Assessing the Impact of Pathogen Decay on Quantitative Microbial Risk Assessment Infection Estimates

Quantitative microbial risk assessments (QMRAs) infrequently include pathogen and fecal marker decay. However, this necessitates the assumption that pathogens and indicators decay at similar rates or that decay prior to human exposure is negligible, which may misestimate the infection risks. Here, we created a QMRA model incorporating pathogen (Cryptosporidium, Giardia, Enterovirus, Rotavirus, Salmonella spp., Campylobacter jejuni, and E. coli O157:H7) and fecal indicator (HF183) decay to assess the impact of decay on the probability of gastrointestinal illness estimates in a recreational water QMRA model. Neglecting to account for pathogen decay rapidly resulted in an overestimation of risk; for example, not incorporating decay resulted in a statistically significant overestimation of risk after 30 min for Campylobacter jejuni and after 13 h for all pathogens. Substituting the fecal indicator HF183 decay rates for pathogen decay rates also rapidly resulted in a statistically significant over- or underestimation of risk. HF183 best represented the decay rate of Salmonella spp. but still resulted in an underestimation of risk after 13 h. Overall, including decay rates resulted in statistically different risk estimations on realistic time scales following pollution events, suggesting that QMRAs that neglect decay overestimate the probability of illness, and the indicator ratio approach could yield overestimations or underestimations depending on the difference between indicator and pathogen decay rate constants.