2015 Waterborne Disease Outbreaks Annual Surveillance Report

Outbreaks reported

82

Cases reported

1,276

Hospitalizations

185

Deaths

27

More than half of reported waterborne disease outbreaks were linked to recreational water in 2015

Linked to recreational water exposure

57% of outbreaks

62% of cases

Linked to drinking water exposure

28% of outbreaks

34% of cases

Linked to other water exposures, such as industrial water or flood waters

7% of outbreaks

4% of cases

Linked to unknown water exposures

7% of outbreaks

1% of cases

Legionella caused nearly half of reported waterborne disease outbreaks and more than 90% of hospitalizations and deaths

Top 5 causes of reported outbreaks
  1. Legionella (43%)
  2. Cryptosporidium (28%)
  3. Giardia (5%)
  4. Norovirus (5%)
  5. Pseudomonas (4%)
Legionella outbreaks caused the most cases of severe disease

18% of reported cases

94% of hospitalizations

100% of deaths

Background

Each year, pathogens transmitted through water are estimated to cause 7.15 million cases of waterborne illness (95% credible interval [CrI], 3.9–12.0 million), 118,000 hospitalizations (95% CrI, 86,800–150,000) and 6,630 deaths (95% CrI, 4,520–8,870) in the United States (1).  Summaries of waterborne disease outbreak data can inform prevention efforts for infectious and non-infectious etiologies.

Methods

For an event to be defined as a waterborne disease outbreak, two or more cases of similar illness must be epidemiologically linked by location and time of exposure to contaminated water, contaminated aerosols, or gases volatilized from water. Cases are defined as lab-confirmed or probable using outbreak-specific case definitions. The epidemiologic evidence must implicate water exposure as the outbreak source and can be strengthened by environmental health and laboratory data.

 

CDC has conducted national surveillance for waterborne disease outbreaks since 1971 via the Waterborne Disease and Outbreak Surveillance System (WBDOSS). Public health officials in U.S. jurisdictions (the 50 states, the District of Columbia, territories, and freely associated states) have voluntarily reported waterborne disease outbreaks through the web-based platform, National Outbreak Reporting System (NORS) since 2009. WBDOSS has captured data regarding outbreaks associated with recreational water, drinking water, and other or unknown types of exposures to water. Water from an identified source other than a recreational venue or drinking water system is referred to as “other water” (e.g., industrial water, flood waters). An outbreak with insufficient data to link the outbreak to one specific source is reported as an unknown water exposure. Individual outbreaks associated with multiple types of water exposures are also reported as unknown.

 

Agencies use a standard form [PDF – 12 pages] to report waterborne disease outbreaks. Data collected include the implicated outbreak water exposure (recreational, drinking, other, or unknown water); earliest illness onset date; etiology; implicated recreational water venue or drinking water system; the setting of exposure (e.g., a hotel, apartment complex, hospital); relevant epidemiologic data; and contributing factors. NORS data are available for download and visualization using the NORS Dashboard, a web-based tool for searching and accessing outbreak data.

 

This summary includes waterborne disease outbreaks reported through NORS as of March 23, 2022, and for which the earliest illness onset was in 2015. Outbreak reports went through a standardized data cleaning process, a collaboration between CDC and reporting jurisdictions. Agencies can submit new outbreak reports and revise or delete reports as additional data become available. Thus, data reported in future analyses for 2015 might differ from the numbers presented here.

Findings

All Water Exposures

During 2015, 82 waterborne disease outbreaks were reported, resulting in at least 1,276 cases, 185 hospitalizations, and 27 deaths (Table 1). Recreational water was implicated in investigations of over half of outbreaks (57%, 47/82), resulting in nearly two-thirds of cases (62%, 787/1,276). Drinking water was implicated in investigations of about one-quarter of outbreaks (28%, 23/82), resulting in about one-third of cases (34%, 429/1,276) and most hospitalizations (67%, 124/185) and deaths (93%, 25/27). Other exposure to water was implicated for 7% (6/82) of outbreaks, resulting in 4% (47/1,276) of cases. In 7% of outbreaks (resulting in 1% of cases) the water outbreak source was unknown.

Outbreak Exposure Outbreaks N (%) Cases N (%) Hospitalizations N (%) Deaths N (%)
Table 1. Number and percentage* of reported waterborne disease outbreaks (N = 82) and associated cases, hospitalizations, and deaths, by outbreak exposure — Waterborne Disease and Outbreak Surveillance System, United States, 2015
Recreational water 47 (57%) 787 (62%) 23 (12%) 1 (4%)
Drinking water 23 (28%) 429 (34%) 124 (67%) 25 (93%)
Other water§ 6 (7%) 47 (4%) 27 (15%) 1 (4%)
Unknown water 6 (7%) 13 (1%) 11 (6%) 0 (0%)
Total 82 (100%) 1,276 (100%) 185 (100%) 27 (100%)

* Percentages might not sum to 100 due to rounding.
§ Water from an identified source other than a recreational venue or drinking water system is referred to as “other water” (e.g., industrial water, flood waters).

