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Dutch Models Indicated the Blackbird–Mosquito Cycle Could Not Sustain Usutu Virus Alone

Male Eurasian blackbird in The Hague, Netherlands
Research · Disease, parasites & microbiology

Eurasian blackbirds (Turdus merula) were the bird species most visibly affected when Usutu virus emerged in the Netherlands, but transmission models indicate that the blackbird–mosquito cycle could not maintain the virus on its own. The study, published in Nature Ecology & Evolution on 3 March 2026, found support for additional, less visibly affected bird hosts contributing to transmission.

Usutu virus is a mosquito-borne virus transmitted in a cycle between birds and Culex mosquitoes. Blackbirds can suffer high infection-related mortality, while other bird species may contribute to transmission with much less visible disease, which is why identifying the full reservoir community matters.

The researchers reconstructed Usutu-virus emergence from 2016 to 2022 using mechanistic models of transmission between birds and Culex pipiens mosquitoes. The models combined several kinds of blackbird surveillance and population data, including PCR testing, antibody surveys, testing of dead birds, population monitoring and public reports of dead blackbirds.

Models With Additional Hosts Fitted the Outbreak Better

Models that included an additional reservoir population — one or more other bird populations able to take part in transmission and help maintain the virus — reproduced the observed outbreak patterns better than models based on blackbirds alone. In the best-fitting model, these reservoir birds had little or no infection-related mortality, lived longer and dispersed farther than blackbirds.

The model indicated that transmission through the blackbird–mosquito cycle was far too weak to sustain the virus on its own, whereas the inferred reservoir–mosquito cycle was strong enough to maintain transmission. Juvenile blackbirds were estimated to contribute about 50% more to transmission than adults, but the blackbird contribution remained too small to explain persistence.

Reservoir Immunity Was Much Higher

Mean modelled seroprevalence — the proportion estimated to carry antibodies indicating previous infection — reached 68.1% in the reservoir population, compared with 13.4% in blackbirds. As immunity accumulated in the longer-lived reservoir hosts, the model reproduced the reduction in transmission after the first outbreak years. The authors interpret this as population immunity in other hosts helping to limit later virus circulation and further blackbird declines.

The Reservoir Birds Remain Unidentified

The study does not identify a single reservoir species. Instead, it infers the properties of one or more additional host populations from the observed surveillance patterns. The proportion of mosquito bites received by each host depends on both mosquito preference and host abundance, which the analysis could not separate.

The model also assumed the same transmission probabilities for the reservoir population as for blackbirds, and estimates of reservoir lifespan and immunity remained uncertain. The authors therefore describe the reservoir as an inferred multi-host component rather than a named species.

Sources and image credit

  1. de Wit, M. M., Beaunée, G., Dellar, M., Münger, E., Krol, L., Atama, N., van Irsel, J., van der Jeugd, H., van den Brand, J. M. A., Reusken, C. B. E. M., Koopmans, M., de Jong, M. C. M., Sikkema, R. S., & ten Bosch, Q. A. (2026). Silent reservoir species are shaping the emergence of Usutu virus. Nature Ecology & Evolution, 10(4), 721–732. https://doi.org/10.1038/s41559-025-02973-4Primary peer-reviewed study published and version of record dated 3 March 2026. Models fitted to five Dutch blackbird surveillance datasets from 2016–2022 indicated that the blackbird–mosquito cycle alone could not sustain Usutu virus and supported an additional reservoir host population. The article is CC BY-NC-ND 4.0.

Image: Ferdi De gier — wild male Eurasian blackbird in The Hague, Netherlands, photographed 13 April 2013; Wikimedia Commons/Flickr; representative Dutch species image · CC BY-SA 2.0 · Source

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