Wednesday, April 24, 2024

Viral disease to watch out for

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How is the scanning rate in your cows this season? Have you had a higher than expected empty rate? BVD is one possible cause. BVD is a viral disease that can cause pregnancy losses, a general increase in disease, and abnormal or ill thrifty calves. It is one of the most important viral diseases of cattle in New Zealand. It is widespread in the country, with about 80% of beef cows having been exposed to BVD. At any one time, at least 65% of beef herds have active virus infections. Estimates in NZ put the losses at $3000-$9000/100 cows in infected herds a year. It is spread by Persistently Infected (PI) individuals. A PI can be created when a cow that is in early pregnancy (and has never been exposed to the virus before) is then exposed to the virus. The resultant calf is a PI. He/she may look perfectly normal when born or may be a runt. This PI excretes the virus and is infectious, and is key to spreading and maintaining the disease on the farm. Roughly 50% of PI animals die before two years of age of the disease (commonly called mucosal disease) or from other secondary diseases such as pneumonia.
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Some survive to become adults. PI animals comprise <1% of the adult population.

A PI cow will always have a PI calf, so the disease continues to the next generation.

A bull PI produces semen that is infected with the BVD virus. If that bull is used in a group of previously unexposed females, it can halve the conception rate and result in PI calves being born. Infected bulls usually have decreased fertility.

Affected beef herds have a continual recycling of the disease because younger and older animals are kept together. Carrier animals are in constant contact with susceptible pregnant cows, heifers and bulls.

Potentially, any bought-in animals can be a PI and introduce the disease to a farm. It can also be introduced if a neighbour’s PI animal jumps into your land.

BVD is not only spread by PIs. It is also spread by transiently infected cattle. This occurs when an individual is exposed to the virus and mounts an immune response to the disease, becoming immune. This process can take two to three weeks. The animal may be ill in this time and will excrete the virus for this short period. Thus these transiently infected (TI) animals are infective for a short time.

In carrier animals, the virus is present in most bodily secretions including nose secretions, saliva, urine, semen and occasionally in faeces.

The key to BVD control is to prevent the formation of PI calves by making sure that pregnant cows do not come in contact with the virus.

A BVD steering committee was formed in 2005 under the umbrella of the New Zealand Veterinary Association, to examine the feasibility of controlling BVD in NZ. They now believe that voluntary BVD control is feasible on many beef farms. A useful website for farmers and vets alike is at controlBVD.org.nz with practical steps to investigate a BVD problem.

A range of tests can investigate the immunity of your cattle and also to check whether you have any PIs on your farm. A BVD vaccine can provide protection, not only to the pregnant cow but also to her unborn calf. Most of you are probably familiar with it if you have breeding bulls. Speak to your local veterinarian if you have any concerns about the reproductive performance or general calf health among your cattle. 

  • Teresa Orioden is with Clutha Vets.
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