For 2015, public health officials reported 82 waterborne outbreaks resulting from exposures in 32 states. (Figure 1). No outbreaks involving multi-state exposures were reported.

* These numbers are largely dependent on public health capacity and reporting requirements, which vary across jurisdictions, and do not necessarily indicate the true occurrence of outbreaks in each state.

* These numbers are largely dependent on public health capacity and reporting requirements, which vary across jurisdictions, and do not necessarily indicate the true occurrence of outbreaks in each state.

Outbreaks most frequently occurred June–August (51%, 42/82) (Figure 2).

Bacteria were implicated in 54% (44/82) of outbreaks with a confirmed or suspected etiology; parasites were implicated in 35% (29/82) of outbreaks (Figure 3).

Figure 3. Percentage of reported waterborne disease outbreaks: Bacteria 54% Parasites 35% Viruses 5% Chemicals/Toxins 2% Multiple 2% Unknown 1%

* Percentages do not sum to 100 due to rounding.
§ Outbreaks of confirmed or suspected etiologies are summarized together. Multiple etiology outbreaks include one outbreak suspected to be caused by Shigella sonnei, Campylobacter jejuni, and norovirus; and one outbreak caused by Campylobacter jejuni and suspected to be caused by Giardia duodenalis.

Figure 3 data table
Etiology Type n %
Bacteria 44 54
Parasites 29 35
Viruses 4 5
Chemicals/Toxins 2 2
Multiple§ 2 2
Unknown 1 1

At least one etiology was confirmed in 85% (70/82) of investigations of outbreaks (Table 2). Among outbreaks with a confirmed or suspected etiology, Legionella was most frequently reported, accounting for 43% (35/82) of outbreaks. This was followed by Cryptosporidium, which accounted for 28% (23/82) of outbreaks. Cryptosporidium caused 28% (362/1,276) of cases among outbreaks, followed by norovirus (21% [263]) and Legionella (18% [234]). Legionella accounted for 94% (174/185) of hospitalizations and all 27 deaths.

Table 2. Number* and percentage§ of reported waterborne disease outbreaks (N = 82) and resulting cases, hospitalizations, and deaths, by etiology — Waterborne Disease and Outbreak Surveillance System, United States, 2015
Etiology Outbreaks Cases Hospitalizations Deaths
n % Confirmed Suspected n % Confirmed Suspected n % Confirmed Suspected n % Confirmed Suspected
Legionella 35 43 33 2 234 18 224 10 174 94 164 10 27 100 26 1
Cryptosporidium 23 28 22 1 362 28 355 7 6 3 6 0 0 0 0 0
Giardia 4 5 4 0 14 1 14 0 0 0 0 0 0 0 0 0
Norovirus 4 5 4 0 263 21 263 0 1 1 1 0 0 0 0 0
Pseudomonas 3 4 1 2 22 2 13 9 0 0 0 0 0 0 0 0
Chemicals/Toxins 2 2 0 2 51 4 0 51 1 1 0 1 0 0 0 0
Escherichia 2 2 2 0 15 1 15 0 2 1 2 0 0 0 0 0
Mycobacterium 2 2 1 1 13 1 9 4 0 0 0 0 0 0 0 0
Acanthamoeba 1 1 1 0 2 0 2 0 0 0 0 0 0 0 0 0
Avian schistosomes 1 1 0 1 22 2 0 22 0 0 0 0 0 0 0 0
Campylobacter 1 1 1 0 4 0 4 0 0 0 0 0 0 0 0 0
Salmonella 1 1 0 1 3 0 0 3 0 0 0 0 0 0 0 0
Multiple† 2 2 1 1 259 20 9 250 1 1 1 0 0 0 0 0
Unknown ‡ 1 1 12 1 0 0 0 0

* Confirmed and suspected totals might not sum to outbreak, case, or hospitalization count due to exclusion of unknown etiologies.
§ Percentages might not sum to 100 due to rounding.
Chemical/toxin exposures include suspected chlorine and chloramine exposures.
† Multiple etiology outbreaks include one outbreak suspected to be caused by Shigella, Campylobacter, and norovirus and one outbreak caused by Campylobacter and suspected to be caused by Giardia.
‡ Outbreaks of unknown etiology cannot be classified according to confirmed or suspected etiology for outbreak, cases, hospitalizations, or deaths. In 2015, there was one reported outbreak with an unknown etiology.

The most frequently reported outbreak settings were hotels or motels (22%, 18/82 outbreaks), outdoor recreational areas (10%, 8/82), hospitals or healthcare facilities (9%, 7/82), and communities or municipalities (7%, 6/82) (Table 3).

