We Need to Prepare for Rat-Borne Disease
As the climate changes, infectious diseases of tropical origin increasingly threaten the northern hemisphere. In May 2026 the EU's Joint Research Centre (JRC) published the first Europe-wide spatio-temporal modelling of leptospirosis risk (JRC, 2026).
The underlying research, published in The Lancet Regional Health – Europe by Estévez-Reboredo and colleagues (2026), processed 14 years of epidemiological data and projected the disease's likely spread under two climate scenarios. The conclusion is clear: climate change increases the risk in every case examined, and some factors have a surprisingly strong effect.
Leptospirosis rarely makes headlines. It has no name familiar to a wide audience, and annual case numbers in Europe are still modest. Nevertheless – precisely because it spreads through water, surges after floods and thrives in favourable climatic conditions – more and more researchers count it among the climate-sensitive zoonoses that deserve attention.
What is leptospirosis?
Leptospirosis is an infection transmitted from animals to humans, caused by spirochaete bacteria of the genus Leptospira. The pathogen's main natural hosts are rodents, above all rats, which shed the bacteria into the environment in their urine. People are most often infected through contact with contaminated surface water, soil or food – especially through broken skin or mucous membranes.
The disease takes two main clinical forms. The mild form causes flu-like symptoms: high fever, muscle pain, headache and weakness. The severe form – known as Weil's disease – progresses rapidly, with jaundice, liver and kidney damage and haemorrhagic complications, and requires intensive care. Early antibiotic treatment is crucial, yet diagnosis is regularly delayed: according to the EEA Climate-ADAPT factsheet, only one in ten cases is correctly identified globally, because the symptoms can be confused with many other infections (EEA, n.d.). The global burden of leptospirosis is therefore greatly underestimated – the 1 million diagnosed cases recorded each year are only a fraction of the real number.
Europe's infection map
The study by Estévez-Reboredo et al. (2026) in The Lancet Regional Health – Europe analysed monthly case data from the European surveillance system (TESSy) between 2010 and 2023. Over the 14-year period, a total of 3,930 cases were reported in Europe, an average of 281 infections a year. The affected areas cover some 140 of the EU's nearly 1,200 NUTS-3 regions each year.
The numbers may seem low on their own, but the trend and spatial distribution are telling. Both annual case numbers and the number of affected regions increased over the period studied. The risk is significantly higher in warm coastal regions and densely populated areas. The JRC analysis also stresses that the real burden is much higher than reported.
Climate as a risk multiplier
The methodological backbone of the Lancet study is a hierarchical Bayesian model linking case data with climatic, environmental and demographic factors. The results reveal the outstanding role of two climate variables.
Temperature is the most decisive factor. With a one-month lag, rising average temperatures result in up to a 5.28-fold increase in relative risk (95% credible interval: 2.65–11.12). That is exceptionally high among climate-sensitive infections. Behind it lies a well-understood process: Leptospira bacteria survive longer and multiply faster in warmer soil and water, and the infectious season covers an ever larger part of the year.
Moisture conditions also have a significant effect. A rise in the three-month Standardised Precipitation-Evapotranspiration Index (SPEI-3) is associated with a 1.62-fold increase in risk (95% credible interval: 1.20–2.10). With higher soil moisture, the pathogen survives longer in surface layers, and with extreme rainfall it is easily mobilised towards watercourses, groundwater and stormwater networks.
Floods: a danger that spreads in a chain
Floods are one of the most important routes for the spread of leptospirosis. According to a 2021 review in The Lancet Regional Health – Europe, leptospirosis was among the most frequently documented pathogen-related diseases in studies of infectious outbreaks following natural disasters in Europe (Semenza & Paz, 2021).
The mechanism forms a chain. A flood simultaneously mobilises soil contaminated with rodent urine, fills cellars, basements and sewer networks, carries contaminated water into spaces where people come into contact with it, and disrupts healthcare – just when it should be working most effectively. A 2026 review in microLife (Oxford Academic) particularly stresses that blocked roads and poorer access to healthcare worsen outcomes, because early antibiotic treatment is crucial and the time window is narrow (Herbreteau & Picardeau, 2026).
Recent extreme weather events support this link with concrete cases. Storm Daniel caused extensive flooding in Greece in 2023, followed by a documented series of leptospirosis cases (Poulakida et al., 2024). The 2024 DANA storm in Valencia also brought severe flooding to southern Europe. The authors of the Lancet study see both events as harbingers of future risks.
Rodents, biodiversity and the dilution effect
The study also points to a little-known but ecologically important relationship. Leptospirosis risk is negatively associated with mammal species richness: the minimum relative risk is 0.63 (95% credible interval: 0.40–0.98), meaning that the more species-rich an area, the lower the risk. The literature describes this as the dilution effect.
The essence of the mechanism: where biodiversity is high, the pathogen is spread across many hosts, reducing the probability of infection for individuals. But when species richness declines – through urbanisation, deforestation or habitat degradation – the share of resistant rodent populations rises, and with it the infection pressure. Climate change has an effect here too: rising temperatures and habitat changes are redrawing the distribution of rodents, potentially extending the risk to previously colder, less affected regions.
