Variation among Bm86 sequences in Rhipicephalus (Boophilus) microplus ticks collected from cattle across Thailand

Anti-tick vaccines based on recombinant homologues Bm86 and Bm95 have become a more cost-effective and sustainable alternative to chemical pesticides commonly used to control the cattle tick, Rhipicephalus (Boophilus) microplus. However, Bm86 polymorphism among geographically separate ticks is reportedly associated with reduced effectiveness of these vaccines. The purpose of this study was to investigate the variation of Bm86 among cattle ticks collected from Northern, Northeastern, Central and Southern areas across Thailand. Bm86 cDNA and deduced amino acid sequences representing 29 female tick midgut samples were 95.6-97.0 and 91.5-93.5 % identical to the nucleotide and amino acid reference sequences, respectively, of the Australian Yeerongpilly vaccine strain. Multiple sequence analyses of these Bm86 variants indicated geographical relationships and polymorphism among Thai cattle ticks. Two larger groups of cattle tick strains were discernable based on this phylogenetic analysis of Bm86, a Thai group and a Latin American group. Thai female and male cattle ticks (50 pairs) were also subjected to detailed morphological characterization to confirm their identity. The majority of female ticks had morphological features consistent with those described for R. (B.) microplus, whereas, curiously, the majority of male ticks were more consistent with the recently re-instated R. (B.) australis. A number of these ticks had features consistent with both species. Further investigations are warranted to test the efficacies of rBm86-based vaccines to homologous and heterologous challenge infestations with Thai tick strains and for in-depth study of the phylogeny of Thai cattle ticks.