Outbreaks of illness associated with recreational water use result from exposure to
chemicals or infectious pathogens in recreational water venues that are treated (e.g.,
pools and hot tubs or spas) or untreated (e.g., lakes and oceans). For 2011–2012,
the most recent years for which finalized data were available, public health officials
from 32 states and Puerto Rico reported 90 recreational water–associated outbreaks
to CDC’s Waterborne Disease and Outbreak Surveillance System (WBDOSS) via the National
Outbreak Reporting System (NORS). The 90 outbreaks resulted in at least 1,788 cases,
95 hospitalizations, and one death. Among 69 (77%) outbreaks associated with treated
recreational water, 36 (52%) were caused by Cryptosporidium. Among 21 (23%) outbreaks
associated with untreated recreational water, seven (33%) were caused by Escherichia
coli (E. coli O157:H7 or E. coli O111). Guidance, such as the Model Aquatic Health
Code (MAHC), for preventing and controlling recreational water–associated outbreaks
can be optimized when informed by national outbreak and laboratory (e.g., molecular
typing of Cryptosporidium) data.
A recreational water–associated outbreak is the occurrence of similar illnesses in
two or more persons, epidemiologically linked by location and time of exposure to
recreational water or recreational water–associated chemicals volatilized into the
air surrounding the water. Public health officials in the 50 states, the District
of Columbia, U.S. territories, and Freely Associated States* voluntarily report outbreaks
of recreational water–associated illness to CDC. In 2010, waterborne outbreaks became
nationally notifiable. This report summarizes data on recreational water–associated
outbreaks electronically reported by October 30, 2014 to CDC’s WBDOSS (http://www.cdc.gov/healthywater/surveillance/)
for 2011 and 2012 via NORS.† Data requested for each outbreak include the number of
cases,§ hospitalizations, and deaths; etiology; setting (e.g., hotel) and venue (e.g.,
hot tub or spa) where the exposure occurred; earliest illness onset date; and illness
type. All outbreaks are classified according to the strength of data implicating recreational
water as the outbreak vehicle (1).¶ Outbreak reports classified as Class I have the
strongest supporting epidemiologic, clinical laboratory and environmental health data,
and those classified as Class IV, the weakest. Classification does not assess adequacy
or completeness of investigations.** Negative binomial regression (PROC GENMOD in
SAS 9.3 [Cary, NC]) was used to assess trends in the number of outbreaks over time.
For the years 2011 and 2012, public health officials from 32 states and Puerto Rico
reported 90 recreational water–associated outbreaks (http://www.cdc.gov/healthywater/surveillance/rec-water-tables-figures.html)
(Figure 1), which resulted in at least 1,788 cases, 95 (5%) hospitalizations, and
one death. Etiology was confirmed for 73 (81%) outbreaks: 69 (77%) outbreaks were
caused by infectious pathogens, including two outbreaks with multiple etiologies,
and four (4%) by chemicals (Table). Among the outbreaks caused by infectious pathogens,
37 (54%) were caused by Cryptosporidium. On the basis of data reported to CDC, 37
(41%) of the 90 outbreak reports were categorized as class IV.
Outbreaks associated with treated recreational water accounted for 69 (77%) of the
90 outbreaks reported for 2011–2012, and resulted in at least 1,309 cases, 73 hospitalizations,
and one reported death. The median number of cases reported for these outbreaks was
seven (range: 2–144 cases). Hotels (e.g., hotel, motel, lodge, or inn) were the setting
of 13 (19%) of the treated recreational water–associated outbreaks. Twelve (92%) of
these 13 outbreaks started outside of June–August; ten (77%) were at least in part
associated with a spa. Among the 69 outbreaks, 36 (52%) were caused by Cryptosporidium.
The 69 outbreaks had a seasonal distribution, with 42 (61%) starting in June–August
(Figure 1). Acute gastrointestinal illness was the disease manifestation in 34 (81%)
of these summer outbreaks, with Cryptosporidium causing 32 (94%) of them. Since 1988,
the year that the first U.S. treated recreational water–associated outbreak of cryptosporidiosis
was detected (2,3) (Figure 2), the number of these outbreaks reported annually (range:
0–40 outbreaks) has significantly increased (negative binomial regression; p<0.001).
Incidence of these cryptosporidiosis outbreaks has also, at least in part, driven
the significant increase (negative binomial regression; p<0.001) in the overall number
of recreational water–associated outbreaks reported annually (range: 6–84).
Summary
What is already known on this topic?
Treated and untreated recreational water–associated outbreaks occur throughout the
United States and their incidence has been increasing in recent years. CDC collects
data on waterborne outbreaks electronically submitted by the 50 states, the District
of Columbia, U.S. territories, and Freely Associated States to CDC’s Waterborne Disease
and Outbreak Surveillance System via the National Outbreak Reporting System.
What is added by this report?
