It was first seen in France in 2000, and entered the country via Corsica. The sub-Saharan origin of Culicoides imicola, the main vector of the disease in the Mediterranean, prompted researchers to look into the environmental characteristics that favour its establishment and the appearance of the disease that it spreads within Mediterranean ecosystems.
They set out to study this issue in the case of Corsica, where diseased animals were seen in 2000, 2001 and again in 2003 and 2004. The virus is transmitted by female Culicoides imicola that feed on the blood of animals. These tiny midges (1 to 2 mm) are poor fliers, and thus have to establish themselves close to livestock farms if they are to find food. The researchers thus looked closely at the immediate surroundings of 80 sheep farms in the South of the island. To characterize these different environments, they used spatial images supplied by the French SPOT* satellite. The aim was to identify variables that could be used to distinguish between infected and healthy farms. The researchers studied various parameters linked to height above sea level, for instance slope or mean height, to hydrography, particularly the length of any rivers in the area, and to vegetation, through the abundance of the different types of land occupation or landscape structure. This last type of indicator was what made the study so original: "The high spatial resolution images from the SPOT satellite, with a resolution of around ten metres, provided us with landscape indicators, whereas previous studies, based on low-resolution images, only provided information on climatic factors", stresses Hélène Guis, who wrote a thesis on the subject. "This was a first in studies of bluetongue disease."
Sardinia, a transit zone for midges heading for Corsica
An analysis of the data provided two main results. Firstly, the number of farms affected was much higher in the far South of the study zone than elsewhere. Was this because of a higher temperature due to their geographical position? Apparently not: in Corsica, the temperature gradient varies according to altitude rather than latitude. According to the researchers, it is the proximity of Sardinia, which is just 12 kilometres from the Corsican coast in this area, that accounts for the preferential emergence of the disease there. "We know that midges can be blown long distances by the wind", Hélène Guis points out, "and the viral serotypes responsible for the epidemics in Corsica in 2000, 2003 and 2004 had all been seen in Sardinia a few months beforehand."
The results also show that landscape fragmentation, which increases the area of contact between different types of vegetation, also increases the risk of the disease. That risk is higher in the case of extensive transitional areas, particularly wooded zones (scrub, garrigue and broad-leaved trees), open prairies and bare soils. Zones with extensive transitional areas, with significant intermingling of different types of vegetation, may contain various essential elements of the midge's habitat, such as egg-laying and rest sites and host animals. To know more, it will be necessary to carry out entomological studies.
The study also produced a third result: mixed sheep-cattle-goat farms are more susceptible to the disease than farms with sheep alone. It was previously thought that cattle attracted more midges than sheep, and thus protected the sheep from midge bites. The results obtained here seem to suggest that cattle, which are healthy carriers of the disease in Corsica, may act as virus reservoirs and in fact increase the risk of the disease in the sheep alongside which they are reared. However, this result needs to be confirmed on a larger number of farms.
Still to come: a combined study of climatic and landscape factors
The model developed based on these results has been validated in Corsica on 134 farms around Ajaccio. However, it is not applicable to the problem in northern Europe, since while the disease is also found there, the viral serotype involved is different and, furthermore, the vectors, which are also different from those in Corsica, have yet to be categorically identified. In particular, Culicoides imicola is not found in the zone. Nevertheless, a similar modelling approach could be taken there so as to identify the environmental risk factors linked to the emergence of the disease in the region. As regards Corsica, a new study is due to begin in October 2007. It will be a combined study of climatic, environmental and landscape factors, and is to be conducted in partnership with the University of Liverpool, United Kingdom.
* Satellite Pour l’Observation de la Terre (Earth observation satellite)
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