Usutu virus: an emerging flavivirus in Europe

Abstract

Usutu virus (USUV) is an African mosquito-borne flavivirus belonging to the Japanese encephalitis virus serocomplex. USUV is closely related to Murray Valley encephalitis virus, Japanese encephalitis virus, and West Nile virus. USUV was discovered in South Africa in 1959. In Europe, the first true demonstration of circulation of USUV was reported in Austria in 2001 with a significant die-off of Eurasian blackbirds. In the subsequent years, USUV expanded to neighboring countries, including Italy, Germany, Spain, Hungary, Switzerland, Poland, England, Czech Republic, Greece, and Belgium, where it caused unusual mortality in birds. In 2009, the first two human cases of USUV infection in Europe have been reported in Italy, causing meningoencephalitis in immunocompromised patients. This review describes USUV in terms of its life cycle, USUV surveillance from Africa to Europe, human cases, its cellular tropism and pathogenesis, its genetic relationship with other flaviviruses, genetic diversity among USUV strains, its diagnosis, and a discussion of the potential future threat to Asian countries.

Figure 1

Geographic locations of USUV-related epidemiological studies on birds and mosquitoes in Europe. These studies were conducted using immunohistochemistry, reverse transcription-PCR, indirect immunofluorescence assay, ELISA, and plaque reduction neutralization assay [10,11,25,29,30,31,32,33,38,39,40,41,42,43,44]. Each epidemiological study is indicated by ●.

Figure 2

USUV model, its gene structure, and the proteins encoded by its genome. The polyprotein precursor is cleaved by cellular and viral proteases to yield three structural proteins (C, prM, and E) and eight non-structural proteins (NS1, NS2A, NS2B, NS3, NS4A, 2K, NS4B, and NS5).

Figure 3

(A) Phylogenetic tree based on complete genome sequence analysis. This figure illustrates the close genetic relationship between USUV and MVEV, WNV, and JEV. The relative accession numbers of selected flavivirus sequences are listed in Table 2. The number at each node is the percentage of 1000 bootstrap replicates. (B) Phylogenetic tree based on polyprotein precursor sequence analysis. This figure illustrates the close genetic relationship between USUV and MVEV, WNV, and JEV. The relative accession numbers of selected flavivirus sequences are listed in Table 2. The number at each node is the percentage of 1000 bootstrap replicates.

Figure 3

(A) Phylogenetic tree based on complete genome sequence analysis. This figure illustrates the close genetic relationship between USUV and MVEV, WNV, and JEV. The relative accession numbers of selected flavivirus sequences are listed in Table 2. The number at each node is the percentage of 1000 bootstrap replicates. (B) Phylogenetic tree based on polyprotein precursor sequence analysis. This figure illustrates the close genetic relationship between USUV and MVEV, WNV, and JEV. The relative accession numbers of selected flavivirus sequences are listed in Table 2. The number at each node is the percentage of 1000 bootstrap replicates.

Figure 4

Comparison of polyprotein precursors of fully sequenced USUV strains. Anchored C protein (1–126), prM protein (127–293), E protein (294–793), NS1 protein (794–1145), NS2A protein (1146–1372), NS2B protein (1373–1503), NS3 protein (1504–2122), NS4A protein (2123–2248), 2K protein (2249–2271), NS4B protein (2272—2529), and NS5 protein (2530–3434). All amino acid substitutions are shown with bolded and enlarged letters.

Figure 5

Comparison of 5ʹ UTRs and 3ʹ UTRs of fully sequenced USUV strains. 5ʹ UTR (1–96), open reading frame (ORF;97–10,401), 3ʹ UTR (10,402–11,064). All nucleotide substitutions are shown with bolded and enlarged letters. Nucleotide deletions are indicated by an asterisk (*).