Summary
Critical tests were completed on foals (n = 15) naturally infected with benzimidazole (bz)-resistant population-B strongyles during the period between 1981 and 1987. Thiabendazole at a dosage of 44 mg/kg was tested in 8 foals, oxfendazole at 10 mg/kg was tested in 4 foals, and phenothiazine at 55 mg/kg, cambendazole at 20 mg/kg, and fenbendazole at 5 mg/kg were tested in 1 foal each.
Efficacies of thiabendazole, cambendazole, and fenbendazole against the 5 species of bz-resistant small strongyles (Cyathostomum catinatum, Cyathostomum coronatum, Cylicocylus nassatus, Cylicostephanus goldi, and Cylicostephanus longibursatus) were partial. Removals in the aggregate averaged 48%, 52%, and 68%, respectively, indicating continued resistance of the 5 species of small strongyles to thiabendazole, cambendazole, and fenbendazole. In contrast, oxfendazole was efficacious, removing an aggregate average of 95% of the 5 bz-resistant species of small strongyles in the 4 foals. This apparent reversion to susceptibility to oxfendazole may be attributable to paucity of reference data (1 test foal) done in 1976.
All 5 species of the bz-resistant small strongyles had cross resistance to phenothiazine (38% aggregate removal), whereas 9 of the bz-susceptible species were effectively (99%) removed. This suggests a similar mode of action between phenothiaznie and the bz compounds.