Susceptibility of Ornithodoros parkeri (Cooley) (Acari: Argasidae) and Dermanyssus gallinae (DeGeer) (Acari: Dermanyssidae) to Ivermectin

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Abstract
Ivermectin was injected into hosts and evaluated for effectiveness against the argasid tick Ornithodoros parkeri (Cooley) and the chicken mite Dermanyssus gallinae (DeGeer). O. parkeri second stage nymphs (N2s) showed a marked increase in mortality when fed on mice injected intraperitoneally with ivermectin at a dose between 0.1 and 0.2 mg/kg host body weight. For adult O. parkeri and D. gallinae, 0.4 and 0.5 mg, respectively, per kilogram of host body weight were necessary for an increase in mortality over controls. These latter findings are comparable to those reported for other tick species but differ from those reported for the northern fowl mite, Ornithonyssus sylviarum (Canestrini & Fanzago). Further testing narrowed the effective dose range for O. parkeri adults to between 0.425 and 0.450 mg/kg host body weight. The time interval (4, 8, and 24 h) between ivermectin injection of the host and tick feeding had only a slight influence on the overall effectiveness of the drug. In O. parkeri, doses of 0.0125 to 0.1000 mg/kg body weight did not affect fecundity, hatchability, gross morphology of the reproductive system and synganglion, or histology of the reproductive system. Contrary to reports of irreversibility of effects of ivermectin on gamma aminobutyric acid-mediated neurotransmission, many ivermectinparalyzed ticks recovered partial mobility over time.