Provision of safe water in the United States is vital to protecting public health
(
1
). Public health agencies in the U.S. states and territories* report information on
waterborne disease outbreaks to CDC through the National Outbreak Reporting System
(NORS) (https://www.cdc.gov/healthywater/surveillance/index.html). During 2013–2014,
42 drinking water–associated
†
outbreaks were reported, accounting for at least 1,006 cases of illness, 124 hospitalizations,
and 13 deaths. Legionella was associated with 57% of these outbreaks and all of the
deaths. Sixty-nine percent of the reported illnesses occurred in four outbreaks in
which the etiology was determined to be either a chemical or toxin or the parasite
Cryptosporidium. Drinking water contamination events can cause disruptions in water
service, large impacts on public health, and persistent community concern about drinking
water quality. Effective water treatment and regulations can protect public drinking
water supplies in the United States, and rapid detection, identification of the cause,
and response to illness reports can reduce the transmission of infectious pathogens
and harmful chemicals and toxins.
To provide information about drinking water–associated waterborne disease outbreaks
in the United States in which the first illness occurred in 2013 or 2014 (https://www.cdc.gov/healthywater/surveillance/drinking-surveillance-reports.html),
CDC analyzed outbreaks reported to the CDC Waterborne Disease and Outbreak Surveillance
System through NORS (https://www.cdc.gov/nors/about.html) as of December 31, 2015.
For an event to be defined as a waterborne disease outbreak, two or more cases must
be linked epidemiologically by time, location of water exposure, and illness characteristics;
and the epidemiologic evidence must implicate water exposure as the probable source
of illness. Data requested for each outbreak include 1) the number of cases, hospitalizations,
and deaths; 2) the etiologic agent (confirmed or suspected); 3) the implicated water
system; 4) the setting of exposure; and 5) relevant epidemiologic and environmental
data needed to understand the outbreak occurrences and for determining the deficiency
classification.
§
One previously unreported outbreak with onset date of first illness in 2012 is presented
but is not included in the analysis of outbreaks that occurred during 2013–2014.
Public health officials from 19 states reported 42 outbreaks associated with drinking
water during the surveillance period (Table 1) (https://www.cdc.gov/healthywater/surveillance/drinking-water-tables-figures.html).
These outbreaks resulted in at least 1,006 cases of illness, 124 hospitalizations
(12% of cases), and 13 deaths. At least one etiologic agent was identified in 41 (98%)
outbreaks. Counts of etiologic agents in this report include both confirmed and suspected
etiologies, which differs from previous surveillance reports. Legionella was implicated
in 24 (57%) outbreaks, 130 (13%) cases, 109 (88%) hospitalizations, and all 13 deaths
(Table 1). Eight outbreaks caused by two parasites resulted in 289 (29%) cases, among
which 279 (97%) were caused by Cryptosporidium, and 10 (3%) were caused by Giardia
duodenalis. Chemicals or toxins were implicated in four outbreaks involving 499 cases,
with 13 hospitalizations, including the first reported outbreaks (two outbreaks) associated
with algal toxins in drinking water.
TABLE 1
Waterborne disease outbreaks associated with drinking water (N = 42), by state/jurisdiction
and month of first case onset — Waterborne Disease and Outbreak Surveillance System,
United States, 2013–2014
State/ Jurisdiction
Month
Year
Etiology*
Predominant illness†
No. of cases
No. of hospitalizations§
No. of deaths¶
Type of water system**
Water source
Setting
Alaska
Aug
2014
Giardia duodenalis
††
AGI
5
0
0
Community
River/Stream
Community/Municipality
Arizona
Jan
2014
Norovirus (S)
AGI
4
0
0
Transient, noncommunity
Unknown
Camp/Cabin Setting
Florida
Sep
2013
L. pneumophila serogroup 1
ARI
4
4
0
Community
Well
Hospital/Health care
Florida
Nov
2013
L. pneumophila serogroup 1
ARI
4
4
0
Community
Other
Other§§
Florida
Apr
2014
L. pneumophila serogroup 1
ARI
2
2
0
Community
Well
Hotel/Motel/Lodge/Inn
Florida
Jun
2014
L. pneumophila serogroup 1
ARI
3
2
0
Community
Unknown
Long-term care facility
Florida
Aug
2014
L. pneumophila serogroup 1
ARI
6
4
0
Community
Unknown
Hotel/Motel/Lodge/Inn
Idaho
Sep
2014
Giardia duodenalis
AGI
2
0
0
Unknown
Unknown
Hotel/Motel/Lodge/Inn
Indiana
Jul
2013
Cryptosporidium sp.
