Bluetongue

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 Disease Watch Hotline on 1800 675 888.

Cause

An orbivirus virus belonging to the family Reoviridae.

There are 26 types of bluetongue virus (BTV) and 10 of these have been isolated in Australia. Orbiviruses are insect-borne viruses (arboviruses) transmitted by Culicoides midges. Many other non-bluetongue orbiviruses are also endemic in Australia.

Description

Bluetongue is an insect-borne, viral disease that can affect sheep, goats, deer and cattle. Sheep are the most seriously affected species. Clinical disease and mortalities occur, and production and trade losses may result. Most infections in cattle are unapparent.

The severe clinical disease seen in other countries has not occurred here. Disease has been produced in experimentally-infected sheep, with some serotypes present in areas of Australia with few sheep.

Distribution in Queensland

The virus is endemic in northern Australia (Northern Territory, Queensland and Western Australia), and its distribution extends down the east coast into New South Wales, south of Sydney, in years that are favourable for the vector. 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.

Bluetongue virus serotype 1 was first identified in Australia in 1975 from trapped insects in the Northern Territory (NT), but despite its long-term presence, it has not caused any clinical disease. Since that time 9 more serotypes have been isolated in the NT. The distribution of BTV in Australia has been monitored in the National Arbovirus Monitoring Program (NAMP) for many years. The zones of BTV virus transmission and freedom are mapped and updated regularly on the Animal Health Australia website.

The virus is present in most countries of Africa, the Middle East, India, China, the United States, and Mexico. Bluetongue virus infection, without associated clinical disease, is present in South East Asia, Papua New Guinea, northern South America and northern Australia.

Affected animals

sheep; goats; deer; cattle

Hosts

Midges

Symptoms

In countries where bluetongue is found, the disease is characterised by:

  • fever
  • widespread haemorrhages of the oral and nasal tissue
  • excessive salivation, and
  • nasal discharge.

In acute cases the lips and tongue become swollen, and this swelling may extend below the lower jaw. The blue tongue that gives the disease its name, occurs only in a small number of cases.

Lameness occurs due to laminitis. Inflammation of the coronary band - a dark red to purple band in the skin above the coronet - is an important diagnostic sign. Emaciation and weakness develop rapidly, and death occurs about 6 days after first signs occur.

Impacts

Convalescence of surviving sheep is slow. The high fever in sheep results in wool breaks, which adds to production losses.

How it is spread

The virus cannot be transmitted between susceptible animals without the presence of the Culicoides vector. The incidence and geographical distribution of BTV depends on seasonal conditions, the presence of insect vectors, and the availability of the susceptible species of animals.

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. Survival of the virus within a location is dependent on whether the vector can over-winter in that area.

Control measures

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.

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

Vaccines are used in many countries, including Europe, to control the disease.