Bovine viral diarrhea virus (BVDV) is endemic to many countries.
BVDV is managed differently in Scandinavian countries than in other countries.
Protecting herds that are free from BVDV can reduce the incidence of new infections, and biosecurity plays a large part in reducing herd infections.
Infections with bovine viral diarrhea virus (BVDV) are endemic in many countries, including the US. For many farmers, the prevailing option to control BVDV is by vaccination. However, there are countries where the infection is no longer a source of significant economic losses, neither in terms of production losses or costs for prophylaxis. In one of these countries, Sweden, a scheme to eradicate BVDV started on an initiative by the farmers’ organisations in 1993. Today, the prevalence of BVDV infected herds has been reduced from over 50% to less than 4%. Without vaccination. And without legislation. In this paper we tell you how, and why.
Is your herd free from BVDV?
All over the world, the focus is on how to handle those herds that have an ongoing problem with BVDV infection. However, from a disease transmission point of view, non-infected herds and their owners are perhaps an even more important target group. Why is this? It is simple; the incidence of new BVDV infections can only be reduced if herds without the infection are protected from getting it. Therefore, one of the key features in the Scandinavian schemes has been the search for herds without infection, in parallel to finding infected herds.
Farmers with non-infected herds have a lot to lose if the virus is introduced. Producers must know whether or not this is the case, and what to do about it! Any farmer who is not 100% sure whether he has got BVDV in his herd or not should ask his veterinarian about help to find out. If there is no evidence of an ongoing infection, immediate evaluation of the farm’s biosecurity level is necessary to prevent BVDV. An early warning system should also be established, i.e. regular sampling for signs of recent BVDV exposure in the herd.
Early warning systems for non-infected herds
Monitoring BVDV infections in dairy herds is best performed by regular checks of antibody levels in bulk milk samples. This is a very convenient and affordable method but is only an option if vaccination is not used. Herds that vaccinate can also be monitored, but it requires individual testing of a small sample, 5-10 young stock. The test frequency should be decided based on risk of exposure. In Sweden, many farmers have chosen to test bulk milk 4 times per year, at a cost of approximately $10 per test. A cheap insurance! And this is all they spend on BVDV management per year.
Certification – a self-purpose?
Certification may sound like a lot of administration, but its justification is the opposite. The reason for certifying herds is to avoid recycling BVDV infection - to re-route the animal flows between herds so that non-infected herds can trade with other non-infected herds. In other words, to create a pool of BVDV-free herds for a more secure livestock trade.
Don’t pay for the virus!
There are many ways by which BVDV can be introduced. However, direct cattle contacts are by far the most important (like for many other infections). Most farms that become infected get the infection either by introducing new livestock or by contacts with cattle on neighbouring farms, over fences or if they break out from the field. Other types of contacts that can result in BVDV introduction are, for example, heifers returning from a heifer grower or from pasture, dry cows returning from pasture or cows returning from shows or exhibitions. If you focus on controlling your cattle contacts, you are half-way to winning the battle.
If you do not test new introductions today, this is where you have to start. Unfortunately, one can not regard a vaccination plan as a safeguard against BVDV infection if the routine is to introduce animals that are not tested for BVDV virus and antibodies – even if they are accompanied by health certificates that state their vaccination status.
Many dairies that vaccinate may have started doing so after a BVDV outbreak or because they have a lot of contacts with cattle from other farms. If the follow-up after the BVDV outbreak is insufficient they may not have gotten rid of the infection. Also, the virus may have been introduced again after the BVDV outbreak. Therefore, unless you test your new introductions, you run the risk of introducing a vaccinated animal that is persistently infected with BVDV. This is a problem that is avoided within a system where whole herds are certified.
Apart from the risk of buying animals that are virus shedders, you also run the risk of buying cows that are pregnant with persistently infected calves. These ”Trojan cows” have been infected with BVDV during early pregnancy – either by a natural infection, or after being incorrectly vaccinated with modified live vaccines during that period. The calf will start shedding virus to its surroundings soon after it is born. The cow herself is healthy and the only thing that may reveal what she is carrying is her antibody levels. They are extremely high (if she is tested in late pregnancy) – much higher than you would ever get from a natural infection. You would know – if you tested her.
Eradicating BVDV from infected herds
Another thing that differs between how BVDV is battled in Scandinavia and in other parts of the world is how virus eradication programmes are implemented in infected herds. The focus is on identifying and removing persistently infected (PI) animals, but a lot of emphasis is also placed on identifying seronegative (female) animals. Therefore, eradication is always initiated by determining the antibody status of all animals.
