Bovine Viral Diarrhea (BVD)

Many acute infections pass unnoticed. When signs appear, about 6-12 days after exposure, they include fever, dullness, falling milk yield, poor appetite, fast breathing, nasal and eye discharge and diarrhoea. Some type 2 strains cause high fever, mouth ulcers and a bleeding syndrome with pinpoint haemorrhages from low platelets. In breeding herds the first clues are poor conception, abortions, stillbirths, weak or deformed calves, and unthrifty calves that die young. Mucosal disease brings watery, sometimes bloody diarrhoea, drooling, dehydration and erosions on the muzzle, lips, gums and tongue. At necropsy there are erosions and ulcers along the digestive tract and haemorrhagic, necrotic mucosa over the Peyer's patches.

Bovine viral diarrhoea is caused by positive-stranded RNA pestiviruses (family Flaviviridae): Pestivirus bovis (type 1), Pestivirus tauri (type 2) and Pestivirus brazilense (type 3). Field strains are mostly non-cytopathic. Acute infection gives a brief 7-10 day viraemia, transient drops in white cells and platelets, and immune suppression that opens the way to secondary respiratory and enteric infections. The outcome in a pregnant cow depends on timing: before about day 25 the embryo dies; between about days 30 and 90 the fetus becomes immunotolerant and is born persistently infected (PI) and seronegative for life; up to about day 150 congenital brain, eye and skeletal defects occur; after day 180 a normal calf is born. Fatal mucosal disease arises only in PI animals later infected by a closely related cytopathic strain.

Cattle of all ages are susceptible and the virus occurs worldwide, though several European countries have made major progress towards eradication. PI animals are the main reservoir, shedding large amounts of virus in urine, faeces, discharges, milk and semen; spread is mainly by close contact with them, and the mean animal prevalence of persistent infection is about 1-2%, approaching 4% on dairy farms with endemic infection. Acutely infected animals shed less. The virus also moves in semen, embryos and contaminated fetal calf serum. Sheep, goats and pigs can be infected by close contact with cattle, and infection has been reported in Old and New World camelids. Type 1 predominates globally; type 3 is mainly found in South America and Asia. BVDV is a major contributor to bovine respiratory disease in feedlots and calf units.

Severe acute BVD and mucosal disease resemble malignant catarrhal fever, which is usually sporadic and seen in mature cattle, and bluetongue, epizootic haemorrhagic disease, rinderpest (now eradicated) and vesicular diseases. Salmonellosis, coccidiosis, infectious bovine rhinotracheitis and papular stomatitis also enter the list. History, herd morbidity and mortality and lesion pattern narrow it down, but laboratory confirmation is needed: antigen ELISA or RT-PCR for virus, repeated after 3 weeks to confirm a PI animal, and paired sera 21 days apart for acute infection.

Gastrointestinal tract, oral mucosa, Peyer's patches and lymphoid tissue, respiratory tract, immune system, platelets, reproductive tract, fetus, testes and semen, brain (cerebellum), eyes, skeleton

Test and remove PI animals; screen traded cattle, bulls and embryo donors. Quarantine incoming stock. Vaccinate: inactivated vaccines are safe in pregnancy, modified live ones are not given to pregnant cows. Screen serum used in breeding and vaccine work.

Most acute infections are mild, but severe acute outbreaks can kill 25% or more. PI animals have a much shorter life, many dying before maturity. Mucosal disease is invariably fatal, usually within days. Treatment is supportive only.