High-resolution melt (HRM) analysis for detection of SNPs associated with pyrethroid resistance in the southern cattle fever tick, Rhipicephalus (Boophilus) microplus (Acari: Ixodidae).

The southern cattle fever tick, Rhipicephalus (Boophilus) microplus, is the most economically important ectoparasite of cattle worldwide. A limitation for sustainable control and eradication is the emergence of acaricide resistance among tick populations. Molecular diagnostic tools offer the opportunity to detect resistance rapidly, which can be complemented with confirmatory bioassays with larvae and adult ticks that are more resource and time consuming to generate. Synthetic pyrethroid resistance is one of the most prevalent and well-studied forms of resistance in arthropods, being linked with target site alterations in the sodium ion channel gene. Here, we report research on a novel molecular method to detect mutations in the para-sodium channel gene of R. microplus associated with acaricide resistance that is based on quantitative PCR high-resolution melt (HRM) analysis. Genomic DNA fragments of domains II and III of the para-sodium channel gene were amplified by real-time PCR in the presence of EVAGreen dye to test resistant and susceptible reference ticks from the U.S., Brazil, and Mexico. Larval packet tests with discriminating doses and a modified lethal time analysis were performed to confirm resistance to permethrin, cypermethrin, deltamethrin, and flumethrin in laboratory strains. Tick specimens collected from cattle that were inspected at the United States Port-of-Entry at the Texas-Mexico border were also genotyped. Previously described mutations associated with pyrethroid resistance (T170C, C190A, G184C, and T2134A) were successfully detected by qPCR-HRM in different genotypes and confirmed by sequencing. A novel non-synonymous SNP located at domain III (C2136A) and the G215T mutation in domain II, previously described only in Asian R. microplus and R. australis, were also detected with the HRM and confirmed by sequencing. This technique could be adapted for high-throughput screening, detection, and discovery of allele-specific mutations in cattle tick outbreak populations to inform eradication strategies in the USA. This knowledge could also be applied to integrated control programs in other parts of the world where R. microplus is endemic and where similar SNPs have been identified associated with pyrethroid resistance. This study highlights the existence of several mutations in the para-sodium channel gene in different combinations in field populations of R. microplus from Mexico.