Usutu Virus: An Arbovirus on the Rise

Abstract

The Usutu virus (USUV) is a flavivirus that is drawing increasing attention because of its potential for emergence. First isolated in Africa, it was introduced into Europe where it caused significant outbreaks in birds, such as in Austria in 2001. Since then, its geographical distribution has rapidly expanded, with increased circulation, especially in the last few years. Similar to West Nile virus (WNV), the USUV enzootic transmission cycle involves Culex mosquitoes as vectors, and birds as amplifying reservoir hosts, with humans and other mammals likely being dead-end hosts. A similarity in the ecology of these two viruses, which co-circulate in several European countries, highlights USUV's potential to become an important human pathogen. While USUV has had a severe impact on the blackbird population, the number of human cases remains low, with most infections being asymptomatic. However, some rare cases of neurological disease have been described, both in healthy and immuno-compromised patients. Here, we will discuss the transmission dynamics and the current state of USUV circulation in Europe.

Keywords: USUV; Usutu; WNV; emergence; flavivirus.

Figure 1

Maximum-likelihood phylogenetic tree analysis of the NS5 gene of USUV strains (n = 158) circulating worldwide. Strains isolated from birds, mosquitoes and mammals are indicated in green, blue and red, respectively. African and European variants are shaded in pink and gray, respectively. The prototype SAAR-1776 strain is indicated by a star. The bar at the bottom of the tree denotes evolutionary distance, as number of base substitutions per site. The interrupted branches (indicated by oblique lines) were shortened by 50% for better graphic representation. The list of USUV strains used to generate the tree are provided in Supplemental Materials: Table S1.

Figure 2

Geographical distribution of USUV. Countries where USUV was detected in wildlife (birds, mosquitoes, horses, etc.) are indicated in green; countries where USUV was detected in healthy blood donors are represented in blue; and countries where USUV caused symptomatic, acute infections are shown in orange. The arrows represent the presumed bird migration event that led to introduction of USUV in Europe, according to Engel et al. [7].