LAUSR

The survival and transmission of vector-borne pathogens is dependent on the interactions between the pathogen, its vertebrate host and the specific midge vector. Studies on the interactions between the vector and the vertebrate host have examined the host ranges of different vector species and determined the relative roles of different vertebrate species in supporting the haematophagous insect vector population (23). A comparatively new area of research is the study of the vector-host interface, i.e. the site of the midge attachment and feeding. It is the site at which midge-borne pathogens pass from an invertebrate to the vertebrate host system, which is a key step in the transmission dynamics and indeed the survival of the pathogen (37). Bluetongue virus (BTV) is one of the best-known examples of pathogen-vector-host interaction. Changes in the epidemiology of BTV in Europe, particularly since 1998, when the virus spread northwards into the Mediterranean Basin (36), have stimulated research on this subject. BTV (family Reoviridae, genus Orbivirus) causes the hemorrhagic disease bluetongue (BT) in both domestic and wild ruminants. It is an infectious but non-contagious disease that causes high socio-economic and sanitary consequences. The most common signs of the disease include fever, catarrhal, stomatitis, rhinitis, enteritis and lameness (21). BT is endemic in many tropical, sub-tropical and temperate regions of the world (Africa, southern Asia, Australia, the Middle East, and the Americas), between latitudes 40°N and 53°S, during times of the year that are optimal for vector activity (36). In August 2006, for the first time, BTV passed the latitude 50°N and disease caused by virus serotype 8 occurred in north-western Europe: the Netherlands, Belgium, Germany, France and Luxembourg. In the next two years, BTV spread to other regions of Europe, the number of outbreaks increased rapidly (about 50000 cases of disease were reported), and new BTV serotypes (BTV1 and BTV6) were detected (38). To date, twenty seven immunologically distinct serotypes of BTV have been identified worldwide (39). It is a small icosahedral virus (of about 90 nm in diameter) with a genome of approximately 19200 base pairs, composed of ten linear segments of double-stranded RNA (dsRNA) (28), which encode ten distinct virus proteins (35). Seven of these BTV-vector-host interaction: the epidemiological triangle in disease transmission