The effect of simulated rainfall on the efficacy of doramectin pour-on against nematode parasites of cattle

Abstract

Two studies were conducted with doramectin topically administered at 500 μg/kg body weight to assess retention of therapeutic efficacy against nematode infections of cattle before, and after, simulated rainfall. In the first study, 50 heifers, with patent nematode infections, were allocated to one of five treatment groups. An untreated control group and one doramectin-treated group were not exposed to simulated rainfall. Simulated rainfall was applied at a rate of 25.4 mm of water in 35 min to three of the five groups: one group immediately before treatment, the second group 90 min after treatment, and the third group 24 h after treatment. Fecal samples were collected for determining egg counts 14 days after treatment. Percentage efficacy ranged from 97.3% to 100% in all treated calves, regardless of exposure to simulated rainfall. The second study involved 40 mixed-sex cattle that were allocated to one of four treatment groups (one saline control and three doramectin-treated groups). All cattle were confirmed to be free of nematode infections prior to oral gavage with infective larvae of Dictyocaulus viviparus, Cooperia oncophora, and Ostertagia ostertagi. Twenty-six days after infection, three groups were treated with doramectin pour-on and exposed to 20 mm of simulated rainfall over 40 min: one group 60 min before treatment, the second 20 min after treatment, and the third 40 min after treatment. Approximately two weeks after treatment, all cattle were necropsied for worm counts. In all treated groups, the percentage efficacy against O. ostertagi and D. viviparus was >99% to 100%. Percentage efficacy against Cooperia ranged from 97% to 98%. Results indicated that doramectin pour-on remains efficacious against nematodes of cattle when administered immediately before or after rainfall.