Five billion people now live in dengue-mosquito country, 26 years ahead of forecast

By David Ogilvy, Chief Marketing Officer at Mosticare Global | Published 2026-06-02

A team at Germany's Robert Koch Institute has spent the last year rebuilding the map of where the world's two most dangerous mosquitoes can live. They covered fifty years, every month, the whole planet. The headline number that fell out of the model is the kind a public-health minister remembers: the territory now suitable for Aedes aegypti and Aedes albopictus overlaps regions home to more than five billion people — a threshold that earlier modelling did not expect the world to cross until 2050.

The work is a preprint, posted to bioRxiv on 18 April 2026 by Tahmina Siddiqui, Christopher Irrgang and colleagues at the RKI. It has not yet been peer-reviewed, and we will say so plainly. But the method is careful, the data are public, and the single statistic at its centre is the cleanest summary of the climate-and-mosquito story anyone has produced this year.

For Mosticare's readers, who have absorbed "mosquitoes are moving north" as background noise for a decade, the news is not the direction of travel. It is the speed. The frontier arrived a quarter of a century early.

What the model actually measures

The two species matter because of what they carry. Aedes aegypti and Aedes albopictus are the primary vectors of dengue, Zika and chikungunya — three arboviruses whose combined burden has climbed steeply as the mosquitoes have spread. Knowing where they can live, month by month, is the foundation under every outbreak forecast, every vector-control budget, and every decision about where to put a surveillance trap.

The RKI group built what they call the Climademic Suitability Model: a machine-learning system that predicts global mosquito habitat suitability at 0.25-degree resolution — roughly 28-kilometre grid cells — for every month between 1975 and 2024. That monthly cadence is the unusual part. Most habitat maps give you a single annual snapshot, which hides the fact that suitability switches on and off with the seasons. A model that resolves months can tell you not just whether a place can host these mosquitoes, but for how long each year — the window that decides whether an imported dengue case turns into a local outbreak.

The model is fed four streams of data: climate, land use, human population, and actual mosquito surveillance records. Crucially, the team used an explainability technique called SHAP to ask the model which inputs were doing the work. The answer was unambiguous: temperature and dew-point temperature — that is, heat and humidity — dominate. The mosquitoes are tracking the climate, not the concrete.

The number, and the honest version of it

Run across fifty years, the model shows what the authors describe as "a complex global redistribution of expanding and contracting vector habitats." This is the part worth holding onto. The story is not a simple uniform spread. Some regions are becoming less suitable — too hot, too dry, the climate overshooting the mosquitoes' tolerance — even as far more become newly habitable. The net movement is outward and, in the northern hemisphere, upward in both latitude and altitude.

The aggregate is the five-billion figure. As of 2024, suitable habitat for the two species overlaps areas containing more than five billion people, and — in the authors' words — this coincides with "the world's most pronounced population growth." The mosquitoes are not just reaching more land. They are reaching the land where the most people are being added fastest.

Now the caution, because a number this large invites over-reading. "Living in suitable habitat" is not the same as "living with the mosquito at your door," still less "living with dengue." Suitability is a measure of climatic possibility — the conditions under which a population could establish if introduced and unmanaged. Plenty of the five billion live in places with good water infrastructure, effective vector control, or a cool season long enough to keep transmission sporadic. The figure is a measure of exposure to risk, not a count of people under active threat. It is the size of the board, not the state of the game.

The comparison with 2050 is where the force of the finding lives. Reaching this threshold in 2024, against earlier projections that placed it at mid-century, means the expansion has run at least 26 years ahead of the timeline modellers had penned in. Forecasts of this kind are routinely revised, and one preprint does not overturn a literature. But the direction of the surprise is consistent with what field entomologists across southern Europe have been reporting in person: the maps drawn five years ago already feel out of date.

What it changes, and what it does not

Here is the part a serious mosquito publication has a duty to get right. A bigger, faster-moving suitability map changes the planning. It does not change the defences.

The mosquitoes are still the mosquitoes. Aedes albopictus still breeds in the few millimetres of water sitting in a plant saucer, a blocked gutter, a discarded tyre, a child's forgotten bucket. It still bites by day. It still responds to coordinated source-reduction — emptying standing water, screening windows, sleeping under a treated net in the months the season is open — in exactly the way it did when the suitable zone was smaller. Climate change is enlarging the hazard. It is not inventing a mosquito that ignores a barrier.

What the RKI map does change is the arithmetic of preparation. If a German Land, a French department, or a town in the English south-east now sits inside the suitable zone for a longer slice of the year than the old maps showed, then surveillance traps, public-information campaigns and clinician briefings stop being precautionary and start being overdue. That the corresponding author works at the Robert Koch Institute — Germany's federal public-health agency, not a tropical-medicine outpost — is itself the quiet message. This is now a northern-European institution's problem to model.

What to watch next

Two things. First, peer review: this is a preprint, and the five-billion figure will carry more weight once it has cleared a journal's referees. Watch for the published version and any revision to the headline number.

Second, the monthly dimension. A model that resolves suitability month by month is built to answer the question that actually matters for Europe — not "can the mosquito live here?" but "for how many weeks a year, and is that window lengthening?" The next papers in this line should start putting hard numbers on season length, city by city. That is the statistic that will tell a family in Lyon or Bologna or Kent whether the change on the map has reached their own calendar.

Mosticare will be reading the peer-reviewed version closely.

Sources cited

1. Siddiqui T, Malysheva N, Hartner A-M, Butyrin S, Parreira D, Genger J-W, Irrgang C, Suitable seasons: Global monthly habitat suitability for the arbovirus vectors Aedes aegypti and Aedes albopictus in 1975–2024, bioRxiv preprint (Robert Koch Institute, 18 April 2026) — https://www.biorxiv.org/content/10.64898/2026.04.17.719149v1