Emergence of Usutu virus, an African mosquito-borne flavivirus of the Japanese encephalitis virus group, central Europe

Abstract

During late summer 2001 in Austria, a series of deaths in several species of birds occurred, similar to the beginning of the West Nile virus (WNV) epidemic in the United States. We necropsied the dead birds and examined them by various methods; pathologic and immunohistologic investigations suggested a WNV infection. Subsequently, the virus was isolated, identified, partially sequenced, and subjected to phylogenetic analysis. The isolates exhibited 97% identity to Usutu virus (USUV), a mosquito-borne Flavivirus of the Japanese encephalitis virus group; USUV has never previously been observed outside Africa nor associated with fatal disease in animals or humans. If established in central Europe, this virus may have considerable effects on avian populations; whether USUV has the potential to cause severe human disease is unknown.

Figure 1

Histology and detection of viral signals in paraffin-embedded tissue sections of birds infected with Usutu virus (USUV). A–C, Eurasian Blackbird; D–F, Great Gray Owl; A, no histologic lesions present, hematoxylin and eosin staining; B immunohistochemistry, using a polyclonal antibody to West Nile virus, shows numerous positive neurons; C, in situ hybridization with USUV-specific oligonucleotide probe shows a staining pattern comparable with that in B; D, microglial nodule within the cerebral cortex, hematoxylin and eosin staining; E, immunohistochemistry shows single positive neurons within a glial nodule; F, in situ hybridization shows several positive neurons next to a glial nodule. Original magnification, x 130 (A–C), x 200 (D–F).

Figure 2

Phylogenetic analysis of several members of the Japanese encephalitis virus (JEV) group and selected other mosquito-borne flaviviruses demonstrates the close genetic relationship of the Austrian Usutu virus (USUV) isolate with the South African USUV (red underlay); well-known members of the JEV group are highlighted in red; distinct branches are formed by Saint Louis encephalitis virus, Dengue virus (type 3), and Yellow fever virus. The partial nucleotide sequence of the Austrian USUV isolate used in the phylogenetic tree has been deposited in the GenBank database under accession no. AF452643.