Recreational water–related illness

Epidemiology

Acute gastrointestinal illness (AGI) is common, occurring in 8% to 9% of Canadians in any 4-week period.^7,8 In one urban children’s emergency department (ED) it accounted for 9% of all visits.⁹

The greatest risk of bacterial, protozoal, and viral gastroenteritis during the swimming season is likely not from exposure through food consumption, drinking water, or at day care, but rather from exposure to recreational water.^10,11

Illness from recreational water exposure is common. In field studies of swimmer illness, diarrhea rates of 3% to 8% are found in follow-up health surveys.^2,6 The same organisms associated with illness from drinking water (ie, Campylobacter, Salmonella, and Giardia species, and enteric viruses) are found in swimming water but in much higher concentrations. The average water ingestion with swimming is estimated to be 10 to 150 mL per hour.¹²

Children younger than 10 years of age contract more illness from recreational water because they play in the shallow water and sand, which are most contaminated, have hand-to-mouth exposure, stay in the water longer, immerse their heads more often, and swallow more water while swimming.^2,13,14

At public beaches, swimmer health is supposed to be protected by beach water sampling and beach closure when levels of indicator bacteria (ie, Escherichia coli and coliforms) exceed regulatory levels. However, high sampling and testing costs and long laboratory turnaround times (48 to 72 hours) make it difficult for many municipalities and health units to monitor beaches comprehensively or make real-time closure decisions. Current monitoring methods are not adequately protective against illness from swimming,¹⁵ and most swimming in Canada occurs in unmonitored water. Beach signage indicating that water will be unsafe after heavy rainfall, when waves are high, and when turbid is also used to prevent the public from swimming in unsafe water.

Important sources of human illness from beach water contamination include treated human waste water and agricultural run-off, especially from cattle manure.¹⁶ Crowded beaches have higher rates of swimmer illness, suggesting swimmer-to-swimmer illness transmission also plays a role.^12,17 Modeling estimates suggest that norovirus and rotavirus account for 75% of human illness from swimming.¹² As the enteroviruses are mainly species specific, this suggests that human contamination sources account for a high proportion of swimmer illness.

Although AGI is the most common recreational water–related illness, respiratory pathogens such as adenovirus are also found in swimming water and might cause summer respiratory illness.⁴ There are 2 other groups of water-borne illness at opposite ends of the disease spectrum. Minor conditions such as swimmer’s itch rash, conjunctivitis, and otitis externa might be caused by water-borne pathogens, both bacterial and viral. Swimmer’s itch is a schistosome, found across the entire border region between Canada and the United States, that burrows under the skin and causes papular summer rashes. At the extreme end of the spectrum are rare cases of diseases caused by unusual organisms such as leptospirosis,¹⁸ the duck-borne Naegleria fowleri (primary amebic encephalitis), or liver dysfunction from toxic chemicals such as microcystin-LR released by dying algal blooms in warm inland waters.⁴

Individuals at high risk of illness from recreational water include children younger than 10 years of age, the elderly, and the immunocompromised.^14,19 Children younger than 5 years of age are at higher risk because they have not yet acquired immunity to rotavirus (unless they have been vaccinated). Beach sand is an important reservoir of bacteria and a source of illness for young children; play that includes being buried in sand poses an especially high risk.¹³

In one experimental study,⁶ swimmers (the exposure group) were required to swim for at least 10 minutes with at least 3 head immersions. This exposure, although apparently small, resulted in much higher 1-week AGI rates than in the nonswimming group (8.6% vs 1.3%, P < .001). There was a dose-response relationship: swimming in more contaminated water and swallowing more water both resulted in higher AGI rates. Sports with high water immersion or high water contact that carry high risks of water-borne infection include triathlon,¹⁸ surfing, windsurfing, kite surfing,^20,21 and diving.²² Even limited-water-contact recreation, such as boating and fishing, increases the risk of AGI by 40% to 50% relative to nonwater recreational activities.²³

Table 1 outlines common recreational water illness pathogens and their management.