Table 3. Number and percentage* of reported waterborne disease outbreaks (N = 82) and resulting cases, by setting — Waterborne Disease and Outbreak Surveillance System, United States, 2015
Water Setting Outbreaks n (%) Cases n (%)
Hotel/Motel 18 (22%) 165 (13%)
Outdoor Recreational Area 8 (10%) 265 (21%)
Hospital/Healthcare Facility§ 7 (9%) 65 (5%)
Community/Municipality 6 (7%) 283 (22%)
Long Term Care Facility/Assisted Living 5 (6%) 78 (6%)
Camp/Cabin 4 (5%) 108 (8%)
Private Residence 4 (5%) 19 (1%)
Recreational Facility 3 (4%) 45 (4%)
Waterpark 3 (4%) 61 (5%)
Beach 2 (2%) 18 (1%)
Club (Requires Membership) 2 (2%) 16 (1%)
Resort 2 (2%) 12 (1%)
Multiple† 4 (5%) 20 (2%)
Other‡ 7 (9%) 45 (4%)
Not Reported 4 (5%) 21 (2%)
Unknown 3 (4%) 55 (4%)
Total 82 (100%) 1,276 (100%)

* Percentages might not sum to 100 due to rounding.
§ A healthcare institution providing inpatient medical or surgical treatment or nursing care for sick or injured persons, or a healthcare facility other than a long-term care or assisted-living facility.
Includes recreational water facilities owned by a community/municipality.
† Multiple setting includes: waterpark and child care/daycare center (1), hotel/motel and resort (1), outdoor recreational area and school/college/university (1), and multiple private residences (1).
‡ Other setting includes: amusement park (1), casino (1), church/place of worship (1), factory/industrial facility (1), farm/agricultural setting (1), indoor workplace/office (1), and restaurant/cafeteria (1).

Recreational Water Exposures

Recreational water was the most frequently reported source of waterborne disease outbreaks, associated with 57% (47/82) of outbreaks, resulting in 62% (787/1,276) of reported cases (Table 1). Recreational water–associated outbreaks occurred year round but peaked during June–August (Figure 4). The seasonality of recreational water–associated outbreaks drove the seasonality of waterborne disease outbreaks overall.

Among recreational water outbreaks, 47% (22/47) were caused by Cryptosporidium and 19% (9/47) by Legionella (Table 4). Among 787 reported cases resulting from recreational water exposure and with a confirmed or suspected etiology, 46% (360/787) of cases were caused by Cryptosporidium and 33% (263/787) caused by norovirus. Over three quarters of 23 hospitalizations in those affected by recreational water–associated outbreaks were caused by Legionella (57%, 13/23) or Cryptosporidium (26%, 6/23). One death was reported for the 47 recreational water–associated outbreaks.

Table 4. Number* and percentage§ of reported recreational water–associated disease outbreaks (n = 47) and resulting cases, hospitalizations, and deaths, by etiology — Waterborne Disease and Outbreak Surveillance System, United States, 2015
Outbreaks Cases Hospitalizations Deaths
Etiology n (%) Confirmed Suspected n (%) Confirmed Suspected n (%) Confirmed Suspected n (%) Confirmed Suspected
Cryptosporidium 22 (47%) 21 1 360 (46%) 353 7 6 (26%) 6 0 0 (0%) 0 0
Legionella 9 (20%) 9 0 30 (4%) 30 0 13 (57%) 13 0 1 (100%) 1 0
Norovirus 4 (9%) 4 0 263 (33%) 263 0 1 (4%) 1 0 0 (0%) 0 0
Pseudomonas 3 (6%) 1 2 22 (3%) 13 9 0 (0%) 0 0 0 (0%) 0 0
Chemicals/Toxins 2 (4%) 0 2 51 (7%) 0 51 1 (4%) 0 1 0 (0%) 0 0
Acanthamoeba 1 (2%) 1 0 2 (0%) 2 0 0 (0%) 0 0 0 (0%) 0 0
Avian schistosomes 1 (2%) 0 1 22 (3%) 0 22 0 (0%) 0 0 0 (0%) 0 0
Campylobacter 1 (2%) 1 0 4 (1%) 4 0 0 (0%) 0 0 0 (0%) 0 0
Escherichia 1 (2%) 1 0 9 (1%) 9 0 2 (9%) 2 0 0 (0%) 0 0
Giardia 1 (2%) 1 0 3 (0%) 3 0 0 (0%) 0 0 0 (0%) 0 0
Mycobacterium 1 (2%) 1 0 9 (1%) 9 0 0 (0%) 0 0 0 (0%) 0 0
Unknown† 1 (2%) 12 (2%) 0 (0%) 0 (0%)

* Confirmed and suspected totals might not sum to outbreak, case, or hospitalization count due to exclusion of unknown etiologies.
§ Percentages might not sum to 100 due to rounding.
Chemical/toxin exposures include suspected chlorine and chloramine exposures.
† Outbreaks of unknown etiology cannot be classified according to confirmed or suspected etiology for outbreak, cases, hospitalizations, or deaths. In 2015, there was one reported outbreak with an unknown etiology.

