Imported Dengue -- Florida, 1997-1998

Dengue fever is a viral disease transmitted primarily by the Aedes aegypti mosquito. There are four antigenically distinct serotypes of dengue virus (DEN-1, DEN-2, DEN-3 and DEN-4). Infection with any serotype may lead to an acute illness characterized by fever, headache, bone and joint pain and, occasionally, rash and hemorrhagic manifestations (1). Secondary infection with a different serotype can lead to a more serious form of the disease (i.e., dengue hemorrhagic fever [DHF]). Dengue incidence has recently increased in the Caribbean and Central America (2), including Cuba and the Bahamas, which are within 100 miles of Florida, and might increase the likelihood of its future autochthonous transmission in Florida. This report summarizes information about cases of imported dengue detected as a result of a laboratory-based active surveillance program implemented in Florida from April 1, 1997, through March 31, 1998.

Dengue surveillance program elements included implementation of an education program focusing on county health departments and commercial clinical laboratories, and enhancing the state laboratory's diagnostic capabilities. Dengue information packets were mailed to all 67 county health department epidemiologists in Florida. Packets contained a letter explaining the program and requesting participation, along with instructions for distributing the enclosed materials to hospital emergency departments, clinics, health departments, and infectious disease physicians within the county. The letter included a reminder that dengue is a reportable disease in Florida and that testing would be performed free of charge. The dengue case definition, specimen requirements and transport instructions, and a dengue case investigation form were supplied.

Cooperative agreements were made with commercial clinical laboratories to promote submission of dengue samples. Under the agreements, specimens from suspected dengue cases were forwarded to the state laboratory for testing. In cases where specimens were tested at commercial laboratories only, dengue antibody positive results were forwarded to county health departments and then to the state laboratory for inclusion in this study.

State laboratory capabilities were enhanced to include testing for anti-dengue IgM antibodies. Acute and convalescent serum specimens were tested for dengue antibodies using the hemagglutination inhibition assay and IgM antibody capture enzyme linked immunosorbent assay (3,4). Specimens positive for IgM antibodies were forwarded to the Dengue Branch, CDC, in San Juan, Puerto Rico, for confirmation of serologic results, and acute phase samples were forwarded to CDC for virus isolation or identification by polymerase chain reaction (PCR) (5,6).

During the 12 months of active surveillance, 83 suspected dengue cases were investigated in Florida. Commercial clinical laboratories referred specimens from 36 (43%) of these cases. The remaining specimens were referred through county health departments, hospital laboratories, infection-control practitioners, or directly from physicians. Recent dengue infection was laboratory-diagnosed in 18 (22%) of these cases. Thirteen (72%) of the 18 positive dengue specimens were referred to the study by commercial laboratories. All four dengue serotypes were detected (by virus isolation and/or identification by PCR) in five of the cases studied. Dengue was ruled out as the etiologic agent in 24 (29%) cases. The remaining 41 (49%) cases were indeterminate because of a lack of convalescent serum samples.

The age of laboratory-confirmed case-patients ranged from 8 to 69 years (median: 38 years), and 14 (78%) were male. Antibody titers were suggestive of secondary dengue infection in 10 (56%) of the 18 cases. Two (11%) appeared to be primary infections, and laboratory tests necessary to determine infection status (primary versus secondary) were not available in the other six cases. Hemorrhagic manifestations were reported in seven (39%) of the laboratory-confirmed cases, one of which met the case criteria for DHF.

All case-patients reported recent (i.e., within 10 days before onset of illness) travel from countries with indigenous dengue transmission, and no local transmission was detected in Florida. The origin of travel of case-patients was Haiti (six), Puerto Rico (three), Colombia (two), Venezuela (two), Barbados (one), Nicaragua (one), and Thailand (one). The two other case-patients did not indicate a specific travel destination but reportedly had visited countries where dengue occurs. Dengue cases were detected in Dade (eight), Hillsborough (four), Orange (three), Palm Beach (two), and Broward (one) counties.

Reported by: J Gill, PhD, LM Stark, PhD, S Wiersma, MD, Bur of Laboratories and Epidemiology, Florida Dept of Health. Dengue Br, Div of Vector-Borne Infectious Diseases, National Center for Infectious Diseases, CDC.

Editorial Note:

Local transmission of dengue was last documented in Florida in 1934 (7). Although no local transmission of dengue was detected in Florida during this investigation, many southern states may be at risk for transmission; dengue transmission has been detected in Texas (8). Two mosquito vectors (Aedes aegypti and Ae albopictus) are widely distributed in Florida, and many infected travelers return from areas where dengue is endemic and the resident population has essentially no immunity to dengue viruses.

Autochthonous transmission may result from importation of viremic cases to counties with Ae aegypti or Ae albopictus. This possibility should especially be considered if cases are reported from such localities over several years and if case-patients from these localities report travel to a country where dengue is endemic. Epidemiologic data from imported cases should be shared on a timely basis with mosquito abatement programs to ensure an entomologic evaluation and appropriate control response by the locality where the case-patients reside. On the basis of the results of this study, surveillance efforts should be concentrated in counties with large populations and large numbers of international travelers and should intensify during dengue season (i.e., July-November) in the Caribbean because of the large number of case-patients who travel to this area.

The findings in this report indicate that dengue infections were imported into Florida in 1997 and 1998 more frequently than expected, based on the 10-year mean of 1.3 cases per year. In this and previous investigations, dengue has been underreported (9,10). Underreporting is common with passive surveillance systems. Active surveillance for dengue requires that state health departments educate the medical community and provide appropriate diagnostic laboratory support (8). Surveillance efforts should be enhanced in the high-risk areas identified in this study. Other states should consider enhanced dengue surveillance in areas with widespread mosquito vectors and large numbers of travelers returning from areas with endemic dengue.

References

1. Hayes EB, Gubler D. Dengue and dengue hemorrhagic fever. Pediatr Infect Dis J 1992;11:311-7.
2. Pan American Health Organization. Re-emergence of dengue in the Americas. Epidemiological Bulletin, PAHO 1997;18:1-10.
3. Clarke DH, Casals J. Techniques for hemagglutination and hemagglutination-inhibition with arthropod-borne viruses. Am J Trop Med Hyg 1958;7:561-77.
4. Innis BL, Nisalak A, Nimmannitya S, et al. An enzyme-linked immunosorbent assay to characterize dengue infections where dengue and Japanese encephalitis co-circulate. Am J Trop Med Hyg 1989;40:418-27.
5. Gubler DJ, Kuno G, Sather GE, Velez M, Oliver A. Mosquito cell cultures and specific monoclonal antibodies in surveillance for dengue viruses. Am J Trop Med Hyg 1984;33:158-65.
6. Lanciotti RS, Calisher CH, Gubler DJ, Chang GJ, Vorndam AV. Rapid detection and typing of dengue viruses from clinical samples by using reverse transcriptase-polymerase chain reaction. J Clin Microbiol 1992;30:545-51.
7. Ehrenkranz NJ, Ventura AK, Cuadrado RR, Pond WL, Porter JE. Pandemic dengue in Caribbean countries and the southern United States--past, present and potential problems. N Engl J Med 1971;285:1460-9.
8. Rawlings RA, Hendricks KA, Burgess CR, et al. Dengue surveillance in Texas, 1995. Am J Trop Med Hyg 1998;59:95-9.
9. Karp BE. Dengue fever: a risk to travelers. Maryland Med J 1997;46:299-302.
10. Lyerla R, Rigau-Perez JG, Vorndam AV, et al. A dengue outbreak among camp participants in a Caribbean island, 1995. J Travel Med 1999 (in press).

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