BVD and the constant threat
Bovine Viral Diarrhoea (BVD) is one example of a disease that can persist within a herd/ community but with appropriate measures can also be managed to work towards elimination.
This season we have reported a number of positive bulk milk BVD tests across the district. At some point, these affected herds have had exposure to a BVD-persistently infected (PI) animal, most likely away at a grazing block, which has then allowed a PI animal to enter the milking herd.
Research indicates a lot of dairy herds have cleared BVD but now have low immunity levels (a naïve herd that is highly susceptible) so the risk of a BVD incursion is now even more serious. As during the COVID-19 pandemic, eliminating BVD from your herd is a step in the right direction but is ineffective unless coupled with testing & culling, vaccination and adequate biosecurity.
Health effects of BVD
The effects of BVD within a herd are varied and far-reaching:
In heifers and cows, BVD seriously impacts fertility through early embryonic death, resulting in higher empty rates and long returns to service. Later on, it may cause abortions, stillbirths, mummified fetuses and even birth defects.
In bulls, BVD causes infertility with disastrous consequences on pregnancy rates. Bulls can shed virus in semen if they become transiently infected, so become a major source of transmission during the breeding season.
BVD also causes immune suppression, so young calves can become susceptible to other diseases such as pneumonia, scouring, coccidia.
Production effects include reduced milk yield and reduced growth rates. Research indicates a persistently infected (PI) animal has half the milk production of a non-PI animal, is approximately 70kg lighter at 21 months, has a lower survival rate, has a 17x higher risk of mastitis and a 12x increased risk of severe illness.
The super-spreader – PI animals
BVD persistence within a herd occurs when a PI animal carriers the disease and transmits the virus to other animals through close contact. The concept of ‘close contact’ can include comingling with a PI animal, a brief nose-to-nose encounter over a neighbour’s fence line or grazing in an environment contaminated with faeces from a PI animal.
A PI animal can only be created when the dam is exposed to the virus during the first 4 months of pregnancy when the developing foetus has an immature immune system. The cow will become transiently infected (TI), and like catching a cold, perhaps have a high temperature and be off-colour for a short period but will recover and develop antibodies. A transiently infected animal does not transmit the virus. Unfortunately though, if the exposed foetus survives to birth, they become a ‘virus factory’ and will nearly continuously shed the virus throughout their entire lifetime, infecting other animals they have contact with.
PI animals cannot be cured or treated and must be culled
Surveillance testing for BVD
Bulk milk sampling the milking herd - bulk milk sampling is extremely sensitive and effective in detecting BVD within the milking herd. There are 2 components to BVD bulk milk testing.
- BVD bulk milk antibody ELISA – Monitors for signs of ongoing BVD exposure. A spike between the start and end of season indicates the cows have been exposed during the risk period for generating PI calves. Testing all replacement heifer calves born next season is essential to identify any PI animals.
- BVD bulk milk PCR test – Screens the milkers for the presence of a PI animal. This test is essential BEFORE mating to ensure no PI animals have entered the milking herd since last season. Note: Bulk milk test results only reflect the BVD status of animals contributing to the vat on the day of sampling, so calves, heifers, colostrum or lame cows will need to be individually tested.
Screening heifer replacements - It only takes 1 cow to become exposed to get a PI animal. Calves should be treated as new introductions to the farm, so screening heifer replacements ASAP after birth will catch any breakthroughs. Ear notch sampling can be done at the time of disbudding and can differentiate between a persistently-infected, transiently-infected and a noninfected animal. Stopping BVD is all about cutting off the supply of PI animals, so identifying them early allows quick removal!
- Screening bulls - Bulls are a high-risk group due to the timing of their introduction into the herd. A large number of AB calves are likely to be within the risk period for developing into a PI calf if exposed to BVD. Ensure you only purchase BVD-tested bulls that have been vaccinated prior to arrival.
- Individual blood testing - While a scouring, poor-doing animal is a common sign for BVD infection, this is not always the case. Blood sampling individual animals can identify the presence of virus and confirm if the animal is persistently infected.
Biosecurity
Don’t become complacent if there is no evidence of infection within your herd. Maintaining good biosecurity measures is important to prevent future disease introduction. These measures include;
Knowing the BVD status of any introduced stock onto the property (bulls, herd replacements, bought in stock)
Ensuring good boundary fencing with neighbouring properties to prevent nose-to-contact. Double-fencing or not grazing adjacent stock at the same time are effective means of social distancing..
Maintaining good levels of hygiene. The virus doesn’t survive well in the environment but can be easily spread in faeces, saliva, milk, urine.
• If you’re sharing yards, spell these for at least a week before re-using.
• If you’re milking the neighbours herd due to breakdowns, ensure proper washdown between milking herds.
Quarantining stock of unknown status until they are tested or vaccinated.
Testing calves is the best way to see if your biosecurity plan is working and to monitor for breakthrough.
BVD Vaccination
Vaccination remains the most effective means of breaking the cycle of transmission by preventing the formation of those pesky super-spreader PI animals. Although vaccination cannot completely stop animals from becoming infected, it will provide foetal protection by preventing virus spread across the placenta and formation of a PI calf. Vaccination will also reduce the severity of infection and minimizes production losses.
Note: Vaccination will have no effect on a PI animal as they cannot mount an immune response. They will continue to shed the virus even if vaccinated, so it is important to promptly remove PI animals and screen prior to vaccination.
Timing of vaccination is critical to break the transmission cycle.
RECOMMENDED VACCINATION SCHEDULE:
Previously vaccinated animals : Annual booster, 2 weeks prior to PSM.
Previously unvaccinated animals (heifers) : 2 injections, 4 weeks apart, 1 month prior to PSM.
Bulls : 1 month prior to PSM : Vaccination of bulls is ESSENTIAL as the consequence of bulls becoming sick with BVD are severe.
Carry-over cows/heifers leaving farm or : Vaccinate before leaving the farm because often they don’t away at grazing return until they’re ready to calve. : OR, vaccinate at least 1 month prior to PSM.
STOP BVD IN ITS TRACKS
The most devastating impacts of BVD are on pregnant cattle and their unborn calves, so protecting heifers and cows from infection during mating and gestation is critical.
Protect the health of your herd by keeping BVD off your farm for good with Ultravac BVD® the premium foetal protection vaccine proven in New Zealand.