Health Letter, June 2014
By Michael Carome, M.D.
With the recent Memorial Day weekend signaling the traditional start of the summer season, many people soon will be spending a significant amount of leisure time swimming at the neighborhood pool, local lake or ocean over the next few months. While swimming is a great way to maintain physical fitness and have fun, recent data from the Centers for Disease Control and Prevention (CDC) warn us that sometimes pool, lake or ocean water contains an invisible danger.
In a pair of studies published in Morbidity and Mortality Weekly Report on Jan. 10, 2014, CDC researchers found a significant increase over the past three decades in the number of documented disease outbreaks caused by exposure to infectious organisms or toxins in recreational water., Fortunately, such outbreaks are rare, and the resulting illnesses usually resolve without serious long-term adverse consequences.
The CDC’s monitoring system for recreational-water-associated disease outbreaks 
The CDC defines a recreational-water-associated disease outbreak as “the occurrence of similar illnesses in two or more persons … linked by location and time of exposure to recreational water or water-associated chemicals [released as a vapor] into the air surrounding the water.”
The CDC divides recreational-water disease outbreaks into two categories. Those in the first category are associated with treated recreational water venues, which include pools and hot tubs or spas. Treated recreational water has chemicals added to kill bacteria, viruses and other disease-causing germs. The second category encompasses outbreaks associated with untreated recreational-water venues, which include freshwater lakes and oceans.
In 1971, the CDC established the Waterborne Disease and Outbreak Surveillance System (WBDOSS) as a component of its National Outbreak Reporting System. Using the WBDOSS, public health officials in U.S. states, the District of Columbia and U.S. territories can voluntarily report, among other things, recreational-water-associated disease outbreaks to the CDC. Prior to 2009, such reporting was accomplished by submission of paper reports. Subsequently, reports have been submitted electronically. Information requested in the reports includes:
- number of people affected;
- number of affected people hospitalized;
- number of affected people who died;
- type of illness;
- cause of the illness, if known;
- venue (for example, pool or lake) and setting (for example, a hotel, water park or community recreational center) where the outbreak exposure occurred; and
- onset date of earliest illness.
Ten states also received funding from the CDC to conduct enhanced surveillance for illnesses and deaths associated with exposure to cyanotoxins, a poisonous chemical produced by certain bacteria found in freshwater (discussed in more detail below).
1978 to 2010 trends 
Although there has been significant year-to-year variation in the total number of reported waterborne disease outbreaks associated with recreational water (ranging from 6 to 84 outbreaks per year), the CDC data reveal an overall upward trend in such outbreaks between 1978, when disease outbreak reports were first submitted to the agency, and 2010, the most recent year for which final complete data are available. (The CDC researchers noted that the data for 2007 with respect to the number of outbreaks reported – 84 – may represent an outlier because one multistate recreational-water-associated outbreak was counted as multiple separate outbreaks in adjacent states, thus inflating the outbreak count for that year.)
For example, for the years 1978 and 1979, the CDC received a total of 12 reports of such disease outbreaks. More recently, for the years 2007 and 2008 combined, a total of 134 outbreaks involving nearly 14,000 cases were reported, and for 2009 and 2010 combined, 81 outbreaks involving 1,326 total cases were reported.
The CDC researchers also noted one important trend in the causes of recreational-water-related disease outbreaks: In 1988, the first such outbreak linked to the microscopic parasite cryptosporidium was reported to the agency. Since 1988, the number of outbreaks linked to this parasite has significantly increased, peaking at 40 in 2007. (Again, the count for 2007 may represent an outlier because one multistate recreational-water-associated cryptosporidium outbreak was counted as multiple separate outbreaks in adjacent states, thus inflating the outbreak count for that year.)
Cryptosporidium has a protective outer shell that allows it to survive in the environment for long periods of time and makes it very tolerant to chlorine disinfection. The most frequent sources of exposure to this parasite are drinking water and recreational water.
Cryptosporidium causes intestinal infections, symptoms of which include diarrhea and abdominal cramping. Both the parasite and the disease are commonly known as “Crypto.”
2009-2010 outbreaks 
For the 81 reported outbreaks of recreational-water-associated disease in 2009-2010, 62 (5 percent) of the 1,326 total cases resulted in hospitalization, but there were no reported deaths. Additional key statistics reported by the CDC researchers regarding these 81 reported outbreaks included:
- The cause was confirmed for 60 percent of the outbreaks.
