IDENTIFICATION AND CHARACTERIZATION OF A PYRANTEL PAMOATE RESISTANT CYATHOSTOME POPULATION

Three fecal egg count reduction assays (FECR) and one critical trial w ere performed to determine the efficacy of pyrantel pamoate (PP) at 6. 6 mg base kg(-1) on a well managed stud farm in Louisiana where a loss of efficacy was suspected. Efficacy of PP based on FECR varied from 2 5% in mares to 83% in yearlings. Second treatments with PP 2 weeks fol lowing an initial treatment failed to reduce eggs per gram (EPG). A cr itical trial was performed to determine the cyathostome species resist ant to PP. Three strongyle-naive ponies which acquired infections on t he farm were used for this purpose, Following treatment with PP at the recommended dose, 11 species of cyathostomes remained in the intestin e of the tracer ponies. Reduced efficacies (62%-88%) were noted for se ven species. Resistance to oxibendazole (OBZ), which was > 90% effecti ve on this farm in 1982, was also evaluated by FECR and found to exist . The results of one experiment indicate that dual resistance of paras ites to PP and OBZ also exists.