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An Outbreak of Norovirus Gastroenteritis at a Swimming Club --- Vermont, 2004

John Snow's historic investigation of a severe epidemic of cholera traced the cause of infection to a common water source (1). 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 (January 31--February 1). This report summarizes the results of an investigation conducted by VDH and CDC, which 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. The findings underscore the importance of correct pool maintenance for rapid identification of water-quality problems to prevent outbreaks of swimming pool--associated illness.

Pool attendance records were available for review for the period from Friday evening, January 30, to Monday noon, February 2, 2004. During this time, seven private groups used the pool, including three mother-infant swimming classes, two groups from a local girls' organization, a birthday party of children aged 5--10 years, and a preschool class. In addition, members of the club used the pool during two defined open-swim sessions. The seven private groups ranged in size from four to 31 persons. The group leader for each event provided a roster of attendees. An adult in each household was contacted by telephone and asked to identify family members who attended events at the swimming club and to question them about recent gastrointestinal illness by using a standardized questionnaire. Family members who reported recent gastrointestinal illness were asked to submit stool specimens for laboratory testing. Respondents also were asked to describe the appearance of the pool water at the time of their visit. A case was defined as vomiting or diarrhea (i.e., >3 loose stools within a 24-hour period) that occurred in a person within 72 hours of visiting the swimming club.

Of the 189 persons for whom information was collected and who visited the pool during the outbreak period, median age was 13 years (range: 5 months--73 years); 53 (28%) reported an illness consistent with the case definition. Among these 53 persons, onset of symptoms began a median of 30 hours (range: 8--62 hours) after attending an event at the club and included vomiting (89%), diarrhea (50%), nausea (77%), stomach cramps (68%), chills (58%), and a fever of >100.4º F (>38º C) (53%). The median age of patients was 7 years (range: 5 months--61 years); 31 (58%) were female. Six persons (five children and one adult) sought medical care from their physicians, and one adult was hospitalized with severe vomiting. Of the 10 stool specimens tested, five were positive for norovirus by reverse transcription-polymerase chain reaction (RT-PCR). Three strains were characterized further and determined to share identical nucleotide sequences. The highest attack rates were observed among persons who visited the pool on Saturday or Sunday (Figure). No one who attended Friday's event became ill, and by Sunday afternoon, the attack rates had declined sharply. 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 2 weeks before visiting the pool. Attending an event at the club on Saturday or Sunday (versus Friday or Monday, relative risk [RR] = 7.7; 95% confidence interval [CI] = 2.0--30.0; p = 0.003) and going into the pool (RR = 6.0; 95% CI = 1.6--23.0; p = 0.009) increased risk for illness.

Interviews with swimmers and staff indicated that the water was visibly cloudy throughout Saturday and on Sunday morning (Figure), when the regular maintenance person was not on duty and pool usage was the highest. No action was taken until Sunday afternoon, when the pool was hyperchlorinated (i.e., "shocked") twice. Analysis of a water sample collected on Monday morning demonstrated low free residual chlorine (0.5 parts per million [ppm]; normal range: 1--4 ppm) and low pH (6.8; ideal range: 7.4--7.6), indicating suboptimal disinfection. A kink in the tube that supplies chlorine to the pool was subsequently identified and repaired by the pool-maintenance manager. The pool was hyperchlorinated again Monday night, and the pH was corrected to optimize chlorine efficacy.

The pool was equipped with an automated chlorine feeder and filtration system and was monitored and maintained by lifeguards and a maintenance worker. On Tuesday, February 3, a comprehensive environmental health systems review of the pool equipment, maintenance, and operations was conducted. At the time of the review, although disinfection equipment was working properly and pool chlorine, pH, and temperature were consistent with recommended national standards (2), 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 pool-chemistry monitoring results or pool maintenance.

