Pestivirus (BVDV) infection of cattle can lead to complex, potentially expensive disease. Many infected properties suffer such low level ongoing losses from pestivirus infection that it is hard for them to see the impact of the disease.
Australian veterinarians and producers have the tools available to them to both cost effectively and systematically control bovine pestivirus at the herd level in most management situations. There are many management strategies available to the producer including: improving biosecurity, testing for and culling carriers, and strategic or herd vaccination. Each beef enterprise has a different level of biosecurity and risk of exposure to the virus, different level of endemic herd infection and differing herd size, structure and enterprise type. Consequently pestivirus management recommendations can involve a lot more than recommending blanket herd vaccination 3,4.
This paper describes various economic models that show the cost of pestivirus management to be worthwhile, or at worst cost neutral 2,5. It then discusses the various control methods being recommended in the field and aims to present a methodical approach to recommendations.
Very few economic analyses of the cost of pestivirus in Australia have been completed. McGowan et al2 presented a preliminary estimate of the 'Economic impact of bovine pestivirus infection in beef cattle herds in central Queensland' in 1997. It used a serological survey and modeling to conclude that losses were conservatively between 1 and 2% of overall profit. Phil Holmes in conjunction with Pfizer6 presented a road show of power point and video presentations demonstrating the economic impact of BVDV on commercial beef herds. Holmes work was not published, thus the principles under the modeling cannot be analyzed. However the estimates seem reasonable and it allows for multiple herd economic values and infection rates. It showed the cost of vaccination to be financially beneficial for profitable herds and at worst cost neutral if only 10% of the heifers were getting infected and this was only occurring every 5 years.
Two studies are currently underway in Australia and will become available this year. An economic modeling study was commissioned by Dairy Australia. Richard Shepherd has completed this work using dynamic herd modeling to look at the long term economic effects of BVDV in endemically infected dairy herds. Another study is being undertaken by Geoffrey Fordyce who has been modelling the cost of endemic infection of BVDV in beef herds in Queensland. A number of articles have been published in New Zealand in recent years showing the economic benefit of pestivirus control, most of this modeling has been done on dairy herds5. In 2007 a large study of randomly selected dairy herds in New Zealand showed a significant and substantial cost of BVDV in infected herds, estimated at NZ$87 per cow. Costs varied largely with antibody levels and the number of PI (persistently infected) animals in the herd14.
In terms of recommending vaccination to endemically infected beef herds I was keen to show simply that the cost of vaccination was less than the cost of ongoing losses.Some modeling has been done to demonstrate the benefits of vaccination to protect a naive herd. Holmes modeled that it would take 60-70 years of vaccination without a challenge for the cost of vaccination to outweigh the cost of a new infection to a naive herd6, as it takes more than 7 years to recover financially. Pestivirus infection within a herd is dynamic and often changing from year to year. In around 1/3 of endemically infected smaller to medium sized seasonally calving herds, pestivirus has been shown to clear its self naturally from a mob in mobs over a 10 year period, leaving these herds naive again for pestivirus introduction.
Survey data published by Bruce Watt7 showed that only 1 in 5 mobs had 80% or greater exposure and therefore protection in their heifers. Morton et al 8 in a larger trial showed that in 66% of mobs, less than half the heifers were seropositive some months before mating start date. In a British survey, Done et al9 estimated that approximately 39% of cows became infected with pestivirus during their first 3 pregnancies. McGowan's work in Queensland concluded very similar seroconversion results 2. During this time the foetus is vulnerable to infection with pestivirus for the first 2 trimesters of each pregnancy, which is 180/365 days (49%) of each year. Thus you could estimate that 19% (49%of 39%) of the new calvers are at risk over these 3 years, or 6%(19% divided by 3) per year. With the cost of vaccination assumed to be $15 (cost plus labour) in the first year, and at calf prices around $600 (in 2015), the cost of vaccination equates to 2.5% losses in the first year and 1.25% losses in following years. The actual cost of BVDV infection is likely to be higher than this, as this basic calculation does not take into account the multiplier effect of losses due to PI cows and calves within the herd, or episodic larger losses. PI cows are estimated to be at 0.5-1 % prevalence within herds and PI calves at 1.5-2.5% prevalence1. A PI cow within the herd is costing producers' money in lost income every year, from the annual loss of her calf alone.
