Multiple anthelmintic resistance in Trichostrongylus colubriformis

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Abstract
Following the failure of anthelmintic treatment to control an outbreak of trichostrongylosis in sheep, multiple resistance to levamisole and oxfendazole was confirmed in field strains of Trichostrongylus colubriformis at the CSIRO Pastoral Research Laboratory, Armidale [Australia]. Resistance in Trichostrongylus spp. the levamisole was also confirmed on an adjoining farm. From the results of an experiment where lambs were treated at the recommended dose rate with one or both anthelmintics, it was estimated that 32% of T. colubriformis were resistant to levamisole 19% to oxfendazole and 12% to both drugs. Simultaneous administration of levamisole and oxfendazole resulted in the additive anthelmintic effect. Naphthalophos (36.6 to 51.2 mg/kg) was 93% efficient against the multiple resistant strain. The similar histories of anthelmintic usage of the CSIRO Pastoral Research Laboratory and on the northern tablelands of New South Wales generally, suggest that multiple anthelmintic resistance in T. colubriformis may soon emerge as a problem on the northern tablelands.