The efficacy of moxidectin 0.5% pour-on against Hypoderma bovis in naturally infested cattle: parasitological and serological data.

The field efficacy of moxidectin 0.5% pour-on formulation was assessed in heifers naturally infested with Hypoderma bovis. After their first grazing season in an infested area 34 heifers were selected on the basis of results of an ELISA test performed in January 1992. Twenty-five animals were treated with moxidectin 0.5% pour-on at a dose of 0.5 mg kg-1 bodyweight and nine heifers were left untreated. No local or general side-effects associated with the treatment were noticed. In April, seven of the nine untreated heifers were affected by warble fly, and in May all were infested; none of the treated animals harboured warble fly at any time during the trial. All the extracted larvae were identified as Hypoderma bovis. On the basis of the warble counts, efficacy was 100%. The antibody kinetics, studied on blood samples collected on Days 0 (14 January), 6, 27, 48, 90, 119 and 150, showed that in the untreated animals, the antibody titres increased until May (Day 119) and then declined sharply, whereas in the treated animals there was a drop from Day 30 onwards. The mean ELISA titres of the control and treated groups became significantly different from Day 48 onwards (P < 0.05). In conclusion, moxidectin 0.5% pour-on formulation at a dose of 0.5 mg kg-1 bodyweight can effectively treat cattle naturally infested with Hypoderma bovis first instar larvae.