Outbreaks Associated with Treated Recreational Water - United States, 2000-2014.

Outbreaks associated with exposure to treated recreational water can be caused by pathogens or chemicals in venues such as pools, hot tubs/spas, and interactive water play venues (i.e., water playgrounds). During 2000-2014, public health officials from 46 states and Puerto Rico reported 493 outbreaks associated with treated recreational water. These outbreaks resulted in at least 27,219 cases and eight deaths. Among the 363 outbreaks with a confirmed infectious etiology, 212 (58%) were caused by Cryptosporidium (which causes predominantly gastrointestinal illness), 57 (16%) by Legionella (which causes Legionnaires' disease, a severe pneumonia, and Pontiac fever, a milder illness with flu-like symptoms), and 47 (13%) by Pseudomonas (which causes folliculitis ["hot tub rash"] and otitis externa ["swimmers' ear"]). Investigations of the 363 outbreaks identified 24,453 cases; 21,766 (89%) were caused by Cryptosporidium, 920 (4%) by Pseudomonas, and 624 (3%) by Legionella. At least six of the eight reported deaths occurred in persons affected by outbreaks caused by Legionella. Hotels were the leading setting, associated with 157 (32%) of the 493 outbreaks. Overall, the outbreaks had a bimodal temporal distribution: 275 (56%) outbreaks started during June-August and 46 (9%) in March. Assessment of trends in the annual counts of outbreaks caused by Cryptosporidium, Legionella, or Pseudomonas indicate mixed progress in preventing transmission. Pathogens able to evade chlorine inactivation have become leading outbreak etiologies. The consequent outbreak and case counts and mortality underscore the utility of CDC's Model Aquatic Health Code (https://www.cdc.gov/mahc) to prevent outbreaks associated with treated recreational water.

Outbreaks associated with exposure to treated recreational water can be caused by pathogens or chemicals in venues such as pools, hot tubs/spas, and interactive water play venues (i.e., water playgrounds). During 2000–2014, public health officials from 46 states and Puerto Rico reported 493 outbreaks associated with treated recreational water. These outbreaks resulted in at least 27,219 cases and eight deaths. Among the 363 outbreaks with a confirmed infectious etiology, 212 (58%) were caused by Cryptosporidium (which causes predominantly gastrointestinal illness), 57 (16%) by Legionella (which causes Legionnaires’ disease, a severe pneumonia, and Pontiac fever, a milder illness with flu-like symptoms), and 47 (13%) by Pseudomonas (which causes folliculitis [“hot tub rash”] and otitis externa [“swimmers’ ear”]). Investigations of the 363 outbreaks identified 24,453 cases; 21,766 (89%) were caused by Cryptosporidium, 920 (4%) by Pseudomonas, and 624 (3%) by Legionella. At least six of the eight reported deaths occurred in persons affected by outbreaks caused by Legionella. Hotels were the leading setting, associated with 157 (32%) of the 493 outbreaks. Overall, the outbreaks had a bimodal temporal distribution: 275 (56%) outbreaks started during June–August and 46 (9%) in March. Assessment of trends in the annual counts of outbreaks caused by Cryptosporidium, Legionella, or Pseudomonas indicate mixed progress in preventing transmission. Pathogens able to evade chlorine inactivation have become leading outbreak etiologies. The consequent outbreak and case counts and mortality underscore the utility of CDC’s Model Aquatic Health Code (https://www.cdc.gov/mahc) to prevent outbreaks associated with treated recreational water.

An outbreak associated with recreational water is the occurrence of similar illnesses in two or more persons, epidemiologically linked by location and time of exposure to recreational water or to pathogens or chemicals aerosolized or volatilized from recreational water into the surrounding air. Public health officials in the 50 states, the District of Columbia, U.S. territories, and Freely Associated States* voluntarily report outbreaks associated with recreational water to CDC. This report focuses on data in two groups of outbreaks associated with treated recreational water: 1) those that started during 2000–2012 and were previously summarized () and 2) those that started during 2013–2014 and were electronically reported to the Waterborne Disease and Outbreak Surveillance System (WBDOSS) by December 31, 2015 (https://www.cdc.gov/healthywater/surveillance/rec-water-tables-figures.html). Data on each outbreak included case count, number of deaths, etiology, setting (e.g., hotel) and venue (e.g., pool, hot tub/spa) where the exposure occurred, and earliest illness onset date. Poisson regression analysis was conducted to assess the trend in the annual counts of outbreaks, except when overdispersion required the use of negative binomial regression analysis.

