Status of ivermectin resistance in Rhipicephalus (Boophilus) microplus (Acari: Ixodidae) populations from north-western Himalayas, India.

Status of ivermectin resistance was studied in 16 isolates of Rhipicephalus (Boophilus) microplus ticks collected from north-western Himalayan region of India using larval immersion test (LIT). Regression graphs of probit mortality of larval ticks plotted against log values of concentrations of ivermectin were utilized for the determination of slope of mortality, lethal concentrations (LC50 and LC95) and resistance ratios (RRs). Out of 16 isolates examined, 15 were found resistant against ivermectin. Level II resistance was detected in eleven isolates (RR50 ranged from 5.72 to 7.62), whereas four isolates exhibited level I resistance (RR50 ranged from 1.58 to 2.77). Only one isolate was found susceptible (RR50: 1.23). Higher RR values were observed in tick isolates from low altitude areas where intensive animal husbandry activities are practiced as compared to tick isolates of high altitudes accredit the fact that managemental practices, availability of drug and its frequency of application play an important role in development of resistance.