Clinical and virological findings in patients with Usutu virus infection, northern Italy, 2018

BACKGROUND: Usutu virus (USUV) is a mosquito-borne flavivirus, which shares its transmission cycle with the phylogenetically related West Nile virus (WNV). USUV circulates in several European countries and its activity has increased over the last 5 years. AIM: To describe human cases of USUV infection identified by surveillance for WNV and USUV infection in the Veneto Region of northern Italy in 2018. METHODS: From 1 June to 30 November 2018, all cases of suspected autochthonous arbovirus infection and blood donors who had a reactive WNV nucleic acid test were investigated for both WNV and USUV infection by in-house molecular methods. Anti-WNV and anti-USUV IgM and IgG antibodies were detected by ELISA and in-house immunofluorescence assay, respectively; positive serum samples were further tested by WNV and USUV neutralisation assays run in parallel. RESULTS: Eight cases of USUV infection (one with neuroinvasive disease, six with fever and one viraemic blood donor who developed arthralgia and myalgia) and 427 cases of WNV infection were identified. A remarkable finding of this study was the persistence of USUV RNA in the blood and urine of three patients during follow-up. USUV genome sequences from two patients shared over 99% nt identity with USUV sequences detected in mosquito pools from the same area and clustered within lineage Europe 2. CONCLUSIONS: Clinical presentation and laboratory findings in patients with USUV infection were similar to those found in patients with WNV infection. Cross-reactivity of serology and molecular tests challenged the differential diagnosis.