Bovine viral diarrhoea virus (BVDV) may be having a significant impact on your herd. Within Australia 60% of cattle and 90% of all herds have evidence of exposure to BVDV.
BVDV is a Pestivirus which belongs to the Flaviviridae family of viruses. Two different types of BVDV are recognised in the world. Type 1 is found worldwide, including within Australia, while Type 2 is only found in the USA and Europe.
The outcome of infection depends upon the age at which an animal is infected. Naïve animals that are infected as adults suffer from an acute infection that causes only mild or subclinical disease. Indeed, it is estimated that 70-90% of infections occur without any signs of clinical disease. If clinical signs are seen, they will usually only last for a few days and can include pyrexia, depression, mild inappetence and diarrhoea. Nasal and/or ocular discharge and erosive ulcerations in the mouth can develop in some animals. Affected animals are also susceptible to secondary infections due to immunosuppression. Infection, with or without clinical signs of disease, is accompanied by shedding of the virus for approximately 10 days in bodily secretions and excretions i.e. respiratory secretions, saliva, milk, faeces and semen.
Problems arise when naïve animals are infected during pregnancy as the virus is able to cross the placenta and infect the developing embryo or foetus. The outcome of infection depends upon the age of the embryo/foetus when infected. If less than 80 days of age, early embryonic death is likely i.e. foetal death, abortion or mummification. If infection occurs between 80 and 125 days of age, defects in the brain and eyes are common, with calves typically being born underweight. Most importantly, if the calf survives s/he may never develop an immune response to the virus, as infection has occurred prior to development of the immune system. The result is that the virus is not recognised as a pathogen; it is recognised as a component of the calf and s/he remains infected for life shedding large amounts of the virus in their bodily secretions. These calves are known as persistently infected calves/cattle. If infection occurs after 125 days of gestation, the foetus is able to develop neutralising antibodies and eliminate the virus as the immune system of the calf is functional. Despite this, the animal may be born with congenital defects or abortion may still occur.
Persistently infected calves remain a permanent source of infection for other animals in the herd and are at increased risk of developing mucosal disease. Mucosal disease depends upon infection with BVDV and the consequent mutation of this virus to a more virulent strain. The clinical signs of mucosal disease are more severe than those seen with BVDV and include fever, anorexia, watery diarrhoea, erosive or ulcerative inflammation of the mucous membranes in the mouth, weight loss and consequently death. The time course of these clinical signs may be sudden or take a number of weeks to occur. Affected animals usually die within 3-7 days but can survive for up to 18 months.
Persistently infected animals are typically described as poor doing animals, being stunted in their growth and unthrifty. However, this is not always the case and many can be indistinguishable from uninfected herd mates. It is therefore possible for a persistently infected animal to be present in a herd, causing significant economic loss. This may be in the form of low conception rates, irregular inter-service intervals, the birth of calves with congenital defects or simply animals that grow poorly.
Economic loss due to BVDV can be minimised through the development of an effective risk management plan with your veterinarian. This will be tailored to your situation and may include:
- Establishing the immune status of your herd.
- Identifying persistently infected animals in your herd.
- Establishing appropriate biosecurity protocols i.e. maintaining a closed herd or screening all incoming animals for evidence of exposure to BVDV or establishing a quarantine protocol.
- Vaccinating at risk groups i.e. naïve heifers prior to their first pregnancy.