Why spend money on antibody tests? Well, natural infection with BVDV gives, in practice, lifelong protection against fetal infection. Therefore, the only new animals that could become infected (and give birth to another virus shedder) are dams who lack antibodies at the start of the eradication. This is why an eradication programme should always be concluded by a follow-up test of such cattle. If they have antibodies when you test them the second time, you know they could be carrying infected foetuses. In this way you make sure you find all virus shedders that were not yet conceived when the animals were tested the first time.
At the time when herd clearance is initiated, persistently infected animals can either be present ”alive” in the herd, or in the womb of one or more cows. Therefore, all calves that are born within 9-10 months after the first screening have to be tested to determine whether they are virus shedders.
Be aware that it takes time - a year or more - to get rid of BVDV from an infected herd. Don’t finish the eradication too quickly! Be patient.
Was the eradication sucessful?
All calves born after the last shedder has left the herd will lack antibodies to BVDV, after their maternal antibodies have declined. If a small sample of young stock is tested, 6-8 months old and born after the last shedder was culled, the expected result is that they should all lack antibodies. In this way, it is easy to tell whether the eradication has been succesful, or if there are still undetected shedders in the herd.
Why no vaccination? What about the risk of re-infection?
The reason why none of the Nordic BVDV schemes rely on the use of vaccines is many-fold:
- The use of vaccines is regarded as economically unsound – it is a measure that has to be implemented continuously and repeatedly whereas eradication is time-limited and the subsequent monitoring cheap.
- Vaccines thwart the use of rational screening procedures (e.g. bulk milk sampling) because these procedures require that one can differ between immunity after natural infection and immunity from vaccination.
- There are risks associated with vaccination. The worst risk is that a vaccination programme can give a false sense of security whereas the truth is that biosecurity is still a necessity. Also, you run the risk of perceiving a vaccinated animal as a ”safe” animal, in terms of trade. However, unless you check for it, the vaccinated animal may in fact be a persistently infected, vaccinated animal.
- Another risk is associated with the use of modified live vaccines (MLV), where wrong timing of the vaccination can cause as much damage as the disease itself. Also there is the risk is that any live vaccine (not only BVDV) can be contaminated with wild strains of BVDV, as a result of the use of foetal calf sera in the production process. However, vaccination is indeed an excellent tool to limit virus spread in herds that are newly infected or to immunize susceptible breeding females in order to break the infecious circle. In other words - vaccination should be used for therapy, not prophylaxis! In any case, the use of BVDV vaccines as prophylaxis without implementing any other biosecurity measure is not sufficient – as indicated above, no vaccination plan will give 100% protection.
What you gain if you don’t vaccinate, apart from being able to use convenient monitoring tools, is that you facilitate the interpretation of any antibody- and virus tests in your herd. You also ensure that eradication programmes can be carried out in a quality assured manner, should there be need for it. You also save time and money.
In our experience, the annual incidence rate is low if basic biosecurity routines are in place - below 1 case per 650 herd-years-at-risk. Herds that have had the infection and gotten rid of it have an even lower risk of becoming reinfected.
Are we that different?
In Sweden, the average dairy has got approximately 40 cows (range 1-340) plus replacement heifers and calves, with an average annual milk yield of 8600 kg/cow and year. The replacement rate is around 38.5 % and the replacements are either purchased or home raised. Cows are housed during the winter and on pasture from May to September/October. In some parts of the country, common pastures are abundant. This includes the more densely populated areas in the Southern part of the country (average cattle density » 100 head per km2) where most of the above-mentioned averages are higher.
At the start of the BVD scheme in 1993 there were over 18,000 dairy herds and around 17,000 beef herds. Of these, more than 50% had experienced BVDV infection recently or had ongoing infection. Today the number of herds are 11,000 and 14,000 respectively. The number of cattle is approximately the same because the average herd size has increased.
Denmark who is also close to eradicating BVDV has got an even more intense dairy production. In other words, there is no reason why this can’t be done in other countries too. So stop wasting money! You don’t have to live with BVDV.
Lindberg, A. L. E. & Alenius, S. (1999) Principles for eradication of bovine viral diarrhoea virus (BVDV) in cattle populations. Veterinary Microbiology 64, 197-222
The National Animal Disease Center