Epidemiological and clinical characteristics of West Nile virus infections in Istanbul, Türkiye: a population-based cohort study.

BACKGROUND

West Nile virus (WNV) infection is a mosquito-borne infection that is usually asymptomatic but can also cause serious neurological diseases. This study aimed to determine the incidence and seasonal distribution of WNV infections in Istanbul, Turkiye, as well as risk factors for neuroinvasive disease and mortality.

METHODS

A population-based cohort study was conducted using surveillance data collected by the Health Directorate of Istanbul between 2010 and 2024. Cases meeting the national and EU case definitions for confirmed or probable WNV infection were included. Clinical, demographic, laboratory data, and outcomes were extracted from medical records. Relative risks (RR) and 95% confidence intervals (CI) were calculated using univariate analyses and log binomial regression to identify risk factors for neuroinvasive disease and mortality.

RESULTS

A total of 159 reported WNV cases were included in the study, with incidence rates of 0.53 and 0.48 per 100,000 person-years calculated for the 2019 and 2024 outbreaks, respectively. All infections were reported between June and October. The mean patient age was 52.9 years; 65.4% were male. West Nile neuroinvasive disease (WNND) was diagnosed in 50.9% of cases, with encephalitis as the most common manifestation. The overall case fatality rate was 5.7% in all case, and 9.8% among WNND cases. Age was identified as an independent risk factor for WNND (RRadj=1.016, p < 0.001) according to the log binominal regression model. In univariate analyses, non-survivors were older (RR = 1.08; 95% CI: 1.02-1.14) and more frequently had diabetes mellitus (RR = 3.98; 95% CI: 1.13-14.08) and chronic obstructive pulmonary disease (RR = 7.29; 95% CI: 1.90-27.91) compared with survivors.

CONCLUSIONS

This study highlights the epidemiological and clinical characteristics of WNV infections over a 15-year period in Istanbul, Turkiye, identifying two major outbreaks in 2019 and 2024, with seasonal peaks in summer and early fall. The findings highlight the importance of increased clinical awareness and the inclusion of WNV in the differential diagnosis of encephalitic presentations, particularly in elderly patients during periods of high transmission.