Pestivirus (Bovine Viral Diarrhoea Virus - BVDV)
This disease is widespread throughout the Australian cattle population (about 70% of herds are actively infected with the virus) and can cause significant economic loss. It causes early-term abortions or embryonic loss, temporary infertility, ill-thrift in young, increased susceptibility to other diseases, diarrhoea and respiratory disease to name a few. Infection may not become apparent until well after the initial transmission of the virus.
Spread and persistence of Pestivirus
When a naive animal encounters pestivirus for the first time, it is susceptible to the infection. The immune system responds, producing antibodies and clears the virus. Under normal conditions and in non-pregnant animals, this viral spread is usually unnoticed as signs of infection (if any) are mild. The animals infected and cleared of the virus are now immune.
If a naive heifer/cow is infected during the first 3-4 months of gestation, the calf is born as a “persistently infected (PI)” animal and is a carrier of the virus for life. This is because the foetus is too immature at that stage to recognize the virus as a virus, it simply accepts the virus as a normal part of itself. The virus is multiplied in the tissues of the PI calf while they live with no defence being mounted against it, making them spreaders of the virus. PIs generally have a reduced life expectancy and may be small at birth too.
If infection occurs after 4 months of gestation, the foetus is able to produce antibodies, so does not become a PI. However, it may still develop other abnormalities due to the infection. The heifer/cow develops immunity normally and eliminates the virus from her own tissues as per normal, regardless of gestational stage.
PIs are the key to the virus’s survival in the herd. Pestivirus is shed in all body secretions of a PI animal (saliva, tears, nasal discharge, milk, urine, semen and faces). Transmission is via direct or close contact with a carrier animal or their secretions. Pestivirus itself dies out quickly with no close contact as it is destroyed by sunlight and high temperatures, usually in less than one day.
Diseases due to Pestivirus
Severe reproductive loss — losses can occur if animals are exposed to the virus for the first time from 2 weeks before joining up to about 7 months of gestation. Depending on the stage of pregnancy and the strain of virus heifers/cows may fail to conceive, reabsorb a very early pregnancy (and then return to service), abort, produce a stillborn, produce an abnormal, deformed calf, produce a PI or an apparently normal, but small, stunted calf.
Ill thrift and diarrhoea (mostly seen in PIs) — stress of weaning usually precipitates ill health in carrier calves. They are noticeably ill thrifty, scruffy and stunted, often with a chronic scour or intermittent lameness, tend to get affected by worms more severely than the rest of the mob and are more susceptible to any diseases. Think of the animal that has just “never done any good”. However, not all carriers are recognised at this point, some remain apparently normal into adulthood.
Mucosal disease — a PI animal that gets infected by another, more damaging strain of Pestivirus. The animal cannot recognise the virus as foreign, so the virus has no influence from the body against it. This allows the development of a fatal condition – mucosal disease. Signs include profuse, watery diarrhoea, unresponsive to drenching, chronic weight loss, ulcers in the mouth, gums, dental pad and muzzle, excessive salivation and nasal discharge. Sometimes the animal shows brief periods of improved health, but death usually occurs from one week to months after the onset of symptoms.
Respiratory disease — the combination of intense management, stress and Pestivirus results in respiratory disease. Pestivirus also lowers the immune system, allowing other viruses and bacterial infections to create a more severe respiratory disease. The respiratory disease can occur during weaning, transport, mixing of stock or entering a feedlot. There is always a greater likelihood of exposure to multiple diseases when stressed animals from several different sources are mixed.
Suppression of the animal’s immune system — leading to increased risks of other diseases.
Severe bleeding disorders and rapid death — not observed in Australia
Diagnosis of Pestivirus
Testing is recommended in suspect animals or herds with poor pregnancy rates as part of disease investigation. It is easily done with either an ear notch or blood test.
(Note these are simply general suggestions. For property-specific management plans, please contact the clinic).
Control for the positive herd:
- ID all carriers in the herd and dispose of them
This is done by testing every animal in the herd and all calves born in 9 months following the test
The virus may be reintroduced from an outside source, so this option is usually not recommended on its own (a good one to use in conjunction with vaccination)
- Manage the herd to maximize infection of young heifers well before joining
This early exposure pre-joining allows the heifers to get infected, create antibodies against the disease and therefore become immune (while not affecting pregnancies)
Done by running heifers with a carrier (if you have one identified)
Steers that may enter feedlots should be exposed too
Given the highly infectious nature of the virus, all animals in herds that are yarded and handled frequently are likely to be infected if a carrier animal is present
If in doubt of immunity, blood tests of a proportion of heifers pre-joining can be done to check if they are antibody positive (therefore have been exposed)
- Commence a regular vaccination program against Pestivirus
If you decide to vaccinate, the program must be ongoing (as discontinuation after a short time would be a disaster, as all-natural immunity due to ongoing exposure will be lost – making the whole herd susceptible to infection)
Ideally the whole herd would be vaccinated
2 doses of vaccine are required at an interval of at least 4 weeks (but up to 6 months is possible)
The full vaccination program should be completed at least 2 weeks prior to joining, or 2-4 weeks before stock meet animals from other sources
Control for the apparently negative herd:
If pestivirus has not been recognised, the virus could still be present and widely spread (meaning there is a high level of immunity). But often, the PI animals die out, and leaves susceptible breeders present.
Some closed herds will not have been exposed. Maintaining a closed herd would be the best way to negate the risk of any introductions of the virus. Otherwise testing animals before arrival and investigating any suspicious animals as soon as possible. Vaccination may be a good option here too, as a closed herd will have no natural immunity, so any introduction of the virus could be catastrophic.
There is very likely a much wider range of Pestivirus animals than we appreciate. It is possible to manage the disease and still have a high level of production. Give the clinic a call if you would like to discuss it further.