First seroprevalence study of West Nile Virus (WNV) infection in blood donors after abrupt increase in West Nile Neuroinvasive Disease (WNND) cases in Southern Italy in 2023

Introduction

WNV is an emerging arbovirus commonly transmitted by the Culex species. Human WNV infection can be asymptomatic (80% of cases) or presents with a wide range of clinical manifestations. Less than 1% of cases evolve into a WNND. Until 2023, the Apulia region was classified as a non-endemic area. In 2023 eight autochthonous cases of WNV infection were recorded in Apulia (six manifested WNND and two were identified through blood donor screening). The aim of the study was to assess the prevalence of anti-WNV antibodies in a wide sample of blood donors in order to obtain a more accurate estimate of the virus circulation in the Apulia region.

Methods

A prospective observational study was conducted on blood donors (aged ≥ 18 years), who attended one of the seven blood donation centers located in five of the six provinces in the Apulia region, between November 2023 and February 2024. All patients answered a short anonymous questionnaire. An anonymized serum sample was collected from each subject. All sera were tested for anti-WNV IgG by ELISA (EUROIMMUN, Lübeck, Germany) and by CLIA (VIRCELL, Granada, Spain). All positive samples were confirmed through the Plaque Reduction Neutralization Test (PRNT).

Results

A total of 1,579 subjects were enrolled. The median age was 47 years (IQR:37-53), and 75% were male. The questionnaires administered showed that 68.6% had not made any trips in the past few weeks prior to blood sampling and 30.5% remembered being bitten by mosquitoes. While 17 samples tested by ELISA were positive for anti-WNV IgG, only six were also positive when tested by CLIA. Analysis by PRNT confirmed 5 cases, representing 0.32% (IC95%:0.07-0.59) of the sera. Of the subjects who tested positive, only one reported having traveled in the past week. None recalled being bitten by mosquitoes or habitually using mosquito repellents.

Discussion

Mosquito-borne diseases are an emerging threat in Europe. In recent years, global climate change has been identified as a significant driver of the increased spread of the virus. In particular, increased temperatures and precipitation have played a crucial role in WNV spreading in horses and humans. The average temperature in the Apulia region in 2023 was higher than in previous years. The neuroinvasive form of WNV infection occurs in less than 1% of cases. Based on the prevalence data in blood donors (0.32%) and the Apulian population (about 4milion), it could be estimated that more than 12 thousand infections have occurred.

Conclusion

Our study confirmed the wide circulation of WNV in Apulia and the potential human health concern of this emergent virus. Strengthening the integrated surveillance system is crucial to address a potential massive spread of WNV in Southern Italy.