Friday, October 26, 2012

Bluetongue disease

Blue-tongue infected sheep may also
bend their necks in a strange manner
(Photo: Onderstepoort Veterinary Institute)
Blue-tongue disease is a non-contagious, viral disease of ruminants, mainly sheep and less frequently cattle, goats, buffalo, deer, dromedaries and antelope. It is caused by the Bluetongue virus which is transmitted by the midge Culicoides imicola and other culicoids. Major signs are high fever, excessive salivation, swelling of face and tongue as well as cyanosis of the tongue. Swelling of the lips and tongue gives the tongue its typical blue appearance, though this sign is confined to a minority of the animals. Nasal symptoms may be prominent, with heavy discharges at times. The mortality rate is normally low, but it is high in susceptible breeds of sheep. In Africa, local breeds of sheep may show no mortality, but in imported breeds it may be up to 90 percent.

Culicoides obsoletus
Blue-tongue has expanded its range in recent decades, probably due to the expansion of its main vector and the presence of other competent vectors such as Culicoides obsoletus and Culicoides pulicaris both of which are spread widely throughout Europe. Consequently the disease has been travelling through Europe and many countries there are trying to be proactive with respect to the spread of the disease.

A good example is a new study from Spanish researchers that utilizes DNA Barcoding to map the distribution of biting midges and to understand their feeding patterns in areas susceptible to bluetongue. They confirmed the presence of the potential bluetongue vector Culicoides obsoletus on two of the islands in the Canary archipelago (Spain) and demonstrated the susceptibility of ruminants (goats and sheep) to the attacks of this biting midge species by analysing their bloodmeal. 

Given that Culicoides obsoletus has been shown to be one of the main vectors of this viral disease in continental Europe and given the high density of ruminants that are fed on by this midge species on these islands all necessary elements for an outbreak are given on the Canary Islands. It hasn't happen yet but it seems paramount to set up an active surveillance program on these remote islands as infected midges can drift all the way from Africa or continental Europe.

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