Bluetongue virus
Friday, September 28th, 2007
Bluetongue is a highly infectious virus disease of ruminants. Cattle and goats are major hosts of the virus, but in these species infection is usually asymptomatic despite high virus levels, allowing the disease to circulate in the absence of any symptoms. Sheep and deer are usually the only species to exhibit symptoms of infection. Bluetongue infections are marked by a high fever, excessive salivation, swelling of the face and tongue and cyanosis of the lips and tongue (turning blue). Infected animals become lame and listless. Ulcers appear around the mouth, nose and eyes. Then the neck may start to swell, followed by the head. The animal becomes lame, starts bleeding internally and breathing becomes difficult. The incubation period for bluetongue is 5-20 days. The mortality rate is normally low, but infected animals lose condition and there is a high mortality rate of 70% or more in susceptible breeds of sheep (due to secondary bacterial infections). While infected animals can recover, productivity is reduced with milk yields in dairy herds dropping by about 40%.
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Bluetongue virus (BTV) is a Reovirus of the genus Orbivirus. The virus is transmitted by midges, in particular Culicoides imicola and a few other species. Of more than 1,400 species of midges known world-wide, only around 20 culicoid species are known to be involved in transmission of bluetongue virus. Bluetongue can also be transmitted directly from one animal to another through semen and transplacentally. Bluetongue occurs in Australia, the USA, Africa, the Middle East, Asia and Europe, generally between latitudes 35°S and 50°N. It occurs around the Mediterranean in summer, subsiding when temperatures drop in winter. In Europe the disease has been spreading north since October 1998, possibly as a result of climate change. In August 2006 bluetongue spread to the Netherlands, then Belgium, Germany, Holland, and Luxembourg. The first ever case of bluetongue in the UK was reported in Suffolk on 23rd September 2007. On 28th September 2007 Defra confirmed that bluetongue is now endemic in the UK.
Unlike foot and mouth disease, bluetongue cannot be controlled by culling of infected livestock alone. Since midges form a reservoir of infection in endemic areas, you would also need to kill all the midges to eradicate the disease. Another complication is there are at least 24 distinct serotypes of the virus (based on the lack of cross neutralisation). Vaccination against one serotype does not usually confer protection against any of the other serotypes. The antigenic diversity of Bluetongue virus is due to both antigenic drift (accumulation of point mutations) and antigenic shift (reassortment of individual gene segments). The virus which has affected northern Europe and the UK is known as BTV8.
Live attenuated BTV vaccines containing a weakened form of the live virus are cheap, easy to produce and can be administered in a single dose. They are effective in controlling clinical outbreaks of bluetongue. However, the disadvantages of attenuated BTV vaccines are:
- Risk of reassortment with virulent wild viruses which potentially could give rise to new virulent strains.
- Potential for reversion to virulence both in the vertebrate host and in vector insects.
- Attenuated BTV can cross the placenta and pregnant ruminants vaccinated with attenuated vaccines may suffer foetal loss.
- Existing vaccines are designed for sheep; there is little data on their safety and effectiveness in other species.
There have been attempts to develop inactivated (killed) whole virus vaccines for BTV for the past 25 years, but none have yet been produced commercially. Inactivated vaccines are they more expensive to produce than attenuated vaccines and also require at least two doses with an adjuvant to generate a protective immune response. Bluetongue virus is not usually contagious for humans, and meat and dairy products pose no hazard. However, there is some concern over the potential spread via blood from infected people.
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