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Bluetongue

Alert

Bluetongue is category 1 restricted matter and prohibited matter (clinical disease).

Under Queensland legislation, if you suspect the presence of this disease in any species of animal, you must report it to Biosecurity Queensland on 13 25 23 or contact the Emergency Animal Disease Watch Hotline on 1800 675 888.

Bluetongue is an insect-borne viral disease affecting domestic and feral ruminants. Sheep are the most severely affected of the domestic species. Some breeds of sheep appear more susceptible to clinical disease than others, and these differences present challenges for surveillance activities.

The wide global distribution of bluetongue is directly linked with habitats supporting the presence of competent insect vectors capable of transmitting the disease. While the severity of the disease can vary with location and serotype, bluetongue virus (BTV) activity has been found on all continents except Antarctica.

Although bluetongue viruses are present in northern and eastern Australia, clinical disease has never been reported in cattle and is rarely observed in sheep.

The outcome of infection varies from inapparent (in most animals) to fatal in a proportion of susceptible species. Infection causes damage to small blood vessels and this forms the basis of presenting clinical signs.

Scientific name

Bluetongue

Cause

BTV belongs to the genus Orbivirus in the family Reoviridae. Orbiviruses are arboviruses, being able to replicate in both their arthropod vectors and their vertebrate hosts.

The insect hosts for the bluetongue virus are a few species of biting midges within the genus Culicoides. BTV is transmitted between susceptible ruminants by the insect vectors feeding on viraemic animals and subsequently spreading the infection.

The BTV serogroup contains 26 serotypes and 13 of these have been isolated in Australia.

Description

Disease caused by infection with bluetongue virus has 2 main manifestations: an acute form and an inapparent (or subclinical) form (see Symptoms below). The virulence of different strains of bluetongue virus varies significantly. The outcome of infection depends on an interplay between the virus (serotype and genotype), the host and the environment.

The acute form is more likely to occur in sheep and some species of deer. Infection in cattle is usually subclinical but has great epidemiological importance in constructing surveillance programs.

Experimental infection of sheep using 10 of the Australian serotypes have demonstrated variable pathogenicity. The highly pathogenic strains found in some overseas countries are exotic to Australia.

There are no public health risks associated with bluetongue.

Distribution

Endemic Australian serotypes are distributed throughout much of the Northern Territory and Queensland, and in northern areas of Western Australia. As of September 2020, its southern distribution extended down the east coast and adjacent hinterland areas of New South Wales, as far as Batemans Bay.

Retrospective serological studies have indicated that bluetongue virus has been present in Australia since at least 1958. The first bluetongue virus to be isolated in Australia (BTV serotype 20) was from a mixed species pool of Culicoides trapped at Beatrice Hill, near Darwin, in 1975. Since then, 13 serotypes have been shown to be present in Australia; these serotypes include: 1, 2, 3, 4, 5, 7, 9, 12, 15, 16, 20, 21 and 23. The highly pathogenic strains found in some overseas countries are exotic to Australia. C brevitarsis is the most common and widely distributed vector species in Australia and its distribution correlates with that of bluetongue viruses.

The distribution of BTV in Australia has been monitored through the National Arbovirus Monitoring Program since 1994. The program establishes mapped zones of BTV transmission and freedom. The maps displaying BTV zoning within Australia can be accessed at the Animal Health Australia website.

In recent years the virus has been detected in northern European countries, including England, and has adapted to new Culicoides sp. vectors capable of over-wintering in temperate climates.

Hosts

The bluetongue virus replicates in both its insect and vertebrate hosts.

  • The vertebrate hosts consist of domestic and feral ruminants.
  • The arthropod hosts are species within the genus Culicoides.

Life cycle

Bluetongue is a non-contagious disease of domestic and wild ruminants transmitted by Culicoides biting midges. The number of species in the genus Culicoides is large but the number that are competent to transmit the virus are relatively few. The ability of Culicoides to transmit the virus is influenced by ambient temperature, humidity and seasonal rainfall.

C brevitarsis is an important insect vector in Australia as its distribution determines the southern boundaries of the distribution of BTV. In doing so, it influences the likelihood of exposure of sheep, the vector-free zone and trade negotiations.

Affected animals

  • sheep
  • goats
  • deer
  • cattle

Symptoms

Disease caused by infection with bluetongue virus has 2 main manifestations: an acute form and an inapparent (or subclinical) form.

Acute form (usually in sheep and some species of deer):

  • Fever up to 42°C, excessive salivation, depression, dyspnoea and panting.
  • An initially clear nasal discharge becomes mucopurulent and can form crusts.
  • Oedema, stemming from hyperaemia and congestion, can affect the muzzle, lips, face, eyelids and ears.
  • The coronary bands of the hooves are hyperaemic, with coronitis leading to lameness.
  • The skin of the groin, perineum and axilla is hyperaemic.
  • Torticollis is seen in some cases.
  • Abortion or birth of deformed lambs.
  • The clinical feature that gives the disease its name, a cyanotic tongue, occurs in only a small percentage of cases.
  • Emaciation.
  • Death can occur within 8 to 10 days or the animal can undergo a prolonged period of recovery.

Inapparent infection:

  • This is the usual course in cattle.
  • Subacute and mild disease can occur in sheep with the above clinical features appearing to varying degrees.
  • Subclinical infection can also occur in sheep.

Impacts

Bluetongue is category 1 restricted matter and prohibited matter (clinical disease).

Although bluetongue viruses are present in northern and eastern Australia, clinical disease has never been reported in cattle and is rarely observed in sheep. The highly pathogenic strains found in some overseas countries are exotic to Australia.

Bluetongue virus continues to cause significant interference with export trade in live animals and animal products. Some trading partners will not accept live animals from Australia unless they can be shown to be from an area that is BTV and or insect vector free.

How it is spread

Bluetongue is a non-contagious disease of domestic and feral ruminants transmitted by Culicoides biting midges.

The virus can be introduced to BTV-free regions by movement of infected viraemic animals, incursion of insect activity during favourable seasons and potentially by carriage of infected insects in wind currents.

Animal carcasses and products, such as meat and wool, are not a method of spread.

Risk period

The insect vectors—biting midges—prefer warm, moist conditions, and are in their greatest numbers and most active after rains.

Bluetongue virus does not survive outside the insect vectors or susceptible hosts.

The survival of the virus from one season of vector activity to the next is called ‘overwintering’. The mechanisms involved in this process are poorly understood.

Monitoring and action

The distribution of BTV in Australia has been monitored through the National Arbovirus Monitoring Program since 1994. The program establishes mapped zones of BTV virus transmission and freedom. The maps displaying BTV zoning within Australia can be accessed at the Animal Health Australia website.

Control

In countries where bluetongue outbreaks occur, the strategy is to contain the outbreak and minimise trade impact by:

  • using movement controls to prevent spread
  • using treatments and husbandry procedures to control vectors, reduce transmission and protect susceptible animals
  • tracing and surveillance to determine the extent of virus and vector distribution
  • zoning to define infected and disease-free areas.

In Australia, national freedom from BTV is not possible. Through the National Arbovirus Monitoring Program (NAMP) Australia has internationally recognised BTV zoning and monitoring of vector activity. An emergency response may be required if clinical disease was detected in Australia but would depend on the species involved and the zone in which the disease appeared

There is no justification for stamping out, but some animals may need to be destroyed for welfare reasons. It is not possible to eradicate the bluetongue vectors.

Vaccination

  • Inactivated and live-attenuated vaccines are available in other countries but are not used in Australia.
  • Vaccines are serotype specific and need to be matched to the virus causing disease.

Further information

This information was compiled from the following reference documents: