Molecular evidence of neuroinvasive Sindbis virus infection in humans: detection in cerebrospinal fluid by next generation sequencing.

Sindbis virus (SINV) is a mosquito-borne alphavirus causing seasonal outbreaks in northern Europe, Africa, and Russia. Neurological involvement in humans is poorly documented, and detection in cerebrospinal fluid (CSF) has not previously been reported. This study provides the first unequivocal evidence of human CNS involvement by detecting SINV RNA directly in the cerebrospinal fluid of four autochthonous patients presenting with acute neurological symptoms in south eastern Tuscany, Italy, July-August 2025. Utilizing metagenomic Next Generation Sequencing (mNGS), we reconstructed complete viral genomes, strongly supporting a causal relationship between SINV and neurological manifestations. Phylogenetic analysis revealed a complex epidemiological landscape in Italy characterized by the co-circulation of Clade D lineages. Our findings fundamentally expand the clinical spectrum of SINV, demonstrating that it is not merely an arthritogenic pathogen, but a neuroinvasive threat to humans. This highlights the critical need to include SINV in the differential diagnosis of viral CNS infections in endemic areas and underscores the urgency for enhanced European laboratory surveillance.