Enhanced West Nile Virus Circulation in the Emilia-Romagna and Lombardy Regions (Northern Italy) in 2018 Detected by Entomological Surveillance

Abstract

With several human cases reported annually since 2008 and the unapparent risk of infection of blood donors, the West Nile virus (WNV) is emerging as an important health issue in Europe. Italy, as well as other European countries, experienced a recrudescence of the virus circulation in 2018, which led to an increased number of human cases. An integrated surveillance plan was activated in the Emilia-Romagna and Lombardy regions (Northern Italy) since 2008 in order to monitor the intensity and timing of WNV circulation. A fundamental part of this plan consists in entomological surveillance. In 2018, the surveillance plan made it possible to collect 385,293 mosquitoes in 163 stations in the two Regions. In total 269,147 Culex mosquitoes were grouped into 2,337 pools and tested for WNV, which was detected in 232 pools. Circulation started in the central part of the Emilia-Romagna region in the middle of June, about one month before the previous seasons. Circulation suddenly expanded to the rest of the region and reached the Lombardy region in the middle of July. WNV circulated more intensively in the eastern part of the surveyed area, as confirmed by the highest number of human cases. A relationship between the number of mosquitoes collected and the virus incidence emerged, but the data obtained highlighted that the probability of detecting the virus in a given site was less than expected with a higher number of collected mosquitoes. A significant relationship was observed between the temperature recorded one week before the sampling and the number of collected mosquitoes, as well as between the estimated number of WNV-positive mosquitoes and the temperature recorded two weeks before the sampling. The two weeks delay in the influence of temperature on the positive mosquitoes is in line with the time of the virus extrinsic incubation in the mosquito. This finding confirms that Calzolari et al. Italian WNV Enhanced Circulation (2018) temperature is one of the principal drivers in WNV mosquito infection. The surveillance system demonstrated the ability to detect the virus circulation early, particularly in areas where circulation was more intense. This allowed evaluating the effect of mosquito abundance and weather factors on virus circulation.

Epidemiology of West Nile Virus Infections in Humans, Italy, 2012–2020: A Summary of Available Evidences

“…In other words, Po River Valley should be considered at high risk for WNV and for all arthropod-borne diseases [ 11 , 41 ], as otherwise suggested by available data from blood donors [ 42 ]. As the majority of WNV infections occur as paucisymptomatic or even asymptomatic cases, cases of WNND are usually employed as a proxy for the broader epidemiology of WNV [ 19 , 32 , 43 , 44 , 45 ], with an eventual and extensive underestimation of actual incidence of WNV infections.…”

“…Albeit interesting, our data are affected by significant limits. First, we drew our estimates from national bulletins, and therefore we were able to summarize and analyze only the information that was preventively reported and analyzed by national authorities [ 19 , 20 , 41 ]. As a consequence, not only did we lack significant data about the demographics of both WNV and WNND cases, with resulting uncertainties in eventual estimates, but significant information on the reported signs and symptoms were not available to our analysis [ 17 , 18 , 24 , 30 ].…”

Public Perceptions on Non-Pharmaceutical Interventions for West Nile Virus Infections: A Survey from an Endemic Area in Northern Italy

“…The last decade was characterized by a sustained circulation of WNV in Northern Italy [ 9 , 15 , 43 ]. However, even in residents from areas characterized by a high or very high incidence of WNV infections (i.e., Emilia Romagna Region), nearly half of respondents had not heard of WNV/WNF, and among those who did, most of them exhibited significant uncertainties in terms of understanding of WNV-related issues, including the actual risk perception.…”

Contrasted Epidemiological Patterns of West Nile Virus Lineages 1 and 2 Infections in France from 2015 to 2019

“…The intensity of WNV circulation and transmission is shaped in part by mosquito vector abundance and is influenced by biotic and abiotic factors favorable for mosquito proliferation [34,35]. Mosquito abundance was found to be significantly lower in Hérault than in the other two departments for the period 2015 to 2019 but the number of traps was six for this department compared to eight for the Bouches-du-Rhône and Gard (see Section 4.2 material and methods).…”