AGI
7
0
0
Community
Unknown
Mobile home park
Indiana
Nov
2014
Unknown
AGI
3
0
0
Community
Unknown
Apartment/Condo
Kansas
Jun
2014
L. pneumophila serogroup 1
ARI
2
2
0
Community
Unknown
Hospital/Health care
Maryland
Nov
2012
L. pneumophila serogroup 1
ARI
2¶¶
2¶¶
0
Community
Well
Hotel/Motel/Lodge/Inn
Maryland
Feb
2013
Nitrite***
AGI, Neuro
14
0
Community
Lake/Reservoir/ Impoundment
Indoor workplace/Office
Maryland
Apr
2014
L. pneumophila serogroup 1
ARI
2
2
0
Community
Lake/Reservoir/ Impoundment
Apartment/Condo
Maryland
Jul
2014
L. pneumophila serogroup 1
ARI
2
1
0
Community
Well
Hotel/Motel/Lodge/Inn
Maryland
Aug
2014
L. pneumophila serogroup 1
ARI
2
2
0
Community
River/Stream
Prison/Jail (Juvenile/Adult)
Michigan
Jun
2014
L. pneumophila serogroup 1
ARI
45
45
7
Community
River/Stream
Hospital/Health care, Community/Municipality†††
Montana
Jul
2014
NorovirusGII.Pe-GII.4 Sydney
AGI
62
0
0
Transient, noncommunity
Well
Hotel/Motel/Lodge/Inn
New York
Jul
2013
L. pneumophila serogroup 1
ARI
2
2
0
Community
Lake/Reservoir/ Impoundment
Hospital/Health care
New York
Jun
2014
L. pneumophila serogroup 1
ARI
2
2
0
Community
Well
Hospital/Health care
North Carolina
Dec
2013
L. pneumophila serogroup 1
ARI
3
2
0
Community
Unknown
Long-term care facility
North Carolina
Dec
2013
L. pneumophila serogroup 1
ARI
7
3
0
Community
Unknown
Long-term care facility
North Carolina
May
2014
L. pneumophila serogroup 1
ARI
7
6
1
Community
Other
Long-term care facility
North Carolina
Jun
2014
L. pneumophila serogroup 1
ARI
3
3
0
Community
Unknown
Long-term care facility
North Carolina
Jul
2014
L. pneumophila serogroup 1
ARI
3
2
1
Community
Unreported
Long-term care facility
Ohio
Apr
2013
L. pneumophila
ARI
2
2
1
Unknown
Unknown
Long-term care facility
Ohio§§§
Sep
2013
Cyanobacterial toxin¶¶¶
AGI
6
0
0
Community
Lake/Reservoir/ Impoundment
Community/Municipality
Ohio
Jul
2014
L. pneumophila serogroup 1
ARI
14
4
0
Community
River/Stream
Long-term care facility
Ohio
Aug
2014
Cyanobacterial toxin¶¶¶
AGI
110
Community
Lake/Reservoir/ Impoundment
Community/Municipality
Ohio
Oct
2014
Cryptosporidium sp. (S)****
AGI
100
0
0
Individual
River/Stream
Farm/Agricultural setting
Ohio
Dec
2014
Viral, unknown (S)
AGI
2
0
0
Commercially bottled
Unknown
Private residence
Oregon
Jun
2013
Cryptosporidium parvum IIaA15G2R1
AGI
119
2
0
Community
Lake/Reservoir/ Impoundment
Community/Municipality
Oregon
Sep
2014
L. pneumophila serogroup 1
ARI
4
4
1
Community
Well
Apartment/Condo
Pennsylvania
Dec
2013
L. pneumophila serogroup 1
ARI
2
2
0
Unknown
Unknown
Hospital/Health care
Pennsylvania
Feb
2014
L. pneumophila serogroup 1
ARI
5
5
0
Community
River/Stream
Long-term care facility
Pennsylvania
Oct
2014
L. pneumophila
ARI
2
2
1
Community
Unknown
Long-term care facility
Rhode Island
Apr
2013
L. pneumophila serogroup 1
ARI
2
2
1
Community
Lake/Reservoir/ Impoundment
Hospital/Health care
Tennessee
Jul
2013
Cryptosporidium parvum
AGI
34
0
0
Transient, noncommunity††††
Spring
Camp/Cabin setting
Tennessee
Jun
2014
Clostridium difficile (S); Escherichia coli, Enteropathogenic (S)
AGI
12
0
0
Nontransient, noncommunity
Well
Camp/Cabin setting; Community/Municipality
Virginia
Jun
2013
Cryptosporidium sp.
