To determine resistance of small strongyles to albendazole, 3 female ponies (group 1) were grazed on a pasture from May to November 1985 and were treated with 7.5 mg of albendazole/kg of body weight, PO, 2 days before turnout in May and again in June and in July. Three other female ponies (group 2) grazed on a similar pasture from May to July, were treated with 7.5 mg of albendazole/kg, and were removed to another pasture until November. In December, ponies from both groups were treated with 7.5 mg of albendazole/kg, and 8 days later, they were euthanatized and necropsied for a critical test. Worm egg counts in the ponies' feces revealed that the May treatment of group 1 and the July teatment of group 2 were more effective than were later treatments. Numbers of small strongyles were higher in group 1 than in group 2. Efficacy of treatment against all developmental stages of small strongyles were higher in group 2 than in group 1. Efficacy was low in both groups against parasitic 3rd- and 4th-stage larvae. Fifteen species of small strongyles were identified at necrospy. Efficacy was limited against adult Cyathostomum coronatum, Cya labratum, Cylicostephanus calicatus, and Cyl poculatus in both groups; Cylicocyclus nassatus, Cyl minutus, and Cyl longibursatus in group 1; and Cya labiatum in group 2. Efficacy was 100% against Cya catinatum, Cyl goldi, and 5 other species that were found in low numbers.

Anthelmintic efficacies of fenbendazole, ivermectin, and levamisole were evaluated against naturally acquired infections of Nematodirus battus in lambs. Four groups of 10 lambs each were used. Oral administration of 8 mg of levamisole/kg of body weight or 5 mg of fenbendazole/kg significantly (P < 0.01) reduced the degree of infection by N battus (adults) by > 99%. An oral formulation of 200 microgram of ivermectin/kg was 98% effective (P < 0.01). Numbers of other Nematodirus spp (including N filicolis and N spathiger) were significantly reduced.