Bluetongue virus: What to look out for and how to manage it

Bluetongue virus risk is at a seasonal low, but producers should be ready for that to change in spring.

Bluetongue virus is an insect-borne disease that can affect all ruminants. While cattle are thought to be the main carriers of the disease, sheep are the most severely affected.

It is a notifiable disease because of its impact on movement restrictions and animal trade. It is not zoonotic, which means it cannot be spread from animals to humans.

See also: Could thin ewes be a sign of infectious disease on your farm?

Bluetongue virus is transmitted by biting midges (Culicoides species) feeding on infected animals.

The virus reproduces within the midge, which can then spread the disease to other animals.

Direct animal-to-animal transmission has not been reported, except from mother to foetus.

Midges can be blown up to 200km, and though they are inactive in cooler climates (less than 12C), they can survive overwinter.

This makes controlling the disease very difficult once it is circulating.

Clinical signs

Impact of disease and clinical signs vary depending on the strain (serotype). The current outbreak in the UK is bluetongue virus serotype 3 (BTV-3).

Clinical signs are most severe in sheep and goats and include:

• Mouth ulcers
• Discharge of thick mucus from the mouth and nose, and drooling
• Swelling of the mouth, head and neck, especially around the eyes and muzzle
• Swelling of the coronary band (junction where the skin of the leg meets the horn of the foot)
• Reddening of the skin where blood vessels have burst beneath it
• Fever (elevated temperature)
• Lameness
• Difficulty breathing
• Abortion and the birth of deformed and/or weak lambs
• Death – mortality rates can be as high as 70%.

As the main carriers of bluetongue, cattle provide an important reservoir for disease spread.

However, clinical signs in adult cattle are mild, with some infected animals not showing any signs of disease.

© Tim Scrivener

Where they do show signs, these include lameness, nasal discharge and drooling and fever.

Infected cattle can also be lethargic, stop eating, show reddening of the mouth, eyes, nose and coronary band, and develop crusting erosions around the muzzle, nostrils and teats.

In addition, milk yield can drop, and cows infected during pregnancy may abort or give birth to small, weak, deformed or blind calves, with some dying within a few days of birth.

Monitoring and reporting

Vigilant monitoring of herds and flocks is strongly encouraged, as well as reporting any clinical signs of disease.

Bluetongue cannot be diagnosed on clinical signs alone; any suspicion of disease must be reported to the Animal and Plant Health Agency as soon as possible.

Subsequent veterinary examination and diagnostic testing is required to confirm the disease.

Care must be taken when importing live animals and germinal products, following appropriate pre- and post-export testing protocols. It is especially important to adhere to these regulations.

Treatment

There is no cure for bluetongue.

Infected animals may receive supportive care, however, there is potential for long-term production losses in animals that have recovered from the disease, such as reduced milk yield and infertility.

Vaccination remains the most effective way of protecting flocks and herds from bluetongue.

Vaccinations currently exist for BTV serotypes 1-4, and 8. Immunity against the virus is serotype-specific, with no cross-protection.

This means, for example, that an animal exposed to BTV-3, either naturally or via vaccination, is not protected against BTV-8.

The following vaccination guidelines should be followed:

• All vaccines must be supplied by the prescribing vet
• Records of vaccinated animals must be kept for five years
• Details of individual animals and the vaccine used must be provided to Defra within 48 hours of vaccinating
• Any excess doses must be returned to the prescribing vet.

The course of vaccination required will depend on the species being vaccinated, and the vaccine used. Farmers should check the vaccine data sheet and speak to their vet for further advice.

Keeping animals in areas that experience higher winds may reduce the risk of bluetongue disease risk to an extent, by reducing exposure to midges.

Similarly, housing stock may reduce the accessibility of animals to midges. However, this is not always practical or as effective as vaccination.

Other methods of disease prevention do not have sufficient evidence to prove reduced disease risk.

The information in this article was up to date at the time of publishing, however, information and evidence for the current BTV-3 epidemic are dynamic and changing.

Check the government website for regular updates on changing restriction zones and temporary control zones or speak to your local vet for advice.

If bluetongue virus is suspected on-farm, call 03000 200 301 in England and 03003 038 268 in Wales.