Outbreaks associated with treated recreational water — United States, 2015–2019

Outbreaks associated with treated recreational water can be caused by pathogens or chemicals in aquatic venues such as pools, hot tubs, water playgrounds, or other artificially constructed structures that are intended for recreational or therapeutic purposes. For the period 2015–2019, public health officials from 36 states and the District of Columbia (DC) voluntarily reported 208 outbreaks associated with treated recreational water. Almost all (199; 96%) of the outbreaks were associated with public (nonbackyard) pools, hot tubs, or water playgrounds. These outbreaks resulted in at least 3,646 cases of illness, 286 hospitalizations, and 13 deaths. Among the 155 (75%) outbreaks with a confirmed infectious etiology, 76 (49%) were caused by Cryptosporidium (which causes cryptosporidiosis, a gastrointestinal illness) and 65 (42%) by Legionella (which causes Legionnaires’ disease, a severe pneumonia, and Pontiac fever, a milder illness with flu-like symptoms). Cryptosporidium accounted for 2,492 (84%) of 2,953 cases resulting from the 155 outbreaks with a confirmed etiology. All 13 deaths occurred in persons affected by a Legionnaires’ disease outbreak. Among the 208 outbreaks, 71 (34%) were associated with a hotel (i.e., hotel, motel, lodge, or inn) or a resort, and 107 (51%) started during June–August. Implementing recommendations in CDC’s Model Aquatic Health Code (MAHC) (1) can help prevent outbreaks associated with treated recreational water in public aquatic venues. An outbreak associated with recreational water is the occurrence of similar illness in two or more persons whose illnesses are epidemiologically linked by location and time of exposure to 1) recreational water or 2) pathogens or chemicals aerosolized or volatilized into the air from recreational water. Public health officials in U.S. jurisdictions (the 50 states, DC, U.S. territories, and freely associated states) voluntarily report outbreaks to CDC via the National Outbreak Reporting System. This report examines data on outbreaks that were associated with treated recreational water and reported by February 4, 2021, and for which the first illness occurred during 2015– 2019. Data on each outbreak include earliest illness onset date, count of cases of illness, counts of hospitalizations and deaths, etiology, and setting (e.g., hotel) and venue (e.g., pool, hot tub, or water playground) of the outbreak exposure. This

An outbreak associated with recreational water is the occurrence of similar illness in two or more persons whose illnesses are epidemiologically linked by location and time of exposure to (1) recreational water or (2)   outbreak. Among the 53 outbreaks with a nonconfirmed (i.e., suspected or unknown) etiology, 20 (38%) were suspected to be caused by chemical etiologies (e.g., excess chlorine, one or more disinfection byproducts, or altered pool chemistry) ( Table 1). Hotels (i.e., hotels, motels, lodges, or inns) or resorts were associated with 71 (34%) of the 208 outbreaks; 50 (70%) of these outbreaks were associated with hot tubs. Among the 43 hotel-or resort-associated outbreaks with a confirmed etiology, 31 (72%) were caused by Legionella and were associated with a hot tub.

What is already known about this topic?
Outbreaks associated with treated recreational water in pools, hot tubs, and water playgrounds can be caused by pathogens or chemicals.
Revisions to corresponding National Outbreak Reporting System data fields are underway to improve data quality and as part of data modernization efforts.
In addition to voluntarily adopting the MAHC and Legionnaires' disease prevention recommendations, public health officials and operators of public aquatic venues can help prevent outbreaks associated with treated recreational water by educating the public.
Given Cryptosporidium's extreme chlorine tolerance, "don't swim or F I G U R E 2 Outbreaks associated with treated recreational water* (N = 208), by etiology †, § and year -National Outbreak Reporting System, United States, 2015-2019. *Treated recreational water is water in a pool, hot tub, water playground, or other artificially constructed structure that is intended for recreational or therapeutic purposes. Outbreaks are the occurrence of similar illness in two or more persons who are epidemiologically linked by location and time of exposure to (1) treated recreational water or (2) pathogens or chemicals that were aerosolized or volatilized into the air from treated recreational water. † "Nonconfirmed" includes outbreaks with the following reported etiologies: suspected chemical (e.g., excess chlorine, one or more disinfection byproducts, or altered pool chemistry), suspected Cryptosporidium, suspected Giardia, suspected Legionella, suspected norovirus, suspected Pseudomonas, unknown bacterial, and unknown. § "Other" includes outbreaks with the following confirmed etiologies: Acanthamoeba, Campylobacter, Shiga toxin-producing Escherichia coli, water playgrounds. ¶ https://www.cdc.gov/legio nella/ wmp/contr ol-toolk it/index.html ** https://www.cdc.gov/legio nella/ wmp/toolkit † † pH will determine the relative amounts of hypochlorous acid, the active disinfectant form of chlorine referred to as free available chlorine, and hypochlorite ion, a less active disinfectant form of chlorine. The pH range 7.2-7.8 is one that balances maximizing free available chlorine with swimmer comfort and preventing equipment corrosion. § § For reference purposes, MAHC elements discussed in this report are followed by the specific section number that corresponds to that element. ¶ ¶ https://www.cdc.gov/healt hywat er/swimm ing/aquat ics-profe ssion als/ fecal respo nse.html *** The difference accounts for the depletion of the disinfectant concentration by higher water temperatures and aerosolization of water by hot tub jets. † † † The formula for calculating water replacement is frequency in days = (hot tub volume in gallons/3)/average number of users per day. § § § https://www.cdc.gov/legio nella/ downl oads/hot-tub-disin fecti on.pdf ¶ ¶ ¶ https://www.cdc.gov/legio nella/ downl oads/fs-legio nnair es.pdf **** https://www.cdc.gov/healt hywat er/swimm ing/swimm ers/steps -healt hy-swimm ing.html