The association between frequency of thiabendazole treatment and the development of resistance in field isolates of Ostertagia spp. of sheep |
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Authors: | P J Martin N Anderson T Lwin G Nelson T E Morgan |
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Institution: | 1. CSIRO Division of Animal Health, Animal Health Research Laboratory, Private Bag No. 1, P.O., Parkville, Victoria, 3052, Australia;2. CSIRO Division of Mathematics and Statistics, Private Bag No. 10, Clayton, Victoria, 3168, Australia;3. 4 Kessel Street, Gladstone, Queensland, 4680, Australia |
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Abstract: | Martin P. J., Anderson N., Lwin T., Nelson G. and Morgan T. E. 1984. The association between frequency of thiabendazole treatment and the development of resistance in field isolates of Ostertagia spp. of sheep. International Journal for Parasitology14: 177–181. Isolates of Ostertagia spp. were obtained from grazing sheep 3,4 and 5 years after nil, planned (five per year) and regular (3-weekly) treatments with thiabendazole (TBZ). Levels of resistance to TBZ were measured by an in vitro egg hatch assay and a controlled anthelmintic efficiency assay. Isolates from planned treatment groups showed an increase in the level of resistance; the lethal concentrations of TBZ to 50% of eggs (LC50s) were 3, 3 and 6 times the LC50s of isolates from nil treatment groups for years 3, 4 and 5 respectively. The LC50s of isolates from regular treatment groups were 14 times higher than those from nil treatment groups in each year. To assess the potential for an increase in level of resistance, additional egg assays were done 14 days after treatment with 44 mg kgt?1 of TBZ on sheep infected with the planned group isolates for each year. This treatment raised the LC50S for years 3, 4 and 5 respectively by 3, 2 and 1–5 times the LC50s of the same isolates which had not been exposed to additional TBZ treatment. The controlled anthelmintic efficiency assay using 44 mg kgt?1 of TBZ produced a significant reduction in the number of adult and immature worms from the nil isolate but failed significantly to reduce the number of worms from the planned and regular isolates. A three component analysis resolved the nonlinear trends of the log dose-probit plots in egg hatch assays for isolates from planned treatment groups into subpopulations of susceptible, hybrid and resistant individuals each with different levels of resistance. The proportions of these subpopulations changed in accordance with the level of resistance observed in each year. |
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Keywords: | Sheep anthelmintic resistance thiabendazole egg hatch assay anthelmintic assay |
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