AGI
19
0
0
Individual
Well
Farm/Agricultural setting
West Virginia
Jan
2014
4-Methylcyclohexanemethanol (MCHM)§§§§
AGI
369
13
0
Community
River/Stream
Community/Municipality
Wisconsin
Aug
2014
Giardia duodenalis
AGI
3
0
0
Nontransient, noncommunity
Other
National forest
Wisconsin
Sep
2014
Campylobacter jejuni
AGI
5
0
0
Individual
Well
Private residence
Abbreviations: AGI = acute gastrointestinal illness; ARI = acute respiratory illness;
L. pneumophila = Legionella pneumophila; Neuro = neurologic illnesses, conditions,
or symptoms (e.g., meningitis); S = suspected.
* Etiologies listed are confirmed, unless indicated as suspected. For multiple-etiology
outbreaks, etiologies are listed in alphabetical order.
† The category of illness reported by ≥50% of ill respondents. All legionellosis outbreaks
were categorized as ARI.
§ Value was set to “missing” in reports where zero hospitalizations were reported
and the number of persons for whom information was available was also zero or for
instances where reports are missing hospitalization data.
¶ Value was set to “missing” in reports where zero deaths were reported and the number
of persons for whom information was available was also zero or for instances where
reports are missing data on associated deaths.
** Community and noncommunity water systems are public water systems that have ≥15
service connections or serve an average of ≥25 residents for ≥60 days per year. A
community water system serves year-round residents of a community, subdivision, or
mobile home park. A noncommunity water system serves an institution, industry, camp,
park, hotel, or business and can be nontransient or transient. Nontransient systems
serve ≥25 of the same persons for ≥6 months of the year but not year-round (e.g.,
factories and schools) whereas transient systems provide water to places in which
persons do not remain for long periods of time (e.g., restaurants, highway rest stations,
and parks). Individual water systems are small systems not owned or operated by a
water utility that have <15 connections or serve <25 persons.
†† Classification of all reported Giardia cases has changed from Giardia intestinalis
to Giardia duodenalis to align with laboratory standards.
§§ Setting is listed as “other” because implicated facility houses both independent
living and assisted living facilities.
¶¶ This count was not included in the analysis of the current report. This outbreak
occurred in 2012 and was not reported in the previous drinking water outbreak report.
*** Patients’ methemoglobin levels ranged from 1.6% to 32.3%. Water was determined
to be the source rather than food because all cases had direct exposure to water.
Of the 14 cases, five used the water to make oatmeal or cream of wheat.
††† This report includes both community and hospital-associated cases (27 of 45 patients
reported health care/hospital exposure).
§§§ This is the first drinking water–associated outbreak of this etiology reported
to the National Outbreak Reporting System.
¶¶¶ Microcystin was detected in finished water sampled from a community water system;
levels exceeded state thresholds and resulted in a “Do not drink” advisory.
**** Cryptosporidium was detected in water samples but not in any clinical specimens.
†††† This system was registered as a community system as a result of the outbreak
investigation.
§§§§ Illnesses were associated with exposure to 4-methylcyclohexanemethanol following
a documented industrial spill into water supplying a public water system. However,
individual levels of exposure could not be quantified in clinical specimens. Propylene
glycol phenyl ether was also present in the spill at low concentrations.
The most commonly reported outbreak etiology was Legionella (57%), making acute respiratory
illness the most common predominant illness type reported in outbreaks (Table 2).
