This interview was previously published by the Royal Netherlands Academy of Arts and Sciences (KNAW) (LinkedIn, June 11, 2025)

Tijmen Hartung, currently a PhD candidate at Erasmus MC, developed a fascination with vectors — carriers of disease within ecosystems — during his studies. With a grant he received in February from the KNAW Ecology Fund, he is now focusing his research on the role of mosquitoes and birds in the spread of viruses that can also cause illness in humans. The location of his research? The green spaces in and around Rotterdam.

Water & Green
“My research is partly about mosquitoes and birds, but more importantly, it’s about how landscape changes in response to climate change can impact populations of mosquitoes, birds, and viruses like West Nile and Usutu. It’s nothing new that birds can carry viruses that cause disease, or that mosquitoes can transmit them to humans. But the fact that ‘new’ viruses have now made their way to the Netherlands due to rising temperatures is new. West Nile and Usutu are two such viruses. These viruses rely on birds as hosts.

In my research, I focus on how landscape adaptations — such as water retention areas outside the city — influence bird populations and their movement between natural areas and urban green spaces. For example, between water buffers outside the city and urban parks. This way, I hope to understand whether these adaptations, alongside benefits like flood prevention and improved livability, might also come with certain risks.”

Birds & Mosquitoes
“During migration, birds can bring diseases to Europe, such as West Nile virus and Usutu virus. This can be harmful to the birds themselves — for instance, Usutu is known to be fatal for blackbirds and owls. With West Nile and Usutu, we humans don’t get infected directly by the birds — but mosquitoes can act as the bridge. They are what we call ‘vectors’: organisms that transmit disease within an ecosystem.

These can be exotic mosquitoes, like the Asian tiger mosquito that spreads dengue. But in the case of West Nile and Usutu, it’s the common house mosquito — the one we’re all familiar with. Only a handful of human cases of Usutu have been documented, and these involved people who already had underlying health conditions. That’s why Usutu is mainly seen as a wildlife concern, not a public health concern.

West Nile virus, however, is a different story. About 20% of infected people develop a serious fever, and around 1% develop encephalitis — a potentially deadly inflammation of the brain.”

Bird Shazam
“The impact of climate change on virus spread in the Netherlands — and thus on the risk to human health — is complex. As the country gets warmer, viruses are spreading from southern to northern Europe, carried by migratory birds. At the same time, mosquitoes are thriving in higher temperatures: they reproduce for longer periods and develop faster.

Besides green spaces and water features meant to keep cities livable during heatwaves, more and more green infrastructure is being developed outside cities to buffer heavy rainfall. These green areas, with their plants and water, often become attractive habitats for both birds and mosquitoes.

All of these factors raise important research questions. Mainly: Which bird species frequently move between these outer-city water buffers and inner-city green areas? Which of these birds are good hosts for West Nile and Usutu? How often do they move between city and nature? And does this movement increase the risk of human infection?

To monitor bird activity — which is also what my funding application was partly about — we can use various methods. Classic bird counting by observation or tagging is possible, but these are time-consuming and costly, and traditional monitoring can miss things. That’s why we’re now deploying Passive Acoustic Monitoring Devices at our research locations — they work a bit like the popular app Shazam, but for birds. These devices pick up bird calls and match them to specific species based on sound. That way, we can collect data 24/7.”

From Sandbox to Green Hub
“Last year, we ran a pilot with this ‘bird Shazam’ in the Eendragtspolder, a water buffer area with wetlands just outside Rotterdam. The pilot was successful, and now we’re planning to monitor both city parks and the Eendragtspolder.

Rotterdam is undergoing major greening — think of the new tidal parks. The Rijnhaven, currently still a large sandy area, is set to be transformed with vegetation and water features, aimed at recreation. The same applies to areas outside the city, like the Eendragtspolder, which also serves as a rainwater buffer.

Right now is the perfect time to carry out measurements with support from the fund. We can clearly say: okay, it’s still a sandy area mostly frequented by herons and gulls. But what happens to the ecology — and to the birds and mosquitoes involved — once you radically change the landscape? And what effect could that have on viruses that might also pose a risk to human health?”