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BACKGROUND: Local introductions of dengue have been identified annually in Florida since a sustained outbreak occurred from 2009-2010 in Key West. A cluster of three locally-acquired cases of dengue fever with exposure in Martin County, Florida was reported to the Florida Department of Health in August 2013. Mosquito control efforts were enhanced immediately. The outbreak investigation identified 21 cases in two geographically separate areas, with the earliest onset date in mid-June. A seroprevalence survey was launched in September 2013 to determine the extent of the outbreak, determine whether transmission was ongoing, and identify potential risk factors.
METHODS: The survey employed two different sampling strategies. A random sample of households within a one kilometer radius of each index case exposure address was selected. In addition, a cluster sampling of all businesses within a 100 meter radius of the two index exposure sites was conducted. All residents or employees of selected households and businesses were asked to complete a questionnaire concerning dengue risk factors, as well as participant medical and travel histories. Participants were also asked to provide blood samples for dengue IgM and IgG enzyme-linked immunosorbent assay (ELISA), reverse transcriptase polymerase chain reaction (RT-PCR), and plaque-reduction neutralizing antibody testing (PRNT). Data will be analyzed using Chi-squared or Fisher exact tests, weighted logistic regression modeling to assess risk factors. Variables associated with infection in a univariate analysis (P≤0.05) will be included in the multivariate model.
RESULTS: The serosurvey included 364 individual household participants from 256 households and 41 employees from 11 businesses. Six (1.6%) had evidence of current (positive RT-PCR) or recent dengue infection (positive dengue IgM ELISA). One of the six had recent symptoms compatible with dengue fever, bringing the total clinical case count to 22. PRNT testing of sera from 53 individuals is currently being completed and will likely yield additional cases. Individual, business, and household level environmental and behavioral risk factors will be assessed in the final analysis.
CONCLUSIONS: After completion of the PRNT testing, the estimated incidence of dengue in the two areas of activity will be calculated. No cases have been identified in Martin County since mid-September 2013; however, passive surveillance is still ongoing. This incident illustrates that the potential for sustained outbreaks of dengue exists in Florida outside the Florida Keys.