Bovine virus diarrhea

Bovine virus diarrhea or bovine viral diarrhea (BVD) or bovine viral diarrhoea (UK English) 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 sometimes BVD are notifiable diseases and eradication programs 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.^[1]

Genetics

There are two recognised genotypes—genotype 1 and genotype 2.^[2] These have been subdivided into subtypes: a–p for BVDV-1 and a–c for BVDV-2.

Bovine Serum Contamination

The bovine serum is widely used for cell culture, however, bovine serum or related product may have potential BVDV contanmination due to the BVDV infection.^[3]

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 55 and 100 days of gestation, abortion 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 (180–300 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. However, if a PI dam is able to reproduce then their offspring will be PI calves.

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 cannot be identified serologically as they are seronegative. PI animals are virus positive so they can be detected by antigen test (e.g. PCR).

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.

Hemorrhagic Syndrome

A severe acute form of BVD may occur when cattle are infected with highly virulence BVDV strains. Thus far hemorrhagic syndrome has only been observed when cattle are infected with noncytopathic strains belonging to the BVDV2 species. This syndrome is characterized by pyrexia, leukopenia, thrombocytopenia and hemorrhagic diathesis. Bloody diarrhea may be observed. There is notable depletion of immune organs including the spleen, thymus, lymph nodes and bone marrow.

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.

See also

• Animal viruses

References

• Bovine Viral Diarrhoea Virus, expert reviewed and published by Wikivet at http://en.wikivet.net/Bovine_Viral_Diarrhoea_Virus, accessed 21/07/2011

External links

• New York State Cattle Health Assurance Program BVD Module
• Description of the entity on the Merck Veterinary Manual
• Animal viruses
• Bovine Viral Diarrhea Resource Page
• Specialist BVD site, Royal Veterinary College, London