During 2000–2014, public health officials from 46 states and Puerto Rico reported 493 outbreaks associated with treated recreational water, which resulted in at least 27,219 cases (Table) and eight deaths. Etiology was confirmed for 385 (78%) outbreaks. Among these, 363 (94%) were caused by pathogens (including four caused by both Cryptosporidium and Giardia) and resulted in at least 24,453 cases. Twenty-two (6%) outbreaks were caused by chemicals and resulted in at least 1,028 cases. Among the 363 outbreaks with a confirmed infectious etiology, 212 (58%) were caused by Cryptosporidium, 57 (16%) by Legionella, and 47 (13%) by Pseudomonas. Of the 24,453 cases, 21,766 (89%) were caused by Cryptosporidium, 920 (4%) by Pseudomonas, and 624 (3%) by Legionella. Of the 212 outbreaks caused by Cryptosporidium, 24 (11%) each affected >100 persons; four of these outbreaks each affected ≥2,000 persons. At least six of the eight deaths, which all occurred after 2004, were in persons affected by outbreaks caused by Legionella.

TABLE

Number of outbreaks associated with treated recreational water, total and median number of cases, by etiology — United States, 2000–2014

Etiology	No. (%) of outbreaks		No. (%) of cases	Median no. (range) of cases per outbreak
Total		493 (100)	27,219 (100)	10 (2–5,697)
Bacterium		129 (26)	1,899 (7)	6 (2–119)
Bacillus		1 (0)	20 (0)	20 (—*)
Campylobacter		2 (0)	10 (0)	5 (4–6)
Escherichia coli		6 (1)	86 (0)	12.5 (2–31)
Legionella		57 (12)	624 (2)	3 (2–107)
MRSA		1 (0)	10 (0)	10 (—)
Nontuberculous mycobacteria		2 (0)	14 (0)	7 (3–11)
Pseudomonas		47 (10)	920 (3)	10 (2–119)
Salmonella		1 (0)	5 (0)	5 (—)
Shigella		11 (2)	207 (1)	12 (3–56)
Staphylococcus		1 (0)	3 (0)	3 (—)
Parasite		220 (45)	21,976 (81)	14 (2–5,697)
Cryptosporidium		208 (42)	21,626 (79)	14.5 (2–5,697)
Giardia		8 (2)	210 (1)	8.5 (3–149)
Cryptosporidium, Giardia		4 (1)	140 (1)	37 (3–63)
Virus		14 (3)	578 (2)	36 (6–140)
Echovirus		1 (0)	36 (0)	36 (—)
Norovirus		13 (3)	542 (2)	36 (6–140)
Chemical		22 (4)	1,028 (4)	17.5 (2–665)
Excess chlorine, disinfection by-product, or altered pool chemistry		22 (4)	1028 (4)	17.5 (2–665)
Unidentified		108 (22)	1,738 (6)	7.5 (2–280)

Abbreviation: MRSA = methicillin-resistant Staphylococcus aureus.

* Not applicable because only one outbreak was nationally reported for that etiology.

