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Aseptic Meningitis Outbreak Associated With Echovirus 9 Among Recreational Vehicle Campers --- Connecticut, 2003

Aseptic meningitis is an inflammation of the tissues covering the brain and spinal cord and caused by a virus, most frequently an enterovirus (1). In August 2003, the Connecticut Department of Public Health (CDPH) received a report of three viral meningitis cases among recreational vehicle (RV) campers staying at a campground in northeastern Connecticut. CDPH, assisted by CDC, conducted an investigation, which 1) identified a total of 12 cases of aseptic meningitis and 24 cases of enterovirus-like illness among 201 campers interviewed, 2) demonstrated how transmission of enterovirus from persons with mild illness contributed to the aseptic meningitis outbreak, and 3) determined that crowded conditions inside RVs and in the campground swimming pool likely facilitated spread of enterovirus. Pool operators should check chlorine and pH levels frequently, particularly during peak pool occupancy; adults should take precautions against passing enterovirus to children, who are at greater risk for severe illness.

A retrospective cohort study was conducted of seasonal campers (i.e., campers who rented RV sites for the entire summer) via personal interview. Any person who stayed at least one night at a seasonal campsite during the study period was considered a cohort member. Response rate was estimated by using the mean number of campers per campsite to estimate the number at campsites where campers could not be contacted. A meningitis patient was defined as a seasonal camper with headache and either neck stiffness or photophobia, with illness onset during July 16--August 17, 2003. Laboratory-confirmed cases additionally had >10 white blood cells/µL identified by CSF analysis. Other acute, self-limited illnesses consistent with enteroviral infections also were identified during the outbreak period. A case of enterovirus-like illness was defined clinically as an acute illness with any one of the following symptoms: headache, neck stiffness, photophobia, sore throat, chills, or exanthem (i.e., acute generalized skin rash) in a seasonal camper with illness onset during July 16--August 17. Primary campsite illnesses were defined as the first acute illness of either case type at a campsite. Univariate and multivariate logistic regression analyses were used to identify predictors for primary illness. Enterovirus polymerase chain reaction (PCR) and amplicon sequencing (2) were performed at CDC.

Of an estimated 239 seasonal campers, 201 (84%) completed the study questionnaire. Among the 201 campers, 12 cases of meningitis and 24 other cases of enterovirus-like illness were identified (attack rates: 6% for meningitis and 18% for all illness). Four meningitis patients were hospitalized and had CSF analysis that confirmed aseptic meningitis. At CDC, PCR tests on CSF from three of these patients detected enterovirus serotype echovirus 9 in two samples. The CSF sample for the fourth patient was positive for enterovirus by PCR at a clinical laboratory.

The median age was lower for meningitis patients (14 years; range: 3--42 years) and enterovirus-like illness patients (15 years; range: 3--64 years) than for well campers (38 years; range: 8 months--80 years). Patients were more likely to be female among both patient groups (67%) than among well campers (40%). Illness duration was longer for meningitis patients (median: 7 days; range: 2--28 days) than for enterovirus-like illness patients (median: 3 days; range: 1--21 days). Among the 12 meningitis patients, the most common symptoms were headache (12), stiff neck (10), nausea (10), and photophobia (eight). Among the 24 patients with enterovirus-like illness, the most common symptoms were sore throat (15), headache (12), cough (seven), and diarrhea (six).

Dates of illness onset for meningitis and other enterovirus-like illness cases were similar and clustered in four peaks, 6--8 days apart (Figure). Two enterovirus-like illness cases from a single campsite preceded the first illness peak by 8 days. Four children hospitalized with laboratory-confirmed aseptic meningitis came from four different campsites. Mothers of three of these children had an enterovirus-like illness with onset 6--8 days before their child's illness onset. The secondary campsite attack rate (29%) was greater than the primary rate (9%), and attack rates were higher at campsites with more campers per site: one to two campers per site, 6% (two of 36); three to four campers, 16% (12 of 74); five to six campers, 17% (eight of 47); seven to eight campers, 21% (six of 28); nine to 10 campers, 50% (eight of 16) (age-adjusted trend analysis; p<0.05). Primary illness was associated with younger age (odds ratio [OR] for each additional decade of age = 0.77; 95% confidence interval = 0.59--0.99).

Increasing frequency of submerging one's head in the campground pool during the outbreak period was associated with increased risk for primary illness of either case type (age-adjusted OR = 3.3 for one--five times, 5.9 for six--15 times, 6.1 for >16 times; p<0.05). Campers reported that the pool often was crowded at midday (e.g., "wall-to-wall" swimmers), particularly during weekends. An automated chlorine feeder with stabilized cyanurated chlorine was in use at the pool throughout the day. Chlorine levels were checked twice a day (i.e., at approximately 7 a.m. and 8 p.m.) with a handheld test kit. According to written records, chlorine levels were low (0.5--1.0 mg/L versus the required level of >1.5 mg/L) almost every evening throughout late July and August. The pool operator was not certified by a national certification group.

At the time of the initial outbreak report, campground staff were advised to ensure adequate pool chlorination and to clean and disinfect common areas (e.g., bathrooms, bathhouses, and game room). Through printed bulletins and informational postings, all campers were directed to 1) wash their hands frequently, especially after bathroom use, diaper changes, and before eating or preparing food; 2) avoid sharing eating utensils and drinking containers; and 3) shower before using the swimming pool. Parents were instructed to keep children with febrile illness at their campsites until fever and other symptoms resolved.

