Cases within the Berlin urban area have been reported during the summer months for several years, but until now it was not known which urban areas are conducive to the presence of WNV. Over a two-year period, a research team collected mosquitoes during the summer months from five closely neighbouring but distinct sites in Berlin and tested them for the presence of the virus. They found high infection rates in some of the mosquitoes, but the number of virus detections varied significantly between the sites. Urban planning has a significant influence on the infection rate of mosquitoes, the team concluded in a paper in the journal “Nature Communications”.
The study was conducted under the leadership of scientists from Charité – Universitätsmedizin Berlin in collaboration with the Leibniz Institute for Zoo and Wildlife Research (Leibniz-IZW), Freie Universität Berlin and the Technical University of Berlin. The study areas comprised sites that differed in terms of vegetation cover, impervious surfaces and the presence of water. A former industrial site was also included, which had been renaturalised according to the so-called ‘sponge city’ concept with a view to climate adaptation and rainwater management. These different urban structures have a significant influence on the prevalence of the West Nile virus: the highest infection rates were found in mosquitoes at a cemetery and in a park-like residential area, whilst significantly lower infection rates were observed in mosquitoes in a nature conservation area, in a residential backyard and on the sponge-city site.
WNV originated in Africa and was first detected in Europe in the 1960s, primarily in the Mediterranean region. Climate change is facilitating the spread of the virus, which occurs mainly via migratory birds. Since 2018, the virus has also been detected in Germany and has become established there. Locally acquired WNV infections in humans occur primarily in Berlin, Brandenburg, Saxony-Anhalt and Saxony. In most cases, the infections are asymptomatic; however, in rare cases, they can lead to fever and severe neuroinvasive diseases. “The virus is sporadically detected in blood donors and seriously ill patients. In animals, it primarily affects birds and horses, which can become seriously ill or die from a WNV infection. To find out where the virus is particularly able to multiply and how high the risk of infection is in different locations, we have carried out standardised and systematic monitoring”, say study leaders Dr Corinna Patzina-Mehling and Prof. Sandra Junglen from Charité.
To gain a better understanding of the factors influencing the prevalence of WNV infections in mosquitoes, the Charité team collected around 25,000 mosquitoes between June and September over two consecutive years at five neighbouring sites in Berlin that differed significantly in terms of urban structure: in a park-like residential area with detached and terraced houses, a semi-natural urban cemetery, a nature conservation area, an industrial wasteland renaturalised according to the ‘sponge city’ concept with green spaces and water features, and in a residential backyard. The scientists identified the mosquito species and tested them for WNV infection. Where viruses were detected, their genomes were sequenced to determine whether the viruses had replicated locally or had been introduced from outside. If the mosquitoes still contained blood from their last meal, this was also genetically analysed to identify the species (bird, mammal or human) from which the blood had been taken.
At the same time, the Leibniz-IZW team determined the composition of songbird communities following standardised song monitoring across the five study sites. “To analyse the factors influencing the infection rate in mosquitoes, we collected data on a range of environmental variables, such as water availability, the degree of impervious surfaces, and the density of tree and shrub cover”, says Dr Conny Landgraf of the Leibniz-IZW. “In addition, we included the abundance and diversity of mosquito and bird species in the analysis.”
Infection rates vary from place to place and depend on the urban environment
The analyses revealed that the Northern or Common house mosquito (Culex pipiens), the vector for WNV, was the predominant species found across all study sites. “We found no difference in the abundance of the Northern house mosquito across the study sites that could explain the significant variations in WNV infection rates. Mosquito density could also be ruled out as an explanation, as, surprisingly, the lowest infection rates were associated with the highest mosquito densities”, says Prof. Sandra Junglen. High infection rates were recorded at certain locations and in certain months, for example at the cemetery and the park-like residential area. In the nature conservation area and the sponge-city area, infection rates were significantly lower. Although the nature conservation area had the highest number of mosquitoes in both years, these had the lowest infection rates.
“We correlated the infection rates with all the factors we had recorded and calculated, and identified two interesting connections”, explains Stephanie Kramer-Schadt, Head of Department at the Leibniz-IZW and Professor at the Technical University of Berlin. “As mosquitoes can only become infected via birds, the presence and species composition of birds are crucial. We observed that the infection rate in mosquitoes was high at sites where we found a particularly large number of bird species adapted to humans and the urban environment. The nature reserve is inhabited by other bird species, and these are presumably unable to transmit the virus, or do so to a significantly lesser extent.”
On the one hand, high infection rates are linked to the availability of water, which mosquitoes need as a reproduction site. On the other hand, the combination of water and suitable habitats for certain bird species within urban areas is important. This suggests that urban planning decisions, particularly in the context of cities adapting to heat, drought and heavy rainfall in the wake of climate change, can have a significant influence on the occurrence of mosquito-borne infectious diseases. “The combination of natural green spaces, water and suitable habitats for a diverse range of wildlife is a key factor in preventing infections in the future”, says Prof. Sandra Junglen. “Natural green spaces and a diverse range of wildlife appear to have a positive influence on mosquito infection rates.”
