Cyanobacterial blooms at Canadian freshwater beaches: A prospective cohort study of exposure, health outcomes, and One Health implications

Cyanobacterial blooms are an emerging One Health concern in freshwater environments, yet limited evidence exists on how humans and companion animals interact with bloom-prone waters or how these exposures align with water-quality indicators. This study characterizes recreational behaviours, environmental conditions, and self-reported illness patterns across multiple Canadian beaches to inform integrated monitoring and risk-management approaches. A prospective cohort study was conducted at seven freshwater beaches in Manitoba, Nova Scotia, and Ontario during the 2024 and 2025 summer seasons. Participants completed on-site surveys detailing demographics and water-contact behaviours, followed up 3-7 days later with an illness survey. Dog exposures and illnesses were also recorded. Cyanobacterial cell counts, pigments, and E. coli counts were measured on recruitment days. Descriptive analyses and exploratory visualizations examined patterns across human, dog, and environmental data. Among 2,324 participants, water exposure patterns varied widely across sites and demographic groups, with children and teenagers demonstrating the highest water-contact behaviours. Dogs exhibited similar high-contact patterns. Cyanobacterial concentrations showed substantial spatial and temporal variability and did not correlate with E. coli counts. Approximately two-thirds of participants (66.6%) completed the illness follow-up survey. Exploratory analyses revealed a basic association between cyanobacteria and acute gastrointestinal illness (AGI), although the incidence of AGI was low in both humans and dogs. Freshwater recreational environments represent shared human–animal–microbial interfaces. Multi-indicator monitoring, including cyanobacterial metrics, and targeted communication for high-contact groups are essential. A One Health approach integrating human, veterinary, and environmental surveillance will be critical as cyanobacterial blooms intensify under changing climatic conditions.