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. 2025 Sep;30(35):2500634.
doi: 10.2807/1560-7917.ES.2025.30.35.2500634.

Outbreak of autochthonous West Nile virus infection in Lazio region, Italy, July to August 2025: preliminary investigation

Ilaria Mussetto  1 , Andrea Bongiovanni  1 , Francesca Colavita  1 , Cristina Giambi  2 , Marcello Giovanni Sala  3 , Cosmo Del Borgo  4 , Fabrizio Carletti  1 , Maria Teresa Scicluna  3 , Alessandro Zerbetto  2 , Angela Corpolongo  1 , Federico Romiti  3 , Maria Beatrice Valli  1 , Stefania Vaglio  5 , Roberto Giammattei  6 , Paola Scognamiglio  1   7 , Gabriella De Carli  1 , Alessandro Agresta  1 , Claudio De Liberato  3 , Giuseppe Di Luzio  8 , Florindo Micarelli  2 , Emanuele Nicastri  1 , Andrea Siddu  7 , Valeria Ficarelli  7 , Enrico Girardi  1 , Fabrizio Maggi  1 , Francesco Vairo  1 ; Lazio WNV Outbreak Group  9 ; Lazio WNV outbreak group
Collaborators, Affiliations

Outbreak of autochthonous West Nile virus infection in Lazio region, Italy, July to August 2025: preliminary investigation

Ilaria Mussetto et al. Euro Surveill. 2025 Sep.

Abstract

In July-18 August 2025, 171 autochthonous cases with West Nile virus (WNV) infection were confirmed in Lazio, Italy: four asymptomatic blood donors, 110 with WNV fever, 57 with neuroinvasive syndrome and nine deaths. WNV lineage 2 was detected in two neuroinvasive cases. Infection with WNV was confirmed in 28 horses, five crows and a Culex pipiens pool. We present the preliminary epidemiological and phylogenetic analysis of the outbreak and the public health measures taken within a One Health approach.

Keywords: Arbovirosis; Autochthonous; Italy; One Health; Outbreak; West Nile virus.

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Conflict of interest statement

Conflict of interest: None declared.

Figures

Figure 1A shows the epidemic curve of confirmed symptomatic human WNV cases by date of symptom onset, subdivided by clinical syndrome (WNV fever and WNV neuroinvasive disease), with first cases starting on 3 July and the last on 16 August 2025. Figure 1B shows the timeline of confirmed and suspected WNV cases by date of sampling, detected through entomological monitoring, active and passive surveillance of sentinel equids, and investigations in synanthropic birds, with the first positive mosquito pool on 9 July and the last suspected equine cases on 11 August 2025.
Figure 1
Timeline of symptom onset of human cases with West Nile virus infection (n = 167) (A) and sampling of animals (B), Lazio region, Italy, July–18 August 2025
The figure presents the map of Lazio, Italy, divided by municipalities (the city of Rome is outlined in red, divided into districts), showing probable places of exposure for 167 confirmed symptomatic human cases with West Nile virus infection between July and 18 August 2025. Municipalities with no notified cases are marked with grey colour. Municipalities with notified cases range from white (one case) to deep red (60 cases). Latina municipality was the area with the highest number of cases (55 cases), followed by Cisterna di Latina (30 cases), and additional cases in surrounding municipalities. Coloured boundary lines indicate the provincial borders of Rome (blue), Latina (green), and Frosinone (purple).
Figure 2
Probable places of exposure of confirmed symptomatic human cases of West Nile virus infection, Lazio region, Italy, July–18 August 2025 (n = 167)
Phylogenetic tree showing the sequences from two autochthonous cases of West Nile virus infection in Lazio, July 2025, and sequences previously reported (from 2013 to 2024) from other human cases marked with green square (fifteen squares), from mosquitoes with blue triangle (eight triangles), sequences from birds with yellow circle (11 circles) and sequences from horses with orange rhombus (two rhombi).
Figure 3
Phylogenetic analysis of full-length genome sequences from two autochthonous cases of West Nile virus infection in Lazio, July 2025 and sequences from humans (n = 15), birds (n = 11) and horses (n = 2)
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References

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