Pool, Hot Tub and Water Playground Outbreaks in the United States, 2000–2014

Treated recreational water — pools, hot tubs and spas, and interactive water playgrounds — can make people sick when it carries germs or chemicals. A CDC report pulled together the outbreaks that state and territorial health officials reported from 2000 through 2014.

An outbreak here means similar illness in two or more people linked by place and time to the same water, or to germs or chemicals carried from it into the air. Reporting to CDC is voluntary.

The toll

Public health officials in 46 states and Puerto Rico reported 493 outbreaks, which caused at least 27,219 cases and eight deaths. The cause was confirmed for 385 of them:

• 363 were caused by germs, with at least 24,453 cases;
• 22 were caused by chemicals (too much chlorine, disinfection by-products or altered pool chemistry), with at least 1,028 cases.

Among the 363 outbreaks with a confirmed germ:

Cryptosporidium outbreaks could be very large: 24 of them each sickened more than 100 people, and four each sickened 2,000 or more. At least six of the eight deaths, all after 2004, were in Legionella outbreaks; the other two were in an outbreak suspected, but not confirmed, to be Legionella.

Where and when

• Hotels (including motels, lodges and inns) were the leading setting, with 157 outbreaks (32%). Among hotel outbreaks with a confirmed germ, Pseudomonas led, followed by Legionella and then Cryptosporidium; 65 were tied to hot tubs or spas.
• Outbreaks peaked twice a year. 275 (56%) began in June–August, mostly caused by Cryptosporidium; a smaller peak of 46 (9%) came in March, mostly from other or unidentified causes. The authors suggested the March peak may reflect swimming shifting from a summer-only to a year-round activity as indoor venues become more common.

Outbreaks by month of onset and cause. CDC figure.

Trends

• Cryptosporidium outbreaks rose by an average of 25% a year in the early part of the period, then showed no significant trend after 2007.
• Legionella outbreaks increased significantly year on year across 2000–2014.
• Pseudomonas folliculitis outbreaks (41 in all) fell by an average of 22% a year — which may reflect a real fall, or health departments fixing hot tubs rather than investigating and reporting outbreaks.

Outbreaks by year and cause. CDC figure.

Why these germs

Chlorine is the main barrier. At CDC-recommended free chlorine levels of at least 1 ppm, most germs die within minutes, but Cryptosporidium is extremely chlorine-tolerant and can survive for more than 7 days. It spreads when someone has diarrhea in the water and others swallow it.

Legionella and Pseudomonas are controlled by chlorine or bromine in well-kept water, but they can shelter and multiply in biofilm, the slimy film on wet surfaces, when disinfectant levels slip. About 20% of 13,864 routine inspections of public hot tubs in 16 jurisdictions in 2013 found improper disinfectant levels. Warm water, jets that aerate the water and use up chlorine, and scale and sediment all help these bacteria grow. Legionella spreads when people breathe in mist from the water; Pseudomonas spreads through skin contact.

Prevention

For pool operators and health officials, the report pointed to:

• CDC's Model Aquatic Health Code, voluntary guidance on designing, building, running and maintaining public pools and hot tubs;
• hyperchlorination after a diarrheal incident or a suspected Cryptosporidium outbreak, and ultraviolet or ozone systems at venues at higher risk, such as those meant for children under 5;
• a trained operator or responsible supervisor for every public venue, hotels included;
• water management programs for buildings and hot tubs at risk of Legionella growth.

For swimmers and parents:

• Don't swim, or let children swim, while sick with diarrhea.
• Check a venue's inspection score, and test the water yourself with chlorine or bromine and pH strips before getting in.
• Know whether you are at higher risk of Legionnaires' disease — for example, being aged 50 or older, smoking or having smoked, or having chronic lung disease or a weakened immune system — and consider avoiding hot tubs if so.

The authors suggested that Healthy and Safe Swimming Week, held the week before Memorial Day, may have helped halt the rise in Cryptosporidium outbreaks, and that the campaigns could broaden to other germs.

Limitations

The counts likely understate the true number, because health departments' capacity and reporting rules differ. Reporting procedures also changed over time, which makes year-to-year comparison harder.

Sources

• Hlavsa MC, Cikesh BL, Roberts VA, et al. "Outbreaks Associated with Treated Recreational Water — United States, 2000–2014." Morbidity and Mortality Weekly Report 67(19), Centers for Disease Control and Prevention.
• The report is inconsistent about two trends: its summary puts the Cryptosporidium rise over a period ending one year earlier than its results section does, and it gives the yearly rise in Legionella outbreaks as 14% in one place and 13% in another. This page states neither of the disputed figures.