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Bovine virus diarrhea

Bovine virus diarrhea

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Introduction


Bovine virus diarrhea or bovine viral diarrhea (BVD) is a disease of cattle which reduces productivity and increases death loss. It is caused by a Pestivirus
Pestivirus
Pestivirus is a genus of viruses that belong to the family Flaviviridae. Viruses in the genus Pestivirus infect mammals, including members of the family Bovidae and the family Suidae .-Virus Genetics and Structure:Pestivirus viruses have a single strand of...

 from the family Flaviviridae
Flaviviridae
The Flaviviridae are a family of viruses that are primarily spread through arthropod vectors . The family gets its name from Yellow Fever virus, a type virus of Flaviviridae; flavus means yellow in Latin...

. Classical swine fever
Classical swine fever
Classical swine fever or hog cholera is a highly contagious disease of pigs and wild boar.-Clinical Signs:...

 (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
Molecular biology
Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...

 of pestiviruses shares many similarities and peculiarities with the human hepacivirus
Hepacivirus
Hepacivirus is a genus of the Flaviviridae family of viruses. The only member of the genus is the type species, Hepatitis C virus. Hepaciviruses, like other Flaviviridae, infect mammals.-External links:* *...

es. Pestiviruses have the ability to establish persistent infection during pregnancy. Persistent infection with pestiviruses often goes unnoticed; for BVDV frequently nonhomologous RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

 recombination events lead to the appearance of genetically distinct viruses that are lethal to the host.

Clinical Signs


Clinical signs depend on the timing of infection and status of the animal:

Pregnant animals


If infection is acquired by a cow between 50 and 100 days of gestation, abortion
Abortion
Abortion is defined as the termination of pregnancy by the removal or expulsion from the uterus of a fetus or embryo prior to viability. An abortion can occur spontaneously, in which case it is usually called a miscarriage, or it can be purposely induced...

is likely to occur.
Another possible outcome of infection before 120 days of gestation is the production of Persistently Infected (PI) calves. These animals will be discussed further below.

Infection in the third trimester (180-200 days) can still cause abortion but most commonly produce a normal but seropositive calf due to the immunocompetency of the fetus.

Non-Pregnant Animals


BVD in non-pregnant immunocompetent animals is generally mild and usually presents as respiratory disease consisting of conjunctivitis, nasal discharge, lethargy, decreased milk yield in cows, depression and a cough.

If acute, animals will be leucopaenic and immunosuppressed and mortalities may be significant.

PI Animals


PI animals are produced when a fetus is infected while partially immunocompetent, thus recognising the viral cells as self and not mounting an immune response. They are therefore antigen positive and antibody negative. PI animals tend to never reach their productive potential, exhibiting stunted growth, reduced fertility and increased susceptibility to other diseases.

These animals are a constant source of virus for other vulnerable members of the herd and therefore a significant risk and a vital target for BVD control programs. They can be identified serologically.

Mucosal Disease


BVD virus can also mutate within a PI animal into the cytopathic form of BVD. This causes mucosal disease to form, which features ulcers, blisters and erosions on the oral mucosa and snout. How the mutation occurs is poorly understood but this form of disease is invariably fatal. Death occurs over days or weeks and is usually in cattle 6-18 months of age.

Type II BVD


Genotype 2 BVD is a severe, haemorrhagic form of disease found in the USA and Canada. Clinical signs include haematochezia, haemorrhagic nasal and ocular discharges, epistaxis and clotting dysfunction causing petechiation. Mortalities are high.

Diagnosis


There are several tests available for detecting both BVD antigens, viruses and antibodies from a variety of samples including blood, milk, aborted fetuses and faeces.

Control


Vaccinations are available for protection against BVD. The goals of vaccination are to minimise respiratory disease secondary to BVD exposure and to prevent the occurrence of PI animals. Cross-protection to BVD-Type II is unfortunately poor. 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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