BVD in Cattle: Welfare and Disease Control
Disease Overview
Bovine viral diarrhoea virus (BVDV) causes a complex of diseases in cattle. Acute BVD: most infections are subclinical or cause mild disease; some strains cause severe haemorrhagic disease. Reproductive BVD: infection of pregnant cows causes embryonic death, mummification, abortion, congenital defects, and crucially, the birth of persistently infected (PI) calves. Mucosal disease: occurs when PI calves are superinfected with a cytopathic BVDV strain; causes fatal haemorrhagic gastroenteritis.
Persistently Infected Calves — the Welfare Core
PI calves are the welfare and epidemiological core of BVD. Born to cows infected in the first 120 days of pregnancy, PI calves are immunotolerant to BVD and shed massive quantities of virus throughout their life. PI calves may appear normal or may be poor-doers with ill-thrift, respiratory disease, and scours. Many die young from mucosal disease or secondary infections. They are the primary source of herd BVD infection. Identifying and removing PI animals is the foundation of BVD control.
Welfare of BVD-Infected Herds
BVD causes widespread, often hidden welfare harm in endemic herds: increased respiratory disease incidence (BVDV causes immunosuppression — 'the virus of viruses' — predisposing to BRD); increased calf mortality; poor fertility (late returns to service, embryo death); reduced milk production; and increased susceptibility to other diseases. Herds with BVD endemic show significantly worse performance across all welfare and productivity indicators.
Testing and Control Strategy
BVD control is based on testing and removal. Options: ear tissue sampling (TAG-EARS) of all calves at birth for antigen testing (identifies PI calves rapidly, tags as the sampling device); blood testing of calves 3-6 months old; bulk milk ELISA testing of dairy herds (detects antibody in tank milk — indicates recent exposure); and individual animal serology. Removing PI animals immediately breaks the transmission cycle. National BVD eradication schemes (Scotland — effectively BVD free; England/Wales — voluntary schemes) provide frameworks.
Vaccination Role
BVD vaccines (inactivated and modified live) reduce reproductive losses and virus replication but do not eliminate infection from herds with PI animals. Vaccination is most beneficial where: PI removal is not immediately possible; herd has had recent BVD exposure; or as interim protection while completing PI testing. Vaccination cannot substitute for testing and PI removal as the primary control strategy. The welfare goal is BVD-free herd status through testing, which eliminates the welfare burden of endemic BVD entirely.