Climate change and leptospirosis: a growing environmental and zoonotic threat

Abstract

Climate change is increasingly recognized as a key driver in the emergence and re-emergence of infectious diseases, including leptospirosis, a globally distributed bacterial zoonosis. The bacterial agents of leptospirosis infect humans through contact with soil or water contaminated by the urine of animal reservoirs. As a primarily waterborne disease, leptospirosis is strongly influenced by climatic conditions, including rainfall, flooding, and extreme events such as hurricanes and cyclones. Here, we present current knowledge on the links between climate change and leptospirosis, as well as perspectives on effective strategies to combat this disease, which disproportionately affects the poorest populations.

This review highlights how climate-driven environmental change is accelerating the global re-emergence of leptospirosis.

Introduction

Our planet is facing rapid climate change, together with two other interconnected crises: pollution and biodiversity loss. Together, they form the “Triple Planetary crisis,” each with distinct causes and impacts [United Nations Framework Convention on Climate Change (UNFCCC) 2022]. Scientific evidence indicates that human activities are the main drivers of these multiple changes [Intergovernmental Panel On Climate Change (IPCC) 2023]. The Earth’s capacity to absorb these changes is limited and, beyond these limits, the Earth may lose its capacity for self-regulation, leading to radical and irreversible changes (Richardson et al. 2023). Among these global changes, climate has been accelerating rapidly since industrialization, with already dramatic impacts on human, animal, and ecosystem health (Zinsstag et al. 2018).

Climate change is driving more frequent and intense extreme events including heatwaves, heavy rainfalls causing floods and landslides, droughts, severe storms and cyclones associated with rainfall, and extreme sea levels causing high-tide flooding [Intergovernmental Panel On Climate Change (IPCC) 2023]. The year 2024 broke multiple temperature records, it was the warmest year since at least 1850 and the first year to exceed the 1.5°C threshold (WMO 2025). It was also marked by exceptional rainfall and flooding (Zhang et al. 2025).

Climate change has numerous direct and indirect health impacts, both short- and long-term (Bianco et al. 2024). It is thus estimated that more than half of human diseases will be worsened by climate change (Mora et al. 2022). Among these, waterborne diseases may increase during extreme wet events or potentially reduced in dry periods, depending on changes in their frequency and intensity (Nichols et al. 2018, Semenza and Ko 2023). This is particularly true for leptospirosis, a bacterial zoonosis endemic in many intertropical countries but also present in temperate countries, where epidemics are often associated to flooding and rainfall (Appassakij et al. 1995, Dufour et al. 2008, Lau et al. 2010, Pijnacker et al. 2016, Muñoz-Zanzi et al. 2020, Cano-Pérez et al. 2021, Rees et al. 2023, Douchet et al. 2024, Ab Kadir et al. 2025, Lian et al. 2025, Obels et al. 2025). Floods increase risk by exposing human and animals to leptospires flushed into the environment (Chadsuthi et al. 2021) (Fig. 1). Leptospirosis is thus considered as a re-emerging zoonotic disease commonly associated with climate change but also, poverty and urbanization (Costa et al. 2015, Mwachui et al. 2015, Torgerson et al. 2015, Goarant et al. 2019). A study carried out 10 years ago estimated 1.03 million cases and 58 900 deaths per year worldwide, with a significantly higher incidence in tropical countries (Costa et al. 2015). However, these numbers are likely underestimated because of undiagnosis, misdiagnosis, and under-reporting in many countries, particularly in regions where other diseases with similar non-specific presentations, such as dengue and malaria, are prevalent (Goarant et al. 2021).

Figure 1.

Effect of climate on leptospirosis infections. Here, we illustrate how climate change, through specific interactions between humans, animals, and the environment, can increase environmental contaminations, exposures, transmissions, and infections.

In light of increasing evidence linking climate change to the re-emergence of leptospirosis, this narrative review synthesizes current knowledge on the effects of climate change on Leptospira environmental dynamics and disease emergence. By examining how climate-driven factors influence bacterial persistence, transmission, and outbreak patterns, we aim to inform future prevention and adaptation strategies.

