From the Experts

BVDV: A pest of a virus

By Dairy News Australia

A complex and challenging disease, bovine viral diarrhoea virus (BVDV) or pestivirus is widespread across Australia and known to cause significant reproductive and economic losses.

Up to 90 per cent of Australian herds have evidence of exposure to the virus

The aim of this article is to provide a brief overview and a reminder of the widespread and potentially catastrophic nature of BVDV.

BVDV replicates in a variety of bovine body tissues and is transmitted via a wide range of bodily fluids.

Transmission may be either horizontal (via close contact with infected animals and subsequent inhalation or ingestion of infected fluids) or vertical (passed from infected cow to calf in-utero).

Horizontal infection usually causes diarrhoea and transient immuno-suppression (leaving defences down for other diseases to wreak havoc).

Otherwise healthy cattle with good functioning immune systems will quickly develop immunity and eliminate the virus.

The trouble arises when we start talking about animals that are persistently infected (commonly referred to as PI) or when cattle are exposed to the virus for the first time during pregnancy.

PI animals excrete the virus at much higher levels and for longer periods (often for their entire lifetimes) than animals that are transiently infected.

Transiently infected animals usually only shed the virus for four to 15 days after infection.

Most PI animals do not survive to adulthood, however if a PI heifer makes it to breeding she will produce PI offspring.

This is a less common route of transmission.

Usually, PI calves are born within a herd after previously unexposed cows or heifers come into contact with the virus during gestation.

Source: BVDV Management Guide (Australian Cattle Vets, 2015).

Infection prior to day 120 of gestation (before the calf’s immune system is properly developed) has a high chance of causing the calf to be born a PI (if it survives to full term).

If exposed early in gestation (before day 40), the embryo may simply die.

Later in gestation (days 100 to 140) calves can develop severe malformations or deformities.

After about day 120 of gestation, calves have enough of an immune system to be able to mount a response to the virus and can be normal (non-PI), however abortion, stillbirth, abnormalities or small weak calves can still result.

Once born, PI calves can sometimes look ill-thrifty, stunted or scruffy as they grow but can also be visually indistinguishable from other non-PI animals.

Occasionally, PI animals will develop a super-infection and severe mucosal disease will result.

Mucosal disease usually affects PI animals between six and 21 months old and can be either acute or chronic.

Depression, fever, profuse watery diarrhoea are all signs, as are ulcerative lesions in the mouth, feet and vulva causing lameness, drooling and anorexia.

In the severe form, death usually results within a week.

If this isn’t ringing any alarm bells, I’ll point out now that some of these signs may resemble emergency or exotic diseases (such as foot and mouth disease) and if you ever notice an animal on your farm with these lesions, you have a community responsibility to make contact with your local veterinarian at once, or phone the Emergency Animal Disease Watch Hotline on 1800 675 888.

Testing for BVDV can be done individually by either ear notching or blood testing, or at a herd level by bulk tank milk testing.

Testing for antibodies will give you an indication of the level of exposure and immunity currently in your herd, while testing for antigens is used to identify the presence of the virus itself (usually in persistently infected animals).

Complete eradication of the virus from your herd is possible but should only be attempted in consultation with an animal health professional.

A vaccine (Pestigard) exists and can be used to protect animals that have never been exposed.

Booster doses are required. Once the virus is eradicated from your herd, without vaccination your animals remain susceptible to re-infection.

Strict biosecurity is essential to prevent BVDV infected animals entering your farm.

Ear notching bulls, blood or milk tests from cows and heifers should all form part of your screening process before welcoming any new arrivals or animals returning from other locations.

BVDV is a very complicated disease.

The Department of Agriculture and Australian Cattle Veterinarians have extensive resources available with more information.

Management of BVDV will vary from farm to farm.

Your regular veterinarian will be able to help you determine your herd’s current level of exposure and risk, and together you can create a management plan unique to your operation.

Lucy Collins is completing her dairy residency with the University of Melbourne. She works as an on-farm veterinarian in Kyabram with Apiam Animal Health, and alongside her partner on his 600-cow dairy farm in Dixie. The author has no affiliations (financial or otherwise) with Zoetis.