Anthelmintic resistance and sheep management practices in south western Victoria

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Abstract
Farms [28] in 7 shires in south western Victoria were selected and tested for presence of benzimidazole-resistant nematodes between Nov. 1979 and June 1981. Mean fecal egg counts of sheep were < 100 strongyloid eggs/g on 11 farms. Fecal egg count reduction tests were conducted on the remaining 17 farms and thiabendazole was < 90% efficient in reducing egg counts in sheep from 5 (29%) of these farms. Thiabendazole-resistant Teladorsagia circumcincta were identified at necropsy of experimentally infected treated sheep. In further studies a survey of 104 farms was conducted in the Mount Rouse and Dundas shires of western Victoria in 1981 and 1982, respectively, to determine the prevalence of anthelminthic resistance in these shires. Mean fecal egg counts among weaner sheep in the winter-spring of both years were < 100 eggs/g which indicated low levels of parasitic nematode populations. A fecal egg count reduction test was conducted on 10 farms and thiabendazole was < 90% efficient on 3; levamisole was > 90% efficient in all 10 tests. Most of the surveyed farms carried Merino or Merino crossbred sheep at 10-15 dry sheep equivalents per ha and weaners were treated with anthelminthics 3-6 times per year. Management procedures based mainly on anthelminthic therapy were effective in controlling nematode populations in weaner sheep, although many producers alternated between different groups of anthelminthics within the same year contrary to current recommendations for long-term preservation of anthelminthic efficacy. Anthelminthic resistance was not of practical importance to the majority of sheep producers in the region.