Detectable human-associated microbial source tracking markers in ambient recreational water strengthen associations between general fecal pollution indicators and gastrointestinal illness in swimmers.

Identifying the source of fecal contamination of recreational water matters because human feces typically pose greater health risk than non-human feces. This study examines whether human-associated microbial source tracking (MST) markers and general fecal indicator organisms (FIOs) are associated with gastrointestinal symptoms and norovirus infections in swimmers at a freshwater beach affected by human fecal contamination. Participants were enrolled at the beach, and gastrointestinal symptom data were collected 10-14 days later. Saliva samples were collected at enrollment and 10-14 days and 30-40 days afterward to measure immunoglobulin G (IgG) responses to norovirus antigens. A steep increase in anti-norovirus IgG (immunoconversion) served as a proxy for infection. Daily water samples were analyzed for FIOs and human-associated MST markers (HumM2 and HF183/BacR287). Among 425 participants with full-body water contact, the presence of both MST markers was associated with an adjusted odds ratio (aOR) of 6.78 (1.61; 28.6) for norovirus immunoconversion. Detection of at least one MST marker significantly strengthened associations between FIOs and diarrhea. When an MST marker was detected, the aOR of diarrhea per doubling of FIO concentration was 1.84 (95% confidence interval 1.20; 2.82) for somatic coliphage, 6.24 (2.06; 18.8) for male-specific coliphage, 1.58 (1.31; 1.91) for culturable enterococci, 2.41 (1.34; 4.31) for culturable E. coli, 1.72 (1.08; 2.74) for Enterococcus spp. by qPCR, and 4.41 (1.86; 10.4) for E. coli by qPCR. FIOs were not associated with diarrhea when both MST markers were not detected. Using two MST markers supplementing FIOs helps to characterize waterborne infection risk in swimmers.