Microbial Risk Assessment

Microbial risk assessment is the science of quantifying the degree of health risk associated with coming into contact with aquatic fecal contamination. Multiple factors determine infection rates from exposure to fecal pathogens, including degradation rates, dispersal dynamics, and whether the contamination is human sewage vs. non-human fecal matter. The coastal water-quality management community relies on microbial risk assessment to adequately protect swimmers and surfers, as well as those who consume shellfish and other seafood.

In recent decades, SCCWRP and its partners have conducted some of the most seminal coastal health risk assessment studies targeting Southern California’s recreational beaches. This work encompasses the two predominant scientific approaches to conducting microbial risk assessments:

  • Epidemiology studies, which involves tracking illness and infection rates for human subjects who swim and surf at a site of interest
  • Health risk modeling via Quantitative Microbial Risk Assessment (QMRA), which uses site-specific measurements of fecal pollution to estimate health risks from water contact
Two members of a SCCWRP field crew wearing yellow safety vests wade ankle-deep into the water at Leo Carillo State Beach to collect water samples.
A SCCWRP field crew collects water samples at Leo Carrillo State Beach near the Los Angeles-Ventura County border for a study estimating illness risks for beachgoers. SCCWRP and its partners have conducted some of the most seminal coastal health risk assessment studies targeting Southern California’s recreational beaches.

Epidemiology vs. QMRA-based risk assessment

Epidemiology studies remain the gold standard for assessing health risks from exposure to fecal contamination. Epidemiology studies report risk by quantifying the number of infection cases per 100,000 for a given site. But these risk assessments are prohibitively costly and time-consuming to conduct at every beach. Epidemiology studies also state only the magnitude of the problem, and don’t offer management insights into root cause.

By contrast, health risk modeling via Quantitative Microbial Risk Assessment (QMRA) has the potential to offer a cost-effective, rapid alternative; QMRA also generates field data that can help identify the source and origin point of the contamination. During a QMRA, researchers use DNA-based microbial source tracking (MST) technology to quantify the number and types of microbes present. This information helps researchers determine how much contamination, if any, is coming from human sources; human sewage is far more likely to be pathogenic than non-human fecal contamination.

Three field researchers standing waist-deep in Newport Bay near a bridge use plastic buoys attached to yellow ropes to aid in collecting water samples.
A SCCWRP field crew deploys oysters in cages in specific locations across Newport Bay in Orange County for a study examining whether the bacterial water-quality numeric objective designed to protect the health of people who consume shellfish from the bay has been appropriately set. Coastal water-quality managers rely on microbial risk assessment studies to ensure swimmers, surfers as well as those who consume seafood are adequately protected.

Test-driving health risk modeling

SCCWRP is test-driving QMRA at the relatively small portion of Southern California beach sites that experience chronically elevated fecal contamination levels in dry weather. Although beach water-quality managers have invested significant resources to improve water quality, fecal contamination levels remain high at these sites.

What isn’t clear is how much of a health risk the contamination at these sites poses to beachgoers. Traditionally, managers have defaulted to assuming that all fecal contamination carries the same health risk as human sewage; however, if the fecal contamination is from non-human sources, the health risk would be much lower.

QMRA has the potential to help beach managers build a more accurate, site-specific picture of health risk. Using source tracking technology, researchers first evaluate whether the contamination at the site is predominantly human; if it’s not, researchers use computer models to estimate the health risk posed by the contamination. Beach managers can use these insights to focus their attention and limited resources more effectively.

A warning sign on the beach reads: “Warning. Bacteria levels exceed heath standards ocean water contact may cause illness. No Swimming.”
Elevated fecal contamination levels at beaches – common during wet weather in Southern California – can trigger public health officials to post warning signs advising beachgoers to stay out of the water. Using source tracking technology, researchers first evaluate whether the contamination at the site is predominantly human; if it’s not, researchers use computer models to estimate the health risk posed by the contamination.