Control of Boophilus annulatus (Acari : Ixodidae) on cattle using injectable microspheres containing ivermectin

The efficacy of an injectable microsphere formulation of ivermectin For con trol of the cattle tick. Boophilus annulatus (Say), was tested on 2 groups of 6 Hereford heifers held on separate 7-ha tick-infested, buffel grass pas tures. Cattle in one pasture were injected subcutaneously in the neck with a controlled-release microsphere formulation of ivermectin at the rate of 2 .4 mg AI/kg body weight: the other group was injected with carrier only. Be ginning 4 wk after injection and continuing throughout the remainder of the test (16 wk), no engorged ticks (greater than or equal to 5.5 mm) were fou nd on any of the treated cattle, whereas large numbers of engorged ticks we re found on the untreated controls During this period. a few ticks were rec overed from untreated sentinel animals placed in the treatment pasture duri ng 7-8 wk after treatment, but none were recovered from animals exposed fro m II-ZZ wk or 14-15 wk. Large numbers of B. annulatus ticks were Found on u ntreated sentinel cattle placed in the control pasture during these same pe riods. Although the cattle, pastures, and tick habitat were approximately e qual, the treated cattle gained an average of 77 kg compared with an averag e of 42 kg for the control group, This technology offers a possible alterna tive to the current official program of dipping and vacating pastures for e radication of Boophilus sp. infestations from the quarantine zone in southe rn Texas. Larger scale testing is needed to determine the potential of the injectable microsphere formulation and to optimize its use in eradication o r control strategies.