CASE NOTES

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Clinical bluetongue virus in sheep in the Lower Hunter in April 2024

Kylie Greentree, Hunter LLS, Maitland

Posted Flock and Herd November 2024

Introduction

Clinical bluetongue disease is rare in Australia. In NSW, there had previously only been two cases, in May 2023. Bluetongue virus (BTV) was detected on multiple properties in NSW from 29 February 2024 with all cases within the bluetongue transmission zone. BTV is an insect-borne (primarily biting midges Culicoides brevitarsis) viral pathogen affecting ruminants and is primarily a disease of sheep. BTV infection in cattle is usually subclinical, but occasionally clinical signs are seen in sheep (rarely in goats). These include lethargy and fever, reddening of mucous membranes of the mouth and nose, swollen face/muzzle, excessive salivation and discharge around mouth and nose. Affected sheep may also have sore feet making them reluctant to stand and walk, laboured breathing, ulcers in the mouth and nostrils and occasionally death. BTV is a notifiable disease, and all suspect BTV animals need to be reported.

History

In late April 2024, a producer in the Lower Hunter notified the local District Veterinarian about the loss of two young Wiltipol sheep (<12months) and a sick adult sheep (>12 months) that was lethargic and hypersalivating. The remainder of the flock had no clinical signs. The producer had noticed the lethargy, increased salivation, abdominal breathing in the two young sheep and one of the sheep had regurgitation and staining of rumen contents around the muzzle. He had thought it was worms and had drenched one of the sick young sheep, but the two sheep died approximately 24 hours apart.

Clinical findings

Figure 1. Green staining around muzzle

The sick adult sheep was salivating from the mouth and had a long string of saliva hanging from the chin. She was listless and had been noticed away from the rest of the flock. She had pink conjunctiva and a temperature of 39.9°C. On examination of the mouth there was ulceration on the top of the tongue 1-2cm from the tip of the tongue. The tongue was oedematous and there was inflammation and reddening of the frenulum of the tongue. There were no lesions on the feet and the sheep was not lame.

Figure 2. Affected ewe (Sheep 3) showing profuse salivation

Figure 3. Sheep 3, with ulceration on tip of tongue

All other sheep appeared normal with no clinical signs.

Post-mortem

A necropsy was performed on both dead sheep 1 and dead sheep 2.

Dead sheep 1 was a juvenile ram that was less than 12 months of age. The ram was in body condition score 3/5. There was noticeable congestion around the head and neck, with oesophageal congestion in the cranial portion and blanching in the caudal section of the oesophagus. The trachea was dark red and haemorrhagic/congested around the larynx. There was no froth in the trachea, some froth in the bronchioles and no obvious sign of aspiration. The lungs were congested and oedematous with a rib pattern on the lung lobes. The cranio-ventral lung lobes were dark red and haemorrhagic looking. All lung lobes appeared swollen. No abnormalities were detected in the abdominal cavity except that the carcass was starting to autolyse.

Figure 4. Sheep 1. Congested oedematous lungs with obvious rib impression

Dead sheep 2 was a juvenile ewe in 3/5 body condition score and less than 12 months of age. Post-mortem findings included congestion around the neck and larynx. Froth filled the trachea and there was increased vasculature, especially on the cranial portion of the inner lining of the trachea. The lungs were diffusely congested, haemorrhagic and oedematous. There was an ecchymotic haemorrhage the full thickness of the pulmonary artery.

Figure 5. Sheep 2. Congested oedematous haemorrhagic lungs

Figure 6. Sheep 2. Ecchymotic haemorrhage on pulmonary artery

Laboratory findings

Histopathology report commented that the pulmonary oedema in both lungs was a non-specific finding seen in many infectious and non-infectious conditions including bluetongue. Haemorrhage at the base of the pulmonary artery is a more specific and relatively common finding in sheep with clinical bluetongue disease. In both lungs of dead sheep 1 and dead sheep 2, there was also a suppurative bronchopneumonia (with significant haemorrhage in dead sheep 2). There was evidence of aspiration of foreign plant material in dead sheep 2, which can also induce a suppurative inflammatory response. The tissues of dead sheep 1 were markedly autolysed.

Nasal and mouth swabs were taken from the sick sheep and placed in PBGS for FMD exclusion. Eleven blood samples were taken from the flock (animals 3-13) to test for BTV exposure and to exclude other Emergency Animal Diseases.

FMD exclusion

FMD and vesicular stomatitis were excluded.

Faecal Egg Count of the two dead sheep

Dead sheep 1 FEC= 1840 (this animal was drenched the day before he died).

Dead sheep 2 FEC=11000.

Larval Differentiation:

86% Haemonchus, 8% Trichostrongylus, 1% Cooperia and 5% Oesophagostomum.

Bluetongue PCR

Five positive PCR results (animals 3,7,9,10,11) out of 11 blood tests.

Bluetongue cELISA

Seven positives (animals 4,6,7,8,9,10,11), one inconclusive (animal 3) and three negatives (animals 5,12,13)

Bluetongue serotype PCR on animals 3,10,11 detected BTV 1.

Discussion

All cases of clinical bluetongue in sheep have been within the bluetongue-transmission zone - National Arbovirus Monitoring Program: Public system home page ( animalhealthaustralia.com.au ) and the serotypes detected have been the same as the serotypes detected in the sentinel herds (BTV 1, 16 and 21). All the clinical cases have been BTV 1 or 16.

All movements of sheep, cattle, goats and their products from BTV-positive properties were restricted, with an Individual Biosecurity Direction, until the end of the vector season. The end of the vector season may coincide with the first frost of the season or when the National Arbovirus Monitoring Program testing for the season concludes and the Culicoides vector is no longer present. This process is to ensure that no ruminants or their products leave the BTV transmission zone.

While there is no specific treatment for BTV, supportive therapy (anti-inflammatories and long-acting antibiotics) was provided to the sick sheep. The sick sheep appeared to improve 24 hours after treatment but unfortunately died the following day.

Acknowledgements

Thanks to the DPI for working around the clock when BTV clinical cases in sheep were detected in NSW. They were constantly providing resources, regular situation reports, clear direction of investigation requirements and fielding questions from District Veterinarians around NSW.

References

1. Bluetongue virus dpi.nsw.gov.au
2. National Arbovirus Monitoring Program: Public system home page animalhealthaustralia.com.au
3. Informing EAD Responses - AUSVETPLAN - Animal Health Australia animalhealthaustralia.com.au