Reported by: D Waite, MD, Day Kimball Hospital, Putnam; P Beckenhaupt, MPH, Northeast District Dept of Health, Danielson; L LoBianco, MPH, P Mshar, MPH, A Nepaul, MA, K Marshall, MPH, T Brennan, JL Hadler, MD, Connecticut Dept of Public Health. WA Nix, M Pallansch, PhD, Div of Viral and Rickettsial Diseases, National Center for Infectious Dieases; EM Begier, MD, EIS Officer, CDC.

Editorial Note:

In this aseptic meningitis outbreak, community spread was associated with swimming in a crowded campground pool. Chlorine levels were low in the evening; hot sun and high occupancy likely reduced chlorine levels during the day (3), allowing the pool water to become intermittently contaminated with enterovirus.

The cohort's high attack rate was likely facilitated by secondary intrahousehold enterovirus spread among residents of the same campsite promoted by crowding; risk for illness was higher in campsites with more campers. Crowding results in more frequent person-to-person contact and possibly less personal hygiene (e.g., hand washing). Meningitis illness began in three hospitalized patients, one incubation period after their mothers' enterovirus-like illnesses, suggesting that intrahousehold spread from adults to children was a source for several more serious infections.

Echovirus 9 was the predominate enterovirus serotype circulating in the eastern United States during 2003 (4) and was identified as the likely etiologic agent in this outbreak. Enterovirus activity typically peaks in temperate climates during the summer and early fall (5). Based on meningitis data from Connecticut's hospital admission syndromic surveillance system, the outbreak occurred in the weeks preceding widespread community enteroviral transmission in eastern Connecticut. Enterovirus infections usually are mild illnesses; a small proportion result in aseptic meningitis (5).

The findings in this report are subject to at least two limitations. First, although the cases of enterovirus-like illness were consistent clinically with echovirus infection and were linked temporally and epidemically to echovirus 9 aseptic meningitis cases, no laboratory confirmation was attempted. Second, because the pool was closed for the season by the time interview results were analyzed, pool water was not tested for enterovirus.

Ongoing contamination of pool water with enterovirus likely facilitated community transmission. Connecticut's public health code requires that water be tested when a pool opens each day and then with sufficient frequency during bather use to ensure that an adequate disinfection level and pH are maintained. The free available chlorine residual should be >1.5 mg/L for stabilized cyanurated chlorine, substantially higher than the 0.8 mg/L required for unstabilized chlorine. Given that an automated chlorine feeder was in use, the frequent low evening chlorine levels suggest little chlorine was available earlier in the day during peak bather usage. This outbreak underscores the importance of testing chlorine and pH during peak pool occupancy, even if levels are appropriate when a pool is opened.

Swimming has been associated with other enteroviral outbreaks (6--8) and other infectious disease outbreaks (9). To reduce swimming-associated illness, CDC recommends that 1) staff frequently check pool water chlorine and pH levels, particularly during periods of heavy bather use, 2) persons with diarrhea avoid swimming, 3) swimmers shower before pool use and avoid swallowing pool water, and 4) children be taken to restrooms frequently (3). In addition, CDC recommends improving pool testing, pool staff training, and public education on appropriate pool use to prevent recreational water-related illness (10).

Children are at greater risk for severe manifestations of enteroviral infection, including aseptic meningitis (1); adults with enteroviral infection are more likely to experience upper respiratory or "cold" symptoms only. Hygienic precautions need to be taken within households as well as among other community members. Enterovirus is shed in the saliva and feces of infected persons (5). To minimize viral spread to children in their care, ill caregivers should wash their hands thoroughly after toilet use and avoid sharing drinks and utensils.


  1. Rotbart HA. Viral meningitis. Semin Neurol 2000;20:277--92.
  2. Oberste MS, Nix WA, Maher K, Pallansch MA. Improved molecular identification of enteroviruses by RT-PCR and amplicon sequencing. J Clin Virol 2003;26:375--7.
  3. CDC. Healthy swimming. Your disinfection team: chlorine & pH. Atlanta, Georgia: U.S. Department of Health and Human Services, CDC, 2004. Available at
  4. CDC. Outbreaks of aseptic meningitis associated with echoviruses 9 and 30 and preliminary surveillance reports on enterovirus activity---United States, 2003. MMWR 2003;52:761--4.
  5. American Academy of Pediatrics. Enterovirus (nonpoliovirus) infections. In: Pickering LK, ed. Red Book: 2003 report of the Committee on Infectious Diseases, 26th ed. Elk Grove Village, Illinois: American Academy of Pediatrics, 2003:269--70.
  6. Kee F, McElroy G, Stewart D, Coyle P, Watson J. A community outbreak of echovirus infection associated with an outdoor swimming pool. J Pub Health Med 1994;16:145--8.
  7. Lenaway DD, Brockmann R, Dolan GJ, Cruz-Uribe F. An outbreak of an enterovirus-like illness at a community wading pool: implications for public health inspection programs. Am J Public Health 1989; 79:889--90.
  8. Hawley HB, Morin DP, Geraghty ME, Tomkow J, Phillips CA. Coxsackievirus B epidemic at a boys' summer camp: isolation of virus from swimming water. JAMA 1973;226:33--6.
  9. CDC. Surveillance for waterborne-disease outbreaks---United States, 1999--2000. In: CDC Surveillance Summaries (November 22). MMWR 2002;51(No. SS-8).
  10. CDC. Surveillance data from swimming pool inspections---selected states and counties, United States, May--September 2002. MMWR 2003;52:513--6.


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