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

Jiménez-Clavero, Miguel Á

But climate change alone is insufficient to explain the whole picture regarding the current rise of BT in Europe. First, I have already mentioned how the wind can drag swarms of infected Culicoides, spreading the disease to considerable distances, and that this is likely route of entry of BT in southern Europe (Ducheyne et al., 2007) , though animal movements could also play a role. I have also mentioned how camels could act as carriers for these viruses from sub-Saharan Africa to North Africa, where BT is often detected some time before affecting Europe (Touil et al., 2012) . But how was BTV8 introduced into northern Europe? Unlike other strains that have invaded Europe in recent years, this BTV-8 strain has not previously been detected in the North of Africa or the Middle East. Among the few sequence data available to date on relevant BTV strains, it has been shown that the most similar virus found is a strain from sub-Saharan Africa (Nigeria) isolated in 1982 (Maan et al., 2008) . This "gap" in space and time indicates clearly that more sequence data from relevant BTV sources are needed to draw any conclusion about the origin of the European BTV-8 strain. As already discussed in the previous section, the trade of animals and animal products, as well as of wildlife (either legal or illegal), may be behind many accidental introductions of infectious diseases, and this hypothesis should be taken into account in this case, although thus far, no evidence has been obtained neither for nor against it (Mintiens et al., 2008) . Other questions remaining where climate change does not provide a full explanation are (1) the virus ability to overwinter in extremely cold conditions such as those found in certain areas of northern Europe, (2) the role that vertical transmission may have on its survival, and (3) phenotypic changes perceived in some recent BTV variants, which are more pathogenic and/or affect other species such as cattle. More research is needed to find answers to these questions.