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Am Fam Physician. 2004;70(10):2026-2028

Norovirus Gastroenteritis Outbreak at a Swimming Club

John Snow’s historic investigation in London, England, of a severe epidemic of cholera traced the cause of infection to a common water source. Today, 150 years later, waterborne diseases remain a public health problem, and similar investigations are used to identify the source of infection. On February 3, 2004, the Vermont Department of Health (VDH) was notified of an outbreak of acute gastroenteritis among children whose only common exposure was attendance at a swimming club the previous weekend.

The VDH and the Centers for Disease Control and Prevention (CDC) determined the cause of the outbreak to be a combination of stool contamination, a blocked chlorine feed tube, and multiple lapses of pool-maintenance procedures.

Attendance records indicated that seven private groups used the pool, including three mother-infant swimming classes, two groups from a local girls’ organization, a birthday party of children five to 10 years of age, and a preschool class. In addition, members of the club used the pool during two defined open-swim sessions. Of the 189 persons for whom information was collected and who visited the pool during the outbreak period, 53 (28 percent) reported an illness consistent with the case definition of gastroenteritis. Among these 53 persons, onset of symptoms began a median of 30 hours after attending an event at the club and included vomiting, diarrhea, nausea, stomach cramps, chills, and a fever higher than 38°C (100.4°F).

No obvious source of contamination was identified: all infants were reported to have worn swim diapers while in the pool, no vomiting or fecal incidents were reported, and no persons, when questioned, reported gastrointestinal illness in the two weeks before visiting the pool. Interviews with swimmers and staff indicated that the water was visibly cloudy throughout the weekend, when the regular maintenance person was not on duty and pool usage was the highest. A kink in the tube that supplies chlorine to the pool was subsequently identified and repaired by the pool’s maintenance manager.

At the time of the review, although disinfection equipment was working properly and pool chlorine and pH levels, and temperature were consistent with recommended national standards, multiple lapses and inadequacies in pool management were identified. Of these, most remarkable were a lack of staff training and response policies, and the absence of records of the pool’s chemistry-monitoring results or pool maintenance.

Although waterborne outbreaks of norovirus gastroenteritis are much less commonly reported than foodborne outbreaks, the recorded incidence of norovirus-associated waterborne disease is likely an underestimate because of the lack of simple diagnostic technology. However, norovirus outbreaks associated with swimming pools rarely are reported.

Although prevention of norovirus outbreaks is difficult, this outbreak investigation suggests that staff training, pool-chemistry monitoring, and maintenance of appropriate disinfectant levels are important prevention strategies. As with John Snow’s Broad Street cholera outbreak (see accompanying box to the right), a series of environmental health failures occurred, creating conditions that could convey almost any waterborne pathogen.

150th Anniversary of John Snow and the Pump Handle

John Snow, M.D. (1813–1858), a legendary figure in epidemiology, provided one of the earliest examples of using epidemiologic methods to identify risk for disease and recommend preventive action. Best known for his work in anesthesiology, Snow also had an interest in cholera and supported the unpopular theory that cholera was transmitted by water rather than through miasma (i.e., bad air).

On August 31, 1854, London experienced a recurrent epidemic of cholera. Snow suspected water from the Broad Street pump as the source of disease. To test his theory, Snow reviewed death records of area residents who died from cholera and interviewed household members, documenting that most deceased persons had lived near and had drunk water from the pump. Snow presented his findings to community leaders, and the pump handle was removed on September 8, 1854. Removal of the handle prevented additional cholera deaths, supporting Snow’s theory that cholera was a waterborne, contagious disease. Snow’s studies and the removal of the pump handle became a model for modern epidemiology.

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