Impact of climate change on a waterborne disease

The term waterborne disease refers to a disease primarily transmitted through contact with, or ingestion of, water contaminated with a pathogenic agent. This definition applies to leptospirosis as it does to other diseases such as cholera (Semenza and Ko 2023). Accordingly, leptospirosis outbreaks are strongly associated with seasonal rainfall, whereas direct contact with animal reservoirs is only rarely reported (Ko et al. 1999). Under favorable physico-chemical, Leptospira can survive for prolonged periods in waterlogged soils and fresh water and may even multiply while retaining their virulence (Bierque et al. 2020, Yanagihara et al. 2022). Moreover, studies have shown that genotypes of pathogenic strains found in the environment are identical to those infecting humans in Brazilian urban slum and rural setting in New Caledonia (Casanovas-Massana et al. 2022, Thibeaux et al. 2024). This is consistent with epidemiological studies identifying contact with mud and water as major risk factors for Leptospira infection (Hagan et al. 2016, Lau Colleen et al. 2016, Govan et al. 2025). However, linking the environment to infections is challenging because (i) studies are often conducted long after outbreaks, (ii) saprophytic or pathogen-related Leptospira in soil and water are present in high levels making detection of highly virulent pathogen species difficult, and (iii) some pathogenic strains or species may not persist in the environment (Bulach et al. 2006, Thibeaux Roman et al. 2017, Stone et al. 2022). Improving the sensitivity of molecular methods should soon overcome the limitations of current molecular and culture-based approaches in detecting at low concentrations of highly virulent pathogens in the environment. All together, these finding highlight that exposure to contaminated freshwater is a key factor in the spillover transmission of pathogenic Leptospira from animal reservoirs to humans.

During the environmental phase, rainfall and hydrological processes such as river flow, soil infiltration, and flooding facilitate the dissemination of Leptospira over long distances, thus increasing human exposure and the risk of leptospirosis (Chadsuthi et al. 2021). Heavy rainfall destabilizes riverbanks and promotes Leptospira resuspension, leading to elevated bacterial concentrations in rivers for several days following the rainfall (Thibeaux et al. 2024). Considering the role of water in the survival and transmission of leptospires, we can identify three major climate change-related consequences: altered seasonal rainfall patterns, changes in surface water availability, and increased frequency and intensity of extreme weather events (Fig. 1).

First, climate change is altering not only the overall amount of precipitation, but also its seasonal distribution and interannual variability (Feng et al. 2013). Feng et al. showed increasing uncertainty for these three parameters, which constitute the seasonality of rainfall in tropical areas, its onset, duration, and intensity. Leptospirosis follows a seasonal pattern, with more people affected late summer to fall in temperate countries, or during the rainy season in tropical regions (Desvars et al. 2011, Habus et al. 2017). In the tropics, disruptions to the seasonality of rainfall may indirectly alter the seasonality of leptospirosis, creating additional challenges for diagnosis when cases occur outside of the usual rainy season.

Second, climate change affects surface water availability, defined as the amount of water flowing or remaining on land (including rivers, streams, lakes, and wetlands), through changes in precipitation, evapotranspiration, runoff, and water storage (Zaitchik et al. 2023). One documented scenario is that wet areas will become wetter while dry areas will become drier (WWDD). However, this scenario requires careful contextualization as it may not be applicable across all regions (Zaitchik et al. 2023). In wet regions, particularly in Central Africa and South and Southeast Asia, run-off is expected to be 20% more intense by 2100, leading to an increased risk of flooding (Gu et al. 2025). It is also important to note that land artificialization could further worsen runoff and flooding. Floods are considered the main risk factors for leptospirosis and several outbreaks have been reported following extreme weather events (Muñoz-Zanzi et al. 2020). We can therefore hypothesize that this increase in runoff and flood risks will increase the risks of leptospirosis transmission and outbreaks in these wet regions where leptospirosis is already highly prevalent. Conversely, the risks of environmental exposure could decrease in regions that are already dry, which may experience even greater aridity, even though the occasional transmission through direct contact with animals would remain unaffected. However, transmission within localized microhabitats—such as irrigated croplands or livestock farms—may still persist in these dried areas (Dobigny et al. 2015, Wainaina et al. 2018).