Recreational water–associated outbreaks were most frequently associated with recreational water venues at hotels or motels (26%, 12/47), outdoor recreational areas (13%, 6/47), or communities or municipalities (11%, 5/47) (Table 5).

Table 5. Number and percentage* of reported recreational water–associated disease outbreaks (n = 47) and resulting cases, by setting — Waterborne Disease and Outbreak Surveillance System, United States, 2015
Water Setting Outbreaks n (%) Cases n (%)
Hotel/Motel 12 (26%) 150 (19%)
Outdoor Recreational Area 6 (13%) 258 (33%)
Community/Municipality§ 5 (11%) 33 (4%)
Camp/Cabin 3 (6%) 104 (13%)
Recreational Facility 3 (6%) 45 (6%)
Waterpark 3 (6%) 61 (8%)
Beach 2 (4%) 18 (2%)
Club (Requires Membership) 2 (4%) 16 (2%)
Private Residence 2 (4%) 13 (2%)
Resort 2 (4%) 12 (2%)
Amusement Park 1 (2%) 7 (1%)
Multiple 1 (2%) 7 (1%)
Not Reported 3 (6%) 18 (2%)
Unknown 2 (4%) 45 (6%)
Total 47 (100%) 787 (100%)

* Percentages do not sum to 100 due to rounding.
§ A recreational water facility owned by a community/municipality.
Multiple setting includes: a waterpark and child care/daycare center.

Treated water (e.g., in pools, hot tubs, or splash pads) was associated with 85% (40/47) of these outbreaks, resulting in 62% (490/787) of cases; untreated recreational water was associated with 15% (7/47) of outbreaks, resulting in 38% (297/787) of cases (Table 6–7). Among the 40 treated recreational water–associated outbreaks, 45% (18/40) were associated with pools. Eight (20%, 8/40) outbreaks were associated with multiple venue types (Table 6). Nearly all untreated recreational water outbreak venues were associated with a lake or reservoir (86%, 6/7); the remaining outbreak was associated with a river or stream (Table 7).

Table 6. Number and percentage of reported treated recreational water–associated disease outbreaks (n = 40) and resulting case, by venue
Venue Outbreaks n (%) Cases n (%)
Pool 18 (45%) 286 (58%)
Hot Tub 6 (15%) 30 (6%)
Splash Pad 3 (8%) 9 (2%)
Inflatable or Plastic Pool/Temporary Water Slide 1 (3%) 4 (1%)
Multiple Venue Types§ 8 (20%) 120 (24%)
Not Reported 3 (8%) 18 (4%)
Unknown 1 (3%) 23 (5%)
Total 40 (100%) 490 (100%)

* Percentages do not sum to 100 due to rounding.
§Multiple venue types includes: pool and hot tub (5); pool and splash pad (1); pool, inflatable or plastic kiddie/wading pool, and splash pad (1); pool, inflatable or plastic pool, and hot tub (1).

Table 7. Number and percentage of reported untreated recreational water–associated disease outbreaks (n = 7) and resulting cases, by venue
Venue Outbreaks n (%) Cases n (%)
Lake/Reservoir 6 (86%) 279 (94%)
River/Stream 1 (14%) 18 (6%)
Total 7 (100%) 297 (100%)

