Bluetongue disease (BT) is a viral disease of cattle, sheep, goats, camelids and wild ruminants.

Bluetongue is found almost worldwide. In 2023, a specific serotype of the virus, BTV-3, was detected for the first time in the Netherlands, Belgium and Germany. Since then, BTV-3 has continued to spread across Europe; Austria has also been affected by BTV infections involving two serotypes of the virus (BTV-3 and BTV-4) since September 2024. At the beginning of August 2025, a third serotype, BTV-8, emerged, initially reaching the southern provinces of Carinthia and Styria. BTV-8 is now widespread in several European countries (France, Spain, Italy, Switzerland, Germany, Austria and several Balkan states). The BTV-12 serotype, which emerged in the Netherlands in 2024, is unlikely to have spread further. In 2025, another BTV serotype entered the European scene with the detection of BTV-5 in Sardinia.

Ruminants (e.g. cattle, sheep, goats), wild ruminants (e.g. deer, red deer, ibex, chamois) and camelids (e.g. alpaca). The sheep is considered the most susceptible animal species, although there are differences in susceptibility between the individual breeds. There is no risk of infection for humans.

The pathogen is transmitted by midges (biting-sucking insects). Direct transmission by smear infection from animal to animal is not known, but transplacental transmission (from the infected mother to the unborn young animal) can occur with certain BTV serotypes. The relevance of this vertical transmission for the epidemiology of the disease is unclear. Infectious BTV has been detected in the semen of infected bulls and rams. Transmission of field virus through mating or artificial insemination has not been confirmed. It has been shown that BTV can adhere to cattle embryos, but transmission through embryo transfer has not been observed. Contaminated needles from vaccination/injection equipment or surgical instruments pose a potential risk of transmission. There is no risk of bluetongue spreading or transmitting through meat or milk.

4-8 days

The clinical picture is mainly characterised by fever and hyperaemic congestion in the area of the head or head mucous membranes (swollen, protruding "blue" tongue) and the limbs. Sheep are usually more severely affected than cattle. In cattle and goats, asymptomatic courses are also common. In sick cattle, there is often a significant drop in milk yield. Deaths in sheep do occur and are more frequent than in cattle. With the exception of sheep, mortality is generally low and also depends on the virus strain. Compared to BTV-8, BTV-3 appears to lead to more severe disease progression, while the BTV-4 strain has hardly been associated with clinical disease to date. The clinical picture can be reminiscent of foot-and-mouth disease (e.g. fever, reddening/inflammation of the mucous membranes, lameness), making the latter an important differential diagnosis to BT.

There is no specific therapy

Bluetongue was first detected in South Africa in 1934. The disease spread further with the export of Merino sheep to many countries across the African continent. There are currently 36 known BTV serotypes, although only serotypes 1–24 are notifiable under European animal health legislation.

The bluetongue virus (BTV) is a non-enveloped double-stranded RNA virus belonging to the genus Orbivirus of the family Reoviridae. The serological relationship between the numerous individual BTV serotypes varies. For this reason, there is a possibility that there may be either a high degree of cross-reactivity (‘close’ relationship, e.g. BTV-8 and BTV-18) or a low degree of cross-reactivity (‘distant’ relationship, e.g. BTV-8 and BTV-15) between two BTV serotypes. Consequently, an animal vaccinated against BTV may develop clinical disease following exposure to a different BTV serotype and produce antibodies against this second BTV serotype. In laboratory tests, BTV was inactivated after 3 hours at a temperature of 50 °C or after 15 minutes at a temperature of 60 °C. Under suitable conditions, the virus can remain infectious for years, e.g. in blood samples at 20 °C. (Source: OIE).

Differential diagnoses include FMD, BKF, BHV-1, BVD, PI-3, VS (vesicular stomatitis), EHD, lip rot, circulatory disorders of other aetiologies, etc.

Transmission

The pathogen is transmitted by midges (Culicoides spp.); there is no direct route of transmission from mammal to mammal. Infection is seasonally linked to midge activity and therefore usually occurs in late summer and autumn. When feeding on an infected animal, the midge ingests BTV-containing blood. The BTV first enters the gut and from there passes into the midge’s salivary glands. During the next feeding, virus-containing saliva is introduced into the host’s bloodstream. There, the virus multiplies and spreads to all organs. Following infection, the mammal mounts an immune response (antibody production) against the pathogen. Once infection has occurred, BTV can be detected in EDTA blood samples from sheep and cattle for several months, even after recovery has taken place. For some BTV serotypes (BTV-3, BTV-8), intrauterine transmission of the virus to the foetus has also been described. This can result in externally visible malformations in newborn calves (e.g. hydrocephalus) and/or behavioural disorders (lack of suckling reflex, coordination disorders); such calves are usually weak and unlikely to survive. Corneal opacities may also occur as a result of intrauterine infection.

Symptoms

Infected animals exhibit low mortality and high morbidity.  Mortality (i.e. the proportion of the susceptible animal population that dies from the disease) is reported to be 1% to 5% in sheep and up to 1.5% in goats and cattle. However, according to reports from the Netherlands, mortality associated with BTV-3 in sheep was significantly higher. Following the bite of an infected mosquito, viraemia develops in the susceptible host, accompanied by fever and clinical symptoms. The most common form of the disease is the inapparent form. Various forms of the disease have been observed in affected animals: acute, subacute and abortive, all of which begin with a rise in temperature.

The symptoms are:

• Fever (40–42 °C)
• Hyperaemia of the oral and nasal mucous membranes
• Lip oedema
• Hoof inflammation: hyperaemia of the coronary band
• Abortions
• Changes in the skeletal muscles

The following are suitable as sample material:

• Blood (EDTA is required for viral detection; serum alone is only suitable for antibody testing)
• Organs
• Mosquitoes (not routinely, only as part of research projects)

Detection of BTV from the above materials is possible using the following methods:

• Serological tests for antibody detection:
  • ELISA (serum): the antibodies detected by ELISA are not serotype-specific! Antibodies may result from either vaccination or infection.
  • Serum neutralisation test (serum): these antibodies are serotype-specific; however, the method is very labour-intensive and not suitable for routine diagnostics.
• Molecular biological identification (EDTA blood, organs and mosquitoes)
• BT virus culture (EDTA blood, organs, possibly mosquitoes)

In all cases, samples should ideally be sent to the laboratory by an authorised logistics company, using coolants and in accordance with the relevant transport regulations (UN3373).