Thirty-five (83%) outbreaks were associated with public (i.e., regulated), community
or noncommunity water systems,
¶
and three (7%) were associated with unregulated, individual systems. Fourteen outbreaks
occurred in drinking water systems with groundwater sources and an additional 14 occurred
in drinking water systems with surface water sources. The most commonly cited deficiency,
which led to 24** (57%) of the 42 drinking water–associated outbreaks, was the presence
of Legionella in drinking water systems. In addition, 143 (14%) cases were associated
with seven (17%) outbreak reports that had a deficiency classification indicating
“unknown or insufficient information.”
TABLE 2
Rank order (most common to least common) of etiology, water system, water source,
predominant illness, and deficiencies associated with 42 drinking water outbreaks
and 1,006 outbreak-related cases of illness — United States, 2013–2014
Characteristic/Rank
Outbreaks (N = 42)
Cases (N = 1,006)
Category
No. (%)
Category
No. (%)
Etiology
1
Bacteria, Legionella
24 (57.1)
Chemical/Toxin
499 (49.6)
2
Parasites
8 (19.1)
Parasites
289 (28.7)
3
Chemical/Toxin
4 (9.5)
Bacteria, Legionella
130 (12.9)
4
Viruses
3 (7.1)
Viruses
68 (6.8)
5
Bacteria, non-Legionella
1 (2.4)
Multiple bacteria
12 (1.2)
6
Multiple bacteria
1 (2.4)
Bacteria, non-Legionella
5 (0.5)
7
Unknown
1 (2.4)
Unknown
3 (0.3)
Water system*
1
Community
30 (71.4)
Community
759 (75.4)
2
Noncommunity
5 (11.9)
Individual
124 (12.3)
3
Individual
3 (7.1)
Noncommunity
115 (11.4)
4
Unknown
3 (7.1)
Unknown
6 (0.6)
5
Bottled
1 (2.4)
Bottled
2 (0.2)
Water source
1
Ground water
14 (33.3)
Surface water
795 (79.0)
2
Surface water
14 (33.3)
Ground water
157 (15.6)
3
Unknown
12 (28.6)
Unknown
39 (3.9)
4
Mixed†
1 (2.4)
Mixed
12 (1.2)
5
Unreported
1 (2.4)
Unreported
3 (0.3)
Predominant illness
§
1
ARI
24 (57.1)
AGI
862 (85.7)
2
AGI
17 (40.5)
ARI
130 (12.9)
3
AGI; Neuro
1 (2.4)
AGI; Neuro
14 (1.4)
Deficiency
¶
1
Legionella spp. in drinking water system**
23 (54.8)
Treatment not expected to remove contaminant
485 (48.2)
2
Unknown/Insufficient information††
7 (16.7)
Unknown/Insufficient information
143 (14.2)
3
Multiple§§
3 (7.1)
Legionella spp. in drinking water system
126 (12.5)
4
Treatment not expected to remove contaminant¶¶
3 (7.1)
Treatment deficiency
119 (11.8)
5
Untreated ground water***
3 (7.1)
Untreated ground water
70 (7.0)
6
Distribution system†††
1 (2.4)
Multiple
42 (4.2)
7
Premises plumbing system§§§
1 (2.4)
Premise plumbing system
14 (1.4)
8
Treatment deficiency¶¶¶
1 (2.4)
Distribution system
7 (0.7)
Abbreviations: AGI = acute gastrointestinal illness; ARI = acute respiratory illness;
Neuro = neurologic illnesses, conditions, or symptoms (e.g., meningitis).
* Community and noncommunity water systems are public water systems that have ≥15
service connections or serve an average of ≥25 residents for ≥60 days per year. A
community water system serves year-round residents of a community, subdivision, or
mobile home park. A noncommunity water system serves an institution, industry, camp,
park, hotel, or business and can be nontransient or transient. Nontransient systems
serve ≥25 of the same persons for ≥6 months of the year but not year-round (e.g.,
factories and schools) whereas transient systems provide water to places in which
persons do not remain for long periods of time (e.g., restaurants, highway rest stations,
and parks). Individual water systems are small systems not owned or operated by a
water utility that have <15 connections or serve <25 persons.
