Resistance to benzimidazole anthelmintics in field strains of Ostertagia and Nematodirus in sheep

Abstract

Resistance to a range of benzimidazole anthelmintics was investigated in 2 strains of Ostertagia spp. One strain (SR80) had been exposed to fenbendazole, oxfendazole and thiabendazole, the other strain (KR79) only to thiabendazole. Both strains showed a high degree of resistance to albendazole, fenbendazole, oxfendazole and thiabendazole, which had efficiencies of 11-38% and 0-60% against all developmental stages of SR80 and KR79, respectively. There was no significant reduction in the KR79 worm count by thiabendazole at 132 mg kg-1 given either as a single dose or divided into 12 equal portions, one administered every 6h. Nematodirus spp were also found to be resistant to all benzimidazoles tested. Levamisole at 7 mg kg-1 and naphthalophos at 30 mg kg-1 had efficiences of 89 and 66%, respectively, against SR80 Ostertagia and 99 and 19% against Nematodirus, all but the last of these being significant reductions in worm burdens. The resistant SR80 Ostertagia occurred on a research station, but possibly originated from a property which 3 years earlier had supplied ewes to the station. The use of benzimidazole anthelmintics and subsequent grazing on worm-free pasture may have enhanced the level of resistance. A relationship was established between egg counts and adult worm counts 10 days post-treatment, which suggested that for Ostertagia the worm count could be predicted from the geometric mean egg count from about 10 animals. Thus, where an Ostertagia population is suspected of being resistant, an anthelmintic efficiency assay using pre- and post-treatment faecal egg counts should provide a satisfactory diagnostic procedure.

Anthelmintic resistance: The state of play revisited

“…One of the important limitations of FECRT is that test results may not estimate anthelmintic efficacy accurately because nematode egg output does not always correlate well with actual worm numbers, and the test only measures effects on egg production by mature worms. A good correlation has been found between faecal egg counts and worm counts for H. contortus, but not for T. colubriformis (Sangster et al, 1979) or T. circumcincta (Martin et al, 1985). Egg counts for Nematodirus spp.…”

“…Egg counts for Nematodirus spp. are generally low and bear little relationship to actual worm burdens (Chalmers, 1985;Martin et al, 1985).…”

A Field Study on the Effect of Some Anthelmintics on Cyathostomins of Horses in Sweden

“…This has been observed for Ostertagia spp. in sheep (Martin et al, 1985). Another possible explanation could be that resistant female cyathostomins that survived the anthelmintic treatment compensated for the loss of susceptible worms.…”

Resistência anti-helmíntica em nematoides gastrintestinais de pequenos ruminantes: avanços e limitações para seu diagnóstico

“…Foi encontrada uma boa correlação para H. contortus (Roberts & Swan 1981, Chagas et al 2013), porém não para T. colubriformis (Sangster et al 1979), Ostertagia circumcincta (Martin et al 1985) e Nematodirus spp. (Martin et al 1985). Palcy et al (2010) relataram uma baixa sensibilidade do teste para a detecção de T. axei, cuja postura de ovos é muito baixa.…”