Other management costs are often not taken into account when calculating the true cost of BVDV. These include the cost of spread out calving patterns caused by early embryonic loss (i.e once a late calver always a late calver, and the cost of late-term pregnancy loss, as these cows are not detected at pregnancy testing and are then run dry for an extra year or until calf marking. BVDV has also been shown to be highly immune suppressive to animals at any age10. In the Yass district in 2014, I saw BVDV contribute to respiratory disease and foot rot outbreaks in steers. This is well documented in feedlots. As well as obvious preventable losses occurring when mucosal disease takes hold, I have also seen the reverse where scouring and ill-thrift has been put down to an animal or group of animals being a PI when it actually has another disease process going on.
Many producers don't start a BVDV management program because of the multitude of management options available and the excess of information presented to them. Often they feel that they have taken the initial cost hit and that once their herd is infected the disease takes care of itself in terms of creating natural immunity. One study shows that for 1 in 5 herds in most years >80% of the heifers will have lifelong immunity7.
For herds that have high levels of exposure, the main risk is that they can still produce a drop of weaners that have very little immunity and that will be exposed to the disease when they enter the main cow herd. One of the reasons for this is protection of the calves via colostral immunity4. Another is social patterns of cattle within a mob as well as mob to mob differences of PI distribution within the herd. In an infected self replacing breeding herd, this reliance on natural immunity was modeled by Holmes to show that it is likely to ensure ongoing low level episodic losses6. He determined the cost of this to be greater than or equal to the cost of vaccinating.
Many producers are also wary of the advice to vaccinate in Australia as a lot of the persuasive promotional material has been sponsored by the vaccine manufacturer Pfizer, now Zoetis, and thus they are suspicious of it and do not trust the trial and modeling results. It may be that complicating pestivirus management by offering a lot of testing and management options rather than recommending blanket vaccination for all herds could unnecessarily prolong production and economic losses from this disease. Certainly, when discussing pestivirus management with many veterinarians and farm advisors in the south east of NSW, they have concluded in more recent years that recommending blanket vaccination is a 'no brainer', especially for the heifer and first calver groups.
For some larger properties however, as vaccination is approximately 80% effective in preventing foetal infection8, BVDV control involves doing more than just vaccinating. For others eradication and maintenance of their biosecurity is a preferable and more cost effective long term control.
In terms of simplifying the message to producers I liked the New Zealand BVD control website www.controlbvd.org.nz . It has toolkits for beef and dairy farmers separately, simplifying and structuring the decision making process and the testing and management options. I found this a similar design to MLAs popular 'More beef from pastures' structure15. It uses a decision tree matrix to prompt the initial decision making, and then a 4 step flow through method to assist producers to arrive at the most appropriate decision outcome for them. It also limits the amount of information and decision making per page. Because the testing recommendations are different in New Zealand, I have adjusted their approach for Australia and incorporated Enoch Bergman's recommendations below.
The first step is to assess the BVDV biosecurity of the beef herd. This is done as the first step as it then has implications for which method of BVDV management would be most appropriate. To assess the biosecurity on the farm the producer fills out a simple questionnaire, which focuses on introduction possibilities. It uses a simple catch phrase "in, out and over". In: buying it in, or bringing it onto the farm. Out: sending cattle out on agistment or to a show. Over: the risk of nose to nose contact over the fence from neighbours.
If it is decided that the farm had a BVD biosecurity risk which cannot be mitigated, then it is recommended that they either proceed to vaccinating the pre-breeding herd pre-mating, or do some herd status profiling first.
If the herd would be able to negate its biosecurity risk their best control measure could be to test and cull all PIs from the herd and then maintain a biosecure herd.
The second step is defining the BVDV status of the herd. This could be done by blood testing 5% or a minimum of 6 randomly selected cows from each distinct management group and 6 heifers to give an idea of the herd's exposure profile10. The primary goal is to ensure that each management group is reasonably immune (>80% have antibodies). This screening could be done at any stage. Screening at pregnancy testing will then allow for management decisions about vaccination to be implemented into other management opportunities and result in a vaccination program completed before the subsequent joining. A study on the economic effects of implementing biosecurity found labour to be a very high cost component of on farm biosecurity implementation12, so integrating testing or vaccination into other management operations reduces the cost to the producer.
