Animal viral diseases and global change: bluetongue and West Nile fever as paradigms

Jiménez-Clavero, Miguel Á

Survival and transmission of BT in an area requires the presence of Culicoides vectors. To be transmitted, BTV must infect a competent Culicoides vector. In this process the vector acquires the infection by feeding on blood from a viremic host, then the virus multiplies and disseminates throughout the vector body, reaching the salivary glands, ready to be inoculated in the next host after biting on a further blood feed. There is no transovarial transmission of the virus. There are approximately 1,500 known species of Culicoides midges, of which about 50 have been shown to be competent for BTV transmission (Wilson and Mellor, 2009 ). In different endemic areas, BTV is transmitted through one or a few distinct local Culicoides species. For example, in North America this species is Culicoides sonorensis, while in southern Europe and Africa Culicoides imicola is the dominant vector. The distribution and abundance of Culicoides species involved in BTV transmission matches with the distribution of the disease in endemic areas (Mellor and Wittmann, 2002) . It also depicts the maps of areas at risk for its introduction. Similarly, seasonality strongly influences vector populations throughout the year, and this determines the periods of occurrence of disease cases (Mellor and Wittmann, 2002) . For instance, in temperate zones of the northern Hemisphere most cases of disease occur between August and November, coinciding with the period of greatest abundance of vectors. Among the key factors linking weather to BTV epidemiology, temperature has a crucial influence on vector survival, which, as noted above, directly affects disease occurrence, transmission, and spread. For example, the mean survival time of a Culicoides midge depends on the temperature, i.e., survive longer at lower temperatures (Veronesi et al., 2009 ). By contrast, higher temperatures promote breeding and feeding of the vectors, which increases virogenesis and transmission of BTV (Wilson and Mellor, 2009 ). The optimal range of temperature lies between 13 and 35 • C. However, excessively high (above 40 • C) or low (<0 • C) temperatures are lethal in a short period of time. Between 0 and 13 • C Culicoides midges remain in a state of dormancy that allows them to survive until more favorable temperatures allow resuming their activity. Relative humidity is also a key factor in Culicoides life cycle . The mentioned temperature ranges are valid only above a minimum relative humidity. Winter survival (overwintering) is crucial for the maintenance and consolidation of the presence of BTV in a geographic area. Midge survival is favored by mild, frost-free, winters. Wind is another important factor that links weather to BTV eco-epidemiology. Wind blows can drag swarms of Culicoides midges, and bring them hundreds of kilometers away. This mechanism has been widely recognized as an important way Culicoides (and BTV, if present in the vectors) spread, bridging distances up to 700 km under favorable conditions (Carpenter et al., 2009; MacLachlan and Guthrie, 2010) . This form of dispersal is the most likely way BTV has reached the shores of southern Europe from Africa in many occasions in the last years (Wilson and Mellor, 2009 ).