Abstract
The control of cattle tick, Rhipicephalus microplus, is focused on repeated use of acaricides. However, due to growing acaricide resistance and residues problem, immunization of animals along with limited use of effective acaricides is considered a suitable option for the control of tick infestations. To date, more than fifty vaccine candidates have been identified and tested worldwide, but two vaccines were developed using the extensively studied candidate, Bm86. The main reason for limited vaccine commercialization in other countries is genetic diversity in the Bm86 gene leading to considerable variation in vaccine efficacy. India, with 193.46 million cattle population distributed in 28 states and 9 union territories, is suffering from multiple tick infestation dominated by R. microplus. As R. microplus has developed multi-acaricide resistance, an efficacious vaccine may provide a sustainable intervention for tick control. Preliminary experiments revealed that the presently available commercial vaccine based on the BM86 gene is not efficacious against Indian strain. In concert with the principle of reverse vaccinology, genetic polymorphism of the Bm86 gene within Indian isolates of R. microplus was studied. A 578 bp conserved nucleotide sequences of Bm86 from 65 R. microplus isolates collected from 9 Indian states was sequenced and revealed 95.6–99.8% and 93.2–99.5% identity in nucleotides and amino acids sequences, respectively. The identities of nucleotides and deduced amino acids were 94.7–99.8% and 91.8–99.5%, respectively, between full-length sequence (orf) of the Bm86 gene of IVRI-I strain and published sequences of vaccine strains. Six nucleotides deletion were observed in Indian Bm86 sequences. Four B-cell epitopes (D519-K554, H563-Q587, C598-T606, T609-K623), which are present in the conserved region of the IVRI-I Bm86 sequence, were selected. The results confirm that the use of available commercial Bm86 vaccines is not a suitable option against Indian isolates of R. microplus. A country-specific multi-epitope Bm86 vaccine consisting of four specific B-cell epitopes along with candidate molecules, subolesin and tropomyosin in chimeric/co-immunization format may provide a sustainable option for implementation in an integrated tick management system.
Highlights
India houses the largest cattle population (193.46 million) in the world [1] and is the highest producer of milk [2]
One of the objectives of the study was to measure the level of polymorphism between Indian (IVRI-I) Bm86 gene with worldwide published full-length Bm86 gene sequences
The multiple sequence alignment (MSA) analysis showed (Supplementary Figure S4) that the specific amino acids of isolates conservation sequences with respect to Indian (IVRI-I) Bm86 differs from the Yeerongpilly vaccine strain at 44 loci, including 42 substitutions and 2 deletions (186, 187) and differs from Camcord (Cuba) vaccine strain at 44 loci (44 substitutions) (Supplementary Table S2)
Summary
India houses the largest cattle population (193.46 million) in the world [1] and is the highest producer of milk [2]. Tick infestation is an important contributor to the low-level of animal productivity. Among the 109 species of ticks reported from India, Rhipicephalus microplus is a widely distributed species that infests livestock, wildlife, and zoo animals and causes significant losses to cattle production [3]. This species inhabits India, South East Asia, Central and South America, northern and eastern Australia, eastern and southern Africa, Madagascar, the Mascarene Islands, New Caledonia, and French Polynesia [4,5]. In India, the cost of controlling ticks and tick-borne diseases (TTBDs) has been estimated at US$498.7 million/annum [8]
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