Since its first description almost 70 years ago bovine viral diarrhea (BVD) has been gradually recognized to be the economically most important infectious disease of cattle. Acute postnatal infections with BVD virus (BVDV) mostly have a mild or even inapparent course, with rare exceptions like the emergence of highly virulent BVDV-2 variants that kill young and adult cattle. The true economic loss is produced by BVDV interfering with pregnancy.
Like all related pestiviruses BVDV can be transmitted to the fetus in pregnant cattle and persistently infected (PI) offspring may generated in those cases, where at time of infection the fetus’ adaptive immune system is not yet developed (about 60 to 100 days of pregnancy). In addition the infecting noncytopathic (ncp) BVDV virus is able to modulate the innate immunity, e.g. by suppressing interferon production. The resulting PI calves are immunotolerant against BVDV and they constantly shed large amounts of virus thus representing the most efficient reservoir for the spread of BVDV in the cattle population. The majority of PI animals die within their first two years of life, however, about 10 percent of them survive and may live for years. By several mutations cytopathic (cp) BVDV may arise in PI cattle. These cp viruses destroy the cells in which they replicate, and – since the animals are immunotolerant to their ncp and cpBVDV – extensive damage is produced in tissues lining the digestive tract, e.g. mucosal surfaces and Peyer’s patches. This condition was named “mucosal disease” and it is always fatal. In other cases of intrauterine infection malformations, abortions or stillbirths can occur. Early infection around mating or the first few weeks of pregnancy can result in transient infertility.
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