Hotels** (i.e., hotels, motels, lodges, or inns) were the leading setting associated with 157 (32%) of the 493 outbreaks. Of the 157 hotel-related outbreaks, 94 (60%) had a confirmed infectious etiology, 40 (43%) were caused by Pseudomonas, 29 (31%) by Legionella, and 17 (18%) by Cryptosporidium. Sixty-five (41%) hotel-related outbreaks were associated with hot tubs/spas, and 47 (30%) started during February–March. Among all 493 outbreaks, a bimodal temporal distribution was observed. The 275 (56%) outbreaks that started during June–August were predominantly caused by Cryptosporidium, whereas the 46 (9%) that started in March were predominantly caused by an unidentified etiology or pathogens other than Cryptosporidium (Figure 1). Negative binomial regression analysis indicated that during 2000–2007, the annual number of outbreaks caused by Cryptosporidium increased by an average of 25% (95% confidence interval [CI] = 7%–45%) per year (Figure 2). No significant trend was found after 2007. Poisson regression analysis indicated that during 2000–2014 the annual number of outbreaks caused by Legionella increased by an average of 13% (95% CI = 6%–21%) per year, and the annual number of Pseudomonas folliculitis outbreaks (a total of 41 outbreaks during 2000–2014) decreased by an average of 22% (95% CI = 14%–29%) per year.***

FIGURE 1

Number of outbreaks associated with treated recreational water (N = 493), by etiology and month — United States, 2000–2014*

* Includes outbreaks with the following etiologies: Bacillus, Campylobacter, Escherichia coli, methicillin-resistant Staphylococcus aureus, nontuberculous mycobacteria, Salmonella, Shigella, Staphylococcus, Giardia, echovirus, norovirus, or excess chlorine/disinfection by-product/altered pool chemistry.

FIGURE 2

Number of outbreaks associated with treated recreational water (N = 493), by etiology and year — United States, 2000–2014

* Includes outbreaks with the following etiologies: Bacillus, Campylobacter, Escherichia coli, methicillin-resistant Staphylococcus aureus, nontuberculous mycobacteria, Salmonella, Shigella, Staphylococcus, Giardia, echovirus, norovirus, or excess chlorine/disinfection by-product/altered pool chemistry.

Discussion

Approximately 500 outbreaks associated with treated recreational water occurred in the United States during 2000–2014. The most frequently reported outbreak setting was hotels. Approximately half of the outbreaks started during June–August, followed by a smaller peak in March. The second peak might reflect swimming’s transition from an only-summertime to a year-round activity, as the relative number of indoor versus outdoor treated recreational water venues increases. The aquatics sector and public health can voluntarily adopt CDC’s Model Aquatic Health Code to improve the design, construction, operation, and maintenance of public (nonbackyard) treated recreational water venues to prevent illness and injury.

Chlorine is the primary barrier to the transmission of pathogens in treated recreational water. At CDC-recommended concentrations of at least 1 ppm, free available chlorine inactivates most pathogens within minutes although extremely chlorine-tolerant Cryptosporidium can survive for >7 days (,). Cryptosporidium is transmitted when a diarrheal incident (i.e., a high-risk Cryptosporidium contamination event) occurs in the water and the contaminated water is ingested. The parasite’s extreme chlorine tolerance enables it to persist in water, cause outbreaks that sicken thousands, and spread to multiple recreational water venues and other settings (e.g., child care settings). Rates of individual cases caused by Cryptosporidium peak in the summer, coinciding with the summer swim season ().

In contrast, Legionella and Pseudomonas are effectively controlled by halogens (e.g., chlorine and bromine) in well-maintained treated venues. However, because these pathogens can persist in biofilm (where microbial cells inhabit a primarily polysaccharide matrix, the cells cannot be removed from a surface by gentle rinsing) (), and they are protected from inactivation and amplify when disinfectant concentrations are not properly maintained. Approximately 20% of 13,864 routine inspections of public hot tubs/spas conducted in 16 jurisdictions in 2013 identified improper disinfectant concentrations (). Legionella is typically transmitted when aerosolized water droplets (e.g., produced by hot tub/spa jets) containing this bacterium are inhaled, whereas Pseudomonas is transmitted when skin comes in contact with contaminated water. Multiple factors contribute to Legionella and Pseudomonas growth in hot tubs/spas, including inadequate disinfectant concentration; warm (77°F–108°F [25°C–42°C]) water temperatures (which facilitate pathogen amplification and make it difficult to maintain adequate disinfectant concentration); water aeration (which depletes halogens); and the presence of biofilm on wet venue surfaces, scale, and sediment (). The increasing annual rate of Legionnaires’ disease cases (286% during 2000–2014) (), and possibly, the significantly increasing annual number of outbreaks caused by Legionella, might be associated with increasing size of susceptible populations (persons aged ≥50 years or those with chronic disease [particularly chronic lung disease] or who are immunocompromised; current or former smokers; or cancer patients), and increased Legionella growth in the environment, as well as increased awareness of the disease with improved testing and reporting (). The significantly decreasing number of annual Pseudomonas folliculitis outbreaks might reflect an actual decrease or possibly focusing on hot tub/spa remediation to prevent further transmission rather than outbreak investigation and reporting.

