Bovine Virus Diarrhoea - BVD

Incidence

BVD virus is a common infection in cattle in the UK. Surveys have shown that it is present on approximately 95% of farms and that approximately 65% of adult cattle have encountered the virus and have a detectable antibody titre.

Transmission

Infection is transmitted in faeces and in nose and eye secretions as well as in the semen of infected bulls. Animals that become infected generally shed the virus for a short period of time before they make antibodies to it and recover. The main spreaders however are animals that have become infected before they were born (when they were a foetus in the womb), so called persistently infected animals (PI's), as these shed the virus continually for the whole of their lifetimes.

Clinical signs

In non-pregnant adult cattle infection usually results in a high temperature and a milk drop - a scour may also be present, hence the name Bovine Virus Diarrhoea. Cows usually recover spontaneously with no treatment. However, the virus is immuno-suppressive and so concurrent infection with another bacteria or virus, for example Salmonella, can be more serious if BVD infection is also present - this is particularly important in calves and young stock and is commonly seen with pneumonias.

The main effects of BVD occur when cows are pregnant at the time of infection as the virus has a particular affinity for the uterus and the developing calf. Embryo loss can occur resulting in return to service at 21 days, unusually long inter-oestrus intervals or later abortion. Developmental abnormalities can also occur such as calves born with cataracts or with deficits in balance and motor control (shaky calves). If unborn calves become infected during the first third of pregnancy, then apparently normal but persistently infected (PI) calves can be born. Most of these animals die of "mucosal disease" in the first two years of life but occasionally they survive for many years and themselves have calves (which are always PI calves). Whilst alive, PI animals continually excrete BVD virus and are responsible for transmitting the infection to other cattle.

Diagnosis

Diagnosis is usually based on either bulk milk samples, individual cow blood samples or preferably both. The results of these tests can be used to decide which control programme would be most suitable for your herd.

A bulk milk sample can be taken to give an approximate idea of the level of BVD infection within the herd. Obviously bulk milk samples only take into account cows contributing milk into the tank at the time of the sample ie not dry cows or heifers. Also, because of differing yields the result is only approximate - it gives a negative or a low, mid or high positive result, which is then correlated to an approximate percentage of cows that have antibody within the herd. The bulk milk result tends to vary with time as new heifers come into the herd and older cows are culled.

Blood samples can be taken from individual animals to determine the status of a particular age group. This is most frequently done with maiden heifers to see if they are being exposed before they enter the milking herd.

Control

The most important aspect of control is to prevent the birth of PI calves as these are the main spreaders of the disease. This means ensuring that cows are not infected in the first third of pregnancy and this is achieved by giving cows two doses of vaccine before putting them in calf. The vaccine actually lasts for the whole of the pregnancy so booster vaccination can take place after calving in each subsequent year, thereby protecting the next pregnancy.

Ideally all breeding animals within the herd are vaccinated every year including maiden heifers. This ensures that all future pregnancies are protected and no further PI replacement heifers are born. Any PI animals within the herd at the commencement of vaccination will either die of mucosal disease or be culled in the following years. Until that time, the spread of infection to other animals is prevented by vaccination.

An economic alternative to full herd vaccination is heifer only vaccination, which is sometimes used when the antibody level in the bulk milk sample is high. Although there may still be some circulation of BVD within the unvaccinated part of the herd, most animals will be immune to infection and all incoming replacements are vaccinated and then boostered in subsequent years. Over a period of years, a greater proportion of the herd is vaccinated and the older unvaccinated animals are gradually culled so that after five or six years whole herd vaccination is taking place.

In the past, PI animals have been identified and used to deliberately spread the infection to maiden heifers before being put in calf. However, this has several disadvantages over vaccination. Firstly, a PI has to be found, secondly it eventually dies and thirdly spread to other animals in this way is not very reliable and needs monitoring by blood sampling, which costs more than vaccination.