Bluetongue

Description

Bluetongue is  an arthropod-borne viral disease of ruminants. Bluetongue virus (BTV) is spread by insect vectors, Culicoides midges, feeding on viraemic animals. 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. In NSW, BTV transmission occurs regularly on the northern coastal plain to the Hunter Valley, and more irregularly in the northern tablelands, northwest slopes, Sydney basin and south coast regions. Thirteen bluetongue serotypes have been recorded in Australia (serotypes 1, 2, 3, 4, 5, 7, 9, 12, 15, 16, 20, 21 and 23). BTV-1, BTV-16 and BTV-21 have been detected throughout the endemically affected area of NSW. BTV-15 has been detected on the far north coast of NSW in a single year only (2022). BTV-2, BTV-3, BTV-4, BTV-5, BTV-7, BTV-9, BTV-12, BTV-20 and BTV 23 have been isolated only in the Top End of the Northern Territory. There has been variable serological evidence for some of these serotypes in northern areas of Western Australia and Queensland. BTV-2 has also been detected in eastern Queensland.

All ruminant species are susceptible to infection with BTV, however bluetongue is primarily a disease of sheep.  Infection in cattle, although of great epidemiological significance, is usually subclinical. The virulence of different strains of bluetongue varies significantly.

Experimental infection studies with ten of the serotypes detected in Australia produced variable pathogenicity in sheep.  The highly pathogenic strains encountered in some overseas countries are exotic to Australia.

Clinical disease is an uncommon occurrence in Australian sheep and has never been reported in any other susceptible animal species in Australia. Cases have been reported in a sentinel sheep flock on a research station near Darwin in 1989, a small outbreak in a non-commercial sheep flock near Darwin in 2001, a small flock south-west of Cairns in autumn 2020, a small flock in southern Queensland and two commercial sheep flocks in the northern tablelands region of NSW in autumn 2023, and on multiple properties within the BTV zone in NSW in Autumn 2024. The Northern Territory has regulated the import of susceptible species into the known bluetongue zone. The mortality rate is very variable in sheep and generally ranges for 0-30% depending on the virus strain and genotype of sheep.

Clinical signs may range from acute to mild and typically involve variable, fluctuating fever, hyperaemia of oral and nasal mucosae, excess salivation and nasal discharge.  Lips and tongue may become swollen and the oedema may extend over the face and intermandibular space.  Haemorrhages occur on oral and conjunctival mucosae.  Ulcers develop on the gums, cheek and tongue 5-8 days after the onset of fever. Feet lesions may appear towards the end of the febrile period.  There is reddening and petechial haemorrhages on the coronary band.  The associated pain causes the animals to stand with arched backs and be reluctant to move.

The differential diagnosis includes the following diseases:

• Foot and mouth disease
• Peste des petits ruminants
• Contagious pustular dermatitis
• Sheep pox
• Photosensitisation
• Footrot
• Oestrus ovis infestation
• Pneumonia
• Acute haemonchosis (with depression and submandibular oedema)

Note: Highest concentrations of virus in the blood usually occur during the early stage of disease before antibodies develop but virus can be reliably detected for at least 7-10 days after the onset of disease and usually much longer when using qRT-PCR.

Control

National freedom from BTV is not possible.  Australia has a recognised BTV zone and monitoring of BTV distribution is undertaken by the National Arbovirus Monitoring Program (NAMP).

Diagnosis and tests available

Diagnosis

Diagnosis is supported by history, clinical signs, confirmation of infection by PCR or virus isolation and the post mortem finding of haemorrhages in the tunica media at the base of the pulmonary artery (regarded as being characteristic of bluetongue).

For disease exclusion from live animals collect 10 mL EDTA blood from each of up to six clinically affected sheep with high temperatures (over 40.5⁰C); 10 mL clotted blood from each of 10-15 convalescent or in-contact sheep (if there are no convalescent sheep); and 10 ml clotted blood from in-contact cattle, ideally yearlings, and from other ruminants on the affected property. Collect spleen and lymph node from recently dead affected animals.

Tests available

Click on + in the tables below for sampling requirements and additional test information

Detailed specimen requirements

Additional Information