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

Bluetongue occurs almost worldwide

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 mosquitoes. There is no risk of bluetongue spreading or being transmitted through meat or milk.

10 days

In cattle, the clinical picture is similar to foot-and-mouth disease (e.g. fever, inflammation of the hooves), while in sheep lameness is usually the most obvious symptom. Mortality is low.

There is no therapy

Bluetongue is a notifiable animal disease according to the Animal Diseases Act. In Austria, susceptible animals are examined throughout the country. Vaccination of cattle, sheep and goats is permitted.

Austria had its first BT case with serotype 8 in 2008. BT freedom was regained as of 17 March 2011. In the second half of 2014, a new BTV-4 epidemic occurred in south-eastern Europe and spread rapidly from Turkey via Greece, Romania, Bulgaria and the Balkan states to Hungary and Croatia. In the course of this spread, serotype 4 was also detected in Austria for the first time on 17 November 2015. A total of four BTV-4 outbreaks were recorded in the provinces of Styria and Burgenland in 2015 and three outbreaks in the provinces of Burgenland and Carinthia in 2016. As of 2017, no BTV case was detected nationwide. In 2020, 6,754 cattle from 1,199 farms from all 28 regional units tested serologically negative for BTV. In addition, a total of 51 samples tested negative in the same year as part of exclusion tests.

Bluetongue was first detected in South Africa in 1934. The disease spread further with the export of Merino sheep to many countries on the African continent.

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 of either high cross-reactivity ('close' relatedness, e.g. BTV-8 and BTV-18) or low cross-reactivity ('wide' relatedness, e.g. BTV-8 and BTV-15) between two BTV serotypes. Therefore, a BTV-vaccinated animal may become clinically ill with another BTV serotype and develop antibodies against this second BTV serotype. In laboratory experiments, 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 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, labial thrush, circulatory disorders of other genesis, etc.

Transmission

The pathogen is transmitted by so-called gnats(Culicoides spp.); there is no direct route of transmission from mammal to mammal. The infection is seasonally linked to the activity of the gnats and therefore usually occurs in late summer and autumn. During the sucking act on the infected animal, the gnat ingests blood containing BTV. The BTV first enters the intestine and from there continues into the salivary glands of the gnat. With the next sucking act, virus-containing saliva is brought into the bloodstream of the host. 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 gnat up to approx. 28 days, in sheep up to approx. 60 days and in cattle up to approx. 220 days.

Symptoms

Infected animals show a low lethality and a high morbidity. The mortality (= proportion of the susceptible animal population that dies of the disease) is described as 1 % to 5 % in sheep, and up to 1.5 % in goats and cattle. Following the bite of an infected mosquito, viremia with fever and clinical signs occur in the susceptible host. The most common course is the inapparent one. Different courses have been observed in diseased animals: acute, subacute and abortive, all starting with an increase in temperature.

The symptoms are:

• fever (40-42 °C)
• hyperaemia of the oral and nasal mucous membranes
• Lip edema
• Claw inflammation: Hyperemia of the coronal area
• Abortions
• Changes in skeletal muscles

Control/Prevention

After the first BTV-4 cases were detected in the south-east of Austria, the surveillance programme was adapted in order to be able to precisely limit the extent of the BT virus circulation. For this purpose, a surveillance scheme was used that was already in use during the BTV-8 epidemic in 2008. Twenty-eight regions were defined, the size of which took into account topographical conditions, livestock density and political districts, and 60 unvaccinated cattle were subjected to serological BTV-AK testing per region - in addition to the surveillance already in progress. After two years without a BTV case in the south-east of Austria, the BTV-4 restriction zone could be reduced and also lifted in December 2018.

In addition, a vector surveillance programme is being carried out in Austria to obtain information on the occurrence and activity periods of virus-transmitting insects (mosquitoes of the genus Culicoides spp.). Based on the results of this program, seasonal vector-free periods can be declared. To be sure that no vector activity is to be expected, mosquito traps are installed at selected locations and temperature monitoring is carried out at the same time. This allows additional movement options for the livestock trade.

Vaccination

There are serotype-specific vaccines against bluetongue. Since 01.08.2008 an inactivated BTV-8 vaccine for cattle and small ruminants has been approved by the EMEA. Currently, no official vaccination programme against BT is carried out in Austria. Vaccination against bluetongue serotype 4 is permitted, and may be carried out on a voluntary basis at the request (and expense) of livestock farmers. However, as bluetongue is a notifiable disease according to the Animal Diseases Act, certain general conditions must be observed.

Suitable sample materials are

• Blood (EDTA/serum)
• Milk
• Organs
• Mosquitoes

The detection of BTV from the above materials is possible with the following methods:

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