BVD is probably the most important viral disease of the New Zealand cattle industry. An estimated 90% of New Zealand dairy farms have been exposed to the virus, and BVD costs the dairy industry approximately $50 million per year.
BVD is a common but complex disease. The costs are often hidden and difficult to calculate and the effects of the disease often insidious.
If you are living with BVD, you are probably accepting poor performance as normal.
The BVD virus may be transmitted between animals in one of three ways:
The consequences of viral infection differ, depending on which classes of animals are infected:
Infection of youngstock, bulls, or cows.
Infection is temporary, death rates are low and animals recover from infection after an average of 2 weeks with subsequent immunity to the virus. There may be no signs that an animal is infected at all, but when clinical signs are present they can include diarrhoea, reduced weight gain, reduced appetite, rough coat, immunosuppression with increased susceptibility to other diseases such as pneumonia, temporary infertility (cows and bulls), and reduced milk yield (cows). BVD in youngstock can appear similar to gastrointestinal parasitism.
Infection of cows in the breeding season.
The virus can have particularly devastating effects when it infects cows that have not acquired immunity, either by vaccination or by natural exposure. Clinical signs depend on the stage of gestation of the cow when she encounters the infection and are listed below:
When an animal is infected with the BVD virus generally it will mount a response, clear the virus within 2 weeks and become immune to it whether or not it shows any of the clinical signs listed above. The length of this immunity is variable. This is known as ‘transient’ infection.
However, when unborn calves are infected in the uterus, between 45 and 180 days gestation as described above, they may be born with a persistent BVD infection. Because they are infected before their immune system is mature they perceive the virus as ‘self’ and become tolerant to it. They never get rid of the virus or develop immunity and they act as ‘virus factories’, shedding huge amounts of virus (from all their body fluids, but especially from their nasal secretions) all throughout their lives.
PIs are much more efficient at spreading BVD virus than transiently infected animals—most BVD is spread within and between herds by PI carriers. Any non-immune animal that comes into contact with a PI is at risk of contracting the infection.
The Persistently Infected or ‘PI’ Animal.
PIs are thought to make up about 1% of the New Zealand cattle population. PIs may die before birth. If they survive they are often unthrifty, poorly grown, and stunted with an increased susceptibility to other diseases such as pneumonia and parasitism.
Sometimes the BVD virus in a PI will mutate. This causes a severe disease in the PI known as mucosal disease. Signs include severe mouth and gastrointestinal ulcers, nasal and eye discharge, weight loss, profuse diarrhoea and eventual death. Mucosal disease is always fatal and only occurs in PIs, usually at 6-24 months of age.
There is no cure for PIs and once identified they should be culled immediately before they continue to spread the virus to other cattle. It is important to remember that sometimes a PI will look perfectly normal and may survive long enough to breed itself. A PI cow will always give birth to a PI calf. A PI bull can have disastrous consequences on fertility in a naïve herd. Bulls must be tested to make sure they are not PIs. They only need to be tested once in their lifetime.
PIs should be identified and removed from the herd.
Behaviour of BVD in the Herd.
BVD behaves differently in dairy and beef herds due to their different natures.
In dairy herds the disease waxes and wanes. A PI carrier is born and the herd’s immunity declines as the calf is raised away from the main herd. When that PI rejoins the now naïve herd as a replacement, widespread BVD infection occurs. The pattern is a regular cycle of infection every few years.
In beef herds younger and older animals are kept together, and so PI carriers remain in constant contact with all age groups of the breeding herd — cows, replacements and bulls. The disease remains active and spreading all the time and losses are ongoing.
Individual cows can be blood tested (or tissue tested) to see if they have been exposed to the virus and have mounted an immune response (antibody test), or to see whether the virus is present (antigen test). Animals that have been exposed to the virus and that have recovered will be antibody positive. PIs will usually be antibody negative due to their lack of immune response, but antigen positive due to their inability to clear the virus. Transiently infected animals may be antigen positive due to temporarily circulating virus, and should be retested in 4 weeks, by which time they should be antigen negative but antibody positive, thus distinguishing them from the PIs. Blood samples can be pooled which makes testing more economical.
Note that if a pregnant bought-in cow tests negative for antigen, it is still necessary to test her calf—she may be carrying a PI calf while being antigen negative herself—the ‘Trojan Cow’.
Bulk milk samples can be tested for antibody to assess the level of exposure of the herd to the BVD virus, or for antigen, to see if a PI is present in the herd (N.B. for this to be reliable, milk from every cow must be going into the vat).
Options for Control.
There is no treatment for BVD, but control is vital. There are a number of options for controlling the virus, ranging from very poor to very good, as listed below.
Heifers and bought-in replacements are vaccinated prior to their entry to the main herd. The primary vaccine course for all animals is two injections three weeks apart. Heifers should be older than three months when first vaccinated and should have their booster at least two weeks before their first service. After this, annual boosters are required to maintain immunity.
The main herd has an annual booster vaccine several weeks before the planned start of mating. If the main herd is being vaccinated for the very first time, two injections, three weeks apart, will be required. The second injection should be given at least two weeks before the planned start of mating.
Bulls must not be forgotten. They are prime candidates for bringing BVD onto a farm. They must be blood tested to check if they are PIs, and should have a certificate to prove this. If proven negative, they should be vaccinated as above and their immunity maintained by annual vaccine boosters.