Flooding has emerged as one of the most consistent triggers of leptospirosis outbreaks across Southeast Asia, according to a systematic review of studies published between 2020 and 2025. The analysis, conducted under PRISMA 2020 reporting guidelines and registered with PROSPERO, found that in most of the eight studies that met the review’s inclusion criteria, outbreaks of the waterborne bacterial disease followed monsoon rains, typhoons, or tropical cyclones with a characteristic lag of one to three months. That delay between flood event and case surge, the authors argue, offers public health authorities a critical operational window in which warnings, protective equipment, and other preventive measures could be deployed before case numbers climb.

Leptospirosis is caused by pathogenic Leptospira bacteria, which are shed in the urine of infected animals and can survive in water and soil. Humans become infected through contact with contaminated environments, and the risk rises sharply when floods spread contaminated surface water across communities. The review, published in the journal Discover Public Health, was motivated by the observation that Southeast Asia faces increasing flood intensity driven by climate change, rapid urbanization, and inadequate drainage infrastructure, and that recurring monsoon-related floods and typhoons disproportionately affect rural and low-income populations.

To capture the most recent evidence, the researchers searched PubMed, Scopus, ScienceDirect, and Google Scholar for studies published between January 2020 and December 2025, with the final search executed on 15 December 2025. Eligible studies had to examine human populations in Southeast Asian countries, report exposure to flooding or heavy rainfall, and report leptospirosis outcomes such as incidence, outbreak patterns, spatial clusters, or temporal trends. Observational designs, mathematical modeling, and spatio-temporal protocol evaluations were all accepted. From 990 records identified, the screening process removed 303 duplicates, excluded 660 records at title and abstract stage, and assessed 27 full-text reports, of which 19 were excluded because they lacked flood exposure, lacked human outcomes, or were not in English. Eight studies ultimately met all criteria.

The included studies came from four countries: three from Malaysia, two from Indonesia, two from Thailand, and one from the Philippines. Five were cross-sectional or ecological studies, one was a mixed-methods study, one was a case-control study, and one was a narrative ecological review. Settings ranged from national surveillance databases and state registries to rural community surveys. All eight studies were appraised using JBI critical appraisal tools and judged to be of moderate to high quality, although the authors note that high methodological heterogeneity across designs, exposure definitions, and outcome measures made a pooled quantitative meta-analysis impossible, so the findings were synthesized narratively.

The temporal pattern was striking. Six studies reported outbreaks temporally linked to monsoon rains, typhoons, or tropical cyclones, with the one-to-three-month lag period appearing repeatedly. In Indonesia, case spikes followed Tropical Cyclone Cempaka, while Philippine surveillance data showed severe case surges after typhoons. Malaysian registries confirmed distinct incidence peaks aligned with seasonal monsoons, and mathematical modeling in Thailand demonstrated that flooding significantly amplified transmission dynamics. Because most included studies were cross-sectional or ecological in design, the authors caution that these associations limit causal inference, even though the consistency of the lag pattern across countries strengthens the case for a genuine link.

Environmental drivers identified across the studies included rainfall volume, stagnant surface water, relative humidity, and river overflow. Time-series analysis in Indonesia confirmed positive lagged correlations between heavy rainfall, humidity, and outbreak spikes, while Thai modeling highlighted the interactive effects of flood extent and ambient temperature. In Malaysia, secondary data analyses and spatio-temporal modeling showed that inadequate municipal drainage infrastructure and rising river levels directly expanded flood-risk zones, creating environmental hotspots where the bacteria could persist. These findings point to infrastructure as a modifiable factor: where drainage fails, contaminated water lingers, and transmission risk extends well beyond the flood itself.

Social and behavioral factors shaped who fell ill. Direct contact with rodents, handling contaminated food, wading through stagnant floodwaters, the presence of open skin wounds, male gender, and lack of personal protective equipment emerged as major risk factors. Indonesian case-control data confirmed that living in flood-prone households with stagnant water and sustaining skin wounds significantly elevated the odds of infection. Qualitative focus groups in Thailand revealed that agricultural activities during flood periods and unsafe food storage increased rodent exposure. Rural, unemployed, and outdoor agricultural workers faced disproportionate burdens, underscoring that flood-related leptospirosis risk is not evenly distributed across affected communities.

Protective measures were less extensively studied than risk factors, but the review identified several promising strategies. Wearing protective footwear and gloves during flood exposure appeared as a protective factor in the Indonesian and Thai studies. Recommended structural approaches included GIS-based spatio-temporal hotspot mapping to guide targeted resource allocation, meteorological early warning systems linked to health registries, and community-level education on food hygiene and flood safety. Only three of the eight studies evaluated or proposed targeted preventive interventions, however, and the authors emphasize that empirical evaluation of such interventions remains scarce, meaning their real-world effectiveness has not been rigorously tested.

The review’s conclusions carry weight for policy. The authors argue that the consistent one-to-three-month post-flood lag period gives authorities time to deploy targeted prophylaxis, issue community warnings, and distribute protective equipment before outbreak peaks occur. They also call for integrating health impact assessments into urban infrastructure planning, given the evidence linking inadequate drainage to prolonged bacterial persistence, and for transitioning from passive disease surveillance to proactive, multi-sectoral One Health frameworks that unite meteorological, veterinary, and environmental data. Investments in diagnostics, infrastructure, and early warning systems, particularly for rural and low-income populations, are described as critical to reducing future outbreaks under worsening climate conditions.

The authors also acknowledge the limits of the evidence base. Only eight studies met the inclusion criteria, reflecting a paucity of published post-flood leptospirosis research in the region, and restricting eligibility to English-language peer-reviewed literature may have excluded relevant work. Protocol deviations included expanding eligibility to encompass modeling and spatio-temporal protocol papers, a decision the authors say was necessary given the scarcity of traditional cohort studies. Looking forward, they recommend prospective cohort studies evaluating long-term health outcomes and mortality after flooding, standardized definitions for flood exposure and environmental contamination metrics, and formal cost-effectiveness evaluations of GIS-based early warning systems and community protective equipment distribution programs. Until such evidence accumulates, the review stands as a clear signal that in Southeast Asia, the aftermath of a flood is as much a public health event as the disaster itself.

Subject of Research: The relationship between flooding and leptospirosis outbreaks in Southeast Asia

Article Title: Impact of flooding on leptospirosis outbreaks in Southeast Asia: a systematic review (2020–2025)

Article References: de Cruz, E., Ayob, N. B., Lamit, R., Mohd Shah, S. B., Jelip, J., Atil, A., Syed Abdul Rahim, S. S., Jeffree, M. S., & Keanjoom, R. (2026). Impact of flooding on leptospirosis outbreaks in Southeast Asia: a systematic review (2020–2025). Discover Public Health, 23(1), Article 1738. https://doi.org/10.1186/s12982-026-03028-0

Image Credits: AI Generated

DOI: 10.1186/s12982-026-03028-0

Keywords: leptospirosis, flooding, Southeast Asia, systematic review, climate change, monsoon, public health, rodent exposure, drainage infrastructure, early warning systems, One Health, protective equipment