e-mail: hihealth.admin@btconnect.com
BOVINE VIRAL DIARROHEA (BVD) SUMMARY
BVD is arguably the most economically significant viral disease affecting British cattle today, with approximately two thirds of farms demonstrating infection or previous exposure. Following introduction of this disease the economic losses may exceed £45,000 in an average 100 cow, commercial beef suckler herd over a ten-year period and will be higher in a similar sized dairy herd. Many effects and losses are hidden, difficult to pinpoint and therefore underestimated.
The disease is one of the most complicated to threaten animal production
in the UK. The virus causes many problems including
Interference with reproduction in cows and heifers
Reduction of calf immunity to other diseases
Contributes to calf pneumonia complex in calves
Affects development of foetus resulting in a range of abnormalities
May result in “Mucosal” disease
BVD virus can also cause diarrhoea during acute infection that is usually mild but occasionally takes a fatal form (known as Type II BVD) that has now been described elsewhere in Europe.
Infection immediately before or during the breeding season will reduce conception rates and cause early death of embryos. Infection at any stage of pregnancy can result in abortion. The virus can also cause deformities of the calves. However of particular importance is infection during the first third of pregnancy when calves developing in the womb which survive remain persistently infected with the virus (PI calves) and it is these calves, once born, that provide the major route of spread for the virus. They often appear normal and always shed virus throughout their lives. Many develop fatal enteritis known as mucosal disease before they reach maturity, however significant numbers of PI’s survive well into adulthood.
Semen from acutely (transiently) infected bulls can also spread infection. Theoretically other ruminant species such as sheep and deer can be a source of infection. Contaminated needles and other equipment can also spread virus from animal to animal from herd to herd. High levels of herd bio-security and isolation are an essential part of control.
Exclusion or eradication of BVD virus from the herd therefore appears to be a logical option given the enormous losses associated with herd infection. There is the potential for huge savings to many farm businesses through the reduction in abortion/reproductive losses, improved calf health, fewer deaths and reduced veterinary costs.
BVD SCREENING AND ERADICATION PROGRAMME
Initial Assessment
Where the BVD infection status ins unknown we recommend that samples from
10 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 infection is likely to be active in
the herd. If negative it is the first step to accreditation. If animals
are sold off the herd before 9 months of age, calves at the age range 6-9
months should be sampled instead. In the dairy herd bulk milk screens and
first lactation screens can offer additional information.
Eradication
Once BVD infection if confirmed in the herd all animals over 4 months of
age are screened once and then all animals born subsequently for a period
of twelve months after the removal of the last virus positive animal. This
is usually achieved through two tests 12 months apart. Animals are individually
tested for antibody and any with low or negative levels are then tested
for virus. Once virus positive animals are confirmed by a second test they
are removed from the herd.
BVD Accreditation Programme
Testing is essentially the same procedure as the initial assessment, repeated
at annual intervals and, after tow clear tests, the herd is awarded the
status “accredited free of BVD infection”.
For dairy herds 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 initial assessment of the dairy herd shows no evidence of infection. In this latter case, three bulk milk tests at 7-month intervals allow accreditation of freedom of BVD.
Annual Testing
To maintain accreditation the same test procedure described in the initial
assessment is repeated each year. Bulk milk samples or samples from first
lactation cows are tested three times a year in the dairy herd.
BVD Bio-security Measures
All brought in animals must be isolated and tested as directed the CHeCS
rules. Bio-security measures must be implemented as per the CHeCS rules.
Ideally animals should also have been tested for the BVD virus on the farm
of origin because if virus positive they cannot enter the herd and there
will have been considerable savings in time and expense. Pregnant cattle
with positive antibody results cannot enter the herd because they may give
birth to a persistently infected calf. They would need to be kept in isolation
until the calf could be tested.
Vaccination Programme
Testing is essentially the same procedure as the initial assessment, repeated
at annual intervals and after two clear tests the herd is awarded accredited
status. These screen tests are carried out on the young animals before
they are vaccinated. All animals in the breeding herd must be vaccinated
before service/insemination and according to the manufacturers recommendations
in the datasheet. When vaccinating there is no requirement for double fencing
with a three-metre gap as bio-security.