If a diarrheal incident occurs in treated recreational water or an outbreak at least suspected to be caused by Cryptosporidium occurs, CDC recommends hyperchlorination, i.e., chlorinating water to achieve 3-log10 (99.9%) Cryptosporidium inactivation (https://www.cdc.gov/healthywater/swimming/aquatics-professionals/fecalresponse.html). Alternatively, ultraviolet light or ozone systems can be added to inactivate Cryptosporidium, particularly in venues at increased risk for contamination (e.g., those intended for children aged <5 years, who might have limited or no toileting skills). As in any public setting, treated venues in the hotel setting should be operated and maintained by a trained operator or responsible supervisor. These and other recommendations can be found in CDC’s Model Aquatic Health Code. CDC also provides specific recommendations for disinfecting hot tubs/spas contaminated with Legionella (https://www.cdc.gov/legionella/downloads/hot-tub-disinfection.pdf). Investigations of Legionnaires’ disease outbreaks indicate that effective water management programs for buildings and treated recreational water venues (e.g., hot tubs/spas) at increased risk for Legionella growth and transmission can reduce the risk for Legionnaires’ disease (,).

The findings in this report are subject to at least two limitations. First, the outbreak counts presented likely underestimate the actual incidence, in part because of variation in public health capacity and reporting requirements across jurisdictions. Second, reporting and review procedures (e.g., increased completeness of data on outbreaks caused by Legionella) changed over time, which affects the ability to compare data across years.

Addressing the challenges presented by chlorine-tolerant and biofilm-associated pathogens require sustained attention to improving design, construction, operation, and management of public treated recreational water venues. This includes educating the public. Preventing Cryptosporidium contamination is critical to preventing transmission. Thus, the key message to the public, particularly parents of young bathers, is “Don’t swim or let your kids swim if sick with diarrhea.” Preventing transmission of Legionella, Pseudomonas, and other chlorine-susceptible pathogens means educating bathers and parents of young bathers to check the inspection scores of public treated recreational water venues and conduct their own mini-inspection before getting into the water (e.g., measure bromine or free chlorine level and pH with test strips, which can be purchased at pool supply, hardware, and big-box stores). Potential hot tub/spa users should know whether they are at increased risk for Legionnaires’ disease, so they can choose to avoid hot tubs/spas, as indicated (https://www.cdc.gov/legionella/downloads/fs-legionnaires.pdf). The halting of the substantial increase in annual numbers of outbreaks caused by Cryptosporidium might, at least in part, be because of local, state, and federal Healthy and Safe Swimming Week (the week before Memorial Day) campaigns (). Thus, the focus of these campaigns could regularly be expanded beyond preventing Cryptosporidium transmission in an effort to prevent other recreational water outbreaks.

What is already known about this topic?

Outbreaks associated with treated recreational water can be caused by pathogens or chemicals.

What is added by this report?

During 2000–2014, 493 outbreaks associated with treated recreational water caused at least 27,219 cases and eight deaths. Outbreaks caused by Cryptosporidium increased 25% per year during 2000–2006; however, no significant trend occurred after 2007. The number of outbreaks caused by Legionella increased 14% per year.

What are the implications for public health practice?

The aquatics sector, public health officials, bathers, and parents of young bathers can take steps to minimize risk for outbreaks. The halting of the increase in outbreaks caused by Cryptosporidium might be attributable to Healthy and Safe Swimming Week campaigns.