BVD is a viral disease associated with two distinct disease syndromes, known respectively as Bovine Viral Diarrhoea and Mucosal Disease. It is also associated with non-specific disease, because the virus can suppress the immune response, allowing other diseases to strike, or exacerbating the effect of existing disease. The virus is called Bovine Viral Diarrhoea Virus (BVDv). BVDv’s effects on the infected animal depend upon the age at which the animal is infected - whether that is as a foetus, a calf or an adult.
There is a high prevalence of BVD antibodies in the national dairy herd (up to 95%)1 hence a majority of cows will have BVD antibodies in their milk. If the calf takes in colostrum, it will absorb these BVD antibodies. Such antibodies, because they come from the calf’s mother, are known as Maternally Derived Antibodies (MDA), and these may protect the calf against BVDv for up to four to six months. However this protection is not absolute. If a calf becomes infected, this can give rise to IMMUNOSUPPRESSION, decreasing the calf’s ability to deal with a variety of infections, so the calf is more likely to develop pneumonia, scours and other diseases. Of course, if a calf is born to a cow that does not have colostral antibodies against BVD, it will be completely vulnerable to infection from the moment of birth.
Once any MDA have waned, or if a calf didn’t get BVD antibodies, the calf is open to BVDv infection. The most common source of BVDv infection is from Persistently Infected (PI) cattle (discussed later), which constantly shed BVDv. However, acutely infected calves may also shed BVD virus for a short while after infection.
If the calf encounters BVDv at this age this may result in acute BVD infection, which causes a range of clinical signs:
Infection is often mild and may even be sub-clinical, but the most important thing to remember is that this mild disease can cause IMMUNOSUPPRESSION, decreasing the calf’s ability to deal with a variety of infections. Hence infection with BVDv increases the incidence and severity of other diseases. The immunosuppressive effect reaches a peak about seven days after infection.
Because it is immunosuppressive, BVDv can also adversely affect the response to vaccination against other diseases.
Because BVD is caused by a virus, antibiotics are ineffective. Treatment for acute BVDv infection can only be supportive – it may include antibacterials to control secondary bacterial infections, anti-inflammatories and fluids.
Non-pregnant cows can develop the same mild or subclinical disease that calves and younger cattle can get. In addition, in the early 1990s, a severe form of BVDv infection was reported in dairy herds in the UK. Signs included acute diarrhoea, pyrexia (fever) and decreased milk production, sometimes with oral ulceration. If infection occurs shortly before service in breeding animals, there may be an adverse effect on fertility (see below).
If cattle are pregnant when infected, the outcome is a lot more serious, because BVDv can spread via the placenta to the embryo/foetus, to cause foetal infection.
0-60 days In early pregnancy, infection can lead to failure of conception or early embryonic death. The cow will return to oestrus but fertility is adversely affected.
45-145 days The foetus is developing its own immune system, so it cannot recognise foreign protein such as BVDv and therefore does not produce antibodies to it. Infection in these foetuses may lead to foetal death and abortion. Alternatively, the calf may survive but mistakenly classify the BVDv as “self” and become "immunotolerant", i.e. treat the virus as though it is a normal part of the calf’s own body. This leads to the birth of a Persistently Infected (PI) calf that will never develop immunity to BVDv and, as the name suggests, constantly sheds large quantities of BVDv. PI animals shed the virus throughout their life and are the primary source of new infections within a herd.
90-120 days The foetus now has a functioning immune system, so it can make antibodies to BVDv and get rid of infection. In mid-pregnancy, foetal infection may result in congenital abnormalities.
120 days – Birth Abortion may occur following infection at any stage of pregnancy, but may not happen for up to 50 days following initial infection.
Foetuses infected before 150 days of gestation may die, or be born with birth defects, or be born as PIs; over 150 days most calves develop an antibody response, and will often be born healthy. But even though calves that were infected with BVDv in late pregnancy do not become PIs and appear healthy at birth, they are twice as vulnerable to the various calfhood diseases when compared with uninfected calves.
If not diagnosed and removed from the herd, PI calves will provide a source of BVDv that can cause acute infection in normal calves, or infection in pregnant animals (see above). Many PI calves will fail to thrive, and will appear as “runts” when compared with their peers. Unfortunately, others will appear reasonably healthy, surviving in the herd and constantly shedding BVDv.
Those few PI females that survive to breeding age are always sub-fertile, but they may not be completely infertile. Because BVDv is always circulating in their system during the crucial 45 to 120 day period during which the foetus’ immune system is developing, PI cows will only ever produce PI offspring, resulting in entire PI family lines, which can maintain BVDv within a herd.
As with PI females, PI males may also survive to breeding age. Testing at AI Centres ensures that they don’t enter the AI system, but PI bulls may be used for natural service. Because they always infect the cows at the time of service, they lead to a high rate of early embryonic death and a loss of fertility.
In contrast to acute BVD infection, which is rarely fatal, Mucosal Disease has a case fatality rate of 100% - in other words, affected cattle invariably die of the disease.
The starting point for Mucosal Disease is a PI calf, which is already infected with one of the more common strains of BVDv. Mucosal disease results from superinfection of a PI calf with a cytopathic strain of BVDv. Mucosal Disease usually affects PI cattle between six months and two years of age and inevitably ends in death. Very acute cases will die in five to seven days, but some cases can live for up to three months.
Clinical Signs of Acute Mucosal Disease:
The easiest way to monitor infection in an unvaccinated herd is by analysing bulk milk antibody level, which reflects the numbers of cattle that have been exposed to BVDv. The bulk milk can also be tested for the presence of BVDv: if it is positive, it indicates either that BVDv was actively circulating among the cows on the day of sampling, or that there is a PI among the dairy cows.
BVD-specific vaccines are available to prevent foetal infection. BVD vaccines that offer foetal protection are used especially in dairy herds, because the effects of infection (abortions/ infertility) are more readily monitored in those cattle. Vaccinating adults improves fertility and prevents PI calves being born. However, if cows are already PIs, then vaccinating them will not cure them and they will continue to produce PI calves. BVDv from these PIs infects other calves, causing disease and/or immunosuppression. Also, bought-in replacements could be PIs, although these can be tested for BVD antibodies and/or the presence of the BVD virus. In the worst case of all, a bought-in pregnant heifer may be free of infection, but may be carrying a PI calf, which cannot be detected by testing. Adequate vaccination helps to protect the rest of the herd in these instances.
The ideal vaccine should be:
As BVDv is recognised as a significant factor in respiratory disease, calves should be vaccinated against BVDv to prevent acute BVD infection and the resultant immunosuppression. Rispoval 3 is the only respiratory vaccine available that protects against three key viruses (BVDv, RSV, PI3) associated with pneumonia in cattle. Given the high incidence of BVD in the Irish cattle population, this is relevant to both homebred and bought-in cattle.
Primary vaccination:
Two 2ml doses should be administered subcutaneously three weeks apart. To provide foetal protection from first day of conception, the vaccination regime should be completed at least 14 days before breeding or insemination.
Booster vaccination:
A single booster injection 12 months after the primary vaccination course has been shown to stimulate an immune response similar to that of the primary immunisation
1. Graham, D et al (2001) Vet Record 149:261-265
2. Pfizer study 9131 c-03-00-176
3. Harmeyer, S et al (Research in Veterinary Science 78 (2005) supplement A :15
4. Pfizer study 9131 c-50-01-182