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Project: Mission Bay Epidemiological Study

Background and Objectives

Fecal indicator bacteria are routinely monitored at marine recreational bathing beaches to assess the public health risk of contracting swimming-related illness. There have been numerous epidemiology studies that demonstrated the relationship of indicator bacteria to health risk, but they have been mostly conducted on beaches impacted by point sources with known human fecal contributions. Few studies have examined this relationship at beaches where non-point sources are the dominant fecal input source.

This epidemiology study was conducted in Mission Bay, California, where the dominant fecal source appears to be non-human, and about 20% of historic routine bacterial samples have failed water quality standards. The study focused on three primary questions:

1) Did water contact increase the risk of illness during the two weeks following exposure to water?
2) Among those individuals with water contact, were there associations between illness and measured levels of traditional water quality indicators?
3) Among those individuals with water contact, were there associations between illness and measured levels of non-traditional water quality indicators?


This study was conducted in 2003.


The project was designed as a cohort study. Nearly 8,800 participants were recruited from the six most popular swimming beaches in Mission Bay on weekends and holidays during the summer of 2003. Each participant provided their current state of health and degree of water exposure on their day at the beach. On the same day, water quality was monitored for traditional fecal indicator bacteria (enterococcus, fecal coliforms, total coliforms). A subset of samples was also measured for non-traditional indicators, including new methods for measuring bacteria (chromogenic substrate or quantitative polymerase chain reaction [QPCR]), new bacterial indicators (Bacteroides), and viruses (somatic and male-specific phage, adenovirus, Norwalk-like virus).

Beach locations in Mission Bay where testing was conducted.

Ten to 14 days after exposure, the participants were contacted by phone and interviewed about symptoms of illness that occurred since their visit to the beach. They were queried about multiple types of illness: gastrointestinal illnesses (diarrhea, nausea, stomach pain, cramps, vomiting, highly credible gastrointestinal illness 1 or 2 [HCGI-1 or HCGI-2]); respiratory illnesses (cough, cough with phlegm, nasal congestion or runny nose, sore throat, significant respiratory illness); dermatologic outcomes (skin rash, infected cuts or scrapes); and non-specific symptoms (fever, chills, eye irritation, earache, ear discharge, eye irritation or redness). Multivariate analysis was conducted to assess relationships between health outcomes and degree of water contact or levels of water quality indicators. These analyses were adjusted for confounding covariates such as age, gender, and ethnicity.


• Only skin rash and diarrhea were consistently significantly elevated in swimmers compared to non-swimmers.

• For diarrhea, the risk was strongest among children 5 to 12 years old.

• The risk of illness was uncorrelated with levels of traditional water quality indicators and state water quality thresholds were not predictive of swimming-related illnesses.

• Similarly, no correlation was found between increased risk of illness and increased levels of most non-traditional water quality indicators.

• A significant association was observed between the levels of male-specific coliphage and HCGI-1, HCGI-2, nausea, cough, and fever, but these associations should be interpreted cautiously because so few participants were exposed to the water at times when male-specific coliphage was detected.

For more information on Mission Bay Epidemiological Study, contact John Griffith at (714) 755-3228.
This page was last updated on: 7/2/2014