EFFICACY OF IVERMECTIN POUR-ON AGAINST OSTERTAGIA-OSTERTAGI INFECTIONAND RESIDUES IN THE AMERICAN BISON, BISON-BISON

Sixteen American bison, Bison bison, were artificially infected with 1 0(5) infective stage larvae of Ostertagia ostertagi on 21 April 1993. At 42 days post-infection eight bison were treated with 0.5% ivermecti n pour-on (500 mu g/kg bodyweight) and eight treated with the carrier only. Bison were necropsied 17 and 18 days post-treatment (21 and 22 J une 1993, respectively). Mean (+/-SE) of 5,413 (+/-1,716) adults and 5 65 (+/-305) immature O. ostertagi were recovered at necropsy from biso n treated with the carrier. No O. ostertagi were detected in bison tre ated with ivermectin pour-on. Based on the levels of the ivermectin ma rker metabolite in liver and adipose tissue 18 days post-treatment, th e established bovine withdrawal time of 48 days appears adequate to in sure that violative residues do not occur.