One of the challenges with controlling BVD is that the New Zealand farming system creates many opportunities for the virus to spread within and between cattle herds. However, understanding the risk factors for disease transmission can help you build a plan for keeping BVD out of your business.
There are four main ways that BVD can get onto your farm:
1. Buying infected animals
2. Animals that are pregnant or become pregnant when sent off farm
Another major risk for BVD introductions is when heifers are sent off farm for rearing or when cows are moved to shared grazing pastures and end up in mixing with infected cattle from other herds during the high-risk period of pregnancy. When the animals return to your farm, they may be transiently infected (TI) or recovered and carrying a PI calf (Trojan dams).
3. Contact with neighbouring stock through shared field boundaries
If your fenceline boundaries allow nose-to-nose contact with neighbouring stock or if cattle routinely escape from pasture, you are placing your herd at risk of being exposed to BVD.
4. Bringing contaminated equipment onto the farm
BVD can survive for hours to days in the environment and be brought onto your farm on the surfaces of trucks, tractors, clothing, gumboots, gloves, scanners, dehorners, and any other objects that have been in contact with infected cattle.
How BVD Becomes Established
The consequences of BVD entering your herd depends on both the source and timing of the virus introduction. Outbreaks caused by transiently infected (TI) cattle tend not be as severe because these animals shed much lower amounts of virus for only a few weeks’ time whereas persistently infected (PI) cattle will continue to shed large quantities of virus until they are removed. The worst timing for BVD introductions is during the mating/pregnancy period when animals are at greatest risk of experiencing reproductive problems and creating new PI calves. Unless you can break the cycle of creating new PI calves each year, BVD will become a chronic problem for your herd and you will continue to experience production losses.
Minimising Your Risk
The key to managing your risk for BVD is knowing when pregnant animals will be in the critical stage of pregnancy for developing PI calves and preventing the foetus from getting infected during this time period.
1. Only purchase stock that have tested negative for BVD virus
We can test animals before they arrive on farm to determine if they are actively infected with BVD. The only way to know if the animal is TI or PI is by re-testing them in 3 weeks to see if they are still shedding virus. Animals that are actively shedding BVD should ideally not be brought onto your farm. If this is not possible, keep them strictly quarantined from the rest of the herd to prevent virus from spreading. The calves from any purchased pregnant dams should be tested as soon after birth as possible to make sure they are not PI since there is currently no diagnostic test that can identify Trojan dams.
2. Check the vaccination status of purchased animals
Always check the BVD vaccination status of purchased animals and, when possible, try to source animals that have been vaccinated and certified as BVD free. If the animal has never been vaccinated before, there may not be enough time to complete the booster series before the planned start of mating to give animals maximum protection against BVD during the breeding season. This is particularly important for replacement bulls since they will be mixed with your breeding mob when there is greatest risk of reproductive complications from infections. Vaccination can also interfere with the diagnostic tests for BVD that measure antibody levels to determine exposure and so purchased animals with an unknown vaccination status should never be sampled for these tests to avoid getting false positive results.
3. Manage the risks from grazing cattle off-site
If you send your cattle off-site for grazing, make sure you know if they will be co-grazed with cattle from other herds. If the other cattle originate from BVD positive herds or if you do not know the BVD status of these cattle, there is a risk that they will infect your animals. If your cattle will be pregnant during the offsite grazing, vaccination can help prevent the creation of PI calves. If you choose not to vaccinate, it is important to test any calves born to these animals to make sure they are not PI. Unvaccinated animals should also be isolated from the breeding mob for at least 3 weeks in case they are transiently infected when they return to the farm.
4. Limit nose-to-nose contact with neighbouring cattle
Ideally, animals should be grazed in non-adjacent pastures or the boundaries should be double-fenced to prevent direct contact between animals since this will reduce the transmission of many other infectious diseases in addition to BVD. Vaccination can also be a useful tool to help protect animals from getting infected if contact is unavoidable.
5. Clean all vehicles, equipment, and clothing
Make sure that you clean everything thoroughly with an appropriated disinfectant when moving between cattle farms. This applies for your veterinarians, AI technicians, and scanners too!