Critical tests of new benzothiazole anthelmintic tioxidazole in the horse

The basic-dose confirmation tests of tioxidazole for removal of susceptible populations of gastrointestinal parasites of the horse were made in 10 naturally infected horses, using the critical test method (experiment A). A single dose of toxidazole, given at the rate of 11 mg/kg of body weight, was administered to 5 horses by stomach tube and to 5 horses by mixing the drug with the daily grain ration. In the 5 horses given the drug by stomach tube, aggregate average removals were 90% or more for 5 horses infected with Stronglyus vulgaris, 5 infected with S edentatus, 5 infected with small strongyles, 3 infected with Parascaris equorum, 3 infected with mature Oxyuris equi, and 5 infected with immature O equi. Bots (Gastrophilus intestinalis and G nasalis) in 5 infected horses and tapeworms (Anoplocephala perfoliata or A magna) in 3 infected horses were not removed. Activity against stomach worms (Trichostrongylus axei, Habronema muscae, and Drashia megastoma) was not evidenced. In the 5 horses given the drug in the feed, aggregate average removals were 90% or more for 5 horses infected with S vulgaris, 5 infected with S edentatus, 1 infected with S equinus, 5 infected with small strongyles, 3 infected with P equorum, 3 infected with mature O equi, and 5 infected with immature O equi. Activity against bots, tapeworms, and stomach worms was not detected. Larval count data on fecal samples also indicated liited, if any, activity against Strongyloides westeri by tioxidazole at this dosage (11 mg/kg). In addition to experiment A, tioxidazole (11 mg/kg) was administered via stomach tube in a single dose in a critical test to a foal naturally infected with benzimidazole-resistant small strongyles (population B) and removal was only 27% (experiment B). The same 5 species of small strongyles refractory to the benzimidazoles also exhibited resistance to tioxidazole. Specimens of P equorum and S vulgaris were all removed from the foal. Untoward effects of tioxidazole treatment (11 mg/kg) were not observed in any of the animals.