† Includes outbreaks with mixed water sources (i.e., ground water and surface water).
§ The category of illness reported by ≥50% of ill respondents; all legionellosis outbreaks
were categorized as ARI.
¶ Outbreaks are assigned one or more deficiency classifications. https://www.cdc.gov/healthywater/surveillance/deficiency-classification.html.
** Deficiency 5A. Drinking water, contamination of water at points not under the jurisdiction
of a water utility or at the point of use: Legionella spp. in water system, drinking
water.
†† Deficiency 99. Unknown/Insufficient information.
§§ Multiple deficiency classifications were assigned to three outbreaks. One outbreak
had deficiency 2, 3 one had 3, 4, and one had 5a, 7 (deficiency in building/home-specific
water treatment after the water meter or property line).
¶¶ Deficiency 13a. Current treatment processes not expected to remove a chemical contaminant:
ground water.
*** Deficiency 2. Drinking water, contamination of water at/in the water source, treatment
facility, or distribution system: untreated ground water.
††† Deficiency 4. Drinking water, contamination of water at/in the water source, treatment
facility, or distribution system: Distribution system deficiency, including storage
(e.g., cross-connection, backflow, and contamination of water mains during construction
or repair).
§§§ Deficiency 6. Drinking water, contamination of water at points not under the jurisdiction
of a water utility or at the point of use; plumbing system deficiency after the water
meter or property line (e.g., cross-connection, backflow, or corrosion products).
¶¶¶ Deficiency 3. Treatment deficiency (e.g., temporary interruption of disinfection,
chronically inadequate disinfection, or inadequate or no filtration).
Among 1,006 cases attributed to drinking water–associated outbreaks, 50% of the reported
cases were associated with chemical or toxin exposure, 29% were caused by parasitic
infection (either Cryptosporidium or Giardia), and 13% were caused by Legionella infection
(Table 2). Seventy-five percent of cases were linked to community water systems. Outbreaks
in water systems supplied solely by surface water accounted for most cases (79%).
Of the 1,006 cases, 86% originated from outbreaks in which the predominant illness
was acute gastrointestinal illness. Three (7%) outbreaks in which treatment was not
expected to remove the contaminant were associated with a chemical or toxin and resulted
in 48% of all outbreak-associated cases.
Discussion
Water treatment processes, regulations, and rapid response to illness outbreaks continue
to reduce the transmission of pathogens, reduce exposure to chemicals and toxins,
and protect the public drinking water supplies in the United States. Outbreaks reported
during this surveillance period include the first reports of drinking water–associated
outbreaks caused by harmful algal blooms as well as the continued challenges of preventing
and controlling illnesses and outbreaks caused by Legionella and Cryptosporidium.
Outbreaks in community water systems caused by chemical spills (West Virginia) (
2
), harmful algal blooms (Ohio), Cryptosporidium (Oregon) (
3
), and Legionella (Michigan) demonstrated that diverse contaminants can cause interruptions
in water service, illnesses, and persistent community concern about drinking water
quality. Outbreaks in community water systems can trigger large and complex public
health responses because of their potential for causing communitywide illness and
decreasing the availability of safe water for community members, businesses, and critical
services (e.g., hospitals). These outbreaks highlight the importance of public health
and water utility preparedness for emergencies related to contamination from pathogens,
chemicals, and toxins.
Legionella continues to be the most frequently reported etiology among drinking water–associated
outbreaks (
4
). All of the outbreak-associated deaths reported during this surveillance period
as well as all of the outbreaks reported in hospital/health care settings or long-term
care facilities, were caused by Legionella. A review of 27 Legionnaires’ disease outbreak
investigations in which CDC participated during 2000–2014 identified at least one
water system maintenance deficiency in all 23 investigations for which this information
was available, indicating that effective water management programs in buildings at
increased risk for Legionella growth and transmission (e.g., those with more than
10 stories or that house susceptible populations) can reduce the risk for Legionnaires’
disease (
5
,
6
). Although Legionella was detected in drinking water, multiple routes of transmission
beyond ingestion of contaminated water more likely contributed to these outbreaks,
such as aerosolization from domestic or environmental sources. Cryptosporidium was
the second most common cause of both outbreaks and illnesses, demonstrating the continued
threat from this chlorine-tolerant pathogen when drinking water supplies are contaminated.
