Distribution and abundance of Aedes caspius (Pallas, 1771) and Aedes vexans (Meigen, 1830) in the Po Plain (northern Italy)

Abstract

Background: Knowledge of the distribution and abundance of disease-causing mosquito vectors is fundamental for assessing the risk of disease circulation and introduction. Aedes caspius (Pallas, 1771) and Aedes vexans (Meigen, 1830) have been implicated, to different extents, in the circulation of several arthropod-borne viruses (arboviruses). These two mosquitoes are vectors of Tahyna virus in Europe and are considered potential vectors of Rift Valley fever virus, a virus not present but at risk of introduction on the continent.

Methods: In this work, we analysed abundance data collected during West Nile virus (WNV) surveillance in northern Italy (Po Plain) via 292 CO₂-baited traps to evaluate the distribution and density of these two non-target mosquitoes. We modelled the distribution and abundance of these two mosquito species in the surveyed area using two distinct spatial analysis approaches (geostatistical and machine learning), which yielded congruent results.

Results: Both species are more abundant close to the Po River than elsewhere, but Ae. caspius is present in the eastern and western parts of the plain, linked with the occurrence of rice fields and wetlands, while Ae. vexans is observed in the middle area of the plain.

Conclusions: Presence and abundance data at the municipality level were obtained and made available through this work. This work demonstrates the importance of maintaining and improving entomological surveillance programs with an adequate sampling effort.

Keywords: Aedes caspius; Aedes vexans; Entomological surveillance.

Fig. 1

Sample station locations in the Po Plain (delimited by a white line) with reference to the monitored regions and the Italian map

Fig. 2

Aedes caspius: yearly average collection values for the five regions of northern Italy on a logarithmic scale and reference to the seasonal average (black line)

Fig. 3

Aedes vexans: yearly average collection values for the five regions of northern Italy on a logarithmic scale and reference to the seasonal average (black line)

Fig. 4

Maps of the hotspots derived from Getis Ord analysis of the local autocorrelation of Aedes caspius

Fig. 5

Map of the hotspots derived from Getis Ord analysis of the local autocorrelation of Aedes vexans

Fig. 6

Ordinary kriging interpolation maps of the mean distribution density (log-transformed) of Aedes caspius in the three surveillance years (2018, 2019 and 2020); the black dots denote the traps considered in each analysis, in grey traps not considered

Fig. 7

Ordinary kriging interpolation maps of the mean distribution density (log-transformed) of Aedes vexans in the three surveillance years (2018, 2019 and 2020); the black dots denote the traps considered in each analysis, in grey traps not considered

Fig. 8

Maxent map of high-density Aedes caspius suitability areas (density > 30 females/trap)

Fig. 9

Maxent map of high-density Aedes vexans suitability areas (density > 15 females/trap)