Outbreaks associated with fresh or marine (i.e., untreated) recreational water can
be caused by pathogens or chemicals, including toxins. Voluntary reporting of these
outbreaks to CDC’s National Outbreak Reporting System (NORS) began in 2009. NORS data
for 2009–2017 are finalized, and data for 2018–2019 are provisional. During 2009–2019
(as of May 13, 2020), public health officials from 31 states voluntarily reported
119 untreated recreational water–associated outbreaks, resulting at least 5,240 cases;
103 of the outbreaks (87%) started during June–August. Among the 119 outbreaks, 88
(74%) had confirmed etiologies. The leading etiologies were enteric pathogens: norovirus
(19 [22%] outbreaks; 1,858 cases); Shiga toxin–producing Escherichia coli (STEC) (19
[22%]; 240), Cryptosporidium (17 [19%]; 237), and Shigella (14 [16%]; 713). This report
highlights three examples of outbreaks that occurred during 2018–2019, were caused
by leading etiologies (Shigella, norovirus, or STEC), and demonstrate the wide geographic
distribution of such outbreaks across the United States. Detection and investigation
of untreated recreational water–associated outbreaks are challenging, and the sources
of these outbreaks often are not identified. Tools for controlling and preventing
transmission of enteric pathogens through untreated recreational water include epidemiologic
investigations, regular monitoring of water quality (i.e., testing for fecal indicator
bacteria), microbial source tracking, and health policy and communications (e.g.,
observing beach closure signs and not swimming while ill with diarrhea).
California
On July 22, 2019, the California Department of Public Health was notified of three
cases of shigellosis in persons who reported playing in the Santa Ana River, a waterway
spanning 100 miles through southern California. The department identified this exposure
in other shigellosis cases and, in total, identified 24 cases with closely related
isolates (within 0–2 alleles by core-genome multilocus sequence typing) of Shigella
sonnei. Among 19 ill persons for whom epidemiologic data were available, 16 reported
that during July 6–August 5 they played in a swim area in a shallow portion of the
river where water quality was not regularly monitored. Two of the 16 ill persons also
reported swallowing river water. No other common risk factors were identified. The
median age of these 16 ill persons was 7 years (range = 1–20 years); seven were female.
Two of 15 ill persons for whom clinical data were available were hospitalized; none
died. Date of symptom onset ranged from July 6 through August 7. In response to the
outbreak, local public health officials closed public access to the swim area during
August 8–15. Surface water samples were collected upstream, downstream, and at the
swim area and tested for E. coli, a bacterial indicator of fecal contamination. The
concentration of E. coli ranged from 350 through 1,600 most probable number/100 mL
at these sites.* Investigation into possible sources of fecal contamination upstream
and at the swim area did not definitively identify an outbreak source. No additional
cases were identified after public access to the swim area was reopened on August
15.
Maine
On July 6, 2018, the Maine Center for Disease Control and Prevention received a report
that multiple persons were ill with gastrointestinal symptoms after visiting Woods
Pond Beach in Bridgton, Maine. Town officials in Bridgton closed the public beach
during July 6–10. The agency used social media to identify persons who visited the
pond during July 1–6, interviewed 34 heads of household, and completed surveys for
148 household members. A total of 139 persons reported visiting the pond during this
period, 97 (70%) of whom reported illness. Among these 97 ill persons, 41 (42%) were
male; among the 95 ill persons for whom age data were available, the median age was
12 years (range = 1–73 years). The median incubation period was 38 hours (range = 8–139
hours); the median symptom duration, reported for 91 cases, was 24 hours (range = 3–96
hours). Vomiting was reported by 78 (80%) of 97 ill persons. Visitors who reported
swallowing pond water or going under water (a potential marker for swallowing water)
were approximately three times more likely to be ill than were those who did not (relative
risk = 3.19; 95% confidence interval [CI] = 1.69–6.05). Two of the stool specimens
collected from four ill persons tested positive for norovirus genogroup I. Based on
these test results and the reported symptomology, norovirus was thought to be the
outbreak etiology. The source of water contamination was undetermined. No additional
cases were reported after the beach reopened to swimmers on July 11.
Minnesota
On August 13, 2019, Minnesota Department of Health (MDH) epidemiologists identified
three cases of STEC infection in persons who reported swimming at a public lake. Illness
onset occurred during August 2–4. MDH notified park and recreation board officials
of the cases on August 13 and advised them to close the lake to swimmers. MDH used
social media to distribute a survey and identified 69 total cases, including four
laboratory-confirmed STEC O145:H28 infections with closely related isolates (within
0–2 single nucleotide polymorphisms by whole genome sequencing). Dates of symptom
onset ranged from July 18 through August 16. The median age of ill persons was 29
years (range = 1–65 years); 55 (80%) were female. Among the 24 (35%) ill persons who
visited the beach only once, exposure dates ranged from July 16 through August 11.
The two factors significantly associated with illness were swallowing lake water (odds
ratio = 3.80; 95% CI = 1.17–12.38) and age ≤10 years (odds ratio = 2.90; 95% CI = 1.57–5.35).
