sheep; stability; nematode.

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Last Updated 25-01-1997