Bluetongue virus
What is bluetongue disease?
Bluetongue disease is an insect-borne viral disease of ruminant animals caused by bluetongue virus (BTV).
Clinical signs can include inflammation of the mucous membranes, swelling (oedema) and bleeding (haemorrhages). Signs can vary in severity.
Of the 27 known serotypes of bluetongue virus, 13 have been detected in Australia. The highly pathogenic strains associated with some overseas outbreaks are exotic (not naturally present) in Australia.
BTV does not persist in animal carcasses, animal products, outside the host or vector, on equipment, on personnel or in the environment.
The distribution of BTV in Australia is monitored through the National Arbovirus Monitoring Program (NAMP). The program uses insect trapping and regular testing of cattle herds across the country to track BTV activity.
What species are affected?
All ruminants are susceptible, including:
- sheep
- goats
- cattle
- camelids
- buffalo
- deer.
Sheep are often the most severely affected species, with breeds such as Merinos and British breeds being most susceptible. Sheep (and sometimes deer) are usually the only species that develop clinical signs – and the disease can be fatal.
Cattle can carry the virus without showing any obvious signs of illness. Viraemic (infected) cattle provide a reservoir for the virus to be spread by biting midges.
How is the virus spread?
The virus is transmitted to ruminant animals by biting midges (Culicoides midges).
The biting midge Culicoides brevitarsis is considered the main carrier of BTV in Australia. Other Culicoides species, including C. wadai, C. fulvus and C. actoni, can also spread the virus. The distribution of BTV in Australia largely depends on where these insects occur. These species are not known to be present in Victoria.
Signs of disease occur 4 to 8 days after being bitten by an infected midge and are mainly seen in sheep and deer. As cattle don't usually exhibit any signs of infection, but can amplify the virus, disease may not be detected until sheep (or deer) are infected.
Infected animals may remain infective (viraemic) for up to 60 days. An insect vector spreads the virus by biting a viraemic animal and then transferring the virus to other animals it feeds on.
Can the virus spread to humans?
No, BTV cannot infect people.
There are no food safety issues, and meat and dairy products are safe to consume.
What are the clinical signs of BTV?
Clinical signs include:
- fever (40 °C to 42 °C)
- swelling of the lips, tongue and head
- difficulty breathing
- excessive salivation and initially clear nasal discharge, turning thick with mucus and pus
- profuse diarrhoea
- lesions on the feet, with possible reddening of the coronary band (top of the hoof) resulting in lameness
- rapid weight loss and drop in production (abortions and wool break in sheep)
- death – mortality rates in sheep of 30% to 40% are common but can be as high as 70%.
Some animals may show signs of a swollen, bluish-coloured tongue (hence the name ‘bluetongue’). This will not always be seen and should not be relied on to diagnose bluetongue disease.
What do I do if I suspect BTV?
BTV is a notifiable exotic disease. Any suspected cases must be reported immediately to Agriculture Victoria on the Emergency Animal Disease Hotline on 1800 675 888 (24/7) or to your local Agriculture Victoria animal health staff.
How can I reduce the risk of transmission of BTV?
BTV is spread through biting insects or management practices that involve penetrating the skin. Examples include vaccination and blood collection.
Infected animals should be kept isolated.
Insecticide can be applied to reduce insect numbers and minimise further spread of the virus.
Avoid sharing needles between animals and thoroughly clean and disinfect equipment between uses.
BTV does not infect or persist on people, so there are no specific PPE requirements. However, good hygiene practices are recommended when handling animals.
It is also good practice to change your clothing and wash your footwear before handling animals on a different property. This helps to reduce the risk of disease spread between properties.
Monitoring the distribution of BTV in Australia
Some serotypes of BTV are endemic (always present) in northern and eastern Australia, so national freedom from BTV is not possible.
Until recently, clinical disease had only been reported in Australia in 2 minor incidents involving sheep in Darwin in 1989 and 2001.
Clinical disease has more recently (between 2022 and 2024) been reported in sheep in Queensland and NSW. This is the first time clinical bluetongue has been reported in commercial sheep flocks in Australia. Australia has recognised BTV zones.
The distribution of BTV is regularly monitored through the National Arbovirus Monitoring Program (NAMP). This national surveillance program tracks where BTV occurs and identifies the virus serotypes of BTV that are circulating across Australia.
Victoria is classified as a BTV transmission-free zone. A map showing the current BTV transmission zones is available on the Animal Health Australia website.
The impact of BTV on international markets
Export of Australian livestock provides significant economic benefits to Victoria’s livestock industries. Victoria’s reputation for high health status livestock plays a key role in accessing many of these markets.
Some international countries only source livestock from BTV-free zones.
Export companies can advise producers about eligibility requirements for specific international markets.
What is the government’s response to BTV?
The government’s response to an outbreak of BTV will aim to eliminate clinical disease and minimise economic impacts. A combination of strategies can be deployed, which may include:
- quarantine and movement controls for ruminant livestock in declared areas
- treatment and husbandry procedures to control insects and minimise health, production and animal welfare impacts
- tracing and surveillance to determine the source and extent of infection.