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

Jiménez-Clavero, Miguel Á

Bluetongue was first described in South Africa in the eighteenth century, affecting imported merino sheep (MacLachlan and Guthrie, 2010). Outside Africa, BTV was detected for the first time in Cyprus in 1943. Shortly after it was found in North America, Middle East, Australia, and Asia, as an endemic disease affecting tropical and subtropical areas with epizootic incursions in temperate zones where the presence of competent vectors allowed its transmission (MacLachlan, 2004) . Traditionally, its natural geographic range was considered to be located between latitudes 35 • S and 40 • N. In Europe, regions with suitable conditions for incursions of BT were the south of the Iberian and Italian Peninsulas and some islands of the Aegean Sea, where climate and presence of competent vectors, along with the proximity to endemic areas (Middle East and Africa) contributed to create an area at high risk of entry of the disease (reviewed in Mellor and Wittmann, 2002; Mellor et al., 2008) . In 1956, BT burst into the Iberian Peninsula from Africa, causing major losses in sheep. However, apart from this episode and some subsequent incursions in Cyprus and the Greek islands of Lesbos and Rhodes, Europe had remained free of this disease, which was considered "exotic" in this continent. This situation began changing after 1998, with increasingly frequent outbreaks in the Mediterranean islands and in southern continental Europe (Mellor and Wittmann, 2002; Mellor et al., 2008) . A parallel upsurge was observed meanwhile in the Middle East and the North of Africa. Between 1998 and 2001, BT outbreaks were declared in the territories of Greece, Bulgaria, Turkey, Macedonia, Serbia, Croatia, Montenegro, Bosnia-Herzegovina, Albania, Italy, France, and Spain. BTVs involved belonged to at least five different serotypes (1, 2, 4, 9, and 16). Four of them (1, 2, 9, and 16) came from the East (Middle East), and two more introductions from the South (Africa) involved serotypes 2 and 4 . Even during this period the disease showed some expansion to the North, breaking the northern limit of the disease in Europe, represented by latitude 40 • N, with some outbreaks occurring near 45 • N in the Balkans. Since then, BT is considered an emerging disease in Europe (Wilson and Mellor, 2009) . But the quantum leap in the epidemiology of the disease in Europe was to take place in August 2006 when an outbreak of BT was declared in the Netherlands at the Maastricht region. The virus isolated in this outbreak belonged to serotype 8, and was the first occurrence of this serotype in Europe (Wilson and Mellor, 2009) . The disease has since spread with unprecedented speed and virulence, affecting first Holland, Belgium, Luxembourg, France, and Germany, and expanding rapidly into the UK, Austria, Czech Republic, Switzerland, Denmark, Italy, and Spain (Saegerman et al., 2008) . Since then the virus showed its ability to overwinter in unusually high latitudes, remaining present in most of Europe, and even spreading to other European countries such as Hungary, Norway, and Sweden. The number of outbreaks produced by this BTV8 in Europe since its first detection in 2006 is 89,136 (EUBTNET, 2011). The incidence of the disease peaked in 2007 with 50,479 outbreaks declared. Massive vaccination campaigns, initiated in 2008, contributed largely to control the disease, so that the number of outbreaks were drastically reduced since 2009, and practically disappeared in 2010 and 2011 (EUBTNET, 2011)