For 2011–2012, a total of 90 recreational water–associated outbreaks were reported
to CDC, resulting in at least 1,788 cases, 95 hospitalizations, and one death. Cryptosporidium
caused over half of the outbreaks associated with treated recreational water venues
(e.g., pools). Escherichia coli O157:H7 and O111 caused one third of outbreaks associated
with untreated recreational water (e.g., lakes).
What are the implications for public health practice?
Guidance, such as the Model Aquatic Health Code (MAHC), to prevent and control recreational
water–associated outbreaks can be optimized when informed by national outbreak and
laboratory (e.g., molecular typing of Cryptosporidium) data.
For 2011–2012, 21 (23%) outbreaks were associated with untreated recreational water.
These outbreaks resulted in at least 479 cases and 22 hospitalizations. The median
number of cases reported for these outbreaks was 16 (range: 2–125). Twenty (95%) of
these outbreaks were associated with fresh water; 18 (86%) began in June–August; and
seven (33%) were caused by E. coli O157:H7 or O111. One outbreak associated with exposure
to cyanobacterial toxins was reported.
Discussion
Cryptosporidium continues to be the dominant etiology of recreational water–associated
outbreaks. Half of all treated recreational water–associated outbreaks reported for
2011–2012 were caused by Cryptosporidium. Among treated recreational water–associated
outbreaks of gastrointestinal illness that began in June–August, >90% were caused
by Cryptosporidium, an extremely chlorine-tolerant parasite that can survive in water
at CDC-recommended chlorine levels (1–3 mg/L) and pH (7.2–7.8) for >10 days (4). In
contrast, among 14 untreated recreational water–associated outbreaks of gastrointestinal
illness starting in June–August, 7% (one) were caused by Cryptosporidium. The decreased
diversity of infectious etiologies causing treated recreational water–associated outbreaks
is likely a consequence of the aquatic sector’s reliance on halogen disinfection (e.g.,
chlorine or bromine) and maintenance of proper pH, which are well documented to inactivate
most infectious pathogens within minutes (5). Continued reporting of treated recreational
water–associated outbreaks caused by chlorine-intolerant pathogens (e.g., E. coli
O157:H7 and norovirus) highlights the need for continued vigilance in maintaining
water quality (i.e., disinfectant level and pH), as has been recommended for decades
(5).
In the United States, codes regulating public treated recreational water venues are
independently written and enforced by individual state or local agencies; the consequent
variation in the codes is a potential barrier to preventing and controlling outbreaks
associated with these venues. In August 2014, CDC released the first edition of MAHC
(http://www.cdc.gov/mahc), a comprehensive set of science-based and best-practice
recommendations to reduce risk for illness and injury at public, treated recreational
water venues. MAHC represents the culmination of a 7-year, multi-stakeholder effort
and is an evolving resource that addresses emerging public health threats, such as
treated recreational water-associated outbreaks of cryptosporidiosis, by incorporating
the latest scientifically validated technologies that inactivate or remove infectious
pathogens. For example, MAHC recommends additional water treatment (e.g., ultraviolet
light or ozone) to inactivate Cryptosporidium oocysts at venues where WBDOSS data
indicate there is increased risk for transmission. MAHC recommendations can be voluntarily
adopted, in part or as a whole, by state and local jurisdictions.
The number of reported untreated recreational water–associated outbreaks confirmed
or suspected to be caused by cyanobacterial toxins has decreased, from 11 (2009–2010)
to one (2011–2012) (6). This decrease is likely the result of a decrease in outbreak
reporting rather than a true decrease in incidence. CDC is currently developing a
mechanism for reporting algal bloom–associated individual cases through NORS to better
characterize their epidemiology.
The findings in this report are subject to at least two limitations. First, the outbreak
counts presented are likely an underestimate of actual incidence. Many factors can
present barriers to the detection, investigation, and reporting of outbreaks: 1) mild
illness; 2) small outbreak size; 3) long incubation periods; 4) wide geographic dispersion
of ill swimmers; 5) transient nature of contamination; 6) setting or venue of outbreak
exposure (e.g., residential backyard pool); and 7) potential lack of communication
between those who respond to outbreaks of chemical etiology (e.g., hazardous materials
personnel) and those who usually report outbreaks (e.g., infectious disease epidemiologists).
Second, because of variation in public health capacity and reporting requirements
across jurisdictions, those reporting outbreaks most frequently might not be those
in which outbreaks most frequently occur.
Increasingly, molecular typing tools are being employed to understand the epidemiology
of waterborne disease and outbreaks. Most species and genotypes of Cryptosporidium
are morphologically indistinguishable from one another, and only molecular methods
can distinguish species and subtypes and thereby elucidate transmission pathways (7,8).
Systematic national genotyping and subtyping of Cryptosporidium in clinical specimens
and environmental samples through CryptoNet (http://www.cdc.gov/parasites/crypto/cryptonet.html)
can identify circulating Cryptosporidium species and subtypes and help identify epidemiologic
linkages between reported cases. Molecular typing could substantially help elucidate
cryptosporidiosis epidemiology in the United States and inform development of future
guidance to prevent recreational water–associated and other outbreaks of cryptosporidiosis
(9,10).