Third, as climate change drives more frequent and intense extreme events, the risk of leptospirosis is expected to rise. In particular, outbreaks are more likely following extreme rainfall that triggers sudden and severe flooding. This has been observed in several countries, including Metro Manila in 2009 (Amilasan et al. 2012), Taiwan in 2009 (Su 2011), Western Fiji in 2012 (Togami et al. 2018), Kerala, India in 2018 (Ifejube et al. 2024), Puerto Rico in 2022 (Jones et al. 2024), and several European countries (Suk et al. 2020). Most of these events have been described in urban areas, where precarious living conditions are contributing factors to increased vulnerability (Lau et al. 2010). In urban slums where 1 billion people currently live (projected to double by 2050), inadequate drainage and sewer systems pose significant risks, as heavy rains or flooding can lead to widespread environmental contamination (Reis et al. 2008, Felzemburgh et al. 2014, Picardeau 2015, Costa Federico et al. 2017, Cremonese et al. 2023). Finally, increased heavy rainfall in tropical regions or during summer in temperate regions can also impact the practice of outdoor water-related sports such as triathlon, kayaking and rafting, which are rapidly expanding (Morgan et al. 2002, Stern et al. 2010, Hochedez Patrick et al. 2011, Hochedez et al. 2013, Pagès et al. 2016, Guillois et al. 2018).

Impact of climate change on a zoonotic disease

Nearly three-quarters of emerging and re-emerging diseases during the past decade are zoonoses (Woolhouse and Gowtage-Sequeria 2005). A recent review on zoonotic causes of fever in malaria-endemic countries found that most were bacterial and Leptospira was one of the most frequently reported pathogens (Halliday et al. 2020). Leptospirosis is considered to be the most widespread zoonosis in the world, as nearly all mammals can serve as carriers. Among them, rats represent the primary reservoir for Leptospira transmission worldwide.

Mammalian carriers and reservoirs of leptospirosis are also sensitive to climate change. The overall increase in temperatures could lead to an increase in rat population density, as shown in urban areas (Richardson et al. 2025). Richardson et al. analyzed long-term observational data from 16 large cities worldwide and found a strong association between rising temperatures and increased rat densities in 11 of the 16 cities. The authors commented that rat activity is typically limited by cold temperatures, and that warmer conditions during normally cold seasons may therefore enhance their activity. This increase in rat densities leads to higher frequency of human–rat contacts, thereby influencing pathogen transmission dynamics. Climate change can also affect the habitats of reservoirs and their distribution, through habitat loss from flooding or changes in food availability. Such landscape modifications, characterized by declining habitat diversity, have been shown to promote synanthropic rodent species, thereby increasing the risk of pathogen transmission (Morand et al. 2015, Morand 2018).

The introduction of new pathogens or reservoir hosts may also exert impacts on native wildlife populations, potentially disrupting ecological balances and leading to unforeseen consequences for local biodiversity. Although such impacts have not yet been demonstrated for Leptospira and its reservoirs, the emergence of new invasive reservoirs associated with newly identified pathogenic species has been reported (Nally et al. 2016).

Diagnosis, preventions, surveillance, predictions, and interventions: future perspectives

In its Sixth Assessment Report (AR6) in 2023, the IPCC concluded that the impacts of climate change are occurring more rapidly and severely than previously anticipated, making it increasingly challenging to identify effective mitigation and adaptation strategies [Intergovernmental Panel On Climate Change (IPCC) 2023]. There is therefore an urgent need to consider the impact of climate change on leptospirosis and other environment-related pathogenic bacteria for the implementation of prevention and intervention measures to reduce the risk of transmission.

Strengthening leptospirosis diagnosis and access to healthcare infrastructures

Early diagnosis is vital for effective treatment and preventing severe complications. The existence of a laboratory performing routine diagnostics is an important factor for providing early warning. In high-risk regions, laboratories are often unavailable or quickly overwhelmed during outbreaks. Preparedness requires protocols for diagnosis that are regionally validated and prior knowledge of the regional epidemiology, including incidences, risk factors, circulating strains and reservoirs, before extreme events occur. In addition, guidelines for the prevention, diagnosis, and control of leptospirosis need to be developed or updated at international, national, and regional levels to enhance preparedness. Given that leptospirosis disproportionately affects the most disadvantaged populations (Reis et al. 2008), community-based health systems play a critical role in mitigating its burden. These local healthcare systems must be strengthened, as they also benefit people with little or no social protection who are therefore unable to pay for laboratory diagnostic tests.

Healthcare infrastructures in low-income countries are also facing increasing challenges in terms of working conditions due to rising temperatures and episodes of extreme heat (Nunes 2025). Inadequate infrastructure and insufficient resources can hinder the ability to manage patient surges during heatwaves. Flooded or obstructed roads may also increase travel time or even prevent access entirely (Ihantamalala et al. 2020). This could be an aggravating factor in the treatment of leptospirosis patients in rural areas in these countries. This is particularly detrimental for the most severe forms of leptospirosis, which require management in an intensive care unit. Moreover, it can compromise the timely diagnosis of leptospirosis, as early antibiotic therapy is effective for patient treatment (Muñoz-Zanzi et al. 2025). Climate change may therefore hinder timely intervention, thereby affecting patient outcomes.

