Dengue Outbreak in Key West, Florida, USA, 2009

Elizabeth G. Radke, Christopher J. Gregory, Kristina W. Kintziger, Erin K. Sauber-Schatz, Elizabeth A. Hunsperger, Glen R. Gallagher, Jean M. Barber, Brad J. Biggerstaff, Danielle R. Stanek, Kay M. Tomashek, and Carina G.M. Blackmore

Author affiliations: Florida Department of Health, Tallahassee, Florida, USA (E.G. Radke, K.W. Kintziger, E.K. Sauber-Schatz, D.R. Stanek, C.G.M. Blackmore); Centers for Disease Control and Prevention, San Juan, Puerto Rico (C.J. Gregory, E.A. Hunsperger, G.R. Gallagher, K.M. Tomashek); Centers for Disease Control and Prevention, Atlanta, Georgia, USA (E.K. Sauber-Schatz); Monroe County Health Department, Key West, Florida, USA (J.M. Barber); Centers for Disease Control and Prevention, Fort Collins, CO, USA (B.J. Biggerstaff)

Abstract

After 3 dengue cases were acquired in Key West, Florida, we conducted a serosurvey to determine the scope of the outbreak. Thirteen residents showed recent infection (infection rate 5%; 90% CI 2%–8%), demonstrating the reemergence of dengue in Florida. Increased awareness of dengue among health care providers is needed.

Dengue is the most common mosquito-borne viral infection worldwide (1); however, it has been eliminated from the continental United States. No locally acquired cases have been reported outside the Texas–Mexico border (2,3) in >60 years, despite global increases in incidence and severity. Concern exists that dengue virus (DENV) may be reintroduced (4) because it is the most frequent cause of febrile illness among travelers returning from the Caribbean, South America, and Asia (5). Immigrants and visitors from dengue-endemic countries also provide opportunity for its reintroduction (6).

In September 2009, the Florida Department of Health (FDOH) was notified that a person with suspected dengue had recently traveled to Key West, Florida. The Centers for Disease Control and Prevention (CDC) (Atlanta, GA, USA) confirmed the diagnosis. Subsequently, dengue was confirmed in 2 Key West residents without a history of recent travel. FDOH and CDC conducted an investigation to determine size and scope of the outbreak. Cases identified through passive surveillance were reported (7).

The Study

Key West is a tourist destination with >2 million visitors annually (8). Old Town, the area with reported cases, has a population of 19,846 (9). Aedes aegypti mosquitoes, the usual vectors of dengue, are widespread in Key West, but Ae. albopictus mosquitoes are uncommon.

In September 2009, we surveyed Old Town residents to estimate the infection rate and identify risk factors using stratified, 1-stage cluster sampling to randomly select 911 (15%) households within 1 km of the residence of the index case-patient. The area around the residences of the index case-patients was divided into 3 strata: strata 1 and 2 (within 200 m of each case-patient) and stratum 3 (201–1,000 m). Investigators asked household members (>5 years of age) for a blood sample and information on recent illness, travel, foreign residence, and risk factors for dengue. One adult per household completed a questionnaire concerning the household. Investigators revisited unresponsive households 3 times unless the homes were empty.

Serum specimens were screened by ELISA for dengue-specific IgM and IgG (10,11). IgG-positive samples were tested by plaque reduction neutralization test with 90% cutoff (PRNT₉₀) against DENV serotypes 1–4 and West Nile virus (12). A >4-fold difference in titer between viruses was used to identify the infecting virus. Participants reporting febrile illness within a week were tested by reverse transcription PCR for DENV and West Nile virus and by nonstructural protein 1 ELISA for DENV antigen.
We classified participants with laboratory-positive DENV infection as follows: acute, if positive with reverse transcription PCR or nonstructural protein 1 ELISA; recent, if IgM-positive ELISA and PRNT₉₀ results were consistent with DENV infection; and presumptive recent, if they had dengue-like illness within 3 months, IgG-positive ELISA, and PRNT₉₀ results consistent with DENV infection. We classified participants as having previous DENV infection if they had IgG-positive ELISA and PRNT₉₀ results without recent febrile illness.

We weighted responses to account for sampling design using different probabilities of inclusion across strata and within-household participation rates, allowing for population inference (13). CIs accounted for sampling design and finite population correction factors. We used weighted logistic regression to assess risk factors for infection, and resulting inferences accounted for sampling design. Tests were performed in SAS version 9.2 (SAS Institute, Cary, NC, USA); p = 0.10 was significant.

Informed consent was obtained from all participants >18 years of age. Assent from the minor and informed consent from a parent were obtained for minors.

Of 911 selected households, 200 (22%) had been vacated, 387 (42%) did not have a resident at home, and 324 (36%) had a resident contacted; 170 (52%) households and 240 persons participated. Median age was 53 years (range 15–95), slightly older than the median population (41 years) of Old Town (14). Most participants were non-Hispanic white (78%) and male (58%), similar to Old Town’s population (14). Forty-three (18%) had lived in dengue-endemic countries, and most (148, 62%) had previously traveled to dengue-endemic areas.