Amitraz is a rapidly acting acaricide that has been in use for the control of cattle ticks for more than 30 years. Resistance against amitraz was first reported in Boophilus microplus in Australia in 1980 but has been slow to spread in comparison to resistance against synthetic pyrethroids. The most recent estimate of prevalence of amitraz resistance in Australia is 10.8%. In Mexico, the development and distribution of amitraz appears to have been more rapid and the prevalence has been estimated to be 19.4% in Yucatan state. In New Caledonia, about 10% of properties were confirmed to have amitraz resistance. There is little reliable information on the prevalence of amitraz resistance in southern Africa. Risk factors have been identified, but the small sample sizes in the studies that have attempted to identify risk factors using survey data suggest caution in their interpretation. Regional variation in prevalence has been reported, as has a positive relationship with frequency of acaricide application. There is evidence to suggest that in Australia, amitraz resistance might have emerged on a small number of properties and been disseminated by cattle movements. There is also some evidence to suggest that amitraz resistance can diminish in the field when selection pressure is not applied. The mode of inheritance of amitraz resistance is uncertain and it has been suggested that it is a polygenic trait. The mechanism of amitraz resistance is unknown. Two possibilities have been proposed: octopamine receptor and monoamine oxidase. There is some equivocal support for both possibilities. The larval packet test bioassay is the most reliable method of diagnosing amitraz resistance in B. microplus, and this test has been modified by Miller to provide more accuracy and repeatability. Molecular tests are in development but will not eliminate the need for the bioassay.

Progress in the epidemiology and diagnosis of amitraz resistance in the cattle tick Boophilus microplus in Australia

http://www.cell.com/article/S2405844022000068/pdf

Development of acaricide resistance in tick populations of cattle: A systematic review and meta-analysis

Characterization of acaricide resistance in Rhipicephalus microplus populations infesting cattle in northeastern India and assessment of local plant extracts for tick management

The intensive usage of chemical acaricides for the control of the cattle tick Rhipicephalus microplus has resulted in the development and establishment of multi-acaricide resistant populations. Fipronil, a phenylpyrazole insecticide, is currently marketed in India for the management of this important veterinary tick species. Here, we tested Indian isolates of R. microplus which have developed multi-acaricide resistance, for their susceptibility to fipronil. Twenty-five field isolates from five agro-climatic zones of the country were collected and tested by adult immersion test (AIT) and larval packet test (LPT). Sixteen isolates with resistance factor (RF) in the range of 1.56–10.9 were detected using LPT, whereas only 11 isolates with RF ranging from 1.05 to 4.1 were detected using AIT. A significant variation of RF between both tests was found, which raises doubt about the suitability of larva-based assays in screening of fipronil resistance. The data indicated possible cross-resistance between groups of acaricides in fipronil-resistant tick populations.

Emergence of fipronil resistant Rhipicephalus microplus populations in Indian states

A retrospective review on ixodid tick resistance against synthetic acaricides: implications and perspectives for future resistance prevention and mitigation

The isolates of Rhipicephalus microplus collected from Madhya Pradesh (MP), Punjab (PJB) and Uttar Pradesh (UP) states of India were characterized using laboratory standardized adult immersion test (AIT) against macrocyclic lactone (ivermectin), synthetic pyrethroids (cypermethrin and deltamethrin), organophosphates (coumaphos and diazinon) and phenylpyrazole compounds (fipronil). Out of the six isolates tested, five isolates except MTH were resistant to deltamethrin and cypermethrin at level II with RF ranging from 16.4 to 24.02 and 7.05 to 13.2, respectively. In case of organophosphates, coumaphos was less effective showing resistance level II (RF 8.52–11.2) in all the six populations compared with diazinon to which three isolates (MHW, RWA and AGS) were resistant at level II. Except MTH, other five isolates were categorized at level I with RF ranging from 1.53 to 3.02 against ivermectin. The phenylpyrazole compound however was found effective, and none of the isolates could survive at a discriminating concentration. The possible strategy for the management of multi-acaricide-resistant ticks in the surveyed districts was discussed in the present study.

Shanmuganath C.

Papers

Acaricide resistance in Rhipicephalus microplus collected from selected districts of Madhya Pradesh, Uttar Pradesh and Punjab states of India.