The removal of inhibited early fourth stage Ostertagia ostertagi from yearling cattle by MK 933, an ivermectin formulation

An ivermectin formulation, MK 933, when administered subcutaneously at the rate of 200 g/kg to steers aged 12-14 months, gave a significant (P<0.01) removal of adult, inhibited early fourth stage and later fourth stage Ostertagia (predominantly O. ostertagi] spp. The lowest, median and highest post-treatment counts of early and later fourth stage worms combined were: controls 33,000, 143,500, 233,000; animals treated with MK 933; <500, 2,500, 12,000. For adult worms, the lowest, median and highest counts in the controls were 23,000, 43,000, 68,000, whereas in treated steers no count exceeded 500. The treatment was also highly effective against Trichostrongylus axei.