BY: REBECCA SMITH
BVD virus is an infectious disease that costs New Zealand cattle farmers more than $150 million every year in direct production losses. The prevalence of actively infected dairy herds is sitting at about 15% to 25%, with about two-thirds of herds getting annual screening done in their bulk milk test and as a consequence we have seen a reduction in the number of actively infected dairy herds.
In contrast, a shocking 45% to 55% of beef herds are actively infected with BVD; less than 15% of beef farmers are screening annually and we still have the same percentage of herds with active BVD infections as we did 10 years ago.
Also concerning are the results from a recent survey conducted by Carolyn Gates* and others in the BVD Steering Committee where it was found that:
Approximately 30% of beef farmers surveyed who thought their herd was BVD negative actually had strong evidence of BVD exposure.
Farmers were generally unwilling to implement preventative measures until they experienced the negative impacts of an outbreak.
Many beef farmers were unaware that impacts of BVD extend beyond poor reproductive performance to include poor growth rates and higher incidence of other diseases due to impaired immune function.
Impacts on beef breeding herds
- Reduced bull fertility - Transiently infected bulls can have poor semen quality due to the combined effects of virus and the presence of BVD in the semen leading to fewer cows getting pregnant. They may also be more susceptible to other diseases.
- Poor conception rates - Infections during the mating period can cause damage to the eggs and create a poor uterine environment, which results in a failure to conceive.
- Early embryonic death - Even if fertilisation occurs, the resulting embryo may not be viable leading to early embryonic death and prolonged period before the next cycle (25 to 35 days).
- Abortions, mummies, and stillbirths - If the virus does too much damage to the calf up to 180 days into pregnancy, it can be fatal and result in an abortion. The immunosuppression caused by BVD may also trigger abortions from Neospora or fungal agents. Mummified fetuses and stillbirths can also occur if the fetus is not expelled from the uterus after death.
- Persistently infected calves - If the foetus is exposed to BVD from 40 to 120 days into pregnancy, the calf may be born with a persistent infection (PI). These calves shed virus for life. They are often stunted and grow poorly with many dying or being culled from the herd before reaching 12 months of age. If the strain which infected a PI animal evolves into a more severe form of BVD then this can develop into fatal mucosal disease in these PI animals.
- Developmental defects - If the pregnant cow is infected from days 90 to 150 days into pregnancy, the calf may be born with severe developmental defects that result in early death or that require humane euthanasia.
- Weak or stunted calves - If the pregnant cow is infected greater than 180 days into pregnancy, there is a chance that the calf may be born small or weak. These calves have poor growth rates and reduced fertility due to the damage that BVD can cause by replicating in the calves' ovarian tissue.
- Decreased milk production - When lactating cows become infected, they may have reduced feed and water intake for up to 3 weeks, which can cause a significant drop in milk production levels. This can have a dramatic impact on the growth rates of suckling beef calves.
- Increased disease in calves - When calves under 12 months old become acutely infected with BVD, it suppresses their immune system and makes them more susceptible to other health problems like scouring, poor growth, coughing, ulcers in the mouth, and lameness. These calves may require additional veterinary treatment and additional time to reach appropriate weaning weights.
A shocking 45% to 55% of beef herds are actively infected with BVD.
Impacts on beef finishing
Obviously a finishing unit avoids the reproductive impacts of BVD, however, they are at high risk for transient infections, especially if cattle are bought from multiple source properties and then mixed. You also risk buying persistently infected animals which can be a waste of money.
- Transient infections in mob - When naive cattle are exposed to the virus they will develop a transient infection which lasts about two weeks. During these two weeks these individuals will experience scouring, fever and an impaired immunity to other diseases while their immune system is busy mounting a response to the BVD virus. This leads to decreased growth rates, which can have a significant impact depending on the number of animals the disease needs to progress through.
- Persistently infected individuals - As described above these animals are often stunted and grow poorly with many dying or being culled from the herd before reaching 12 months of age. They can, however, appear normal through the sale yards and it is not until the strain of BVD they are infected with evolves to a more severe form that they rapidly develop a fatal mucosal disease. Persistently infected animals also pose a constant immune challenge to their peer-group which is an energy cost which they are diverting from growth.
Co-grazing with sheep a risk
Just to throw another spanner in the works, there is evidence from overseas research that BVD can infect sheep and that BVD-infected sheep can experience the same negative reproductive impacts seen in cattle, along with the production of persistently-infected lambs. The level of this impact is difficult to establish in sheep and beef co-grazing scenarios in New Zealand due to the presence of another disease called Hairy Shaker which is from the same family and is difficult to differentiate in the laboratory tests. However, the risk is there, providing just another reason to be aware of the BVD status of your cattle on farm and take measures to control BVD on farm.
A simple pooled BVD antibody ELISA test on 15 young stock aged between 10 and 18 months of age can easily determine whether the herd has recently been exposed to the virus. This can easily be done when vets are out on a farm doing another job. It has then been modelled that the most cost-effective way to control BVD was to vaccinate replacement heifers and mixed-age cows annually to prevent foetal infections. This method of control delivered a benefit:cost ratio of two for the average beef herd.
For more information on BVD speak with your local vet or head over to www.bvdfree.org.nz
*Gates MC, Evans CA, JH Han, Heuer C, Weston J (2019) Practices and opinions of New Zealand beef cattle farmers towards bovine viral diarrhoea control in relation to real and perceived herd serological status. New Zealand Veterinary Journal 68(2): 92-100.
*CA Evans, J-H Han, JF Weston, C Heuer & MC Gates (2020) Serological evidence for exposure to bovine viral diarrhoea virus in sheep co-grazed with beef cattle in New Zealand, New Zealand Veterinary Journal, 68:4, 238-241
- Rebecca Smith is a VetEnt vet based in Ranfurly.