Usutu virus (USUV) is a mosquito‑borne flavivirus that primarily infects birds and can occasionally cause neuroinvasive disease in humans. It is an enveloped, positive-sense RNA virus with a genome of about 11 kilobases that encodes a single polyprotein subsequently cleaved into three structural proteins—capsid, precursor membrane and envelope—and seven nonstructural proteins. The virion exhibits icosahedral symmetry and has a lipid envelope covered with E glycoproteins responsible for receptor binding and membrane fusion.
Genome and Virology
USUV shares structural and genetic features with other Flavivirus members such as West Nile virus. Its 5′-capped genomic RNA lacks a poly(A) tail and is translated into a polyprotein at endoplasmic reticulum membranes. Host signal peptidases and the viral NS2B‑NS3 serine protease process the polyprotein into functional components. Replication occurs on virus-induced membranous structures in the endoplasmic reticulum, where the NS5 polymerase synthesises negative-sense intermediates and new genomes. Virions assemble by budding into the endoplasmic reticulum and mature during passage through the Golgi apparatus. USUV circulates in distinct phylogenetic lineages (African lineages 1–3 and European lineages 1–5) that have emerged through multiple introductions and local evolution. The virus replicates efficiently in Culex mosquito midgut and salivary gland tissues and in avian cells, particularly those of passerine birds. Experimental studies show infection of mammalian neurons and astrocytes, consistent with its neurotropic potential.
Ecology and Human Impact
USUV was first isolated from Culex neavei mosquitoes in South Africa in 1959. It is maintained in a bird‑mosquito enzootic cycle, with blackbirds, thrushes and owls serving as major amplifying hosts. Since the early 2000s, USUV has repeatedly emerged in Europe, causing mass mortality among birds in Austria, Germany, Switzerland and other countries. Serosurveys indicate widespread circulation in African birds and sporadic spillover to humans. Human infections are usually asymptomatic or mild; however, several cases of meningoencephalitis and neuroinvasive disease have been reported, mainly in immunocompromised patients. Co-circulation of USUV with West Nile virus complicates surveillance and diagnostic interpretation due to serological cross-reactivity. There is no specific treatment or vaccine for USUV infection. Public health efforts focus on monitoring bird die-offs, vector control and differential diagnosis in patients with neuroinvasive disease. Usutu virus underscores the importance of surveillance for emerging arboviruses, particularly in areas where closely related flaviviruses co-circulate. Related Terms: West Nile virus, Japanese encephalitis virus, Saint Louis encephalitis virus, Zika virus, Flavivirus