Table 8. Reported treated recreational water–associated disease outbreaks (n = 40) and resulting cases, hospitalizations, and deaths
Jurisdiction Month Etiology* Cases Hospitalizations§ Deaths Water Venue(s)§ Water Setting(s)§
Alabama January Cryptosporidium sp. 7 0 0 Pool Recreational Facility
Alabama June Cryptosporidium sp. 3 0 0 Pool Waterpark
Alaska February Chlorine (S) 19 0 0 Pool Community/Municipality
Colorado March Legionella pneumophila serogroup 1 2 2 1 Hot Tub Hotel/Motel
Florida June Legionella pneumophila 2 2 0 Hot Tub Resort
Florida July Cryptosporidium parvum 21 0 0 Pool Waterpark
Florida July Cryptosporidium sp. 15 1 0 Pool Hotel/Motel
Florida July Cryptosporidium sp. 7 0 0 Splash Pad, Pool Child Care/Daycare Center, Waterpark
Florida August Cryptosporidium hominis IfA12G1 10 0 0 Hot Tub, Pool Resort
Florida August Cryptosporidium sp. 4 0 0 Temporary Water Slide Private Residence
Florida December Legionella pneumophila serogroup 1 2 2 0 Hot Tub Community/Municipality
Florida December Legionella pneumophila serogroup 1 2 1 0 Hot Tub Hotel/Motel
Georgia April Legionella pneumophila serogroup 1 2 2 0 Hot Tub, Pool Club (Requires Membership)
Hawaii July Cryptosporidium hominis IgA20 7 0 0 Pool Amusement Park
Illinois June Pseudomonas sp. 13 0 0 Hot Tub Hotel/Motel
Kansas December Cryptosporidium sp. 2 0 0 Pool Hotel/Motel
Kentucky June Norovirus Genogroup II 14 0 0 Pool Club (Requires Membership)
Kentucky July Cryptosporidium hominis 11 2 0 Pool Recreational Facility
Kentucky August Legionella pneumophila 3 2 0 Hot Tub, Inflatable or Plastic Kiddie/Wading Pool, Pool Hotel/Motel
Louisiana May Legionella pneumophila 9 NR 0 NR NR
Louisiana June Legionella pneumophila 6 1 0 NR NR
Louisiana June Giardia duodenalis 3 0 0 NR NR
Minnesota March Pseudomonas sp. (S) 2 0 0 Hot Tub, Pool Hotel/Motel
Missouri July Cryptosporidium sp. 37 0 0 Pool Waterpark
Nebraska August Cryptosporidium sp. 23 0 0 Unknown Unknown
North Carolina July Cryptosporidium sp. 3 0 0 Pool Community/Municipality
North Carolina November Cryptosporidium sp. (S) 7 0 0 Pool Community/Municipality
North Dakota September Legionella pneumophila serogroup 1 2 1 0 Hot Tub, Pool Hotel/Motel
Ohio July Chloramines (S) 32 1 0 Pool Hotel/Motel
Ohio August Cryptosporidium sp. 27 0 0 Kiddie/Wading Pool Recreational Facility
Ohio August Cryptosporidium sp. 3 0 0 Splash Pad Community Park
Oregon June Mycobacterium avium 9 0 0 Hot Tub Private Residence
South Carolina November Acanthamoeba sp. 2 0 0 Pool Community/Municipality
Tennessee January Bacterial – Unknown (S) 12 0 0 Pool Hotel/Motel
Tennessee July Cryptosporidium hominis IfA12G1 58 1 0 Pool Hotel/Motel
Virginia May Campylobacter jejuni 4 0 0 Splash Pad Community Park
Virginia July Cryptosporidium hominis IfA12G1 87 0 0 Inflatable or Plastic Kiddie/Wading Pool, Pool, Splash Pad Camp/Cabin
Virginia August Cryptosporidium sp. 2 0 0 Splash Pad Community Park
Wisconsin March Pseudomonas aeruginosa (S) 7 0 0 Hot Tub, Pool Hotel/Motel
Wisconsin August Cryptosporidium sp. 9 0 0 Pool Camp/Cabin

Table 9. Reported untreated recreational water–associated disease outbreaks (n = 7) and resulting cases, hospitalizations, and deaths, by jurisdiction
Jurisdiction Month Etiology* Cases Hospitalizations Deaths Water Venue Water Setting
Idaho May Avian schistosomes (S) 22 0 0 Lake/Reservoir Unknown
Minnesota August Norovirus Genogroup II GII.2 18 0 0 River/Stream Outdoor Recreational Area
Missouri July Cryptosporidium parvum 8 0 0 Lake/Reservoir Camp/Cabin
Ohio July Escherichia coli, Shiga toxin-producing 9 2 0 Lake/Reservoir Beach
Pennsylvania July Norovirus unspecified 107 1 0 Lake/Reservoir State Park
Tennessee June Cryptosporidium sp. 9 2 0 Lake/Reservoir Beach
Vermont August Norovirus Genogroup II GII.6 124 0 0 Lake/Reservoir State Park

* (S) = Suspected Etiology.

Drinking Water Exposures

Drinking water exposure was associated with 28% (23/82) of outbreaks, resulting in 34% (429/1,276) of reported cases, nearly 70% (124/185) of reported hospitalizations, and 93% of deaths (25/27) (Table 1). Drinking water–associated outbreaks did not display seasonality in when they occurred (Figure 5).

Among drinking water–associated outbreaks, 78% (18/23) of outbreaks were caused by Legionella (Table 10). Outbreaks with multiple etiologies accounted for 9% (2/23) of drinking water outbreaks. The remaining outbreaks were caused by Escherichia, Giardia, and Mycobacterium. Drinking water outbreaks associated with multiple etiologies caused 60% (259/429) of cases. Legionella caused 37% (157/429) of cases, 99% (123/124) of hospitalizations, and all drinking water–associated outbreak deaths.