Strengthening leptospirosis prevention

Effective prevention of leptospirosis involves improving sanitation by reducing rodent infestations, closing open sewers, and enhancing drainage systems, promoting health education and risk communication, and improving access to healthcare, diagnosis, treatment, and preventive measures in high-risk communities. A One Health approach can further strengthen leptospirosis prevention by identifying animal reservoirs and characterizing soil types and environmental conditions that favor Leptospira survival and persistence (Antoniolli et al. 2024).

Additional research is essential to provide clear, evidence-based recommendations for leptospirosis outbreak control, including assessing whether antibiotic prophylaxis is beneficial for populations at elevated risk (Win et al. 2024).

Strengthening leptospirosis surveillance through climate-informed early warning

To model the impact of climate on leptospirosis, robust estimates of prevalence, incidence, and disease burden are essential. Conducting epidemiological studies and establishing surveillance systems to generate reliable baseline data, as well as reanalysing and updating the incidence and burden estimates reported by Costa et al. 10 years ago (Costa et al. 2015), would be valuable for evaluating the impact of climate change on leptospirosis. Indeed, few studies have modelled the long-term impact of climate change on human leptospirosis. This has been achieved in Thailand based on surveillance of leptospirosis, a disease reported by all health centers since 2003 (Douchet et al. 2022). The authors used the monthly reported cases by province and environmental variables to model incidence in Thailand, before extrapolating to neighboring countries (Cambodia, Laos, Myanmar, and Vietnam). Then, they modeled how the current regional distribution of leptospirosis is likely to change according to future climate change scenarios under CMIP6 projections until the end of the 21st century. Surprisingly, the models predicted a reduction in the incidence of leptospirosis, mainly under the influence of the expected sharp rise in temperatures. However, the study also highlighted key limitations that must be addressed to improve future predictions. First, climate models provide averages of temperature and precipitation over defined periods, but cannot predict the timing or frequency of extreme weather events such as floods that are critical in triggering leptospirosis outbreaks. Continued advancements in climate forecasting are therefore essential to refine leptospirosis risk models. Second, we lack of complementary projections for factors such as land use and occupation (urbanization, agricultural practices), human population dynamics (demographic and epidemiological transitions), healthcare system capacity (accessibility and diagnostic capabilities), and animal population dynamics limiting the ability to model leptospirosis evolution at fine spatial and temporal scales.

Healthcare managers need short-term forecasts to reduce epidemic impacts. Rainfall and flooding, key triggers of leptospirosis, can be monitored via satellites and ground stations. Real-time detection enables early warnings, public awareness, and protective measures for at-risk populations. This is particularly important given the one- to two-week lag between environmental exposure, symptom onset, and patient consultation. The possibility of constructing such models based on satellite-acquired climate data has been demonstrated in various island contexts (Douchet et al. 2024). The predictions of the El Niño–Southern Oscillation (ENSO), a primary driver of climate variability over the tropical Pacific Ocean, could also be used to implement forecasting models. Indeed, it has been shown that the cooling phases, known as La Niña, are associated with high rainfall and leptospirosis outbreaks in New Caledonia, Fiji, and Colombia (Weinberger et al. 2014, Arias-Monsalve and Builes-Jaramillo 2019, Rees et al. 2023). The implementation of such climate-based leptospirosis alert systems remains a promising solution as long as the diagnosis of leptospirosis remains rare and the disease not notifiable in most countries.

Conclusion

Climate change is increasingly recognized as a critical driver in the re-emergence of leptospirosis, a zoonotic and neglected tropical disease. Currently, key knowledge gaps in leptospirosis transmission and prevention coupled with insufficient investment are limiting the implementation of effective control measures. However, leptospirosis can be effectively addressed through enhanced efforts in awareness, education, surveillance and modelling, improved sanitation and rapid diagnostics, enabling timely treatment with simple and effective antimicrobial therapy. Given its high incidence and the growing burden associated with climate change, countries and health organizations must strengthen leptospirosis surveillance and mitigation strategies within national climate adaptation plans to reduce population vulnerability.

Funding

None declared.

Conflicts of interest

None declared.