Determination and validation of discriminating concentration of ivermectin against Rhipicephalus microplus

Rhipicephalus microplus, the major cattle tick species of India is prevalent all over the country and causes huge economic loss directly or indirectly to the dairy industries. Chemical acaricides are playing an important role in managing tick infestations on livestock for many years and consequently, resistance to commonly used organophosphate (OP) and synthetic pyrethroid (SP) compounds has been reported. Subsequently, ivermectin (IVM) has been emerged as an alternative to manage OP and SP resistant ticks. However, with the increase of use during the last 5-8 years, there is a possibility of development of resistance and thus there is an urgent need to develop a robust resistance monitoring tool to safeguard the drug. Lethal concentrations for 50 and 95% mortality of treated ticks were determined to work out discriminating concentration (DC) in order to diagnose resistance in the field situation. The DC (2 x LC₉₅) was determined as 93.54 ppm using an established reference susceptible IVRI-1 line of R. microplus adopting adult immersion test. For validation of DC, the resistance status was checked in seven tick isolates of R. microplus collected from northern and eastern regions of India. The RR₅₀ and RR₉₅ values of the field isolates against ivermectin were determined and were in the range of 1.56-8.25 and 1.93-27.58, respectively. All the collected isolates were found to have higher lethal concentration and resistance ratio in comparison to reference susceptible IVRI-1 tick line (LC₅₀ = 21.68, LC₉₅ = 46.77 ppm, RR = 1.0). Amongst the field isolates, the isolate collected from Fatehgarh Sahib district (FTG) of Punjab state showed highest RR₅₀ of 8.25 indicating high level of resistance to IVM. The generated DC will be used for IVM resistance characterization of ticks infesting cattle in different parts of the country.