Table 10. Number and percentage* of reported waterborne disease outbreaks (n = 23) and resulting cases and hospitalizations associated with drinking water exposure, by etiology
Etiology Outbreaks Cases Hospitalizations Deaths
n (%) Confirmed Suspected n (%) Confirmed Suspected n (%) Confirmed Suspected n (%) Confirmed Suspected
Legionella 18 (78%) 16 2 157 (37%) 146 10 123 (99%) 113 10 25 (100%) 24 1
Escherichia 1 (4%) 1 0 6 (1%) 6 0 0 (0%) 0 0 0 (0%) 0 0
Giardia 1 (4%) 1 0 3 (1%) 3 0 0 (0%) 0 0 0 (0%) 0 0
Mycobacterium 1 (4%) 0 1 4 (1%) 0 4 0 (0%) 0 0 0 (0%) 0 0
Multiple§ 2 (9%) 1 1 259 (60%) 9 250 1 (1%) 1 0 0 (0%) 0 0
Total 23 (100%) 19 4 429 (100%) 165 264 124 (100%) 114 10 25 (100%) 24 1

* Percentages might not sum to 100 due to rounding.
§ Multiple etiology outbreaks include one outbreak suspected to be caused by Shigella, Campylobacter, and norovirus and one outbreak caused by Campylobacter and suspected to be caused by Giardia.

Outbreaks associated with drinking water most frequently occurred in hospitals or healthcare facilities (30%, 7/23), long term care or assisted living facilities (17%, 4/23), and hotels or motels (13%, 3/23).

Table 11. Number and percentage* of reported waterborne disease outbreaks (n = 23) and cases associated with drinking water exposure by setting
Water Setting Outbreaks n (%) Cases n (%)
Hospital/Healthcare Facility§ 7 (30%) 65 (15%)
Long Term Care Facility/Assisted Living 4 (17%) 72 (17%)
Hotel/Motel 3 (13%) 8 (2%)
Private Residence 2 (9%) 6 (1%)
Community/Municipality 1 (4%) 250 (58%)
Multiple 1 (4%) 9 (2%)
Other 4 (17%) 16 (4%)
Not Reported 1 (4%) 3 (1%)
Total 23 (100%) 429 (100%)

* Percentages might not sum to 100 due to rounding.
§ A healthcare institution providing inpatient medical or surgical treatment and nursing care for sick or injured persons, or a healthcare facility other than a long-term care facility.
A city, town, or other settlement where a large group of people live and work served by a community or municipal water system.
Multiple setting includes: a hotel/motel and resort.
Other setting includes: casino (1), factory/industrial facility (1), farm/agricultural setting (1), restaurant/cafeteria (1).

Among drinking water outbreaks, about 70% (16/23) were associated with community water systems. Individual or private water systems (e.g., private wells) accounted for four outbreaks (17%, 4/23).  In 2015, there were no reported drinking water outbreaks associated with non-community water systems. By definition, a community water system is a public water system that supplies water to the same population year round; community, individual/private, and non-community water systems are mutually exclusive (3).

Table 12. Number and percentage of reported waterborne disease outbreaks (n = 23) and cases associated with drinking water exposure, by water system
Water System Outbreaks n (%) Cases n (%)
Community 16 (70%) 351 (82%)
Individual/Private 4 (17%) 69 (16%)
Not Reported 1 (4%) 3 (1%)
Unknown 2 (9%) 6 (1%)
Total 23 (100%) 429 (100%)

Ground water was the reported source for 43% (10/23) of drinking water outbreaks, resulting in 65% (277/429) of cases, while surface water was associated with 17% (4/23) of outbreaks, resulting in 26% (110/429) of cases.  Nine percent (2/23) of outbreaks were associated with both ground water and surface water, and 4% (1/23) of outbreaks were associated with ground water under the influence of surface water.

Table 13. Number and percentage* of reported waterborne disease outbreaks (n = 23) and cases associated with drinking water, by water source
Water Source Outbreaks n (%) Cases n (%)
Ground Water 10 (43%) 277 (65%)
Surface Water 4 (17%) 110 (26%)
Ground & Surface Water 2 (9%) 14 (3%)
Ground Water Under Influence of Surface Water 1 (4%) 6 (1%)
Not Reported 2 (9%) 7 (2%)
Unknown 4 (17) 15 (3%)
Total 23 (100%) 429 (100%)

* Percentages might not sum to 100 due to rounding.

