Reduced efficacy of ivermectin treatments in gastrointestinal nematode infections of grazing cattle in Japan
2014
Kudo, N. (Kitasato University, Towada, Aomori (Japan). School of Veterinary Medicine, Department of Veterinary Parasitology) | Yoshioka, T. | Watanabe, Y. | Terazono, Y. | Takenouchi, S. | Donomoto, T. | Nakajima, K. | Hitosugi, K. | Tsukada, R. | Ikadai, H. | Oyamada, T.
Fecal egg count reduction tests (FECRT) and larval migration inhibition tests (LMIT) were conducted to assess the efficacy of ivermectin (IVM) against gastrointestinal nematodes on 2 cattle farms in northern Japan in 2009 and 2010. Twelve to 20 calves on each farm were treated topically with 0.5 mg IVM/kg 2 (Farm 2) or 4 times (Farm 1) during the grazing season (May-October). On Farm 1, fecal egg count (FEC) reduction at 14 days post-treatment ranged from 16 to 87% in 2009 and from 24 to 96% in 2010, with relatively low reductions in August and October (16-53%). Conversely, IVM treatment on Farm 2 reduced FEC by 97% in September 2009. Larvae obtained from fecal cultures and identified by PCR-RFLP analysis revealed that the dominant species on both farms prior to IVM administration was Cooperia oncophora. In 2009, the FEC reduction of C. oncophora on Farm 1 decreased from 85% in May to 56% in August. In 2010, the reduction in C. oncophora in August was 28%. In the LMIT using larvae collected from the fecal cultures on Farm 1 in May and August 2009, the ECsub(50) value of IVM in C. oncophora in August (0.892 micro g/ml) was 3 times higher than that in May (0.296 micro g/ml). The results of the LMIT corroborated the FECRT data, indicating the presence of IVM-resistant C. oncophora on Farm 1, at least in August. This is the first report of IVM-resistant nematodes in Japanese cattle.
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