- Cryptosporidium was by far the leading cause (confirmed in 27 outbreaks). A variety of different bacteria, the norovirus (a very common cause of outbreaks of acute “stomach flu” – leading to nausea, vomiting and diarrhea – that are often seen on cruise ships and in schools and other community settings) or a parasite other than cryptosporidium was identified as the cause in 18 other outbreaks.
- Fifty-seven (70 percent) of the outbreaks were associated with treated recreational-water venues. These outbreaks resulted in at least 1,030 cases of illness and 40 hospitalizations. More than half of these outbreaks were associated with hotel (33 percent) or water park (25 percent) settings.
- Twenty-four (30 percent) of the outbreaks were associated with untreated recreational-water venues. These outbreaks resulted in at least 296 cases and 22 hospitalizations.
- The number of cases reported per outbreak ranged from two to 280.
- The months with the largest number of outbreaks during 2009 and 2010, in decreasing order, were August (21), July (16), June (13) and March (11). The months with the smallest number of outbreaks were January, September, November and December (one each).
Of the 24 outbreaks associated with untreated recreational water, 11 – nearly half – were associated with harmful blooms of algae in freshwater lakes. Harmful algae blooms occur when excess nutrients in the water stimulate the growth of certain microscopic organisms (called phytoplankton) that can produce toxins. One such toxin, cyanotoxin, can cause human illness. These blooms often produce a visible algal scum on the surface of the water.
The 11 reported outbreaks associated with these freshwater harmful algae blooms all occurred in just three states: New York, Ohio and Washington. All occurred between June and August. They resulted in at least 61 illnesses and two hospitalizations.
Signs and symptoms of these illnesses varied among the outbreaks and included:
- skin changes, such as rash, irritation, swelling or sores (eight outbreaks);
- gastrointestinal symptoms, such as abdominal cramps, diarrhea, nausea, vomiting or loss of appetite (eight outbreaks);
- respiratory symptoms, including nasal congestion, cough, wheezing, shortness of breath or chest tightness (six outbreaks);
- fever (five outbreaks);
- ear symptoms, such as earaches (five outbreaks);
- headache (four outbreaks);
- neurological symptoms, such as confusion (four outbreaks); and
- eye symptoms, including eye irritation and visual disturbances (three outbreaks).
Of note, prior to 2009, disease outbreaks linked to freshwater harmful algae blooms were very infrequent, with only three such outbreaks reported to the WBDOSS for 1978-2008. The recent increase may be due to increased water pollution, particularly from fertilizers used on farms and lawns that stimulate algae growth.
The data presented in the two CDC studies likely significantly underestimate the true incidence of recreational-water-associated disease outbreaks. The CDC researchers noted that multiple factors are barriers to the detection, investigation and reporting of outbreaks, including:
- limited illness severity;
- small outbreak size;
- long incubation period of illness, resulting in a long delay between exposure and onset of symptoms;
- wide geographic dispersion of ill swimmers following exposure;
- tran¬sient nature of contamination of the water;
- setting or venue of outbreak exposure that is less likely to attract attention of public health officials (for example, residential backyard pool); and
- potential lack of communication between those who respond to outbreaks of chemical etiology (for example, hazardous materials personnel) and those who usually report outbreaks (for example, infectious disease epidemiologists).
What you should do
The data released by the CDC regarding recreational-water-associated disease outbreaks should not discourage you from swimming in pools, lakes and oceans. Given the number of people who swim each year, the risk of developing a serious illness related to swimming and playing in recreational water is exceedingly low, even taking into account the likelihood that there is significant underreporting of these outbreaks.
Here are some simple steps you can take to minimize your chances of becoming ill due to exposure to infectious organisms or toxins in a recreational-water setting:
- Obey any public health alerts warning against swimming or playing in potentially contaminated water.
- Avoid swallowing water in pools, lakes and oceans.
- Rinse off in the shower after swimming, particularly after swimming in a freshwater lake or pond.
- Before swimming or playing in a freshwater lake that has algae scum on the surface, contact local public health officials to inquire whether there have been any recent concerns about harmful algae blooms in the lake.
If you, a family member or friend become ill after swimming in recreational water – particularly if you develop a rash, other unusual skin symptoms or gastrointestinal illness – contact your primary health care provider and notify the local health department nearest to the recreational-water venue (or nearest to where you live).
 Centers for Disease Control and Prevention. Parasites – Cryptosporidium (also known as “Crypto”). Last updated January 16, 2013. http://www.cdc.gov/parasites/crypto/. Accessed April 26, 2014.