Existing drinking water regulations and filtration systems targeted to control Cryptosporidium
help protect public health in community water systems that are primarily served by
surface water sources or groundwater sources under the influence of surface water
(
7
). Through the Epidemiology and Laboratory Capacity for Infectious Diseases (ELC)
Cooperative Agreement, CDC has recently begun a laboratory-based cryptosporidiosis
surveillance system in the United States, CryptoNet, to better track Cryptosporidium
transmission and rapidly identify outbreak sources through molecular typing (
8
). The cyanobacterial toxin microcystin caused the largest reported toxin contamination
of community drinking water in August 2013 and September 2014 and was responsible
for extensive community and water disruptions. In June 2015, the Environmental Protection
Agency released specific health advisory guidance for microcystin concentrations in
drinking water (
9
). The contamination of a community drinking water supply with 4-metholcyclohexanementanol
(MCHM) also illustrates the importance of source water protection from chemicals and
toxins (
2
).
The findings in this report are subject to at least three limitations. First, 17%
of drinking water–associated outbreak reports could not be assigned a specific deficiency
classification other than “unknown or insufficient information,” because of a lack
of information. Furthermore, the deficiency classification most frequently reported
(“presence of Legionella in drinking water systems”) does not provide insight into
the specific factors contributing to Legionella amplification and transmission. Second,
the detection and investigation of outbreaks might be incomplete. Because of universal
exposure to water, linking illness to drinking water is inherently difficult through
traditional outbreak investigation methods (e.g., case-control and cohort studies)
(
10
). Finally, reporting capabilities and requirements vary among states and localities.
Therefore, outbreak surveillance data likely underestimate actual occurrence of outbreaks
and should not be used to estimate the actual number of outbreaks or cases of waterborne
disease.
Public health surveillance is necessary to detect waterborne disease and outbreaks,
and to continue to monitor health trends associated with drinking water exposure.
Despite resource constraints, 19 states reported drinking water–associated outbreaks
for 2013–2014 compared with 14 for the previous reporting period (
4
). In this reporting cycle, more reported outbreaks and cases were caused by parasites
and chemicals than by non-Legionella bacteria, and more cases were reported from community
systems than from individual systems. Most of the outbreaks and illnesses reported
in this period were in community systems, which serve larger numbers of persons; outbreaks
in these systems can sicken entire communities. Although individual, private water
systems likely serve fewer persons than community systems, they can still result in
relatively large numbers of illnesses. One outbreak reported during 2013–2014 in an
individual system led to 100 estimated illnesses associated with a wedding. The public
health challenges highlighted here underscore the need for rapid detection, identification
of the cause, and response when drinking water is contaminated by infectious pathogens,
chemicals, or toxins to prevent and control waterborne illness and outbreaks.
Summary
What is already known about this topic?
Waterborne disease and outbreaks associated with drinking water continue to occur
in the United States. CDC collects data on waterborne disease outbreaks submitted
from all states and territories through the National Outbreak Reporting System.
What is added by this report?
During 2013–2014, a total of 42 drinking water–associated outbreaks were reported
to CDC, resulting in at least 1,006 cases of illness, 124 hospitalizations, and 13
deaths. Legionella was responsible for 57% of outbreaks and 13% of illnesses, and
chemicals/toxins and parasites together accounted for 29% of outbreaks and 79% of
illnesses. Eight outbreaks caused by parasites resulted in 289 (29%) cases, among
which 279 (97%) were caused by Cryptosporidium and 10 (3%) were caused by Giardia
duodenalis. Chemicals or toxins were implicated in four outbreaks involving 499 cases,
with 13 hospitalizations, including the first outbreaks associated with algal toxins.
What are the implications for public health practice?
Continued public health surveillance is necessary to detect waterborne disease and
monitor health trends associated with drinking water exposure. When drinking water
is contaminated by infectious pathogens, chemicals, or toxins, public health agencies
need to provide rapid detection, identification of the cause, and response to prevent
and control waterborne illness and outbreaks. Effective water management programs
in buildings at increased risk for Legionella growth and transmission can reduce the
risk for disease from drinking water pathogens.