Persistent activity of topical ivermectin against artificial infestations with Hypoderma lineatum (Diptera: Oestridae)

In controlled experiments utilizing artificial infestations, a topical formulation of ivermectin (IVOMEC Pour-On for Cattle) was 100% effective against migrating first instar Hypoderma lineatum for 3 weeks following treatment. Larvae were apparently killed early in the infestation as no significant level of specific antibodies was found in the treated calves. At 4 weeks post-treatment the efficacy remained high at 96%; mortality of larvae in the one calf in which warbles were observed and from which mature third instars were collected, was higher than that seen in the untreated calves, indicating some level of treatment induced effect. No specific antibodies were detected in calves that did not develop palpable warbles. Antibody kinetics in those calves from which viable larvae emerged were typical. The length of activity of this product against early stages of the cattle grub makes it practical to apply treatment up to 3 weeks before the end of fly activity.