INTRODUCTION
HUGH REID∖ HERBERT WEISSENBOCK2 AND KAROLY ERDELYI3
1 The Moredun Foundation, Pentlands Science Park, Bush Loan, Penicuik, Midlothian, Scotland
2Pathology and Forensic Veterinary Medicine, Department of Pathobiology, University of Veterinary Medicine, Vienna, Austria
3 Central Agriculture Office, Veterinary Diagnostic Directorate, Budapest, Hungary
The arthropod-transmitted flaviviruses were originally classified according to epidemiological and antigenic criteria as Group B arboviruses and subsequently on an ultrastructural basis as Togaviruses.
These taxonomic criteria have been replaced by highly analytical molecular technologies that recognize a distinct group of positive- stranded RNA viruses known as the Flavivirus genus, named after the first identified virus of the group, Yellow fever virus. The genus is classified in the family Flaviviri- dae, which includes two other principle genera: Pestivirus (Bovine viral diarrhea 1 and others) and Hepacivirus (Hepatitis C virus).According to their ecology, the Flaviviridae can be divided into three main clusters: tick-borne viruses, mosquito-borne viruses and viruses with no known arthropod vector. Within the tick-borne viruses, the mammalian tick-borne virus group (which harbours the Tick-borne encephalitis virus) and the seabird tick-borne virus group are distinguished.
Flaviviruses are small (approx 50 nm in diameter), consisting of a dense inner core surrounded by a lipid envelope, which is formed of two major viral proteins: the E (envelope) protein, which is the major viral antigenic determinant; and the M (membrane) protein. Within the envelope is a discrete nucleocapsid consisting of a capsid protein and the genomic RNA.
There are more than 50 identified Flavivirus species, the majority of which are recognized as human pathogens causing fever, encephalitis and haemorrhagic fever.
The majority are transmitted by mosquitoes and are distributed globally, predominantly in the tropics. In Europe, only a comparatively small number of mosquito- borne flavivi- ruses (e.g. West Nile virus, Usutu virus) has been found. There is, however, an important group of tick-transmitted viruses present throughout the northern temperate zones known as the tick-borne encephalitides (TBE). Characteristically, flaviviruses are maintained in an occult vector/ wildlife cycle in the absence of any disease. It is only when human activity interposes into or perturbs this cycle that disease occurs. Serious wildlife mortality can occur when these viruses are geographically translocated. Yellow fever virus continues to cause periodic mortality in South American wildlife, centuries following its introduction from West Africa. In recent years, the West Nile virus has been introduced into North America, where it has been responsible for extensive mortality in the native avifauna and its subtler but distinct effect has also been recognized in Europe. Climate change and the predicted increase in human population will probably contribute to further similar introduction events and gradually modify the distribution of endemic areas.Infectious Diseases of Wild Mammals and Birds in Europe, First Edition. Edited by Dolores Gavier-Widen, J. Paul Duff, and Anna Meredith. © 2012 Blackwell Publishing Ltd. Published 2012 by Blackwell Publishing Ltd.