West Nile Virus Surveillance in 2013 via Mosquito Screening in Northern Italy and the Influence of Weather on Virus Circulation

PLoS One. 2015 Oct 21;10(10):e0140915. doi: 10.1371/journal.pone.0140915. eCollection 2015.

Abstract

West Nile virus (WNV) is a recently re-emerged health problem in Europe. In Italy, an increasing number of outbreaks of West Nile disease, with occurrences of human cases, have been reported since 2008. This is particularly true in northern Italy, where entomological surveillance systems have been implemented at a regional level. The aim of this study was to use, for the first time, all the entomological data collected in the five regions undergoing surveillance for WNV in northern Italy to characterize the viral circulation (at a spatial and temporal scale), identify potential mosquito vectors, and specify relationships between virus circulation and meteorological conditions. In 2013, 286 sites covering the entire Pianura Padana area were monitored. A total of 757,461 mosquitoes were sampled. Of these, 562,079 were tested by real-time PCR in 9,268 pools, of which 180 (1.9%) were positive for WNV. The largest part of the detected WNV sequences belonged to lineage II, demonstrating that, unlike those in the past, the 2013 outbreak was mainly sustained by this WNV lineage. This surveillance also detected the Usutu virus, a WNV-related flavivirus, in 241 (2.6%) pools. The WNV surveillance systems precisely identified the area affected by the virus and detected the viral circulation approximately two weeks before the occurrence of onset of human cases. Ninety percent of the sampled mosquitoes were Culex pipiens, and 178/180 WNV-positive pools were composed of only this species, suggesting this mosquito is the main WNV vector in northern Italy. A significantly higher abundance of the vector was recorded in the WNV circulation area, which was characterized by warmer and less rainy conditions and greater evapotranspiration compared to the rest of the Pianura Padana, suggesting that areas exposed to these conditions are more suitable for WNV circulation. This observation highlights warmer and less rainy conditions as factors able to enhance WNV circulation and cause virus spillover outside the sylvatic cycle.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Culex / virology*
  • Disease Outbreaks
  • Epidemiological Monitoring*
  • Humans
  • Insect Vectors / virology*
  • Italy / epidemiology
  • Weather
  • West Nile Fever / epidemiology*
  • West Nile virus / isolation & purification*

Grants and funding

The Regione Emilia-Romagna (www.regione.emilia-romagna.it) had partially funded this work to IZSLER (www.izsler.it) and CAA (www.caa.it) by the Regional project on insect borne diseases (COMM07001). The Regione Veneto (http://www.regione.veneto.it) and the Regione Friuli Venezia Giulia (http://www.regione.fvg.it) had partially funded this work to IZSVe by the Regional projects on surveillance of vector-borne diseases. No other external funding was utilized for this study. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Entostudio snc provided support in the form of a salary for author MS, but did not have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. The specific role of this author is articulated in the ‘author contributions’ section.