Table 14. Reported waterborne disease outbreaks (n =23) associated with drinking water, by jurisdiction
Jurisdiction Month Etiology* Cases Hospitalizations§ Deaths Water System§ Water Source Description§ Water Setting§¶
Arizona October Campylobacter jejuni (S), Norovirus unspecified (S), Shigella sonnei subgroup D (S) 250 0 0 Community Well Community/Municipality
Florida February Legionella sp. 3 1 0 Community Unknown Private Residence
Florida June Legionella pneumophila serogroup 1 7 7 2 Community Unknown Assisted Living Facility
Florida September Legionella pneumophila serogroup 1 2 2 0 Community Well Hotel/Motel
Florida November Legionella pneumophila 5 4 0 Community Unknown Assisted Living Facility
Florida November Legionella pneumophila 2 2 1 Community Well Hospital/Healthcare Facility
Florida December Legionella pneumophila serogroup 1 2 2 0 Community Well Hotel/Motel
Georgia July Legionella pneumophila serogroup 1 3 3 0 Community NR Hospital/Healthcare Facility
Georgia December Legionella pneumophila serogroup 1 (S) 6 6 0 Community NR Factory/Industrial Facility
Georgia December Legionella pneumophila (S) 4 4 1 Community NR Hospital/Healthcare Facility
Illinois July Legionella pneumophila serogroup 1 58 36 13 Individual/Private River/Stream Long Term Care Facility
Illinois October Legionella pneumophila serogroup 1 2 1 0 Unknown NR Long Term Care Facility
Maryland October Legionella pneumophila serogroup 1 4 1 0 Community Well Hotel/Motel
Michigan May Legionella pneumophila serogroup 1 43 43 5 Community River/Stream Hospital/Healthcare Facility
Minnesota December Legionella pneumophila serogroup 1 2 2 0 Community Unknown Restaurant/Cafeteria
Missouri March Legionella pneumophila serogroup 1 3 NR 1 NR NR NR
New York January Legionella pneumophila serogroup 1 and 6 4 4 1 Community Lake/Reservoir Hospital/Healthcare Facility
New York March Escherichia coli, Shiga toxin-producing 6 0 0 Individual/Private NR Farm/Agricultural Setting
New York March Legionella pneumophila serogroup 1 and 4 5 3 1 Community Lake/Reservoir Hospital/Healthcare Facility
New York December Giardia duodenalis 3 0 0 Individual/Private Well Private Residence
Ohio January Mycobacterium sp. (S) 4 0 0 Unknown NR Hospital/Healthcare Facility
Pennsylvania July Legionella pneumophila 2 2 0 Individual/Private Well Casino
Utah June Campylobacter jejuni, Giardia duodenalis (S) 9 1 0 Community Well, Other Private Residence

* (S) = Suspected Etiology.
§ NR = Not Reported.
A Community/Municipality is a city, town, or other settlement where a large group of people live and work served by a community or municipal water system. A Hospital/Healthcare Facility is as a healthcare institution providing inpatient medical or surgical treatment and nursing care for sick or injured persons, or a healthcare facility other than a long-term care facility.

Other Exposures to Water

Non-drinking, non-recreational exposures to water (referred to as “other exposures to water”—e.g., industrial water exposures, flood waters) were associated with 7% (6/82) of outbreaks and 4% of cases (47/1,276) (Table 1).

 

Cooling devices were reported in half (3/6) of the other exposures to water outbreaks (Table 15).  Untreated water consumed from river/streams (33%, 2/6) and a spring (17%, 1/7) were the remaining reported water exposure types.

Table 15. Number and percentage* of reported waterborne disease outbreaks (n = 6) associated with other exposures to water, by water type
Water Type Outbreaks n (%) Cases n (%)
Cooling Device§ 3 (50%) 36 (77%)
River/Stream 2 (33%) 7 (15%)
Spring 1 (17%) 4 (9%)
Total 6 (100%) 47 (100%)

* Percentages might not sum to 100 due to rounding.
§ Cooling device includes: cooling tower (2) and evaporative condenser/air conditioner (1).

Legionella caused 50% (3/6) of other water-associated outbreaks, 77% (36/47) of cases, and all hospitalizations. There was one death caused by Legionella. The remaining outbreaks were caused by Giardia (33%, 2/6) and Salmonella (17%, 1/6).

Table 16. Number and percentage of reported waterborne disease outbreaks (n = 6) and resulting cases and hospitalizations associated with other exposures to water, by etiology
Etiology Outbreaks Cases Hospitalizations Deaths
n (%) Confirmed Suspected n (%) Confirmed Suspected n (%) Confirmed Suspected n (%) Confirmed Suspected
Legionella 3 (50%) 3 0 36 (77%) 36 0 27 (100%) 27 0 1 (100%) 1 0
Giardia 2 (33%) 2 0 8 (17%) 8 0 0 (0%) 0 0 0 (0%) 0 0
Salmonella 1 (17%) 0 1 3 (6%) 0 3 0 (0%) 0 0 0 (0%) 0 0
Total 6 (100%) 5 1 47 (100%) 44 3 27 (100%) 27 0 1 (100%) 1 0

Table 17. Reported waterborne disease outbreaks (n = 6) associated with other exposures to water, by jurisdiction
Jurisdiction Month Etiology* Cases Hospitalizations Deaths Water Type Setting
Kansas September Salmonella enterica Paratyphi B (S) 3 0 0 River/Stream Outdoor Recreational Area
New Mexico July Giardia duodenalis 4 0 0 River/Stream Camp/Cabin
New York September Giardia duodenalis 4 0 0 Spring Outdoor Recreational Area
Ohio May Legionella pneumophila serogroup 5 20 11 0 Cooling Tower Indoor Workplace/Office
Ohio June Legionella pneumophila serogroup 1 6 6 1 Evaporative Condenser/Air Conditioner Assisted Living Facility
Washington October Legionella pneumophila serogroup 1 10 10 0 Cooling Tower Unknown

* (S) = Suspected Etiology.

