Bluetongue epidemiology in the European Union: current status and perspectives.

Bluetongue epidemiology in the European Union: current status and perspectives Bluetongue (BT) is a World Organization for Animal Health notifiable disease and is of considerable socioeconomic concern and of major importance for the international trade of animals and their products. Before 1998, BT was considered an exotic disease in Europe. Between 1998 and 2005, at least 6 viral strains of bluetongue virus (BTV) belonging to 5 distinct serotypes (1, 2, 4, 9 and 16) have been constantly present in the Mediterranean Basin. Since August 2006, BTV serotype 8 emerged and caused a severe epizootic in northern Europe. BTV-8 showed a higher tropism for bovines and an increased virulence characterized by the development of severe clinical signs and reproductive disorders. Endogenous vectors have been involved and in particular the Culicoides obsoletus, C. dewulfi and C. chiopterus complexes. During 2007 the re-emergence and the recrudescence of BTV-8 created the conditions for the establishment of an endemic infection in this region. Furthermore, the radial extension of BTV-8 throughout Europe and the progression of BTV-1 to the South-West of France, increases the risk of an encounter between these serotypes and others, particularly those occurring in the Mediterranean Basin. During its southern progression, BTV might encounter C. imicola, a vector with a longer activity throughout the year. These conditions increase the risks of reassortment events between the genome segments of the different BTV serotypes. A new reassortant virus may be characterized by differences in the virulence compared to its parental viruses. Two prophylactic measures can be applied to face an endemic situation: vaccination with inactivated vaccines and use of insecticides to reduce the contacts between vectors and susceptible animals. Understanding the BT epidemiology depends in the first place on a full understanding of the virology, pathogenesis and immunology involved. This necessitates continued scientific research and vigilance. In addition to the preponderant vector transmission, other transmission routes have been observed and documented in cattle with serotype 8, such as transplacental infection during vector-free period and of lesser importance horizontal (oral) transmission. These aspects promote the maintenance of the disease in Europe and need to be also considered in the development of control strategies.