In vitro acaricide efficacy evaluation on Ambylomma variegatum and Rhipicephalus (Boophilus) decoloratus ticks infesting cattle in the Gondar Zuria District, Northwest Ethiopia

Abstract

Ticks are major ectoparasites affecting the health and productivity of livestock in Ethiopia. However, there exists limited information on the current efficacy of widely used acaricides including ivermectin 1% and amitraz 12.5%. Experimental randomized controlled study design was conducted from February 2023 to June 2023 to evaluate in vitro susceptibility of field-collected Amblyomma variegatum (A. variegatum) and Rhipicephalus (Boophilus) decoloratus (R. decoloratus) in cattle in Gondar Zuria District Northwest Ethiopia to ivermectin1% and amitraz 12.5% at varying concentrations. Collected engorged female ticks ( n = 540) were exposed to four concentrations of ivermectin 1% (0.00125, 0.005, 0.01, and 0.025%) for 24 h and three level concentrations of amitraz 12.5% (0.0125, 0.025, and 0.05%) for 48 h. Ivermectin 1% produced rapid and concentration-dependent mortality, with maximum efficacy of 96.66% for A. variegatum and 90% for R. decoloratus at the highest dose (0.025%). Similarly, amitraz 12.5% showed strong dose-dependent activity after 48 h, achieving mortality of up to 80.0 and 90% for A. variegatum and R. decoloratus , respectively. Two-way ANOVA confirmed that ivermectin 1% concentration, observation time and their interaction significantly affected mean mortality rate of A. variegatum (F = 3.34, P = 0.0184; F = 12.51, P < 0.001; F = 4.94, P < 0.001) and R. decoloratus (F = 21.02, P < 0.001; F = 36.81, P < 0.001; F = 3.45, P = 0.0016), respectively. In addition, amitraz 12.5% concentration and exposure time significantly affected mortality in A. variegatum (F = 26.72, p < 0.001; F = 4.20, p = 0.0129) and R. decoloratus (F = 44.41, p < 0.001; F = 12.82, p < 0.001), although the interaction effect was not significant for A. variegatum ( P > 0.05) but was significant for R. decoloratus ( P < 0.05) . These findings indicate that both acaricides remain effective against adult ticks in the study area, with ivermectin 1% acting more rapidly and amitraz 12.5% demonstrating a clear dose–response relationship. Continued surveillance and integrated tick management are recommended to mitigate the risk of emerging acaricide resistance.