Use of oxibendazole for control of cambendazole-resistant small strongyles in a band of ponies: A six-year study

Abstract

SUMMARY Oxibendazole ( obz ; 10 mg/kg of body weight) was administered to ponies at 8-week intervals to control strongylosis in a breeding band of Shetland-type ponies (n = 29 to 50) from October 1978 through September 1984. A similar use of cambendazole ( cbz ; 20 mg/kg of body weight) in this band of ponies during the preceding 4-year period resulted in the survival of a cbz -resistant population (S) of small strongyles. Effectiveness of obz treatments was monitored by pre- and posttreatment counts of the number of strongyle eggs per gram of feces (epg) and of the number of strongyle larvae per gram of feces (lpg). For the First 4 years of the 6-year period, epg and lpg counts were determined for 3 to 5 of the obz treatments; during the last 2 years, counts were determined for each treatment. For the 1st year, the mean reduction in epg was 96%. During the 2nd and 3rd years, mean reductions in epg were 80% and 81%, respectively. For the 4th and 5th years, mean reductions in epg were 69% and 43%, respectively. For the 6th year, the mean reduction in epg was 37%. Statistical analyses of the epg data, using the linear plateau method, indicated a significant increase in residual (surviving) strongyle infections after the periodic obz treatments, with a mean annual increase in survival of 10.3% for the 6 years. Changes in lpg were variable, but an overall loss of effectiveness of the obz treatments was evident. Only infective larvae of small strongyles were observed in the cultures; larvae of 2 species of large strongyles, Strongylus vulgaris and Strongylus edentatus , were conspicuously absent. Dynamics of the process of resistance selection by obz was gradual and in a stair-step fashion. A mean appreciable activity of 40% was retained for the 5th and 6th years; in the previous 4-year period in which cbz was used, strongyle activity of cbz rapidly decreased during the 2nd year and was nearly inactive during the 3rd and 4th years. This ineffectiveness of cbz persisted after a hiatus of nearly 3 years of obz use, when cbz was evaluated once again in the present, study. Foaling rates in the brood mares were 32% to 58% during the 5 seasons evaluated. Four abnormal foals were born, 1 each in 1980, 1982, 1983, and 1984. These deformities were categorized as genetic in origin, in contrast with the teratologic-type deformities previously associated with the use of cbz in this band.