Cambendazole for strongyle control in a pony band: selection of a drug-resistant population of small strongyles and teratologic implications.

Cambendazole (CBZ) treatments (20 mg/kg) given at 8-week intervals were used for parasite control in a breeding band of ponies (n = 33 to 43) during the period July 1974 to August 1978. Pre- and posttreatment worm egg counts on feces were used to evaluate efficacy of treatments after every 2nd treatment interval by monitoring changes in strongyle egg counts. Initially, effective reductions (expressed as 92% to 96% fewer worm eggs) were lessened to 70% by the 9th treatment (18 months) and to 28% by the 12th treatment, and thereafter, they fluctuated between 0% and 38%. Critical tests on animals culled during the 3rd year revealed that (i) large strongyles (Strongylus vulgaris and S edentatus) were efficaciously removed, (ii) 5 species of small strongyles (Cyathostomum catinatus, Cyathostomum coronatum, Cylicocyclus nassatus, Cylicostephanus longibursatus, and Cylicostephanus minutus) exhibited some degree of resistance, and (iii) 11 species of small strongyles from 6 genera, including the 3 already named, were highly susceptible. Cylicostephanus minutus, in this population exhibited benzimidazole resistance for the first time, whereas Cylicostephanus goldi, previously recognized as resistant in other populations (B and C) in Kentucky, was susceptible to CBZ. A 2-fold increase in the drug-dose rate did not increase efficacy against the CBZ-resistant species of small strongyles, but a 4-fold increase small stronglyles, efficaciously removed 93% of the resistant small strongyles. Foaling rates in the brood band ranged between 38% and 61% for the 5 seasons. Three deformed foals occurred--1 in 1976 and 2 in 1977--among the total of 83 births. Present data did not evidence a probable week of early gestation for possible insult of the fetus by CBZ treatment.