Norovirus Outbreak Associated with Swimming in a Recreational Spring Fed Lake

BACKGROUND:  On April 25, 2011 the Glades County Health Department (GCHD) received calls from several groups of concerned citizens that individuals from their groups had become ill with symptoms of diarrhea and vomiting after camping in a local campground with a fresh water swimming area over the Easter holiday weekend.  The GCHD initiated an investigation.

METHODS:  The Food and Waterborne Disease Program (FWDP) developed a standardized questionnaire for a case-control study.  Telephone interviews were conducted with 76 of the 99 registrants.  A case was defined as any person who visited the campground between April 21 and April 24, 2011 and became ill with vomiting and/or diarrhea (defined as 3 or more loose stools within 24 hours) within 72 hours.  Two stool specimens were collected and submitted to the Bureau of Public Health Laboratories.  GCHD conducted an environmental assessment of the campground.  Water samples from the swimming area were collected and the swim area was temporarily closed.  Resulting data were analyzed using Epi Info 3.5.1™.

RESULTS:  Twenty-nine individuals (38.2%) met the case definition.  Seventeen (58.6%) of the cases were male; ages ranged from 8 months – 43 years, median 12 years.  The incubation period ranged from 24 - 72 hours.  Two people reported having gastrointestinal symptoms within 2 weeks prior to visiting the campground and were asymptomatic.  Four people reported someone vomiting in the water and 1 person reported having diarrhea prior to swimming.  None of the cases were hospitalized and 3 (10%) sought medical care.  Results of statistical analysis identified swimming on April 23 as being a statistically significant risk factor (OR=20.7, CI=2.60-165.49, p-value=0.00010).  One stool specimen was positive for norovirus genogroup II.  Water samples of the swimming area were satisfactory and the swim area was reopened.

CONCLUSIONS:  Swimming in the recreational lake was epidemiologically implicated as the source of this outbreak.  No other commonalities were identified among the ill campers.  The reported onset dates was chronologically clustered indicating a common source exposure.  The incubation period, symptomology, duration of illness, and laboratory results from the stool specimen implicate norovirus as the causative agent for this outbreak.  Norovirus is a highly contagious pathogen with a very low infectious dose.   Individuals should refrain from swimming while ill, avoid getting water in their mouth while swimming, and practice good hygiene to reduce the possibility of developing a recreational water illness.