29 September 2022

6 minute read

Bluetongue is an insect-borne, viral disease affecting sheep, cattle, deer, goats and camelids (camels, llamas, alpacas, guanaco and vicuña). Although sheep are most severely affected, cattle are the main mammal reservoir of the virus and are critical in the disease epidemiology. The disease is non-contagious and is only transmitted by insect vectors (midges of the Culicoides species). The disease is caused by a virus belonging to the family Reoviridae. Bluetongue virus is a notifiable disease in many countries.

Species affected

BTV affects sheep, cattle, deer, goats and camelids (camels, llamas, alpacas, guanaco and vicuña). Humans are not affected.

Distribution

Historically, bluetongue virus has been confined to tropical and subtropical areas. However, endemic areas now exist in Africa, Europe, the Middle East, North and South America and Asia as well as on islands such as Australia, the South Pacific and the Caribbean. Multiple serotypes are found in many regions. Outbreaks can occur outside endemic areas, but in most cases, the virus does not persist once cold weather kills the Culicoides (midges) vectors.

More recently, climate change and trade patterns have seen increasing outbreaks in temperate regions (including Northern Europe) in recent years with outbreaks of up to 9 different serotypes occurring in Europe over the last 10 years. The most significant of which was the BTV-8 outbreak in Northern Europe in 2006-2008. Even more recently has been the circulation of BTV-8 in southern France in autumn 2017.

Map: Bluetongue restricted zones in Europe as of the 18th of April 2018. For updated maps please click here

Key symptoms

Clinical signs are most apparent in sheep, where the disease is characterized by fever, widespread hemorrhages 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. Lameness, due to swelling of the cuticle above the hoofs and emaciation, due to reduced feed consumption because of painful inflamed mouths, may also be symptoms of this disease. The blue tongue that gives the disease its name occurs only in small number of cases. Convalescence of surviving sheep is slow. The high fever in sheep results in wool breaks, which adds to production losses.

Goats, cattle, and wild ruminants such as deer can appear healthy when infected. This can lead to silent spread by midges feeding on the infected animals.

Spread

The virus cannot be transmitted between susceptible animals without the presence of the insect carriers. The incidence and geographical distribution of bluetongue depends on seasonal conditions, the presence of insect vectors, and the availability of the susceptible species of animals. The insect carriers, biting midges, prefer warm, moist conditions and are in their greatest numbers and most active after rains.

The virus

Bluetongue virus does not survive outside the insect vectors or susceptible hosts. Animal carcasses and products such as meat and wool are not a method of spread. Survival of the virus within a location is dependent on whether the vector can overwinter in that area.

At least 26 different serotypes of the bluetongue virus have been identified (with potential for still more to be discovered). The BTV genome evolves rapidly via mutations. Some virus variants may be selected as they are better adapted to the environmental conditions. This evolution and selection of variants during the transmission of BTV between susceptible animals and vector (midges) appear to be the main mechanism that leads to the genetic diversity amongst BTV field strains.

Disease control measures

The strategy is to contain the outbreak and minimize trade impact. Activities include:

• Restriction of movement of animals if BTV is suspected
• Confirmation of suspected cases by laboratory tests
• Zoning to define infected and disease-free areas
• Vaccination of susceptible animals
• Surveillance to determine the extent of virus and vector distribution
• Vector surveillance and control strategies

See e.g. GB Bluetongue Virus Disease Control Strategy (Defra) 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.

Diagnostics

If you suspect BTV in your herd, the first step is to contact your veterinarian. The next step is to work with your veterinarian to obtain blood samples from animals for diagnostic testing to confirm whether the animals are infected with BTV. At least 26 different serotypes of BTV have been identified (with potential for still more to be discovered), therefore it is important to have a single test that can detect any of these viruses. If BTV is verified, the next step is to serotype the strain to apply the appropriate vaccine.

Real-time polymerase chain reaction (qPCR) detection tools can quickly and reliably identify whether BTV is present in the herd and identify specific genotypes.

Virus diversity needs to be closely monitored and diagnostic test tools’ capabilities regularly tested to detect new variants. Here, the collaboration with national reference institutes is required to regularly monitor virus sequences and update diagnostic tools if needed.

For more information about BTV diagnostic tools visit the Thermo Fisher Scientific website.

Many countries are establishing surveillance and management programmes to help veterinarians and farmers battle BTV when detected. As always, remain vigilant monitoring for BTV in your herd, especially in warm, humid weather conditions.

From: OIE: Manual of Diagnostic Tests and Vaccines for Terrestrial Animals

Post-Mortem Blue Tongue Pictures

Bluetongue is an insect-borne viral disease to which all species of ruminants are susceptible. The virus is transmitted by a small biting midge of the Culicoides genus rather than from animal to animal. The virus does not affect humans. Bluetongue is a notifiable disease and, where suspected, should be reported to Animal Health.

VLA is involved in the surveillance for the disease and was responsible for the detailed post mortem examinations of the first cases in the UK. The images below illustrate the pathology associated with this disease as shown by the first 3 cattle cases in East Anglia.

Cow 1 bluetongue case – clinically affected Highland cow. Extensive superficial erosion of the muzzle with a mucopurulent nasal discharge

Cow 1 bluetongue case – clinically affected Highland cow. Diffuse reddening of the dental pad with multifocal haemorrhages on the upper lip and dental pad

Cow 1 bluetongue case – clinically affected Highland cow. Ocular discharge visible at the medial canthus (wrinkling of the cornea reflects post mortem change)

Cow 1 bluetongue case – clinically affected Highland cow. 'Coronitis’: skin reddening around and above the coronary band intensifying distally with serous crusting at the coronary band

Cow 1 bluetongue case – clinically affected Highland cow. Multifocal haemorrhages with oedema visible on the cut surface of the submandibular lymph node

Cow 1 bluetongue case – clinically affected Highland cow. Extensive ecchymotic subepicardial haemorrhages

Cow 1 bluetongue case – clinically affected Highland cow. Extensive subendocardial haemorrhages in the left ventricle

Cow 2 bluetongue case – Gloucester cow. Identified as PCR positive. Multiple pinpoint haemorrhages in the skin around the bases of the teats. There were also scattered petechial haemorrhages on the skin of the ventral abdomen. No other lesions distinctive of bluetongue were seen in this cow

Cow 3 bluetongue case – clinically affected Friesian/Holstein cow. Focally extensive necrosis at the mucocutaneous junction of the nares.

Cow 3 bluetongue case – clinically affected Friesian/Holstein cow. Focally extensive area of haemorrhage (with associated ulceration not clearly visible in this image) on the ventral surface of the tongue. There are multifocal pinpoint haemorrhages covering most of the ventral surface of the tongue

Cow 3 bluetongue case – clinically affected Friesian/Holstein cow. Focally extensive haemorrhage involving the bulbar conjunctiva with associated subconjunctival petechiation

Cow 3 bluetongue case – clinically affected Friesian/Holstein cow. Generalised lymph node enlargement with oedema