Bovine Viral Diarrhea Virus (BVDV) is an economically important virus that is distributed world wide. It is seen in every country that raises cattle. Prevalence of this virus is determined by Antibody titers testing. A blood sample is taken looking for the presence of antibodies that react to BVDV. If antibodies are present the animal is seropositive and has been exposed to the virus, either by natural infection or by vaccine. Once an animal has been infected it will test seropositive for the remainder of its life. If the animal has no antibodies it is seronegative, it is a susceptible as has not been exposed to the virus. The virus spreads quickly throughout a herd; therefore if one animal tests positive, there is a high likelihood that all other animals in the hers will contract the virus and become infected. Animals most at risk for showing clinical signs of the dieases tend to be between the ages of 6 months and 2 years of age.
When a seronegative female is contacted with Bovine Viral Diarrhea Virus while she is in early pregnancy, between days 40 and 120 days of gestation, she has a high chance of producing an offspring that is immunotolerant to the virus. This calf does not recognize the virus as foreign and does not mount an immune response, therefore remaining persistently infected (PI) and viremic. Persistently infected calves are infected via vertical transmission (dam to offspring) and can occur every 100 in 1000 births. Dams that come in contact with BVDV in late gestation, the fetus will mount an immune response intrauterine and always show antibody titers towards BVD.
A persistently infected animal is of key importance in spreading the virus to other seronegative animals in the herd. An infected animal will shed virus while infected however a persistently infected host will shed virus in large amount (virus may still be isolated from a PI serum at a dilution of 10^6) for the remainder of its life. Although PI animals are the key factor in spreading the virus by shedding high amounts of virus, there is a slight amount of transmission of the virus through contact with acutely infected animals. Virus is shed through secretions and excretions, this includes nasal discharge, milk, urine, feces and semen. Infected semen from an infected bull or PI bull will infect the cow it mates with but will not necessarily produce a persistently infected calf. However a persistently infected dam will always produce a persistently infected offspring. There is a high mortality in PI calves which may be up to 50% within the first year of life.
The infection caused by BVDV is self limiting. When there is a high rate of infection the correlated rate of immunity is also high reducing the spread of the virus before the cows are bred producing persistently infected calves. When the infection rate is decreased, the rate of persistently infected calves produced will be greater, increasing the chance of spread of the virus. The virus may be spread through insects that bite, semen, fomites, wild ruminants and other domestic ruminants, such as sheep and goats, as well as direct contact between affected animals.
The epidemiological distribution of Bovine Viral Diarrhea Virus has been studied world wide. Whole herd testings have been done, as they are the only way to determine the presence of a persistently infected animal within the herd. Data is available from many countries, however each country has separate criteria for testing and therefore data cannot be directly compared. This data does however show the distribution of the virus.
When performing whole herd tests looking for PI individuals in Denmark, 10 of the 19 herds tested found at least one PI individual. In the United States of America 3 of the 20 herds tested proved to have PI animals. Sweden had 11 of the 15 herds tested having antibody carriers, who have been exposed to the virus. In Finland milk tanks were tested for antibodies resulting in 9 of the 291 tanks having antibodies against BVDV present. Although knowing the prevalence of PI animals is relevant for research and data collection, its relevance on a farm is controversial.
Many things can affect results like these such as herd size. Infection has a tendency to increase when the density of animals is increased. Management practices play a role in the spread of this virus. For instance in North America it is customary practice to separate dairy calves shortly after birth to be housed with other calves. Bull dairy calves are sold, never being introduced to the milking females, eliminating many susceptible and potentially PI calves. Some dairy farms reduce the time that cows are feeding from the pasture, therefore reducing the time that they are interacting with one another spreading the disease.
This information is critical when attempting to implement a control program. Data should be analyzed from the same region as the farm implementing the program for best results.
1) Houe, Hans. Bovine virus diarrhoea virus (BVDV): Epidemiological studies of the infection among cattle in Denmark and USA. 1996. Department of Clinical Studies. The Royal Veterinary and Agricultural University. Copenhagen. p 31-54.
2) Baker, John C. BVDV Infection: Clinical Manifestations. Dept. of Lg. Animal Clinical Sciences. http://www.veterinaria.org/revistas/vetenfinf/bovino/bdv/bvdv_infection.htm
3) The Merck Veterinary Manual. http://www.merckvetmanual.com/mvm/index.jsp