Bluetongue is a disease of ruminants caused by Bluetongue virus (BTV) and it is transmitted by biting midges. In sheep, clinical signs of BTV infection can include fever, vasodilation, swelling and, in severe cases, death, although the severity of symptoms varies with the breed of sheep, the individual animal and the strain of virus involved. Cattle are a major reservoir host for BTV infection, primarily because of the less obvious clinical signs in these animals. After an exhaustive search for a natural agent of transmission, Culicoides midges were shown to be the vectors for Bluetongue virus (Culicoides and the emergence of bluetongue virus in northern Europe. Trends Microbiol 2009 17(4): 172-178).
Although BTV infection was initially centred in Africa, the virus was first detected in Greece in 1989, from where it has spread steadily north. Since the disease is spread exclusively by insects, and because the virus is quite specific about which midge species can be used as vectors, predictions about the spread of the disease were based on known ranges of different midge species, which in turn depends on climate. However, midges can sometimes be carried over very long distances by weather systems, or by ships or aircraft.
Despite widespread speculation regarding the exact origin of BTV-8 as the strain of the virus found in northern Europe, no single convincing hypothesis has been proposed. Although future full-genome sequencing might assist this task (as was the case in the incursion of West Nile virus into North America), the small number of reference strains of BTV-8 from areas of potential origin collected before the incursion into northern Europe makes it unlikely that this approach will provide unambiguous evidence. As long as our understanding of the potential routes of virus introduction remains poor, we will be unable to accurately estimate the potential for future introductions of BTV, as has been illustrated by the more recent detection of BTV-6 in Europe, or of other midge-borne arboviruses, such as African horse sickness virus (AHSV).
Although the technology to produce safe, effective, inactivated vaccines existed, no coordinated action was taken by any Member State of the European Union (EU) to begin production of a BTV-8 vaccine until late 2007, when the full damage began to become evident. This was in part due to the assumption that the virus would not overwinter under northern European conditions (despite the fact that BTV had been documented overwintering successfully in other areas with far cooler winter temperatures). In the absence of an available vaccine, knowledge concerning the entomology of the insects involved in BTV transmission became paramount.
The spread of BTV has provided a severe test of the way in which the movement of vector-borne pathogens is predicted, identified and controlled in Europe. There are many arobovirus diseases (spread by arthropod vectors such as midges, mosquitos and ticks), affecting human as well as animal health. Whether BTV represents a herald for future incursions by other arboviruses into Europe remains difficult to know. It is clear that there exists a similar potential for emergence of other insect-borne pathogens on grounds of climate alone, but where different vectors are used – for example, in the case of AHSV – the dynamics of the current BTV outbreak cannot easily be used to estimate risk. What has been shown by this outbreak is that arbovirus–vector relationships are highly dynamic and extremely difficult to combat. Unless regions that are potentially at risk of transmission are prepared to invest the resources required to provide adequate information regarding vectors and suitable control methods, this will remain the case.