Projections for the future
The study examined two climate–socioeconomic scenarios: SSP 4.5 (moderate emissions) and SSP 8.5 (high emissions). In both cases, leptospirosis incidence is expected to rise. The difference lies in intensity and time horizon.
In the near term (2041–2060), the model expects a more moderate increase. In the long term (2081–2100), especially under the high-emission scenario, the increase is considerably stronger. The number of affected NUTS-3 regions is expected to grow, the infectious season to lengthen and the spatial distribution to change: noticeable risk may appear in colder regions that were rarely affected before.
The authors stress that the projections cannot fully address the structural problem of under-reporting. If the actual number of cases is two to three times the reported figure, the absolute value of the projected increase is proportionally higher – so the model gives a lower estimate rather than an upper limit.
The role of the water sector
Leptospirosis directly concerns the water sector, but not from the usual angle of water quality pollution. Its spread is closely linked to the quality of stormwater drainage systems, floodplains, sewer systems and the protection of surface waters.
According to the EEA Climate-ADAPT factsheet, effective prevention requires systematic rodent control, renovation of open drains and stormwater networks, increased drainage capacity and public health risk communication – especially after floods, when direct contact with contaminated water is most likely. Where a human vaccine is not routinely available, prevention relies mainly on interrupting the routes of infection.
The main policy message of the JRC study is that developing targeted surveillance and intervention strategies for climate-sensitive zoonoses is an urgent task. This calls for closer cooperation between the water and public health sectors.
What should we prepare for?
The study by Estévez-Reboredo et al. (2026) in The Lancet Regional Health – Europe and the related JRC analysis establish on a scientific footing what scattered data had already suggested: leptospirosis is a climate-sensitive public health factor with a growing burden in Europe. Rising temperatures can increase the relative risk of infection by up to 5.28 times, rainfall anomalies also have a significant effect, and case numbers are expected to rise under every climate scenario examined.
The disease's water dimension is clear: floods, rainfall extremes and the state of sewer networks are directly linked to the spread of the pathogen. This is not only an epidemiological question but also one of infrastructure and planning. In this context, climate-adapted water management – aimed at reducing flood risk, managing stormwater and interrupting pollution pathways – also has a direct public health prevention function.
In May 2026 the EU's Joint Research Centre published the first Europe-wide mapping of leptospirosis risk. The study, published in The Lancet Regional Health – Europe, identified 3,930 European cases between 2010 and 2023 – an average of 281 infections a year, affecting some 140 of the EU's nearly 1,200 NUTS-3 regions each year. The climate links are clear: rising temperatures are associated with up to a 5.28-fold increase in relative risk, and increasing rainfall and soil moisture also have a significant effect. Floods – which bring soil and water contaminated with the pathogen into direct contact with people – are one of the most important routes of spread. Under both climate scenarios examined, the model shows rising case numbers, with stronger long-term effects especially on a high-emission pathway. The disease is underdiagnosed: globally, only one in ten cases is correctly identified.
Leptospirosis is a rodent-borne bacterial zoonosis whose pathogen enters the human body through contaminated drinking water, soil and stormwater. Although much rarer in Europe than in the tropics, a 2026 joint analysis by the EU's Joint Research Centre and The Lancet Regional Health – Europe shows that the disease is climate-sensitive and its risk is rising systematically.
Based on data for 2010–2023, a total of 3,930 cases were reported in Europe – an average of 281 infections a year, affecting some 140 of the EU's nearly 1,200 NUTS-3 regions each year. Both case numbers and geographical spread show a rising trend.
The key factor is temperature: with a one-month lag, rising temperatures increase the relative risk of infection by up to 5.28 times (95% credible interval: 2.65–11.12). Moisture conditions (the SPEI-3 index) also have a significant effect, with a 1.62-fold increase in risk. Floods are particularly dangerous – they mobilise soil contaminated with rodent urine and create a direct route of contact between people and the pathogen. Declining biodiversity also increases risk by weakening the dilution effect: where rodents dominate other species, infection pressure is higher.
The model projects rising case numbers under both SSP 4.5 and SSP 8.5; in the long term (2081–2100), especially with high emissions, the increase is stronger. The disease is underdiagnosed globally and in Europe – its true burden is estimated to be considerably higher than currently reported case numbers. Water infrastructure – stormwater management, flood management and the state of sewers – plays a direct role in reducing the risk.
A study of 10,826 Chinese treatment plants finds that big centralised systems are reaching two limits at once, on easing water scarcity and on paying their way, and that a hybrid model could do the job for up to 37% less.
FAO has published draft principles on who may use water and on what terms, a quiet piece of international work that speaks directly to this year's fights over drought, wells and free irrigation.
A new report projects data centres' water footprint reaching 9.32 trillion litres by 2030, but the number is only meaningful once you know what it counts, and who is making the case.
An English coalition has costed what it would take to keep every drop of wastewater out of England's largest lake, £3.6–3.7 billion, and its modelling says nothing less ends harmful algal blooms.
