Molecular epidemiology and characterization of bovine leukemia virus in domestic yaks (Bos grunniens) on the Qinghai-Tibet Plateau, China

Bovine leukemia virus (BLV) is a member of the genus Deltaretrovirus of the family Retroviridae and cause a chronic lymphosarcoma, which is extensive in cattle. In yaks (Bos grunniens), the distribution, strains and genetic characteristics of BLV have rarely been studied. The aim of our study was to investigate BLV infections in domestic yaks and determine the genetic variability of BLV circulating in a region of the Qinghai Tibet Plateau, China. Blood samples were collected from 798 yaks, which were from different farms from Gansu, Qinghai and Sichuan provinces surrounding the Qinghai-Tibet Plateau. Nested PCR targeting BLV long terminal repeats was used to detect the BLV provirus. The highest prevalence of BLV infection was in Gansu province, where it was 18.93% (39/206) in white yaks from Tianzhu City and 19.14% (31/162) in black yaks from Gannan City. In Qinghai and Sichuan provinces, the prevalence of BLV in black yaks was 14.83% (35/236) and 14.94% (29/194), respectively. The prevalence of BLV was not significantly different in yaks up to one year old than in older animals. Phylogenetic analysis was performed using 16 different env-gp51 (497-bp) gene sequences from the three provinces and 71 known BLV strains, which revealed that in both Gansu and Qinghai provinces, genotypes 6 and 10 of the BLV strains were at high levels, whereas only genotype 10 was prevalent in Sichuan Province. Phylogenetic analysis and sequence comparisons revealed 95.7-99.8% sequence identity among the full-length env genes of 16 strains, nearly full-length genome sequences of six BLV strains, and those of the known genotypes 6 and 10 of BLV. This study provides comprehensive information is regarding the widespread infection of domestic yaks with BLV on the Qinghai-Tibet Plateau of China, and shows that at least two BLV genotypes (genotypes 6 and 10) are circulating in this population.