Bluetongue Disease

Bluetongue disease is a viral disease that is non-contagious, non-zoonotic, and insect-borne. It is mainly found in sheep and less commonly in cattle, goats, buffalo, deer, dromedaries, and antelope. It is not transmissible to humans.

The virus is pathogenic, part of the genus Orbivirus, and is a member of the Reoviridae family. With 24 serotypes the virus can cause serious disease in livestock. Due to its seriousness BTV has received extensive research for the last three decades and is now one of the best understood viruses at the molecular and structural levels. Like other members of the Reoviridae family, the virus is a complex non-enveloped virus with seven structural proteins and a RNA genome consisting of 10 double-stranded RNA segments.

At each stage of the virus there has been found roles for host proteins. Through 3d reconstructions it was possible to determine the complex nature of the virion. Bluetongue’s family, Reoviridae, is one of the largest families of viruses and includes major human pathogens as well as other vertebrate, plant, and insect pathogens.

Like others in its family, bluetongue has the characteristic double-stranded and segmented features of its RNA genomes. BTV, bluetongue’s etiological agent, is transmitted by culicoides species. In sheep, BTV causes disease with high morbidity and mortality. When first described in the 18th century the disease was believed to be confined to Africa. However, BTV has been isolated in many tropical, subtropical, and temperate zones. Since the virus can affect livestock it has been given a lot of research. Mildly acidic conditions can easily affect the infectivity of BTV. Due to the large amount of research done on BTV the foundation for a generation of safe BTV vaccines has been laid for the possibility of vaccines to be used in animals.

Even though there have been major medical advances there are still questions about the life cycle of BTV. One major problem with researching BTV is that there is a lack of availability of suitable systems for genetic manipulation of the virus. In order to solve this problem researchers have rescued live viruses from transfection of BTV transcripts.

Bluetongue is observed in many countries although it is usually seasonal in countries where the temperatures drop and hard frosts kill the adult midge vectors. The disease has slowly spread northward through Culicoides obsoletus and C.pulicaris which can aquire and transmit the disease and both of which are widely spread throughout Europe. The original C.imocola vector is limited to North Africa and the Mediterranean. The first suspected case of the disease found in the UK was reported in 2007. This first case was reported in a cow and has since spread to sheep. Although animals can survive the disease through the winter the disease also survives the winter and comes back in the spring.

Symptoms are high fever, excessive salivation, swelling of the face and tongue, and cyanosis of the tongue. This swelling is what gives the tongue is characteristic blue appearance. Some animals develop foot lesions that can lead to lameness. The sickest die within a week, however, for those that don’t die recovery is slow, possibly lasting several months. Incubation period for the virus is 5 to 20 days and all symptoms develop within a month. Mortality rate is low but sheep are highly susceptible. There is no efficient treatment and best prevention is quarantine.