The third step is to decide on the actions used to control BVD. Decisions about vaccination, controlled natural exposure or eradication can be made using the profiling information. This information may help you to decide that vaccinating the older cows is not required because of prior exposure. Vaccination would be recommended if the individual mobs showed less than 50-80% immunity. Many larger producers have high levels of exposure in their adult cattle. Notching the management ear tag of those cattle in mobs that have sufficient natural immunity can serve as an easy practical reminder that these cattle do not need to be vaccinated. As the program progresses some management groups will need vaccination boosters and discerning them from immune animals is a useful and profitable exercise10.
Heifer management in infected herds can involve either a blanket vaccination recommendation, or annual pre-mating screening prior to the decision. Once weaned and comingled for 90 days13, and at least 8 months of age, 5% or a minimum of 6 heifers are antibody tested. These results will usually indicate either a need to vaccinate or to screen the heifers for PIs. Vaccination is recommended if <80% exposure. If less than 50% exposure the group is unlikely to contain a PI. If >80% exposure, it would be recommended not to vaccinate them and to notch the management ear tags to indicate immunity and to screen the whole heifer group using ear notch or tail hair test as it is likely to contain a PI that should be culled10. If between 50 and 80% seroconversion in a group, more testing is recommended to look for the likelihood of a shy PI being present in the mob.
Many prominent cattle veterinarians have been recommending vaccination of the heifers twice pre-breeding with a booster vaccination then given in the following year. This protects the most vulnerable group of cattle in their highest risk years. Provided that there is adequate exposure to virus when they reach the adult herd their vaccinated immunity will be boosted by exposure to the live virus within the adult herd, giving them lifelong immunity11. This is a good recommendation for infected herds that do not want to enter into a heifer screening program. As the pestivirus control program progresses, however, the number of PIs within the herd could be reduced potentially to zero. In that case ongoing booster vaccinations are required to maintain immunity within the herd. Consequently in herds only vaccinating the heifers there is a need for ongoing monitoring. The AGID antibody test can be used to see if there is natural exposure occurring on top of vaccination sufficient to create lifelong immunity. Negative results in cattle that have not had a booster Pestigard® vaccination for years may indicate a need to vaccinate. The AGID cannot be used to indicate vaccinated immunity, instead the virus neutralization test could be used but this is infrequently used and more expensive11.
Herds that can maintain biosecurity after eradication may decide that tail hair or ear notch screening the entire mob to eradicate PIs is their best pathway. Calves would need to be screened as well, prior to the next joining. This could be done at calf marking by using tail hair samples from the calves. False positives are possible in an actively infected mob, but this is uncommon with hair samples compared to ear notches. This could be clarified by a blood sample, if desired by a repeat test after 30 days.
Whichever management program is used all bulls should be pestivirus PI tested negative prior to introduction to the farm or use. It is also recommended that all bulls are vaccinated annually.
The fourth step is to continue to monitor the BVDV status of the beef herd. This is important if vaccination of the cow herd is not being boostered yearly. By monitoring 5% of the heifer group each year, at 8 to 10 months of age, this measures the success of the program as well as acting as a sentinel for reintroduction of the virus. Herds that have decided that their biosecurity is good enough to cease vaccinating are advised to continue to screen their replacement heifers and any herd additions.
Indicative laboratory or material costs of components of a BVDV control program in a 100 heifer herd in 2015: testing 5% or a minimum of 6 of the herd~$110; vaccination of 100 heifers ~$1100 (plus labor); PI screening of 100 heifers~ $830; screening groups of introduced animals for PIs~$166 per 20 animals or $13.90 per individual.
Pestivirus (BVDV) infection of cattle can lead to complex, potentially expensive disease. Many endemically infected properties suffer such low level ongoing losses from pestivirus infection it is hard for them to see the impact of the disease. Infection introduced into a naive herd can have a significant financial and managerial impact. It seems from economic modeling that pestivirus control is economically worthwhile. Essentially though, for many producers the cost of pestivirus control may get close to approximating their losses from the disease. What vaccination or pestivirus biosecurity does do, however is provide, at worst cost neutral, ongoing protection of the herd while at the same time insuring against those years when the losses are more than base line and management is significantly disrupted.