Question: I farm about 300 head of Hereford cows. There have been deaths among my young heifers, and the tests show that BVD is the possible cause. What should I do if BVD is confirmed?
Bovine viral diarrhoea (BVD) is a highly infectious disease that is characterised by the suppression of the immune system.
- Infected animals are therefore extremely susceptible to other diseases.
- The disease can manifest in several syndromes in a herd, such as mild diarrhoea from which most animals recover spontaneously.
- The affinity of the virus for the unborn calf is of great economic importance.
- When a pregnant cow that is not immune to the disease is exposed to the virus, the death of the embryo, abortion or congenital defects in the calf might occur.
SOURCE OF INFECTION
- The main source of the infection in a herd is permanently infected animals.
- When a female animal that is not immune to BVD is exposed to the virus during the period from day 42 to day 125 of pregnancy, the foetus develops an immune-tolerance to the virus.
- The foetus therefore loses the ability to develop immunity to the disease and remains a carrier of the virus for the rest of its life.
- Permanently infected animals are usually – but not always – frail and extremely susceptible to other diseases.
- From time to time, these animals excrete large quantities of viruses, especially when they are stressed, such as when they are in a feedlot.
- Research has shown that about 6% to 8% of the calves in feedlots have to be treated more than once, particularly for pneumonia, are permanently infected animals.
- The occurrence of permanently infected animals in commercial herds is about 1% to 2%.
- BVD is a possible cause when a herd has a history of poor reproduction, including poor conception rates, abortions or calves showing congenital defects.
- It is essential that other diseases, such as contagious abortion (brucellosis), vibriosis and trichmonasis, be eliminated as causes.
Livestock production: Testing your herd for brucellosis (contagious abortion)
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- BVD can be controlled by ridding the herd of infected carriers, as well as by using vaccines correctly.
- Discuss an inoculation programme with a veterinarian.
- There is more than 1 way to identifying permanently infected animals.
- The most commonly used method is to collect a piece of skin, from an ear nick for example, in a 10% formalin solution.
- The sample should be sent to a facility like the Department of Pathology, Faculty of Veterinary Science at Onderstepoort in South Africa as soon as possible.
- This article was written by Prof. Leon Prozesky and appears in Ask the vet: What cattle farmers should know (1), compiled by Dr. Faffa Malan.