Bluetongue

Bluetongue virus (BTV) belongs to the genus Orbivirus in the family Reoviridae.

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

Bluetongue virus has been found on all continents except Antarctica. It occurs in areas where temperature, humidity and rainfall support insect vectors that can transmit the virus. The serotypes that are present vary with location.

The distribution of BTV in Australia correlates with the most common and widely distributed vector species, Culicoides brevitarsis. This includes much of the Northern Territory and Queensland, and northern areas of Western Australia. The interactive Bluetongue Virus Zone Map shows its southern distribution extending down the east coast and adjacent hinterland areas of New South Wales.

Serological studies indicate BTV has been present in Australia since at least 1958. Thirteen serotypes have been detected in Australia; serotypes 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.

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.

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

• ruminants, including sheep, goats and deer and cattle
• Culicoides biting midges

Bluetongue is a non-contagious disease transmitted by Culicoides – biting midges. The virus replicates in both insect and ruminant hosts.

The genus Culicoides includes a large number of species, however, only a small number are able to transmit BTV.

The distribution of C brevitarsis determines the southern distribution boundary of BTV, making it the most important insect vector in Australia.

The survival of the virus from one season of vector activity to the next is called 'overwintering'. The mechanisms that enable BTV to overwinter are poorly understood.

BTV is usually transmitted between susceptible ruminants by insect vectors feeding on viraemic animals and subsequently spreading the infection.

It can also be transmitted:

• in semen from infected bulls and rams to susceptible cows and ewes; this is not a significant means of transmission
• through the placenta to the foetus.

Infection causes damage to small blood vessels and this forms the basis of the clinical signs.

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

Recovered animals:

• clear the virus
• are immune to reinfection by the same serotype
• are not carriers
• remain susceptible to other serotypes.

Subclinical infection in cattle is epidemiologically important and is a significant factor for the design of surveillance programs.

Disease caused by infection with BTV has 2 main forms: an acute form and an inapparent (or subclinical) form.

The severity of disease caused by of different strains of bluetongue virus vary significantly. The outcome of infection depends on an interplay between the virus (serotype and genotype), host susceptibility, and the environment. Experimental infection of sheep using 10 of the Australian serotypes have demonstrated variable pathogenicity.

In Australia, clinical disease has never been reported in cattle and is rare in sheep.

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, mild disease, or subclinical infection can occur in sheep with the above clinical features appearing to varying degrees.

Trade loss due and import restrictions on live animals and animal products, and the costs of surveillance and health testing.

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. The extent of the vector-free zone has implications for trade negotiations.

• movement of infected viraemic ruminants
• extension of the area in which Culicoides midges are active in favourable seasons
• wind dispersal of Culicoides midges.

Bluetongue is not spread by animal carcasses or products such as meat or wool.

All year round.

In endemic areas, disease tends to be seasonal, occurring when warm, moist conditions favour the breeding of Culicoides biting midges, which are most numerous and active after rain.

Owners should monitor their sheep for signs consistent with Bluetongue (see symptoms).

If you believe Bluetongue is present, you must report it to Biosecurity Queensland on 13 25 23 or contact the Emergency Animal Disease Hotline on 1800 675 888.

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 that are recognised internationally.

National freedom from BTV is not possible in Australia and it is not possible to eradicate the bluetongue vectors.

The strategies to contain outbreaks and minimise trade impact include:

• movement controls to prevent spread
• 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.

Depending on the species involved and the zone in which Bluetongue occurred, an emergency animal disease response may be established.

Culling is not a control strategy; however, some animals may need to be destroyed for welfare reasons.

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.

Australia has strict import conditions for ruminants to prevent the entry of bluetongue and other diseases of ruminants.