Piperazine resistance in population-B equine strongyles: a study of selection in Throughbreds in Kentucky from 1966 through 1983

Observations were completed over an 18-year period (1966 through 1983) in Thoroughbred mares (15/year) and yearlings (11 to 24/year) on a farm where benzimidazole-resistant small strongyles had emerged previously (1962 to 1965). This farm was operated as a closed, nonboarding type, which included a racing stable for its home-bred foals. At 2-week intervals, counts of worm eggs per gram of feces (EPG) and larvae per gram of feces were done to monitor strongyle infections and efficacies of bimonthly (every 8 weeks) antiparasitic treatments that were administered by stomach tube or were fed (dichlorvos pellets) to 1 group of yearlings during a 7-year period (1970 through 1976). The study included several drugs or mixtures, including thiabendazole (TBZ), phenothiazine (PTZ) + piperazine (PPZ)-carbon disulfide (CS2) complex, PTZ + PPZ + trichlorfon (TCF), dichlorvos, and pyrantel pamoate. These were used selectively in treated subgroups (usually 3/year) of mares and yearlings. The horses in drug treatment subgroups grazed together on common pastures; thus, efficacy comparisons between drugs were limited or nullified. However, annual mean EPG and larvae per gram of feces counts of mares and yearlings tended to increase over time, and for the yearlings treated with TBZ + PPZ and PTZ + PPZ-CS2, the buildup of these mean counts was statistically significant (P less than 0.05 for regression coefficients). These trends of annual mean counts to increase were a general index of loss of anthelmintic activity of the 3 drug mixtures containing PPZ (TBZ + PPZ, PTZ + PPZ-CS2, and PTZ + PPZ TCF); however, trends were indicated more definitively by 2 other indices: (1) loss of effectiveness in reduction of EPG counts 2 weeks after treatment and (2) decrease in performance for complete clearance of strongyle eggs (EPG, 0) 2 weeks after treatment. Loss of effectiveness over time was statistically significant (P less than 0.001) for the mares only when data was pooled for all 3 treatments (TBZ + PPZ, PTZ + PPZ-CS2, and PTZ + PPZ + TCF); however, the decreasing effectiveness was statistically significant (P less than 0.05) for TBZ + PPZ and PTZ + PPZ-CS2 treatment of yearlings. Statistically, clearance performance in mareswas significant for treatments with TBZ + PPZ (P less than 0.003) and PTZ + PPZ + TCF (P less than 0.005) and approached significance with PTZ + PPZ-CS2 (P greater than 0.06). Clearance performances in yearlings were highly significant (P less than 0.0003) for treatments with TBZ + PPZ and PTZ + PPZ-CS2. These analyses of data indicate that the intense and prolonged (greater than or equal to 18 years) use of PPZ as a common component in several of the drug mixtures had resulted in the selection of a segment of small strongyles resistant to PPZ. Multiple resistance of this population of small strongyles includes several drugs: PTZ, the benzimidazoles (except oxibendazole and albendazole), possibly pyrantel, and PPZ. Side effects of drug treatments were not apparent during the prolonged period of this study.