Infectious disease risk after the October 2024 flash flood in Valencia, Spain: Disaster evolution, strategic scenario analysis, and extrapolative baseline for a One Health assessment.

The flash flood caused by a cold front occurred beside Valencia city, Spain, on 29 October 2024, was of unprecedented devastation, catastrophic effects and health impact. A fast response initiative was launched to assess the risks of individual infections and infectious epidemics. This early release of results is meant to ease the diagnostic work by hospitals dealing with infected patients and helping to improving preventive infrastructures and action protocols. Infectious agents were searched for in environmental samples of flood water, mud and soil collected during the first three post-flood weeks, mainly focusing on affected core areas. The evolving scenario was followed covering previous days, disaster and post-flood days, and throughout subsequent weeks. The affected geography was mapped to assess water flow trajectories and confluent multidisciplinary circumstances related to short-, mid- and long-term infection risks. The appropriate survey planning distinguished four independent river/ravine basins, each one comprising four different types of sylvatic, rural-agricultural-farming, urban, and lake surrounding areas. A schematic workflow for analytical procedures is provided. A wide diversity of infectious agents was found, with higher concentration rates in mud than in flood waters. Environmental samples included viruses, bacteria, protozoans, opportunistic free-living amoebae, helminths, and vector groups of mosquitos, sandflies and freshwater snails. Transmission characteristics, human infection sources and pathogenicity are summarized for each pathogen found. Potentially emerging vector-borne diseases are analyzed. Detected microorganisms are of different transmission types, from direct to zoonotic, infection sources from oral to transcutaneous and wound infection, from short- to long-term life cycles, and from permanent to seasonal transmission. Fecal contamination showed high rates and indicated both human and animal sources, with potential reservoirs such as sheep, goats, pigs, equids, dogs and cats. The very large flooded zone requires a multidisciplinary One Health approach. Many lessons should be learned by both professionals in involved disciplines and related decision makers in governments. The large heterogeneity, complexity and far-reaching health consequences of the disaster furnish an extrapolative baseline model which may be useful to face similar catastrophic climate events in the future, expected to occur according to current climate change trends, both throughout the Mediterranean basin and elsewhere.