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

Bluetongue occurs almost worldwide. In 2023, a specific serotype of the virus, BTV-3, appeared for the first time in the Netherlands, Belgium and Germany. Since then, BTV-3 has continued to spread in Europe, and Austria has also been affected by BTV infections with two serotypes of BTV (BTV-3 and BTV-4) since September 2024. At the beginning of August 2025, a third serotype was added with BTV-8, which presumably reached our country from the south. BTV-8 has so far occurred in several European countries, including Switzerland and Italy. In addition, BTV-8 has recently been increasingly reported from several Balkan countries.

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

There are serotype-specific vaccines against bluetongue. There are currently inactivated vaccines against serotypes 1, 3, 4 or 8 in sheep and cattle that are authorised in the EU (European Medicines Agency, EMA). These vaccines may be used in Austria in accordance with Section 29 of the Animal Health Act 2024 and the Bluetongue Control Ordinance (the attending veterinarian must notify the competent authority of the intended vaccination in good time; veterinarians must send annual reports on the vaccinations carried out to the competent authority). In addition, the Bluetongue Control Ordinance § 11 must be observed (vaccinated animals must have individual animal identification). There is currently no official vaccination programme against BT in Austria. Vaccination against bluetongue serotypes 1, 3, 4 or 8 is officially permitted, although this can be carried out on a voluntary basis at the request (and expense) of the livestock owner as long as the framework conditions of the Animal Health Act and the Bluetongue Control Ordinance are complied with. Section 58 of the Veterinary Medicinal Products Act ("cascade") and Section 8 of the Medicinal Products Import Act must be taken into account.

Ministry of Health: Bluetongue

Bluetongue was first detected in South Africa in 1934. With the export of Merino sheep to many countries on the African continent, the disease spread further. 36 BTV serotypes are currently known, of which only serotypes 1-24 are notifiable under European animal health legislation.

Bluetongue virus (BTV) is a non-enveloped double-stranded RNA virus that belongs to the genus Orbivirus of the family Reoviridae. The serological relationship between the numerous individual BTV serotypes varies. For this reason, it is possible that there is either a high cross-reaction ("close" relationship, e.g. BTV-8 and BTV-18) or a low cross-reaction ("distant" relationship, e.g. BTV-8 and BTV-15) between two BTV serotypes. Therefore, a BTV-vaccinated animal can become clinically ill with another BTV serotype and produce antibodies against this second BTV serotype. In laboratory tests, the BTV was inactivated after 3 hours at a temperature of 50 °C or after 15 minutes at a temperature of 60 °C. The virus can survive for years under suitable conditions, e.g. in blood samples at 20 °C. (Source: OIE).

Differential diagnoses include FMD, BKF, BHV-1, BVD, PI-3, VS (vesicular stomatitis), EHD, cold sores, circulatory disorders of a different origin, etc.

Transmission

The pathogen is transmitted by so-called midges(Culicoides spp.); there is no direct transmission route from mammal to mammal. The infection is seasonally linked to the activity of the midges and therefore usually occurs in late summer and autumn. During the act of sucking on the infected animal, the midge ingests blood containing BTV. The BTV first enters the intestine and from there into the salivary glands of the midge. With the next sucking act, virus-containing saliva is introduced into the host's bloodstream. There the virus multiplies and spreads to all organs. After infection, the mammal develops an immune response (antibody formation) against the pathogen. After infection, BTV can be detected in the EDTA blood of sheep and cattle for several months, even after recovery.

Symptoms

Infected animals have a low mortality rate and high morbidity. 1% to 5% mortality (= proportion of the susceptible animal population that dies from the disease) is reported in sheep and up to 1.5% in goats and cattle. However, according to reports from the Netherlands, mortality associated with BTV-3 was significantly higher in sheep. The bite of an infected mosquito causes viraemia with fever and clinical symptoms in the susceptible host. The most common form is inapparent. Different forms have been observed in infected animals: acute, subacute and abortive, all beginning 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 coronet band area
• abortions
• Changes in the skeletal musculature

Suitable sample materials are

• Blood (EDTA is required for virus detection; serum alone is only suitable for antibody diagnostics)
• organs
• Mosquitoes

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

• Serological test methods for antibody determination: ELISA (serum)
• Serum neutralisation test (serum)
• Molecular biological identification (EDTA blood, organs and mosquitoes)
• BT virus cultivation (EDTA blood, organs, possibly mosquitoes)

In all cases, samples should ideally be sent to the laboratory with the addition of refrigerants and in compliance with the relevant transport regulations (UN3373) by an authorised logistics company.