Bovine virus diarrhoea (bovine pestivirus)

BVDV, also referred to as bovine pestivirus and mucosal disease virus, is a virus that is now recognised as an insidious cause of losses in beef and dairy herds in Australia. BVDV affects all types of cattle, and has probably been present in Australia as long as cattle. In the past it has commonly been misdiagnosed as other cattle diseases. In herds recently infected with BVDV, production losses of between 25 and 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 and 10% commonly occur.

Bovine virus diarrhoea virus is capable of causing a large number of disease 'syndromes' in cattle herds.

Acute bovine virus diarrhoea: Occurs when healthy normal cattle are infected with BVDV. It is rarely diagnosed in Australia, with most pestivirus infections going unnoticed. When seen, signs include a transient fever, a depressed 'hollow' appearance, diarrhoea and sometimes a cough. While never recorded in Australia, profuse bleeding from small wounds, such as ear tagging wounds may also occur. Cattle infected with BVDV may also be more susceptible to other infections due to a depressed immune response while their body copes with the virus infection. Infected cattle develop a strong immunity when they recover.

Foetal infections with BVDV: Bovine virus diarrhoea virus is capable of crossing the cow's placenta and infecting the developing foetus. If cows lack immunity to BVDV and are infected while they are pregnant, the virus can damage the foetus. Cows that have immunity to pestivirus from previous exposure can block this passage and protect their foetus.

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

• If the cow is infected in the first month of pregnancy, the pregnancy is terminated either by abortion or resorption of the foetus by the dam.
• If the foetus is infected in the second to the sixth month of pregnancy, a variety of different syndromes can be observed. 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 (PI) calves may grow well, but generally are unthrifty compared to others the same age and they often suffer from chronic scours or pneumonia.
• If infection occurs around three to five months 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.
• If the cow is infected between 6 and 9 months of pregnancy, the foetus is generally resistant to any adverse effects of the virus although some growth retardation of the developing calf may occur.

The most important outcome of foetal infection with BVDV is the birth of calves that are PI. These animals shed pestivirus for life, constantly infecting other cattle mixed with them, in particular, pregnant heifers and cows.

Mucosal disease: PI cattle may also develop mucosal disease. This is due to the resident BVD virus mutating in the PI animal and becoming more virulent. 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 often bloody diarrhoea, and sometimes a soft cough and lameness (symptoms that 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 a 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.

Blood tests can be performed to identify BVDV in live animals. This testing is aimed at detecting PI cattle. Tissues collected at post-mortem examination from dead cattle and aborted foetuses can also be tested for BVDV. Tests can also be done on skin biopsies (taken from the ear) or even hair samples.

Blood samples can also be collected from a cross section of a herd to determine if cows or heifers have been exposed to BVDV by determining their antibody status. This can give an indication if BVDV infection is active in a herd or not and aid in designing an appropriate control program.

In control programs milk and blood samples (often pooled to reduce costs) are used.

PI animals are usually antibody negative as they are immunotolerant to the virus (consider it as self).

A pestivirus vaccine is now registered for use in Australian cattle herds. It requires two doses of the vaccine 4 to 6 weeks apart 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 as healthy normal bulls can be transiently infected and shed pestivirus in their semen for a short period after natural exposure.