Pestivirus and persistently infected animals

By Coonabarabran District Vet, Kate Atkinson

Our district vets have been recently diagnosing and receiving calls about many weaner cattle persistently infected with Pestivirus and how to manage this disease. Pestivirus is a disease we commonly deal with and is endemic to most cattle herds in Australia with about 70% of herds actively infected.

When healthy cattle contract the virus, it usually goes unnoticed but can sometimes cause mild and transient clinical signs such as inappetance, depression, fever, mild diarrhoea and transient immunosuppression for a few days.   This results in antibody production and elimination of the virus from the body.

The biggest problems producers tend to see in our area, is when previously unexposed (naïve), pregnant females contract Pestivirus.  This can present in different ways depending on the stage of pregnancy, but reproductive losses and consequences include; conception failure, increased embryonic death, abortion, poor pregnancy testing rates, premature births, stillbirths, congenital defects, the birth of stunted weak calves and the birth of persistently infected (PI) calves.

Females infected day 45-125 of pregnancy may end up producing a calf that is persistently infected with the virus.  This occurs during the calf’s development prior to immune system competence.  When the virus is present during this time, the foetal body recognises it as part of itself so does not mount an immune response against it and becomes persistently infected.

The main source and transmission of Pestivirus in the herd is the direct contact with these persistently infected animals.

PIs may appear clinically normal or unthrifty and small for their age.  Eventually all PIs will develop a clinical disease and die. This usually occurs at the age of 6-24 months, usually not surviving more than a couple of years.  Clinical disease signs include weight loss, ill-thriftiness, chronic diarrhoea and sometimes ulcers in the oral cavity and skin (mucosal disease).  Occasionally there are PIs that survive for several years before showing clinical signs and the female PI’s will always give birth to a PI calf as well.

The effects of Pestivirus are usually managed by ensuring your breeding stock obtain immunity to the virus prior to joining.  This can either be through vaccination (Pestiguard) or autovaccination (controlled exposure to the virus).

Antibody levels induced by inactivated vaccination (ie Pestiguard) are short lived and require booster vaccines to maintain immunity.  Once a vaccination program is commenced it must be continued to ensure cattle remain immune.

Autovaccination or controlled exposure is another method that has been used in Australia and involves introducing a known PI animal to non-pregnant heifers on pasture well in advance to joining and then removing the PI after 12 days.  Exposure to the virus results in seroconversion/immunity by around day 20 and antibody development following natural infection is considered to be protective for life.

Finally, elimination of Pestivirus and maintaining a naïve herd through biosecurity measures is also a possibility depending on your situation.  This can be more difficult than it sounds as pestivirus can also be transmitted from flies and airborne transmission up to 10m.  Therefore, this process involves vigilant biosecurity measures such as no contact with neighbouring herds, quarantine measures for introduced cattle, testing and culling PIs in the herd and may involve the aid of vaccination to initially combat the virus.  This method also leaves your herd open to disaster if there is a breakdown in biosecurity and the virus is introduced so can be risky.

Management of the Pestivirus therefore depends on many factors and implementation will depend your individual situation.

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