A field trial was carried out during a summer-fall period on a commercial beef cattle farm in Minas Gerais State, located in the Southeast of Brazil. In order to evaluate the prophylactic effect and the curative efficacy of
fipronil in a 1%
solution, 200 Zebu crossbred bulls, with ages varying from 20 to 30 months and weights from 233 to 362 kg, were selected. The bulls were assigned by ranked pair to an untreated control group (A) or to a treated group (B), resulting in 100 animals per group. All experimental animals were surgically castrated on day 0, following routine procedures. After
castration all animals in the group B were treated with 10 mg/kg bw of a 1%
fipronil solution, topically on the dorsal mid-line. The
wounds were individually inspected on days: 2, 4, 7, 10, 14, 17, 21, 28 and 35. After
castration the animals were naturally exposed to Cochliomyia hominivorax and remained in the same pasture throughout the trial. Among the animals in the control group, 83 were observed to harbor C. hominivorax eggs, with a total of 97 ovipositions, and among those 73 animals had active
myiasis. In group B (
fipronil 1%), 66 animals showed C. hominivorax eggs, with 92 ovipositions and five animals with active
myiasis. Most ovipositions and active
myiasis were detected until seven days post-
castration for both groups.
Wound parasite infestation evidenced
bleeding, serous purulent exudation and presence of active C. hominivorax larvae. Treatment with
fipronil 1% had a prophylactic effect on scrotal
wounds against the development of C. hominivorax larvae in more than 95% of the treated animals for up to 17 days after
castration. The treatment showed partial protection of 66% and 50% on days 21 and 28 post-treatment (pt), respectively. Three animals from the control group and one from the treated group showed active screwworms on day 21 pt, and one animal from the treated group and two from the control group also presented C. hominivorax larvae on scrotal
wounds on day 28 pt. By the end of the observation period (day 35 pt), the
castration wound had healed in all animals. All experimental animals presenting scrotal
wounds infested with C. hominivorax larvae were treated with a 1% pour-on formulation of
fipronil, on the same day that infestation was observed. Active C. hominivorax larvae were not seen during the monitoring period immediately
after treatment. The curative efficacy of
fipronil 1% against C. hominivorax larvae infestation in
castration wounds was 100%.