Evaluation of the persistent efficacy of doramectin and ivermectin injectable against Ostertagia ostertagi and Cooperia oncophora in cattle.

The persistent efficacy of doramectin and ivermectin injectable against moderate and high infection levels of Ostertagia ostertagi and Cooperia oncophora were evaluated in cattle. Calves were allocated to six groups of six animals. On Day 0 animals of Groups I1/I2 and D1/D2 were treated with 0.2mg/kg ivermectin and doramectin injectable, respectively. Animals of the C1, I1 and D1 groups received a daily (moderate) infection of 1000 L3 of O. ostertagi and 1000 L3 of C. oncophora, and animals of the C2, I2 and D2 groups received a daily (high) infection of 10,000 L3 of each species. The animals were infected for 21 days with both species, the infections with C. oncophora and O. ostertagi started from Days 8 and 15 post treatment, respectively. Animals were necropsied on Day 40. The calculation of the persistent activity of ivermectin and doramectin was based on the efficacy against the different developmental and adult stages of both parasites. The present study confirmed that infection levels may influence the duration of persistent efficacy of an anthelmintic. Doramectin had at the moderate infection level a persistent efficacy of at least 35 days against O. ostertagi and at least 28 days against C. oncophora; at the high infection dose persistent efficacy was somewhat shorter i.e. up to 33 days and approximately 28 days, respectively. The duration of persistent efficacy of ivermectin against O. ostertagi at the moderate infection level was between 14 and 25 days, at the high dose level up to 25 days. Persistent efficacy of ivermectin against C. oncophora could, at both infection doses, not be measured, with the present experimental design.