Detection of Ivermectin Resistance Status in Rhipicephalus (Boophilus) microplus Ticks Infesting Cattle from Different Agro-climatic Zones of Gujarat, India

Abstract

Background: Ivermectin is widely used against endo and ectoparasites of cattle, but repeated exposure can select resistant populations of the cattle tick Rhipicephalus (Boophilus) microplus. Information on the geographical distribution of ivermectin resistance in Gujarat is limited. This study evaluated ivermectin susceptibility in R. (B.) microplus collected from seven agro-climatic zones of Gujarat, India. Methods: Engorged female ticks were collected from Southern Hills, Southern Gujarat, Middle Gujarat, North Gujarat, North-West Arid, North Saurashtra and South Saurashtra. Technical-grade ivermectin at 10, 20, 40, 80 and 160 ppm was evaluated by adult immersion test (AIT) and larval packet test (LPT). Adult mortality, egg mass, reproductive index and inhibition of oviposition were assessed in AIT, while larval mortality was recorded in LPT. LC50 values were estimated from concentration-response data and resistance factor (RF) was calculated relative to the susceptible IVRI-1 reference LC50 of 21.68 ppm. Result: Both assays showed the same regional resistance pattern. Southern Hills and Southern Gujarat were susceptible, with AIT LC50 values of 27.63 and 28.90 ppm and LPT LC50 values of 27.03 and 29.21 ppm, respectively. North-West Arid, North Saurashtra and South Saurashtra showed Level I resistance, whereas Middle Gujarat and North Gujarat showed Level II resistance. Middle Gujarat had the highest LC50 in AIT (120.01 ppm; RF 5.53) and LPT (119.91 ppm; RF 5.53). At 160 ppm, adult mortality in Middle and North Gujarat remained 56.67% and 63.33%, while larval mortality was 56.96% and 59.47%, respectively. The concordance between adult and larval assays demonstrates marked regional variation and supports resistance-guided acaricide use in Gujarat.