Role of climate change

Climate change might play an important role in water-related illness patterns. The 2 highest-probability climate-change events are more frequent heavy rainfall occurrences and more frequent and intense heat waves,²⁴ both with probability greater than 90%. Both these climate-change effects can impair water quality. Heavy rain washes pathogens into surface water, and higher temperatures promote the growth of algae and bacteria. A US study³ demonstrated that during the previous 46 years, 68% of water-borne disease outbreaks were preceded by heavy rainfall above the 80th percentile (P < .001). At a Milwaukee pediatric ED, any rainfall in the previous 4 days increased the rate of visits for AGI by 11%.²⁵

Chronic sequelae

Acute gastrointestinal illness might be viewed as minor, but its economic effects and long-term sequelae are substantial. An Ontario study estimated a cost per case for AGI of $1089 when over-the-counter medications, lost patient or parental work time, and costs to the health care system were included.²⁶ The Walkerton Health Study followed up with patients who became infected with E coli or Campylobacter species from drinking water and provided surprising new information about the serious chronic sequelae of AGI. Adults who were most severely affected (measured by seeking health care) had a 38% rate of diarrhea-predominant irritable bowel syndrome (IBS), diagnosed by the Rome criteria.²⁷ The IBS symptoms were mostly resolved by 6 years after the AGI. Children from the same outbreak also developed postinfectious IBS at higher rates (odds ratio 4.6, 95% CI 1.6 to 13.3) than those who were not exposed. Risk factors for development of IBS in the children were increased illness severity and use of antibiotics during the outbreak.²⁸ Other chronic health sequelae of acute bacterial gastroenteritis included hypertension, proteinuria, and glomerular filtration rate below 60 mL/min.²⁹ The latter 2 complications might occur with or without hemolytic uremic syndrome, and affect children and adults.

Exposure history

The most important element of the office management of Emma’s case, after acute care, is trying to determine a possible source of the gastroenteritis. This begins with a food- and water-exposure history. The exposure history can be taken using a standardized approach (Figure 1),³⁵ which should include questions to determine the child’s drinking water sources and whether there has been recreational water exposure. High-risk foods include not only undercooked beef, poultry, and fish, but also raw fruits, nuts, and vegetables.³⁶ Emma had been swimming the previous weekend and had spent 3 to 4 hours a day playing in the shallow water and beach sand at the cottage. This is a potentially large exposure that increases the risk of recreational water–related illness. The family uses a prefilter and ultraviolet light to treat the lake water for cottage use. Emma’s parents have not tested the drinking water at the cottage for 2 years. At home they use municipally treated water for drinking.

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Figure 1

The CH²OPD² exposure history mnemonic device

Data from Marshall et al.³⁵

Two important clues in this case are the season and the father’s comment about returning to the cottage on the weekend. Cottages often use treated surface water with uncomplicated treatment systems. Lakes and rivers can become contaminated in the summer by agricultural run-off following heavy rainfall, faulty septic systems near lakes and streams, and waste water effluent from treatment plants. The frequency of beach closures due to microbial contamination and algal blooms, which might harbour toxin-producing microorganisms, increases in late summer and early fall, and after heavy rainfall events. Private swimming areas at cottages are seldom monitored for water quality.

In this case, the moderate to severe dehydration (clinical dehydration scale score of 5 out of 8) made ED treatment appropriate, and stool culture results, exposure history, and timing of onset pointed to a probable recreational water source.

Stool cultures, although ordered for only 33% of patients who present to doctors with diarrhea,³⁷ are important for diagnosing the occasional cause (such as Giardia) that is treatable with antibiotics.

Inadequately treated surface water used for drinking at the cottage was another possible source of this child’s infection. Increased risk of gastrointestinal illness in populations using private wells is well documented, but even municipal systems that rely on treated surface water might confer a higher risk than purely groundwater sources.³⁸

As well as recognizing recreational water illness, family physicians have an important role in reminding patients who use private water systems to test 2 or 3 times yearly, including after spring run-off and after heavy rainfall events. Children younger than age 5 who will be swimming frequently have a higher risk of rotavirus infection, so immunization should be strongly recommended.