Status of tick-borne haemoprotozoan infection in south Indian sheep in association with variation in Ovar-DRB1 gene

Abstract

A study was carried out to detect Babesia and Theileria spp. in native sheep of Andhra Pradesh, south India and determine the association of Ovar-DRB1 alleles with resistance/ susceptibility to natural infection of these species using 249 blood samples collected from different breeds (Nellore breed, Gaddabolli, Vizianagaram and Macherla brown). Initial blood smear examination revealed 12.9% (32/249) prevalence of Theileia spp. Further Babesia/Theileria genus specific PCR amplified only 430 bp 18 S rRNA specific to Theileria spp. in 34.9% (87/249) of sheep samples. None of the samples yielded amplification specific for Babesia spp. Species specific differentiation of theileriosis samples revealed that 76 were found to be positive for T. luwenshuni and eleven were positive for mixed infection with T. luwenshuni and T. ovis. Age was found to be significantly (P < 0.05) associated with positivity for theileriosis being high in adult sheep, while gender, breed and zone were not associated with the disease occurrence. Amplification and characterization of Ovar-DRB1 exon 2 by PCR-RFLP from DNA samples of healthy and infected sheep generated three genotypes ‘aa’, ‘ab’ and ‘bb’ and two alleles ‘a’ and ‘b’. The overall frequency of ‘a’ and ‘b’ allele was 0.83 and 0.17 (P < 0.01) and the polymorphic information content varied from 0.22 to 0.26 among the different sheep populations. The χ2 differences of genotype and allele frequencies between different breeds was no significant (P > 0.05). Except for Gaddabolli and Nellore sheep populations the observed heterozygosity value deviated from that of the expected heterozygosity. The χ2 test of independence revealed an association (P < 0.05) between genotypes and infection status in sheep. The frequencies of sheep carrying ‘b’ allele (‘ab’ and ‘bb’ genotypes) were higher in the healthy group compared to the theileriosis sheep, which implied that there was an association between ‘b’ allele and theileriosis resistance. This is the first study demonstrating the association between Ovar-DRB1 alleles and resistance/susceptibility to theileriosis.