EPIDEMIOLOGICAL STUDY OF SWIMMER ILLNESS IN MISSION BAY, CA

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EPIDEMIOLOGICAL STUDY OF SWIMMER ILLNESS IN
MISSION BAY, CA
Stephen B. Weisberg Southern California Coastal
Water Research Project www.sccwrp.org
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BACKGROUND

• Current bacterial standards are based on
  epidemiological relationships between bacterial
  density and swimmer illness rates
• Studies to establish these relationships were
  conducted primarily at sites with human, point
  source fecal inputs
• Santa Monica Bay study is the primary exception
• Nonpoint sources are responsible for most beach
  warnings in California
• It is unclear how well the epidemiological
  relationships extrapolate to nonpoint source
  locations

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MISSION BAY AS A STUDY SITE

• 478 Beach postings in 2003
• Almost 20 of samples exceed standards
• Nonpoint sources are believed to be dominant
• City of San Diego spent millions to remove human
  fecal inputs
• MST studies suggest that animal sources (I.e.
  birds) are dominant
• Heavily used aquatic park
• Protected waters with easy highway access

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STUDY QUESTIONS

• Is there a health risk of swimming in Mission
  Bay?
• Comparison of illness between swimmers and
  non-swimmers
• Can we relate health risk to bacterial
  concentration?
• Comparison if illness among swimmers at different
  times and locations
• Can we relate health risk to non-traditional
  microbiological indicators?
• Virus
• Phage
• Bacteroides
• Rapid enterococcus measurement method

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STUDY DESIGN

• HEALTH OUTCOMES
• Gastrointestinal
• Nausea vomiting, diarrhea, cramps, Highly
  Credible Gastrointestinal 1 (HCGI-1), HCGI-2
• Respiratory
• Cough, cough with phlegm, nasal congestion, sore
  throat, Significant respiratory disease (SRD)
• Dermatologic
• Skin rash, scrapes
• Non-specific
• Fever, chills, earache, ear discharge, eye
  irritation

• People interviewed on the beach to assess
  qualifications
• One-time swimmers only
• Follow up interviews ten days later to assess
  illness rate
• 14 health outcomes assessed
• Almost 9,000 participants
• Seven microbial indicators measured throughout
  the day
• Linked to participants swimming location
• Logistic regression models used to estimate odds
  of illness in exposed vs. unexposed groups
• Multiple measures of exposure
• Model adjusted for potentially confounding
  covariates

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STUDY DESIGN

• EXPOSURE METRICS
• Any water contact
• Water contact
• Shoulders in water
• Face under water
• Water in the mouth
• Swallowing water
• Gag or cough from water
• Water quality exposure
• Median
• Mean
• Maximum

• People interviewed on the beach to assess
  qualifications
• One-time swimmers only
• Follow up interviews ten days later to assess
  illness rate
• 14 health outcomes assessed
• Almost 9,000 participants
• Seven microbial indicators measured throughout
  the day
• Linked to participants swimming location
• Logistic regression models used to estimate odds
  of illness in exposed vs. unexposed groups
• Multiple measures of exposure
• Model adjusted for potentially confounding
  covariates

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STUDY DESIGN

• EXAMPLE COVARIATES
• Age, gender race
• Allergies
• Dug in sand
• Contact with shells, algae
• Used insect repellant, sunblock
• Showered after beach
• History of GI, respiratory illness
• Household income level

• People interviewed on the beach to assess
  qualifications
• One-time swimmers only
• Follow up interviews ten days later to assess
  illness rate
• 14 health outcomes assessed
• Almost 9,000 participants
• Seven microbial indicators measured throughout
  the day
• Linked to participants swimming location
• Logistic regression models used to estimate odds
  of illness in exposed vs. unexposed groups
• Multiple measures of exposure
• Model adjusted for potentially confounding
  covariates

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WATER QUALITY RESULTS

• Nearly 8,000 water analyses conducted
• 16 of samples exceeded thresholds for
  traditional indicators
• Virtually no human specific virus detected
• Quantified Fphage at some beaches
• Independent source tracking study found
  predominately non-human bacteria in Mission Bay

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WATER QUALITY RESULTS

• Nearly 8,000 water analyses conducted
• 16 of samples exceeded thresholds for
  traditional indicators
• Virtually no human specific virus detected
• Quantified Fphage at some beaches
• Independent source tracking study found
  predominately non-human bacteria in Mission Bay

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EPIDEMIOLOGICAL RESULTS

• Comparisons of Swimmers vs. Non-swimmers
• Logistic regression models
• Examine model for covariates
• Thousands of model runs
• Model health outcomes to indicator bacteria
  concentrations
• Repeat with nontraditional indicators

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ADJUSTED ODDS RATIO ANY WATER CONTACT
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DIARRHEA BY AGE GROUP ANY WATER CONTACT
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DIARRHEA BY AGE GROUP AND WATER CONTACT
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DID THE MEASURED LEVELS CORRELATE WITH HEALTH
OUTCOMES?

• Traditional bacterial indicators
• Enterococcus No
• Fecal Coliform No
• Total Coliform No
• Non-traditional indicators
• Bacteroides No
• Enterococcus by QPCR No
• Virus No
• Male specific phage Yes, but small sample size

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STUDY CONCLUSIONS

• 16 of water sampled exceeded standards, but rash
  was the only consistently elevated health outcome
• There was also increased diarrhea, but only for
  the 5-12 year old age group
• There was no relationship between traditional
  fecal indicator bacteria density and health
  risk
• The only indicator related to health risk was
  coliphage
• Viruses were absent from the system

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EPILOGUE

• Current water quality thresholds were not
  predictive of illness
• Affected TMDL implementation
• Is Mission Bay a typical system?
• Limited circulation
• Extensive effort to remove human fecal sources
• Difficult to extrapolate to other systems at this
  point in time
• More studies needed to assess whether results
  generalize to other locations

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Study Beaches Bonita Cove Crown Pt
Shores Tecolote Ck/Shores Leisure Lagoon Visitors
Center De Anza Cove