West Nile virus (WNV) is one of the most important zoonotic, vector-borne pathogens in Europe. This virus is mainly transmitted by native mosquitoes of the genus Culex . As part of the "OH SURVector" project, a uniform Austria-wide monitoring programme for the surveillance of WNV in mosquitoes was carried out for the first time in 2024. To this end, traps were set up at 68 locations twice a month from May to October. The individuals caught were identified to their species and analysed for the presence of WNV. A total of 7,927 mosquitoes were caught, most of which were Cx. pipiens/torrentium, the most important vector of WNV in Europe. The mosquitoes were grouped into 1,364 pools and WNV was detected in three pools. The positive samples were taken in early August from Eisenstadt (Burgenland; two detections) and Lobmingtal (Styria).

The year 2024 was the year with the highest WNV circulation in Austria to date. For example, 36 human cases were reported (acquired domestically), as well as 59 cases in horses and 22 in birds; for all three groups, this is the highest reported value to date. The reports came from the east of Austria (mainly northern Burgenland, but also Vienna, the surrounding area of Vienna and the Weinviertel region). In previous years, almost all WNV cases were also limited to eastern Austria. The two positive mosquito pools from Eisenstadt therefore fit well with the data on human and veterinary cases. However, the WNV detection from a mosquito pool in the Lobmingtal valley in Styria is particularly exciting. Although WNV was detected in an alpaca in Styria in 2023, there has not yet been a human case.

The main target species for this monitoring, the common house mosquito Cx. pipiens/torrentium, was by far the most common mosquito species caught, accounting for 75 % of the total. As this species complex is the most important vector for WNV in Europe, high catch numbers of this species are essential for the success of WNV surveillance in mosquitoes. Although they mainly bite birds for their blood meal and thus ensure the circulation of WNV in the bird population, some forms also like to bite humans or other mammals and act as a bridge vector. The common house mosquito is active at dusk and at night and is particularly common in urban areas. The adult females overwinter in hollow trees and holes in the ground in the wild and in basements, attics or other frost-free rooms in urban areas. In spring, the females lay egg boats of around 150-300 eggs on the surface of the water. The banks of ponds and stagnant ditches as well as artificial water points such as rain barrels or even smaller water points such as flowerpot saucers are suitable places for laying eggs. Depending on the prevailing climatic conditions (precipitation, temperature, etc.), Cx. pipiens/torrentium species can produce several generations per year.

Aedes vexans, the second most common species, is a typical representative of the flood mosquito species. The females lay their eggs in dry floodplains, especially in riparian forests, where they can often survive for several years without water. If the eggs are flooded after a flood event, the larvae hatch en masse. This was also shown in this study, where Ae. vexans was caught in large numbers in early July and early October, both after major flooding events. This species normally remains close to its breeding site, but it can be passively dispersed by strong winds, so that it was also found at the trap sites in the more urban areas.

Cx. modestus was also still caught relatively frequently and was more abundant in late summer. This species is also an important vector species for the WNV. Shallow, sunny waters, often with rich vegetation, are favoured as breeding waters, which may also have a slightly higher salt content. This species was found in large numbers in Illmitz, near Lake Neusiedl, where it finds ideal conditions.

In this first year of standardised WNV surveillance in mosquitoes throughout Austria, a network of trap sites was successfully established in cooperation with the participating provincial health authorities and a number of volunteers. The main vectors were caught in a targeted manner. However, a higher detection rate of WNV in the mosquitoes would require a higher overall number of individuals to be caught. An adjustment of the trapping dates is therefore planned for 2025 so that they are more concentrated in the main activity phase of the mosquitoes. Furthermore, trap locations with particularly low trap numbers are to be relocated where possible.