Unknown Exposures to Water

In 7% (6/82) of outbreaks and 1% of cases (13/1,276) the type of water exposure was unknown. Among the six outbreaks where the type of water exposure was unknown, 67% (4/6) had an unknown water exposure and 33% (2/6) had multiple water exposures (Table 18). Outbreaks involving unknown exposures to water most frequently occurred in hotels or other lodging facilities (67%, 4/6) (Table 20).

Table 18. Number and percentage of reported waterborne disease outbreaks (n = 6) associated with unknown exposures to water, by water type
Water Type Outbreaks n (%) Cases n (%)
Multiple* 2 (33%) 4 (31%)
Unknown 4 (67%) 9 (69%)
Total 6 (100%) 13 (100%)

* Multiple exposure outbreaks includes: Hot Tub and Well (1); Pool and River/Stream (1).

Among outbreaks with an unknown water exposure, 83% (5/6) were caused by Legionella and 17% (1/6) by Cryptosporidium (Table 19).

Table 19. Number and percentage of reported waterborne disease outbreaks (n = 6) and resulting cases and hospitalizations associated with unknown exposures to water, by etiology
Etiology Outbreaks Cases Hospitalizations
n (%) Confirmed Suspected n (%) Confirmed Suspected n (%) Confirmed Suspected
Legionella 5 (83%) 5 0 11 (85%) 11 0 11 (100%) 11 0
Cryptosporidium 1 (17%) 1 0 2 (15%) 2 0 0 (0%) 0 0
Total 6 (100%) 6 0 13 (100%) 13 0 11 (100%) 11 0

Table 20. Reported waterborne disease outbreaks (n = 6) associated with unknown exposures to water, by jurisdiction
Jurisdiction Month Etiology Cases Hospitalizations Deaths Water Type Water Setting
Florida January Legionella pneumophila serogroup 1 2 2 0 Treated recreational and drinking water exposure* Hotel/Motel, Resort
Illinois July Legionella pneumophila serogroup 1 2 2 0 Unknown Hotel/Motel
Ohio August Legionella pneumophila serogroup 1 2 2 0 Unknown Church/Place of Worship
Texas November Legionella pneumophila serogroup 1 2 2 0 Unknown Hotel/Motel
Washington June Legionella pneumophila serogroup 1 3 3 0 Unknown Hotel/Motel
Wisconsin August Cryptosporidium sp. 2 0 0 Treated and untreated recreational water exposure* Outdoor Recreational Area, School/College/University

* Illnesses could not be associated with a single type of water exposure.

Limitations

The findings in this summary are subject to at least three limitations. First, only a proportion of waterborne outbreaks are detected, investigated, and reported to the National Outbreak Reporting System (NORS). The counts here are an underestimate of the true burden of waterborne disease outbreaks in the United States and should not be used to estimate the actual number of outbreaks or cases of waterborne disease. Second, these numbers are largely dependent on public health capacity and reporting requirements, which vary across jurisdictions, and therefore do not necessarily indicate the true occurrence in each state. Third, data on outbreaks with a chemical/toxin etiology might be limited because of differences in how these outbreaks are detected and investigated compared with infectious disease outbreaks or characteristics of the contaminants (e.g., persistence in the environment).

Conclusions

Public health surveillance is key to understanding the epidemiology of waterborne disease and outbreaks. Legionella and Cryptosporidium were the most frequently reported outbreak etiologies of waterborne disease outbreaks for 2015. Filtration and disinfection of water have reduced (though not eliminated) the burden of waterborne disease outbreaks caused by disinfectant-sensitive pathogens. Much of the remaining burden is driven by pathogens that are resistant to disinfection, either through their affinity for biofilm (Legionella and Pseudomonas) or intrinsic structural resistance (Cryptosporidium oocysts are extremely chlorine tolerant due to their hard outer shell). Effective prevention strategies beyond traditional filtration and disinfection of water include developing and implementing water management programs and adoption of CDC’s Model Aquatic Health Code.

Acknowledgements

The authors thank local, DC, state, territorial, and freely associated state public health officials and CDC staff for collecting and submitting outbreak data that made this summary possible.

References

  1. Collier SA, Deng L, Adam EA, et al. Estimate of Burden and Direct Healthcare Cost of Infectious Waterborne Disease in the United StatesEmerging Infectious Diseases. 2021;27(1):140-149. doi:10.3201/eid2701.190676.
  2. Detecting and Investigating Waterborne Diseases and Outbreaks. 2019 4/25/2019 [cited 2020 5/4].
  3. Information about Public Water Systems. 2021 11/10/2021 [cited 2021 11/22].

Suggested Citation

Centers for Disease Control and Prevention (CDC). Waterborne Disease and Outbreak Surveillance System (WBDOSS) Summary Report, United States, 2015. Atlanta, Georgia: U.S. Department of Health and Human Services, CDC, 2022.