Population-S benzimidazole- and tetrahydropyrimidine-resistant small strongyles in a pony herd in Kentucky (1977-1999): effects of anthelmintic treatment on the parasites as determined in critical tests.

Population-S small strongyles have been studied since 1974 in central Kentucky in a closed Shetland pony breeding herd. The ponies were treated approximately every 8 weeks with cambendazole (1974-1978), oxibendazole (OBZ) (1978-1992), or pyrantel pamoate (PRT) (1992-1999). Small strongyles in the ponies have shown resistance to these compounds in field and critical tests. One purpose of this presentation was to compare different parameters for determination of effects on the small strongyle species in ponies after treatment, mainly with OBZ or PRT, from data in critical tests (n=112). Also, the objective was to report on relative changes in the composition of species of small strongyles during the period 1977 through 1999. The following entities were compared to evaluate the effect of OBZ- or PRT-treatment on the small strongyles: (1) numbers of specimens with eggs in utero--there were less gravid worms passed in the feces than recovered at necropsy for OBZ but the numbers of gravid worms were similar in both categories for PRT, (2) pre- and posttreatment counts of eggs per gram of feces (EPGs) and larvae per gram of feces (LPGs)--the reductions were greater for the counts of EPGs than LPGs for OBZ but not for PRT, and (3) pre- and posttreatment counts of EPGs versus % of worms removed--reductions of the former were greater than the latter for both compounds. As shown from data in this study, reduction of EPG counts post treatment indicated much greater drug activity than was actually demonstrated by removal of worms. One evident factor was the value of doing critical tests to verify posttreatment counts of EPGs as indicators of anthelmintic activity. Twenty-eight species of small strongyles were found. For the 20 most prevalent species in the study, two decreased, five remained unchanged, three increased and then became stationary, five increased but then decreased, and five increased progressively. Numbers of small strongyles were highest in 1987 and 1999.