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.
Highlights
West Nile virus (WNV) (Flaviviridae: Flavivirus) is an arbovirus circulating among mosquitoes, which serve as the vectors, and wild birds, which serve as the main reservoir hosts
The entomological surveillance system detected a wide circulation of WNV, characterizing the dynamics and extension of the 2013 outbreak in Pianura Padana
Virus detection in mosquitoes, achieved at the beginning of July, clearly precedes the appearance of human cases, which were recorded from the end of July, as reported elsewhere [20, 21, 22]
Summary
West Nile virus (WNV) (Flaviviridae: Flavivirus) is an arbovirus circulating among mosquitoes, which serve as the vectors, and wild birds, which serve as the main reservoir hosts. WNV is characterized by a complex pattern of circulation, involving different vector and reservoir species in different areas and ecological conditions, depending on mosquito and bird species richness and abundance [1, 2]. WNV raises public health and veterinary concerns for its ability to infect horses and humans, which are considered dead-end hosts due to their inability to develop a sufficient viremia to infect mosquitoes [2, 3]. In addition to the direct risk to human health, asymptomatic blood donors represent a recognized problem for the safety of blood transfusions in affected areas [2, 5]
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