Bovine viral diarrhoea virus

A pestivirus in the Flaviviridae family

Infection is widespread throughout Queensland and Australia, with many herds having evidence of infection - even extensively managed beef herds. Antibody prevalence rates in cattle in Australia are around 60%, while over 80% of herds have been infected.

Cattle, goat, deer

BVDV is capable of causing a large number of disease 'syndromes' in cattle herds.

Acute bovine viral diarrhoea occurs when healthy normal cattle are infected with BVDV. It is rarely diagnosed in Australia. Signs include:

• a transient fever
• a depressed 'hollow' appearance
• diarrhoea
• cough in some cases
• profuse bleeding from small wounds, such as ear tagging
• increased susceptibility to other infections due to a depressed immune response while their body copes with the virus infection.

Persistently infected cattle may also develop mucosal disease. This is due to the resident BVD virus mutating and becoming more virulent.

Mucosal disease

Mucosal disease is the most dramatic syndrome associated with BVDV infection and the syndrome most often recognised by producers. Affected cattle:

• drool excessively
• appear depressed and feverish
• have persistent and often bloody diarrhoea, a soft cough and lameness.

These symptoms can easily be confused with foot-and-mouth disease, rinderpest, and malignant catarrhal fever.

The severity of mucosal disease varies from an acute form with death within a few days to chronic wasting disease. With the acute form of mucosal disease, there is profuse diarrhoea with ulcers in the nose, mouth, eyes and between the toes. At necropsy, these ulcers are often found to extend right through the upper and lower intestinal tract.

With the chronic wasting form of mucosal disease, calves just grow poorly. Often there are no visible abnormalities at necropsy, but microscopic changes can be found.

Persistent infection with BVDV should always be considered where some young cattle in a mob are doing very poorly while most of the other cattle are doing very well.

Economic

In herds recently infected with BVDV, production losses of between 25-40% have been recorded due to reduced reproductive performance, death losses and ill thrift. If BVDV stays in the herd, annual production losses between 5-10% commonly occur.

Growth

BVDV is capable of crossing the placenta and infecting the developing foetus. If cows lack immunity and are infected while pregnant, the virus can damage the foetus. Cows that have immunity to pestivirus from previous exposure can block the virus passage.

This in utero infection of the foetus via the naive dam is the most important form of the infection, and results in most losses under Australian conditions. Foetal infections result in a wide range of disease syndromes and production losses. The losses that occur depend on the stage of pregnancy when the foetus is infected.

• Month 1 of pregnancy: the pregnancy is terminated either by abortion or resorption of the foetus by the dam.
• Months 2-6 of pregnancy: the foetus may still be aborted (this does not necessarily occur straight away), but more often the foetus survives full-term with the resultant offspring born malformed, weak, 'dwarfed', stillborn, or clinically healthy but 'persistently infected' with pestivirus. Persistently infected calves may grow well, but generally are unthrifty compared to others the same age, and they often suffer from chronic scours or pneumonia.
• Months 3-5 of pregnancy: the virus affects the developing nervous system of the foetus. Calves may not be able to stand or suck, or may develop convulsions after birth because they are missing parts of their brain (cerebellar hypoplasia). These calves may also have eye abnormalities such as blindness and cataracts, or bent-up front legs. Some calves will survive, but have a wobbly gait and may have a permanent head tremor.
• Months 6-9 of pregnancy: the foetus is generally resistant to any adverse effects of the virus although some growth retardation of the developing calf may occur.

Infected cattle develop strong immunity when they recover.

Persistently infected calves shed pestivirus for life, constantly infecting other cattle mixed with them, in particular, pregnant heifers and cows.

Vaccination

A pestivirus vaccine is available for use in Australia. Two doses of the vaccine 4-6 weeks apart are required for protection of the developing foetus (i.e. both doses of the vaccine must be given prior to breeding for foetal protection to occur). Timing of booster vaccinations is not as critical as initial vaccination. Bulls should be vaccinated as well as cows because healthy normal bulls can be transiently infected and shed pestivirus in their semen for a short period after natural exposure.

Developing immunity for replacement heifers

Mixing a persistently infected calf with replacement heifers before breeding can help build immunity. Care and advice from your veterinarian should be sought before attempting this.

Control programs

Many countries have now introduced voluntary, industry or government-funded BVDV eradication or control programs. There is no such program in Australia at the present time. Producers are encouraged to develop their own biosecurity measures to control the disease.

Herds free of BVDV

Appropriate isolation and strict biosecurity measures should be applied to keep introduced and neighbouring stock away from pregnant females.