|BVDV can resemble emergency diseases|
Certain emergency diseases not present in Australia have similar signs to BVDV infection in cattle. If any of these diseases became established here, market access for animals and animal products could be severely impacted. Early diagnosis of exotic disease is vital to allow rapid eradication and re-establishment of market access.
If you see any unusual signs of disease, abnormal behaviour or unexpected deaths in stock, including signs that look like BVDV, call your local vet, a Department of Priimary Industries and Regional Development (DPIRD) vet or the Emergency Animal Disease hotline on 1800 675 888.
What is BVDV?
Bovine viral diarrhoea virus (BVDV) in cattle is a complex disease that is caused by bovine pestivirus. Bovine pestivirus and its resulting diseases have several interchangeable names including bovine viral diarrhoea (BVD), pestivirus and bovine pestivirus. Mucosal disease is the fatal disease that develops in animals persistantly infected with BVDV.
Two types of BVDV occur across the world, but only BVDV Type 1 (BVDV-1) is present in Australia. The severe BVDV Type 2 (BVDV-2) that is found in Europe and North America has not been diagnosed in Australia. Western Australia's animal health surveillance system helps to show our trading partners that our cattle continue to be free from BVDV-2 and are fit to trade.
BVDV can reduce herd reproductive rates through infertility or abortion, as well as reduce animals immunity to a range of other diseases such as bovine respiratory disease (BRD). BVDV can also infect other ruminants such as deer or goats, but they rarely show signs of the disease.
BVDV is common in Australia and Meat and Livestock Australia (MLA) have identified BVDV as the second most costly disease to Australian cattle herds, after cattle tick infection, and the most important cattle disease southern cattle herds. It is estimated to have an economic impact of $114 million per year.
How does BVDV spread?
Persistently infected (PI) animals discharge large quantities of virus in saliva, urine, respiratory secretions, uterine discharge, milk, semen and faeces and aborted foetuses. BVDV almost always spreads by direct contact with PI cattle through introduction, mixing at a show, sale, feedlot or through a fence.
When a PI animal is yarded with naïve cattle, such as in a feedlot, the virus can spread within days. In paddocks, the virus may take months to infect all cattle. Naïve cattle are animals that have been never exposed, or vaccinated, to the virus previously.
Other cattle that are not PI and infected after birth rarely produce enough virus to infect other naive cattle except during abortion.
What happens when BVDV infection occurs?
The effect of BVDV depends on when the infection occurs in a cow's life. The effect is different in unborn calves, pregnant cows, feedlot or other cattle.
Most of the harm done by BVDV is to unborn calves and depends upon the timing of infection.
- Infection of a naïve cow in early pregnancy can cause a loss of pregnancy.
- Infection of naïve cows in mid-pregnancy can cause abortions, birth defects and live-born calves to be persistently infected with BVDV. These calves spread the disease within and between herds. Most of these calves die within two years due to mucosal disease.
- By the last third of pregnancy, the calf has developed sufficiently to produce immunity. Some of these calves may be aborted, but most are born healthy.
Infection of a naïve cow results in a mild 2–3 week illness that suppresses the cow’s immune system and reduces resistance to other diseases. Most important are the effects on her unborn calf noted above. After recovery, these cows are immune to BVDV.
If a PI animal is introduced to a feedlot, infection will spread to all naive cattle in that pen and adjoining pens within days. It will rarely spread further because usually only PI animals shed an infective dose.
The exposed naïve cattle will develop mild illness and suppressed immunity. As this coincides with transport and social stress, diet change and challenge by other diseases, the health impact of BVDV in affected pens can be significant and often is a significant cause of bovine respiratory disease (BRD).
All other cattle
Any other cattle exposed to BVDV will also experience a 2–3 week illness followed by lasting immunity. Exposing non-pregnant cattle to PI animals can stimulate immunity and protect future pregnancies in those cattle.
What is mucosal disease?
Mucosal disease only occurs in cattle persistently infected with BVDV Type 1 when they become infected with a more virulent strain of BVDV. Animals infected with mucosal disease die.
In feedlots, PI animals under stress can develop mucosal disease as they mix with other PI animals.
Mucosal disease is usually seen in cattle from 6–21 months of age.
Signs of mucosal disease
The signs of mucosal disease include:
- fever and lethargy
- ulcers and drooling from the mouth and nose
- persistent, profuse, watery diarrhoea that may be bloodstained or contain mucus. It does not respond to drenching.
- ulcers of the vulva and skin between the claws of all four feet.
There is no treatment for mucosal disease. Animals diagnosed with mucosal disease should be humanely euthanased as soon as possible.
How is BVDV diagnosed?
There are several laboratory tests available to diagnose BVDV depending on the situation. Your vet can discuss an appropriate testing and management plan for BVDV in your herd.
Samples for testing include blood samples, which can be tested for evidence of past virus exposure and persistent infection. Body tissues, such as ear notches or post-mortem samples can be tested for persistent infection. Bulk milk samples can also be tested in dairy herds.
Managing the BVDV risk
Seek veterinary advice to develop a whole farm plan to manage the risk of BVDV. There is no ‘one size fits all’ approach.
An effective risk management plan should include:
- establishing the immune status of your herd
- establishing the likelihood of introduction of BVDV
- managing cattle movements and introductions
- attention to farm biosecurity, particularly fencing
- identifying PI animals in the herd
- managing exposure to known PI animals
- vaccination of vulnerable groups such as heifers joining the breeding herd.
Managing the BVDV risk in feedlots
In backgrounding operations, consider vaccination for BVDV, and other diseases, in consultation with the feedlot operator and your vet.
Give cattle the full vaccination program and time to develop immunity after the final injection. Details can be found on the product label.
Vaccinating PI cattle has no effect and these cattle will continue to shed virus.
Testing to identify PI animals before they enter the feedlot is possible but practical and financial constraints mean that it is rarely used to select feedlot cattle.
The immune suppression caused by active BVDV infection makes it likely that respiratory disease will occur when naive cattle come in contact with a PI animal in the feedlot. Production losses can be high when BVDV infection passes through feedlots.
Risk management is crucial when mixing cattle of unknown disease or vaccination status, as any mixing has the potential to expose naive cattle to a PI animal. It is recommended to:
- pen cattle of common background together
- assume that mobs of unknown background include naïve animals
- assume that mobs of unknown background contain some persistently infected animals
- create separation between cattle pens
- consider location of unoccupied pens and double fencing.
Managing the BVDV risk in extensive areas
In extensive areas, the risk of contact with PI animals is lower due to the large areas that cattle graze. Areas of congregation such as watering points can produce naive or immune groups within a herd due to contact with PI animals.
It is recommended that you work with your vet to develop a risk management plan.