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Bovine virus diarrhea
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Bovine Virus Diarrhea (BVD) is a disease of cattle which reduces productivity and increases death loss. It is caused by a Pestivirus from the family Flaviviridae. Classical swine fever (CSF) is also caused by a pestivirus. CSF and BVD are notifiable diseases and eradication programms are administered in many countries worldwide. The molecular biology of pestiviruses shares many similarities and peculiarities with the human hepaciviruses. Pestiviruses have the ability to establish persistent infection during pregnancy.
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Bovine Virus Diarrhea (BVD) is a disease of cattle which reduces productivity and increases death loss. It is caused by a Pestivirus from the family Flaviviridae. Classical swine fever (CSF) is also caused by a pestivirus. CSF and BVD are notifiable diseases and eradication programms are administered in many countries worldwide. The molecular biology of pestiviruses shares many similarities and peculiarities with the human hepaciviruses. Pestiviruses have the ability to establish persistent infection during pregnancy. Persistent infection with pestiviruses often goes unnoticed; for BVDV frequently nonhomologous RNA recombination events lead to the appearance of genetically distinct viruses that are lethal to the host.
Clinical signs of mucosal erosions and diarrhea which occur in the acute form of BVD have a significant effect on those animals infected, but much more costly are those animals who are persistently infected animals or PI's.
Typically, PI's fail to reach their genetic potential, exhibiting decreased weight gain, increased disease susceptibility, and reduced fertility. They shed the virus, causing reproductive loss in the unimmunized animals in the herd. The most important components in programs that reduce or eliminate BVD are vaccinations, testing, and biosecurity. MLV(modified-live) vaccinations are one of the more important means of providing superior immunity to BVD, to protect against exposure to the virus, from both acutely infected individuals and PI animals.
PI's result from cows being exposed to the noncytopathic variant of the virus between 45 & 125 days of gestation in which case the fetus fails to recognize the virus as a foreign invader, allowing it to become 'self', and letting the infection persist after birth. The resulting calves shed the virus, often in great numbers, such that even well-vaccinated animals may become infected. PI individuals often do not exhibit any symptoms of disease but are very damaging to a herd. Since the virus is present in the persistently infected animals at birth and throughout their life, and an animal may only become PI prior to birth, an animal only has to be tested once at any age to establish its PI status. Normal serological tests do not work since the PI does not produce a normal immune response. Serum, milk, or a skin biopsy (ear punch) can be tested with polymerase chain reaction or . Immunohistochemistry on an ear punch biopsy is a definitive test for PI animals. Any calf born to a PI animal will also be PI. Exposure of the PI animal to the cytopathic strain of BVD will lead to mucosal disease, which is usually fatal.
Cows that are exposed to the cytopathic variant of the virus during the same time period (45-125 days gestation) will typically abort the fetus. Earlier exposure to either variant leads to early embryonic death. Exposure between days 125-175 days of gestation will lead to birth defects (such as ocular defects and hydrocephalus), and exposure at greater than 175 days will typically lead to the calf being fully immune at birth.
In the nonpregnant animal, exposure to type 1 BVD will often lead to few or no clinical signs, but will cause immunosuppression and susceptibility to other diseases, especially respiratory pathogens. Exposure to a newly recognised type 2 BVD often causes severe disease with thrombocytopenia and high mortality rates.
Vaccinations are available for protection against BVD. The goals of vaccination are to minimise respiratory disease secondary to BVD exposure, to prevent the occurence of PI animals, and to protect from type 2 disease. Both modified live and killed vaccinations have been used. Modified live vaccinations are not suitable for use in pregnant animals, and have the potential to cause mucosal disease if administered to a PI animal.
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