No hospitalizations or cases of hemolytic uremic syndrome were reported. The beach
was monitored weekly for E. coli throughout the summer, but no test results exceeded
Minnesota’s recreational water criteria during April–October.
†
No evidence of a point source of fecal contamination was identified; however, 15 visitors
and four lifeguards reported continuing to swim or work in the lake while ill. No
additional cases were reported after the beach reopened to swimmers on September 5.
Discussion
Shigella, norovirus, STEC, and other enteric pathogens can be transmitted when persons
ingest untreated recreational water contaminated with feces or vomit. Swimmers can
contaminate water in untreated recreational water venues (e.g., lakes, oceans, and
rivers) if they have a fecal or vomit incident in the water. Enteric pathogens can
also be introduced into untreated recreational water venues by stormwater runoff and
sewage system overflows and discharges. Other potential sources of fecal contamination
and enteric pathogens include leaks from septic or municipal wastewater systems, dumped
boating waste, and animal waste in or near swim areas.
Whereas the detection of Shigella and norovirus in untreated recreational water is
indicative of human contamination, the detection of STEC does not necessarily indicate
human contamination. Because E. coli and enterococci are part of the normal intestinal
flora of humans and other animals, beach managers monitor levels of these bacteria
as indicators of fecal contamination as recommended by the Environmental Protection
Agency’s 2012 recreational water quality criteria (
1
). Monitoring is conducted to detect changes in fecal contamination of water and not
to indicate the presence of pathogens (
2
–
4
). For this reason, fecal indicator data alone cannot implicate the water as the route
of outbreak exposure or identify the source of water contamination. This is particularly
problematic for certain pathogenic strains of E. coli, such as E. coli O157:H7, which
can persist in the sediment and be resuspended in the water but is not detected by
most generic E. coli water tests.
In the outbreaks described in this report, the sources of contamination of the recreational
waters were not definitively identified. Molecularly based microbial source tracking
methods can be used to identify the host genus contributing to fecal contamination
detected in water, which can inform more targeted environmental investigations and
control measures (
5
). For example, identifying the host genus (e.g., horses) can help inform and optimize
efforts to mitigate exposure (e.g., redesigning horse trails near untreated recreational
water venues) to prevent outbreaks. Investigations into environmental influences include,
but are not limited to, sanitary inspection of septic systems, identification of agricultural
animal waste runoff or discharge, monitoring of wildlife activity in public areas,
and identification of improper disposal of solid waste.
Multiple factors could hinder detection and investigation of outbreaks associated
with untreated recreational water venues. First, persons often travel >100 miles to
swim in lakes, oceans, and rivers (
6
). If swimmers become ill after returning to homes in multiple public health jurisdictions,
identifying an outbreak can be difficult. Second, not all jurisdictions include questions
about exposure to recreational water in their investigations of cases of illness caused
by enteric pathogens. Third, issues with response activities (e.g., collection of
water samples and decision-making about closures) might arise among agencies within
the same jurisdiction (e.g., public health and natural resources agencies) or among
jurisdictions if the outbreak source (i.e., untreated recreational water venue) is
in multiple jurisdictions.
In addition to monitoring the level of fecal indicator bacteria at beaches, beach
managers can promote healthy swimming by establishing policies that allow lifeguards
to perform alternate duties that do not require them to enter the water if they are
ill with diarrhea. This is equivalent to CDC recommendations for operators of public
treated recreational water venues (e.g., swimming pools)
§
(
7
). Creating a workplace environment where employees feel comfortable disclosing that
they are ill with diarrhea without fearing potential loss of wages or even work is
important to the success of such policies. Because of the multiple potential sources
of fecal contamination, beach managers and public health officials should educate
swimmers and parents of young swimmers about steps they can take to minimize risk
of infection from enteric pathogens (https://www.cdc.gov/healthywater/swimming/oceans-lakes-rivers/visiting-oceans-lakes-rivers.html).
These healthy swimming steps include observing beach closure signs or water quality
advisories because of elevated levels of fecal indicator bacteria, not swimming in
water made cloudier by heavy rain, not swimming while ill with diarrhea, not swallowing
the water, and keeping sand out of mouths. In addition, for the 2020 summer swim season,
CDC has released coronavirus disease 2019 (COVID-19) prevention considerations for
beach managers (https://www.cdc.gov/coronavirus/2019-ncov/community/parks-rec/public-beaches.html).
Summary
What is already known about this topic?
Untreated recreational water–associated outbreaks can be caused by pathogens or chemicals,
including toxins, in freshwater (e.g., lakes) or marine water (e.g., oceans).
What is added by this report?
This report highlights examples of untreated recreational water–associated outbreaks
that occurred during 2018 or 2019, were caused by Shigella (California), norovirus
(Maine), or Shiga toxin–producing Escherichia coli (Minnesota), the leading causes
of such outbreaks, and demonstrate the wide geographic distribution of such outbreaks.
What are the implications for public health practice?
Swimmers should observe beach closure signs and water quality advisories, not swim
in water made cloudier by heavy rain, not swim while ill with diarrhea, not swallow
recreational water, and keep sand out of their mouths.