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 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 Ag Capture Elisa (ACE)
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