Although it is relatively straightforward to eliminate BVD from individual positive herds, these farms will still experience significant ongoing costs associated with keeping BVD from being re-introduced. These risks and these costs will continue to exist for as long as there are infected cattle herds in New Zealand, which is one of many reasons why BVD is such an expensive disease for the industry. By investing money up front to implement a coordinated national eradication programme, New Zealand farmers can end up saving millions of dollars over the long term.
Principles of Disease Eradication
By definition, eradicating BVD from New Zealand means getting rid of every last virus particle from animal populations including cattle and other potential reservoir species. There are four important actions that must take place to achieve BVD eradication:
1. Determine the BVD status of herds
The first step in disease eradication is knowing where the virus is currently located in the country. This involves having every cattle herd perform a screening test to determine if they are positive, exposed, or negative. Since disease status can change over time, screening tests will need to be performed on a regular basis until the prevalence (number of infected herds) drops to low levels. There needs to be a national database to record the status of individual animals and farms, which in New Zealand would likely integrate with the National Animal Identification and Tracing (NAIT) system.
2. Prevent new BVD outbreaks in negative herds
To stop negative herds from getting infected, all purchased animals should ideally be sourced from other negative herds and tested before movement to certify that they are not persistently infected. For herds that are at risk of getting BVD through other transmission pathways, vaccination should be used to prevent the creation of PI animals.
3. Eliminate existing BVD outbreaks in positive herds
Positive herds should work with their veterinarian to determine the optimal vaccination, testing, and culling programme to eliminate PI animals. Efforts should also be made to prevent BVD transmission until the herd has cleared the virus.
4. Monitor for sporadic BVD outbreaks over time
As BVD becomes less common, we will be able to scale back the frequency of screening tests. Molecular epidemiology (using the genetic sequence of BVD virus in positive herds to determine the most likely source of an outbreak) is a useful tool for contact tracing investigations so that we can rapidly stamp out sporadic outbreaks.
The good news is that New Zealand is geographically isolated from other countries with BVD and so once disease has been eradicated, the chances of it being re-introduced are relatively small.
Frameworks for National Eradication Programmes
Many European countries have already launched successful eradication programmes using different regulatory frameworks for testing, vaccination, and cattle movements restrictions.
1. Voluntary Control
This strategy usually involves creating BVD certification schemes that farmers can voluntarily choose to participate in. The primary benefit to farmers is the potential to receive higher market prices for their cattle. To date, no country has been able to get enough farmers on board to make any meaningful progress towards reducing BVD prevalence.
2. Phased Compulsory Control
This strategy usually involves a gradual transition from voluntary BVD control to requiring that farmers with BVD positive herds take action to eliminate the virus. In Scotland, for example, farms were first required to test their herd every year to determine their BVD status. In the next phase, farms were required to disclose their BVD status to any potential trading partners. Finally, BVD positive herds were required to take action to eliminate the virus. Although this strategy takes longer and ends up costing more to eradicate BVD, it is often seen as more politically acceptable to farmers.
3. Fast-Track Mandatory Control
This strategy usually involves putting legislation in place that requires farmers to screen their herd up front for PI animals and then continue to test calves each year to eliminate newly created PIs. Farms with positive herds may be required to take additional actions to prevent disease from spreading to negative herds. Although this strategy has a higher upfront costs from the logistics of testing so many cattle, the prevalence of disease drops quite rapidly, which decreases the ongoing costs of screening and prevention.
Implications of Vaccination on BVD Control
The use of BVD vaccinations has been banned in several countries since it interferes with the ability to use inexpensive antibody tests to screen herds for BVD. However, it is unlikely that we will be able to achieve BVD eradication in New Zealand without the use of vaccination because of the challenges in removing PI calves from beef herds in a timely fashion. Most beef calves are not handled until weaning at 6 to 8 months of age and any PI calves born into the herd will therefore have the opportunity to expose susceptible breeding dams during the mating period. Preliminary findings from our cohort study suggest that the contact between cattle in beef herds in too low for the PI calves to expose all susceptible dams prior to the mating period, which places them at risk of getting infected during the risk period for generating new PI calves. This will change recommendations around what testing strategies we use to monitor the disease status of herds over time.