Possible drivers for the increased West Nile virus transmission in Italy in 2018

Purpose: To assess the consistency of the observed patterns of West Nile virus (WNV) during the last six years and possible drivers for the observed increased incidence in 2018. Methods & Materials: The data on confirmed West Nile Neuro-invasive Disease (WNND) human cases notified in Italy since 2012 indicate an increase of incidence in 2018 (131 cases as of 30 August). An integrated surveillance system is in place in Italy since 2008, which includes RT-PCR testing of mosquito pools, birds belonging to three target species (magpie, hooded crow, jay) and wild birds of other species found sick/dead. Data from veterinary activities are recorded in a national database, which is integrated with the data on WNND human cases. The possible correlation between the monthly numbers of positive mosquito pools and the monthly numbers of WNND cases, of positive birds of target species, of positive wild birds since 2012 have been tested. Results: The monthly numbers of WNND human cases, positive birds of target species and positive wild birds were all significantly correlated to monthly numbers of positive mosquito pools: Kendall correlation coefficients (tau) equal to, respectively, 0.6466 (p = 5.6e-11), 0.6983 (p = 4.3e-13) and 0.5548 (p = 1.5e-8). Conclusion: The variations of WNV infection in mosquitoes led to similar variations in the incidence in both the vertebrate reservoirs of infection (birds) and in the accidental hosts (human beings), thus supporting the hypothesis that the increased number of human cases observed so far in Italy in 2018 is linked to a higher incidence of infection in the mosquito populations. The analysis of climatic patterns in 2018 is indicating that during the first six months of this year the temperatures were higher than usual (+1.1 °C), and also rainfalls were more frequent, especially during June (http://www.meteo.it/clima-italia-2018-pioggia-caldo-temperature/). These differences in temperatures and rainfall patterns in 2018 could explain the observed increased incidence of cases in vertebrate hosts. Further analyses are ongoing to verify this hypothesis using a more consolidate dataset, to be expected in October, after the peak of WNV infection.