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. All ruminant species are susceptible; 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.

Twelve bluetongue serotypes have been recorded in Australia.  Experimental infection with ten of the serotypes produced variable pathogenicity in sheep. Serotypes BTV-1 and BTV-21 extend throughout the endemically affected areas of Queensland, New South Wales and Western Australia.  BTV-2,BTV-3, BTV-5, BTV-7, BTV-9, BTV-12, BTV-15, BTV-16, 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 far north Queensland. BTV-2 has also been detected in eastern Queensland. The highly pathogenic strains encountered in some overseas countries are exotic to Australia.

There has been no evidence of any clinical disease associated with bluetongue infection in any livestock species in the field in Australia, apart from one clinical case in a sentinel sheep flock on a research station near Darwin in 1989 and a small outbreak in a noncommercial sheep flock near Darwin in 2001. 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 andthe 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 infection is undertaken by the National Arbovirus Monitoring Program (NAMP).

An emergency response to BTV is required when clinical disease occurs, which is confirmed by laboratory testing.  The policy is to minimise the economic impact and to eliminate clinical disease if circumstances permit.  Eradication may or may not be feasible.

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).

Tests available

Test

Sample(s) required

Days of the week test is conducted

Turnaround time1

Bluetongue virus AGID

Clotted blood (red top tube)

Batch tested on Friday

Up to 7 days

Bluetongue virus antibody cELISA

Clotted blood (red top tube)

Batch tested weekly

Up to 7 days

Bluetongue virus qRT-PCR

Fresh tissue or EDTA blood (purple top tube)

According to demand

2-3 days

Bluetongue virus isolationEDTA blood (purple top tube), semen or fresh tissueAccording to demand25-6 weeks

Histopathology examination

Fixed tissues

Monday – Friday

Up to 5 days

1 Turnaround times are provided as a guide only. For specific information about your submission please contact Customer Service.

2 Please contact Customer Service to discuss testing requirements before submitting samples for testing.

Specimen requirements

Blood (with anti-coagulant)

  • One full 10 ml vial of blood from each of up to six sheep with high temperatures (over 40.5⁰C)
    • Collected in an EDTA tube or lithium heparin tube
    • Submit chilled and ensure blood samples are not in direct contact with ice blocks as this might cause freezing and inactivation of the virus

Blood (without anti-coagulant)

  • 10 ml of blood from each of 10-15 convalescent or in-contact sheep (if there are no convalescent sheep)
    • Collected into a plain red top tube
    • Submit chilled
  • 10 ml from in-contact cattle, ideally yearlings, and from other ruminants
    • Collected into a plain red top tube
    • Submit chilled

Fixed tissue

  • Cardiac and skeletal muscle and other tissues as indicated by gross examination
    • Submit fixed in neutral buffered formalin at a 10:1 ratio of formalin:tissue

Fresh tissue

  • Spleen and lymph nodes from all post-mortem cases
    • Submit chilled

Other specimens

  • As required for testing of differential diagnoses

Further information

Note

  • Separate needles should be used for each animal to avoid cross contamination of blood samples.  EDTA blood is preferred for PCR, while lithium heparin is used for virus isolation.
  • Fees for tests undertaken to confirm or exclude a diagnosis of Bluetongue are paid by NSW Department of Primary Industries. Extra testing to establish an alternative diagnosis is at submitters’ expense.
  • Fees for testing to establish an alternate diagnosis are not paid by NSW Dept of Primary Industries.