Bovine virus diarrhoea virus is closely related to the viruses that cause classical swine fever in pigs and border disease in sheep. This virus causes a complex of diseases in cattle the most important of which interfere with reproduction, affect the foetus and lead to mucosal disease.
BVD virus can also cause enteritis during acute or transient infection which is usually mild but occasionally severe enough to kill even adult cows. Transient BVD virus infection is also believed to cause significant suppression of disease resistance and may contribute to the pneumonia complex in calves.
Infection immediately before or during the breeding season will reduce conception rates and cause early death of the embryo. Infection at any stage of pregnancy can result in abortion. The virus can also cause deformities in the calf. However of particular importance is infection in the first third of pregnancy when developing calves that survive remain Persistently Infected with the virus (PI calves) and it is these calves, once born, that provide the major route of spread for this virus. They often appear normal but always shed virus throughout their lives. Many develop a fatal enteritis known as mucosal disease before they reach maturity, however, significant numbers of PIs survive well into adulthood. Semen from transiently infected bulls can also spread infection.
Theoretically other ruminant species, such as sheep and deer, can be a source of infection for cattle although sheep are at greater risk from cattle than vice versa.
Contaminated needles and other equipment can also spread virus from animal to animal and herd to herd. Therefore careful herd health security and quarantine are an essential part of control. The economic losses for an uncontrolled outbreak of BVD can be very great.
It has been calculated that in a large beef herd these can exceed £45,000 over a ten year period while losses can be greater in the dairy herd. In most outbreaks reproductive losses are the most significant although mucosal disease cases are the most obvious.
Where possible control by identification and removal of PI animals is advised with subsequent exclusion of any potential sources of reinfection. Vaccination may also be used in the course of an eradication programme.
Where the BVD infection status is unknown, we recommend that samples from five animals are collected from each distinct management group in the age range 9 - 18 months. These are tested for the presence of antibody to BVD and, if positive, it indicates that there has been recent infection in the herd. If negative, it is the first step to accreditation. If animals are sold before 9 months of age, calves in the age range 6-9 months should be sampled instead, in which case ten are required from each group. In the dairy herd, bulk milk screens and first lactation screens can offer additional information.
Once BVD infection is confirmed in the herd, all animals over 4 weeks of age are screened once and then all animals born subsequently for a period of 12 months after the removal of the last virus positive animal are also tested. It is not necessary to test all adult cattle if there are good herd records. Calves and young stock up to calving age should be tested plus any cows that have not contributed an offspring for testing plus the bulls. Animals are blood tested for the presence of virus.
An animal is considered to be a PI if it tests virus positive twice at an interval of at least 21 days. Once virus positive animals are confirmed by a second test, they are removed from the herd. We follow up to check the success of eradication by repeating the testing described under the Initial Assessment (above) on two calf crops. If these show no evidence of infection, the herd is considered free of BVD.
Annual testing is required to maintain accreditation and the same test procedure described in the initial assessment is repeated each year. Bulk milk samples or samples from first lactation cows are also tested quarterly in the dairy herd.
Disease Free Status Accreditation
Accreditation follows on from either a negative initial screen or after the eradication procedures are complete. Testing is essentially the same procedure as the initial assessment, repeated at annual intervals and, after two clear tests, the herd is awarded the status 'accredited free of BVD infection'. Milk can be used to replace blood samples for antibody testing in lactating cows and bulk milk samples can be used as part of the accreditation procedure where the initial assessment of the dairy herd shows no evidence of infection. In this latter case, quarterly clear bulk milk tests and clear young stock screens for a 12 month period allow accreditation of freedom from BVD.