Reported by: L Zanardi Blevins, MD, D Itani, MS, A Burns, C Lohff, MD, S Schoenfeld, MSPH, W Knight, N Thayer, J Oetjen, PhD, N Pugsley, Vermont Dept of Health. C Otto, Environmental Health Svcs, National Center for Environmental Health; M Beach, PhD, Div of Parasitic Diseases; M-A Widdowson, VetMB, J Bresee, MD, R Glass, MD, S Monroe, PhD, L Browne, MPH, S Adams, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases; M Amundson, DVM, LJ Podewils, PhD, EIS officers, CDC.

Editorial Note:

Whereas classical infectious disease epidemiologic methods were used to identify and characterize this outbreak and the risk factors for illness, several failures in the environmental health systems likely led to the outbreak. First, inadequate monitoring of water quality by the pool staff resulted in critical delays in detecting the chlorinator-tube malfunction. Second, although the pool staff and patrons noticed cloudy, turbid water in the pool, the maintenance staff was not notified, which further delayed implementation of control measures, such as hyperchlorination, for more than 24 hours. Third, none of the pool staff had formal training in pool disinfection. Appropriate monitoring, operation, and response protocols could have prevented this outbreak or reduced the duration of virus transmission. Only on Monday, when low free residual chlorine and pH were measured, despite hyperchlorination the previous day, was the defective chlorinator discovered. Repair of the defective chlorination system and return of the pool water to recommended disinfection standards were associated with resolution of this outbreak. The findings underscore the importance of free residual chlorine concentration and proper pH in the prevention of illnesses associated with recreational water use.

Although no obvious source of norovirus was determined, the epidemic curve and laboratory data were consistent with a single contamination event such as fecal incontinence that occurred on either Friday night or Saturday morning. Previous outbreaks of enteric infections associated with recreational water have occurred with no obvious contamination event (3,4).

Norovirus remains the most common cause of epidemic gastroenteritis in the United States, causing an estimated 23 million cases each year (5). Challenges to prevention of norovirus-associated outbreaks include the low infectious dose, the multiple modes of transmission (e.g., person-to-person, foodborne, and waterborne), the absence of long-lasting immunity, and the diversity of strains that do not confer heterotypic protection. Although waterborne outbreaks of norovirus gastroenteritis are much less commonly reported than foodborne outbreaks (6), 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 (7).

Pool-care guidelines are available from the National Spa and Pool Institute (2) and the state of Vermont; however, use of these guidelines is voluntary. In addition, CDC provides guidelines on how to avoid the risk for infectious illness when swimming (8). 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, a series of environmental health failures occurred, creating conditions that could convey almost any waterborne pathogen. Findings from this investigation highlight the need for review of appropriate guidelines and methods to ensure pools are properly maintained and underscore the utility of environmental health investigations for providing data for development of prevention guidelines.


  1. Snow J. On the mode of communication of cholera. 2nd ed. In: Snow on Cholera. (Reprint). New York, New York: The Commonwealth Fund, 1936:11--39.
  2. National Spa and Pool Institute. Standard for Public Swimming Pools. Alexandria, Virginia: National Spa and Pool Institute, 1991.
  3. Friedman MS, Roels T, Koehler JE, Feldman L, Bibb WF, Blake P. Escherichia coli 0157:H7 outbreak associated with an improperly chlorinated swimming pool. Clin Infect Dis 1999;29:298--303.
  4. Hoebe CJ, Vennema H, de Roda Husman AM, van Duynhoven YT. Norovirus outbreak among primary schoolchildren who had played in a recreational water fountain. J Infect Dis 2004;189:699--705.
  5. Mead PS, Slutsker L, Dietz V, et al. Food-related illness and death in the United States. Emerg Infect Dis 1999;5:607--25.
  6. CDC. Norwalk-like viruses: public health consequences and outbreak management. MMWR 2001;50(No. RR-9).
  7. CDC. Surveillance for waterborne-disease outbreaks---United States, 1999--2000. In: Surveillance Summaries, November 22, 2002. MMWR 2002;51(No. SS-8).
  8. CDC. Healthy swimming 2004. Available at


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