Bovine Virus Diarrhoea (BVD) and its financial implications
BVD is an economically important viral disease of cattle that can be quite complicated with carrier animals being the main problem. About 80-90% of NZ farms have had the virus at some time with about 60% of cattle having evidence of past infection. Infection can go unnoticed in adult cattle with the animals experiencing some fever and slightly reduced appetite and milk production for a short period. Disease in calves and weaners is usually more severe with diarrhoea being the main feature.
After infection, animals are usually immune for life, though a serious consequence is that calves born from dams infected during pregnancy will shed the virus in large amounts for their entire life.
The BVD virus remains in the herd by two methods: direct transmission between animals through physical contact and the virus invading the foetus in a pregnant cow. The latter method gives rise to newborn calves that either develop poorly and die relatively young or grow up apparently normal but become life-long shedders known as "persistently infected", or "PI" cattle. These carrier animals act as a major source of infection for other animals.
As with other viral diseases, there is no effective treatment. Affected stock need to have access to plenty of good quality feed and water. Keep stress to a minimum and isolate them from the rest of the mob (though it is likely that new cases will keep showing up for another 5-7 days). Keep affected stock in sheltered paddocks. Most affected stock will recover in a month or so and the death rate will be low (less than 3%) provided they are well cared for.
What are the financial implications of BVD?
Recent estimates for infected dairy herds totalled the loss at $70 per cow resulting in about a $37million annual loss to dairy farmers in New Zealand. These calculations do not yet consider the additional labour costs or the use of pasture for non-productive cows that will be culled later.
The main losses in dairy and beef herds are due to the effect of the virus on the reproductive performance of the cow and on the foetus she is carrying. In adult breeding cows, the disease can cause huge reproductive losses, the costs of which are often hidden and difficult to calculate.
Infection in young cattle, especially those less than 12 months of age, produces a variety of symptoms that may or may not be recognized as BVD.
As well as sick animals, if BVD is introduced into a group of breeding stock around the time of mating and early pregnancy you will see much higher empty rates. Normally, you would expect only 3-4% of heifers not to get pregnant. One year at a grazing property when BVD was inadvertently introduced, three different lines of heifers had empty rates of 18%, 14% and 11%.
BVD in young stock is frequently not diagnosed as symptoms can be similar to those caused by worms and mineral deficiencies. As a lot of stock are recovering a month after becoming infected, you get the false impression that the stock have responded to the drench when in fact they have not had parasites at all.
If non-pregnant adult cattle become infected they only suffer a mild illness followed by lifelong immunity. However, there are about two weeks during which they can infect others. This mild form of infection is not usually noticed. During the short period of infection, cows may suffer a drop in milk production and are prone to a variety of other diseases.
How can we test for BVD?
Excellent tests now exist to identify infected herds and animals using milk or blood samples. A great starting point is to find out if you have any carrier animals or persistently infected animals in your herd. A sample of milk from the bulk tank detects the actual virus in the bulk-tank milk of up to 400 cows per sample and is useful to identify herds that have at least one lactating PI-cow. If you're milking more than 400 cows, we are happy to discuss the logistics of sampling larger herds - this can be done with little disruption to most routines.
Once you are aware of the BVD status of your herd, Totally Vets can devise the best plan to suit your requirements.
