Herd-level prevalence of bovine leukaemia virus infection and associated risk factors in commercial dairies in five provinces of South Africa

Abstract

The immunosuppressive nature of bovine leukaemia virus (BLV) infection is well documented and associated production losses make it one of the most important diseases of cattle, particularly in dairy cows. Any management interventions should be informed by an understanding of the herd-level and within-herd infection prevalence and of risk factors for spread of infection. Costs associated with blood tests have been a prohibitive factor in routine monitoring in domestic herds. A regional cross-sectional study was undertaken to estimate herd-level prevalence of BLV infection in commercial dairy herds and identify associated risk factors. Bulk tank milk (BTM) from 185 dairy herds randomly selected from Limpopo, North West, Mpumalanga, Gauteng and Free State provinces was tested for BLV infection using an indirect enzyme-linked immunosorbent assay (ELISA). A questionnaire was used to collect data regarding management practices and herdspecific factors. Logistic regression was used to assess factors associated with herd seropositivity. Multiple linear regression was used to identify factors associated with the quantitative outcome of the BTM ELISA (E quant), as a proxy for within-herd prevalence of BLV infection. Estimated herd-level prevalence was 96% (149/155; 95%CI 92-99%); it differed between provinces (P=0.001): 77% (17/22) in Mpumalanga, 97% (34/35) in Gauteng and 100% in North West (n=65), Free State (n=26) and Limpopo (n=3). Questionnaire data were obtained from 125 herds. No factors were significantly associated with herd BLV infection, likely due to the small number of negative herds. Use of communal calving camps, reuse of rectal gloves without disinfection and use of artificial insemination in cows were associated with higher E quant. Regular treatment against flies was associated with lower E quant. Infection with BLV is widespread amongst commercial dairy herds in the northern part of South Africa. Various aspects of herd management have been identified where interventions can be made to control of the disease.