Flooding and zoonotic disease: research still overlooks who is most vulnerable

Flooding can increase exposure to environmentally persistent zoonotic pathogens by redistributing contamination through water, soil and damaged infrastructure. But a new scoping review finds that research still does a poor job of explaining why some populations face much greater infection risks than others. The evidence is dominated by leptospirosis, broad rainfall indicators and basic demographic variables, while housing, sanitation, infrastructure and access to healthcare remain comparatively neglected.

The paper analysed 78 epidemiological studies covering flood-sensitive environmentally persistent zoonoses, pathogens capable of surviving outside their hosts for at least 48 hours and whose transmission can increase with flooding or precipitation. The diseases included leptospirosis, cryptosporidiosis, giardiasis, melioidosis, salmonellosis, campylobacteriosis, E. coli infection and toxoplasmosis. Infected animals may contaminate soil or water, while heavy rainfall and flooding can mobilise pathogens and increase opportunities for human exposure through contaminated water, food or direct contact.

Leptospirosis dominates the evidence

More than half of the studies concerned leptospirosis (41 of 78). Cryptosporidiosis and giardiasis each accounted for 11 studies, melioidosis and salmonellosis eight each, campylobacteriosis seven and E. coli only two. No eligible toxoplasmosis study was identified. The methodological base was also relatively weak for studying climate-related changes over time: 28 studies were cross-sectional, 18 ecological and eight case-control, compared with only six time-series and four cohort studies.

444 vulnerability indicators, but mostly age and sex

The researchers extracted 444 vulnerability indicators, grouped into six domains. Almost half (215) were sociodemographic, particularly sex/gender and age. Biological susceptibility contributed 75 indicators, the local environment 65, WASH services 63, housing only 21 and health and social services just five. Only 16 of the 78 studies explicitly used the words “vulnerability” or “vulnerable”, and their definitions varied considerably. In some papers vulnerability meant poverty or marginalisation. In others it referred to age, biological susceptibility, geographic location or simply elevated disease risk.

This narrow approach can miss factors that are especially important for environmentally transmitted zoonoses: poor drainage, inadequate sanitation, unreliable water supplies, housing quality, waste management and healthcare access.

Rainfall is often used as a substitute for flooding

The review identified 138 hazard indicators, but most did not directly measure flooding. Meteorological variables accounted for 52 indicators and seasonal measures for 38, together almost two-thirds of the total. Direct measures were much less common. 27 indicators described flood susceptibility or landscape characteristics and only 21 examined flood events or hydrological exposure, including individual contact with floodwater, household flooding and community flood events.

That distinction matters. The review cites studies in which actual flood susceptibility or household flooding was more strongly associated with leptospirosis than broad rainfall measures alone.

Only four studies formally tested the interaction

Perhaps the most striking finding is that vulnerability and climate hazards are usually studied independently.Only three studies used conceptual frameworks explicitly linking vulnerability and hazards. Six conducted stratified analyses, but just four formally tested whether vulnerability modified the effect of rainfall, season or flooding on disease risk. The result is an important gap for One Health preparedness: current research may identify places with high rainfall or populations with high baseline disease risk, but often cannot determine whose risk increases most when flooding actually occurs, and why.

The authors argue for studies combining direct flood measurements with social, environmental and infrastructural vulnerability indicators, using stronger longitudinal and interdisciplinary designs. This would make zoonotic-disease surveillance more useful for climate adaptation and for targeting prevention towards the populations most likely to be affected.