Dairy cattle with bovine leukaemia virus RNA show significantly increased leukocyte counts

Infection with bovine leukaemia virus (BLV), a retrovirus, causes dysfunction of the immune system and can have a marked economic impact on dairy industries due to decreased milk production and reduced lifespan in affected dairy cattle. The presence of proviral DNA has been the major diagnostic indicator of BLV infection. However in the course of BLV infection, the viral genome can be dormant, without detectable gene expression, resulting in limited impact on infected animals. At present, there is limited knowledge regarding haematological indices in dairy cattle that could indicate activation of the BLV genome and suggest reactivated BLV infection. In this study, BLV infection and BLV genome reactivation were evaluated based on the presence of BLV DNA and BLV env gene transcripts, respectively. BLV RNA transcription was confirmed. Among 93 whole blood samples obtained from asymptomatic dairy cattle, the prevalence of BLV proviral DNA and transcripts was 93.5% (n = 87/93) and 83.9% (n = 78/93), respectively. Between groups with and without BLV, the mean counts of white blood cells and lymphocytes in whole blood were significantly associated with the presence of BLV RNA (P < 0.05), but not with BLV proviral DNA. These results shed light on the activation status of the BLV genome and should be taken into account when evaluating the possible impact of BLV on cattle.