Bluetongue: the disease

In this step we wil be seeing what bluetongue is, one of the most important emerging arboviral diseases in ruminants. Bluetongue virus mainly affects sheep, in which it causes a clinical disease that can be severe. It also affects cattle and goats, either in subclinical or mild forms, although serotype 8 causes a severe disease in cattle.

Camels, deer and other wild ruminants can also become infected, but not all of them develop the disease. The presence of Culicoides insects, especially Culicoides imicola, is essential for the transmission of the virus from one animal to another. For this reason, it is said that it is a non-contagious disease. When an infected Culicoides midge bites an animal, the viruses initially replicate in the regional lymph nodes. After 4-5 days, viruses reach the blood, and they are disseminated throughout the body. The viruses replicate in the endothelial cells of the small blood vessels of various organs, causing increased vascular permeability with leakage of plasma, bleeding and vascular thrombosis, which produce lesions in the animals.

Still more pathogenic viruses are able to cross the placenta and cause abortion. During the second viremia, viruses are introduced into a kind of pocket or invaginations of the blood cells, particularly erythrocytes. In this way, they are hidden from neutralizing antibodies, which develop within two weeks after infection, and thus, virus and high levels of antibodies may coexist for some time. This mechanism of association to cells also enables a long viremia, up to 30 days in sheep and 60 days in cattle. It also enables the infection of another midge that bites the animal during this viremic stage.

The most frequent clinical signs in sheep are: fever, oedema of the head and neck, increased salivation, nasal discharge, ulcers in the oro-nasal cavity and in the hooves, and shortness of breath. In some cases, the tongue is swollen and bluish in colour, which gives the name to the disease. In pregnant sheep the virus can sometimes produce abortion, or the birth of lambs with malformations. The disease can evolve as a mild or acute very severe process, which causes the death of the animal. The animals who survive are weakened, with weight loss and alteration in the growth of wool. The infection in cattle usually is either subclinical or mild. However, serotype 8 can produce severe processes similar to those in sheep, including abortions.

These clinical signs are not exclusive of bluetongue and they may be mistaken with those of other viral diseases, such as foot-and-mouth disease. That is why a laboratory diagnosis is essential to confirm the infection. Blood samples can be used to detect the virus by RT-PCR, or serum samples for antibodies that show that the animal has been in contact with the virus. In the case of dead animals or abortions, RT-PCR is used to confirm the presence of the virus. Why is bluetongue so important? Mainly because it is an emerging disease around the world. In the past, bluetongue was limited to areas with temperature and humidity conditions adequate for Culicoides imicola, located between parallel 40°N and 35°S.

Outbreaks appeared in a seasonal and localized form. However, in recent decades various serotypes of the virus have expanded into areas more to the North.

Thus, from 2006 to 2008, bluetongue extended from the Mediterranean region 00:04:06.500 –> 00:04:09.060 to Central and Northern Europe, reaching the United Kingdom and the Scandinavian countries.

These outbreaks were caused by serotype 8, which is capable of producing disease in cattle. This new distribution triggered a high alert in Europe since it created high economic losses in livestock. Later, outbreaks have decreased with the help of control measures and vaccination. Why is bluetongue expanding? It could be due to the rising temperatures as a consequence of climate change (which would facilitate that Culicoides imicola dwell in more northern areas); to an adaptation of the virus to other species of Culicoides; or to the emergence of new serotypes of the virus. Currently, BTV is present in all inhabited continents, infecting up to 30 different species of culicoides.

Due to all of the aforementioned situations, bluetongue is considered an emerging disease worldwide and one of the main concerns in Animal Health. Due to its great capability to spread and the economic losses that it causes, it is a notifiable disease. In another step, we will see the control and vaccination measures being taken against this disease.