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 spread further in Europe, and Austria is also affected. Due to the absence of protective antibodies, livestock in regions not previously affected by BTV-3 are fully susceptible. A past infection with BTV-8 or BTV-4 or vaccination against these subtypes is not sufficient protection against infection with BTV-3.

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 predominantly characterized by fever and hyperemic signs of congestion in the area of the head or the mucous membranes of the head (swollen, protruding "blue" tongue) and limbs. In most cases, sheep are more severely affected than cattle. In the latter and in goats, symptomless courses are also common. The clinical picture may resemble foot-and-mouth disease (e.g., fever, mucosal redness/inflammation, lameness). With the exception of sheep, mortality is low and also depends on the virus strain.

There is no specific therapy

There are serotype-specific vaccines against bluetongue. There is currently an inactivated vaccine against serotypes 1, 2, 4 or 8 in sheep and cattle that is authorised in the EU (European Medicines Agency, EMA). This means that this vaccine may be used in Austria in accordance with Section 29 of the Animal Health Act 2024 (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, 2, 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.

There is currently no inactivated vaccine authorised in the EU against serotype 3. There are 3 inactivated vaccines that have emergency authorisation in sheep and cattle in other EU countries. These vaccines were included in the Animal Vaccine Application Ordinance (Amendment 2024) and can be used in Austria in sheep and cattle under the conditions of Section 29 of the Animal Health Act 2024 and Section 10 of the Bluetongue Control Ordinance.

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 bluetongue virus became inactive after 3 hours at a temperature of 50 °C or after 15 minutes at a temperature of 60 °C. The virus can remain inactive for years under suitable conditions. 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 builds up an immune response (antibody formation) against the pathogen. BTV can be detected in the midge for up to approx. 28 days, in sheep for up to approx. 60 days and in cattle for up to approx. 220 days.

Symptoms

Infected animals exhibit low mortality and high morbidity, with mortality (= proportion of the susceptible animal population that dies from the disease) described as 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 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 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 coronet band area
• abortions
• Changes in the skeletal musculature

Suitable sample materials are

• Blood (EDTA/serum)
• 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.