BVD Spread
Did you know that over 95% of cattle farms in New Zealand have at least one breach in their boundary biosecurity that can easily allow BVD to enter their herds from outside sources? Protect your cattle by managing the risks of bringing new cattle IN to your herd, moving cattle OFF and ON for grazing, and allowing nose-to-nose contact OVER fencelines.
How can BVD get into my herd?
In order for a herd to be considered actively infected with BVD, there must be at least one PI animal present somewhere amongst the calves, replacement heifers, mixed-age or milking cows, bulls, and/or fetuses. Based on what we have learned from New Zealand farmers who have experienced BVD outbreaks in their herds, the three biggest risk factors for BVD outbreaks are:
1. Bringing new cattle IN to your herd
You wouldn’t buy a new car without checking under the hood so why risk your entire herd’s health by buying in replacement heifers, cows, or bulls with an unknown BVD status. A quarantine period will not protect you against PI animals. All it takes is one simple test on a blood, ear notch, or milk sample to confirm that the animal is not shedding the virus and then it can be certified as being non-PI for life. Bulls should also be fully vaccinated before arriving on farm so that they are protected against transient infections that could cause problems with fertility and conception during mating. Don’t just accept the seller’s word that the animals you purchase are non-PI and vaccinated – always ask to see a signed veterinary certificate. No certificate, no deal.
Another major risk is through purchasing pregnant non-PI cattle that could be carrying PI calves after becoming transiently infected with BVD during early pregnancy. These are called Trojan dams because they look normal from the outside but are carrying a dangerous source of virus on the inside. There are no tests that can accurately tell the BVD status of the fetus so if you must purchase pregnant animals, make sure they are sourced from herds with good BVD control and test any of their calves that are being kept as replacements.
2. Moving cattle ON and OFF farm for grazing
Your replacement heifers are often the most vulnerable animals on your farm because they are least likely to have immunity against BVD and most likely to get exposed by being sent offsite for rearing. Ask your grazier if cattle from other herds will be present on their property at the same time and if so, what measures are in place to protect your heifers against BVD infections so you don’t end up with Trojan heifers calving into your herd the following season. Winter grazing tends to be less of a problem since most cattle will be past the risk period in pregnancy for generating PI calves, although fetal infections during late pregnancy can still cause the birth of weak or deformed calves that may struggle to achieve growth targets. If you run a separate grazing business on your farm, protect your own breeding dams by making sure any cattle brought on are non-PIs, fully vaccinated, and kept separate from your herd.
3. Allowing nose-to-nose contact OVER fencelines
If your fence boundaries allow nose-to-nose contact with neighbouring stock or if cattle routinely escape from pasture, you are placing your herd at significant risk of being transiently infected with BVD and giving birth to new PI calves. Ideally, animals should be grazed in non-adjacent pastures or the boundaries should be double-fenced to prevent direct contact between animals (> 2m separation) since this will also prevent the spread of many other infectious diseases in addition to BVD. Work together with your neighbours to develop a joint plan for keeping BVD out of your bubbles.
BVD can also survive outside cattle anywhere from hours to weeks in the right environmental conditions. Make sure that any vehicles, clothing, or pieces of equipment that have been in contact with other herds have been thoroughly disinfected before they cross your farm boundary.
How does BVD spread within a herd?
Once you have a PI animal on farm, it can easily become a major source of infection for pregnant cattle leading to the birth of more PI calves into the herd every year. This is what is commonly known as the BVD cycle and why BVD control programmes are geared towards interrupting transmission chains on farm as well as preventing ongoing introductions from outside sources. If any of the PI calves are kept as replacement heifers, they may survive to join the breeding herd and will always give birth to more PI calves.
BVD cycle in dairy herds
BVD is generally easier to control in dairy herds for the simple reasons that most calves are separated from their dams within 24 hours of birth and most calves (except for the 15% to 25% that are being kept as replacement heifers) will either be sent to slaughter as bobby calves or sold to other farms to be reared as dairy beef within their first week of life. This means that there is a good chance that any PI calves born into the herd will be sent off farm well before the mating period and even if there is a PI among the replacement heifer calves, there will likely be much less contact with pregnant cattle in the milking herd or pregnant heifers that are being reared offsite. The main reasons why dairy herds have ongoing problems with BVD is from poor biosecurity where the virus keeps coming in from outside sources every year.
BVD cycle in beef herds
One of the main challenges with breaking the BVD cycle in beef breeding herds is that calves stay with their dams until weaning at 6 to 8 months of age. Any PI calves that were born into the herd that season will be in direct contact with pregnant dams, which can easily lead to creation of new PI fetuses and the birth of more PI calves the following season. For farmers that can’t easily yard cows with calves at foot to identify and remove PIs prior to mating, vaccination can be highly effective for reducing the chances that new PI calves will be created.
Check your understanding
Answer the questions below to check your understanding of BVD spread.
Check Answer
The correct answer is (e).
BVD can easily get into your herd through the movements of animals, personnel, and equipment across your farm boundary. In New Zealand, contact with neighbouring stock over shared fencelines is also a major risk factor for BVD spread.
Check Answer
The correct answer is (a).
Any pregnant dam that is not protected against BVD through either previous natural infection or vaccination is at risk of developing a PI calf. There are no tests available that can determine the infection status of the fetus so any pregnant animals you purchase and any susceptible replacement heifers that are returning to the farm pregnant may give birth to PI calves. Testing their calves shortly after birth is important for making sure no PI animals enter your herd.
Check Answer
The correct answer is (b).
Based on bulk milk BVD testing data from LIC, approximately 4% of dairy herds that are negative for BVD in one season will become positive for BVD in the following season. These herds are ones that are already conducting routine bulk milk BVD testing (~65% of all dairy herds in New Zealand) and the rates of infection may different for herds that do not conduct rouinte monitoring for BVD.
Check Answer
The correct answer is (c).
Based on the results from field studies conducted by the BVD Free Research Project in 2017/2018, the rate at which a single PI animals infectes others was estimated at 0.11 animals per day (1 animal every 10 days). This is not high enough to achieve adequate levels of herd immunity prior to the start of mating to prevent new PI calves from being created.
Check Answer
The correct answer is (a).
For a herd to be considered actively infected with BVD, it must contain at least one persistently infected animal. The BVD cycle is maintained in a herd when PI animals continue to infect susceptible breeding dams during the risk period for generating new PI calves and/or if the farm is purchasing Pi replacement animals.