Serological evidence of subclinical Usutu virus exposure among West Nile virus-seropositive individuals in Israel.

Background

Usutu virus (USUV) is a mosquito-borne orthoflavivirus closely related to West Nile virus (WNV), with confirmed circulation in mosquitoes and horses in Israel but no reported human exposure.

Objectives

To determine whether USUV circulates in humans in Israel and to assess its contribution to serological cross-reactivity with WNV.

Methods

A cohort of 219 Sheba healthcare workers (HCW) seropositive for WNV-reactive IgG was tested for neutralizing antibodies against WNV and USUV using a reporter viral particle (RVP)-based neutralization assay. A subset of 41 USUV-exposed samples from this cohort was additionally tested for Japanese encephalitis virus (JEV) neutralizing antibodies to rule out JEV vaccination as a confounder. USUV-specific RT-qPCR was performed on whole blood from 49 WNV-probable, RNA-negative patients to assess for acute USUV infection.

Results

Among 219 HCW seropositive for WNV-reactive IgG, 47 (21%) were exposed to USUV. Among USUV-exposed samples tested for JEV neutralizing antibodies, 80% retained a four-fold or higher USUV-neutralizing titer relative to JEV and none showed a higher JEV-neutralizing titer, indicating that JEV vaccination does not account for USUV exposure in this cohort. USUV RNA was undetectable in all 49 WNV-probable, RNA-negative samples, providing no evidence of acute USUV infection in this clinically symptomatic population.

Conclusions

This study provides the first serological evidence of USUV exposure in humans in Israel. Since USUV likely causes mild or asymptomatic infection, USUV RNA testing of blood donations should be considered.