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 mod erate and high infection levels of Ostertagia ostertagi and Cooperia oncoph ora were evaluated in cattle, Calves were allocated to six groups of six an imals. On Day 0 animals of Groups I1/I2 and D1/D2 were treated with 0.2 mg/ kg ivermectin and doramectin injectable, respectively. Animals of the C1, I 1 and D1 groups received a daily (moderate) infection of 1000 L3 of 0. oste rtagi 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. oncoph ora and 0. ostertagi started from Days 8 and 15 post treatment, respectivel y. Animals were necropsied on Day 40. The calculation of the persistent act ivity of ivermectin and doramectin was based on the efficacy against the di fferent developmental and adult stages of both parasites. The present study confirmed that infection levels may influence the duration of persistent e fficacy of an anthelmintic, Doramectin had at the moderate infection level a persistent efficacy of at least 35 days against 0, astertagi and at least 28 days against C. oncophora; at the high infection dose persistent effica cy was somewhat shorter i.e. up to 33 days and approximately 28 days, respe ctively. The duration of persistent efficacy of ivermectin against 0. oster tagi at the moderate infection level was between 14 and 25 days, at the hig h dose level up to 25 days. Persistent efficacy of ivermectin against C, on cophora could, at both infection doses, not be measured, with the present e xperimental design. (C) 2000 Elsevier Science B.V. All rights reserved.