Bovine Viral Diarrhoea virus (BVDv) is one of the most economically and welfare-significant pathogens in cattle. Its complexity — involving both transient infections and persistent infection (PI) — makes it a particular challenge to control. The welfare consequences of BVD are wide-ranging and severe.
Transient BVD infection typically causes mild illness: fever, nasal discharge, reduced appetite, and temporary drop in milk yield. However, BVD's primary harm is immune suppression — infected cattle become significantly more susceptible to secondary infections including pneumonia, salmonellosis, and other respiratory and enteric diseases. This "BVD effect" on herd health can be devastating.
BVD infection during early pregnancy (0–45 days) often causes embryonic death and repeat breeding. Infection between 45–125 days of gestation creates persistently infected (PI) calves. Later infection can cause congenital defects, abortions, stillbirths, and weakly born calves. Reproductive failure causes significant welfare harm to individual animals and economic loss.
PI animals are the keystone of BVD epidemiology and represent a profound welfare problem. Created when a pregnant cow is infected between days 45–125 of gestation, PI animals:
When a PI animal is superinfected with a cytopathic strain of BVDv, it develops Mucosal Disease. Clinical signs include profuse watery diarrhoea, erosions in the mouth and throughout the gut, lameness, eye discharge, and rapid deterioration. It is invariably fatal. Any PI animal showing signs of Mucosal Disease should be euthanased promptly on welfare grounds.
Ear notch testing (antigen ELISA or PCR on tissue sample) is the standard method for identifying PI animals. Bulk milk testing (antibody ELISA) can assess herd exposure status. Blood testing of calves can identify PIs before movement or sale.
Scotland has achieved near-eradication of BVD through a national compulsory testing scheme. England, Wales, and Ireland have voluntary industry-led programmes. All newborn calves should ideally be tested — this prevents unknowing movement of PI animals between herds.
Vaccines (live attenuated and inactivated) are available and effective. Vaccination of breeding stock before the mating season protects against reproductive losses and prevents PI calf production. Vaccination does not identify or remove existing PIs — testing is still essential.
New cattle purchases are a major route of BVD introduction. All bought-in animals should be tested negative before or immediately on arrival. Nasal-penning purchased animals before testing and segregating them until confirmed negative is best practice.
PI animals that are not yet showing clinical disease may appear relatively normal. Their removal is essential on welfare grounds (preventing Mucosal Disease development and ongoing viral shedding). PI animals should be: