Assessing global flood-induced occurrence and transmission risks of biological contaminants

Flooding can facilitate the occurrence and transmission of infectious diseases caused by biocontaminants, including pathogenic microorganisms and antibiotic resistance genes (ARGs), however, the full extent of flood-induced biocontamination remains poorly quantified. Here, we conducted a systematic analysis of empirical evidence on the impacts of flooding on biocontamination. Combined sewer overflows (CSOs) are the dominant sources of biocontaminants, with reported fecal indicator bacteria (FIB) concentrations ranging from 2.06-5.45 log₁₀ colony forming unit (CFU)/100 mL or 0.56-5.37 log₁₀ most probable number (MPN)/100 mL, and that most studies focus on receiving water bodies. Although FIB are the most frequently monitored, they are rarely the direct cause of flood-associated diseases. In contrast, Leptospira spp. are the most recorded clinically relevant pathogens. Direct contact with contaminated floodwaters and vector-borne transmission are the primary pathways linking environmental biocontamination to infectious disease. Therefore, mitigating the occurrence and spread of biocontaminants in flood-affected environments is critical for disease prevention. Nature-based solutions, such as bioretention ponds, represent a promising mitigation strategy, demonstrating median removal efficiencies of approximately 1.0 log₁₀ (90%) for FIB. As flooding events increase in frequency and intensity under climate change, proactive investment in climate-resilient water infrastructure and comprehensive monitoring systems will be essential to reduce human exposure to biocontaminants and protect public health during extreme events.