A recent outbreak represents the first dengue cases acquired in the continental United States outside of the Texas-Mexico border since 1945 and the first locally acquired cases in Florida since 1934. Concern about the potential for emergence of dengue in the continental United States has increased in recent years. Reported dengue cases in South America, Central America, Mexico, and the Caribbean increased fourfold, from 1,033,417 during 1980–1989 to 4,759,007 during 2000–2007. Rapid urbanization with a proliferation of man-made containers able to serve as mosquito-breeding sites, increased international travel, and lack of effective vector-control measures likely have been major factors in the spread of dengue. Since 1980, seven localized outbreaks have occurred along the Texas-Mexico border. The most efficient mosquito vector, Ae. aegypti, is found in the southern and southeastern United States. A secondary vector, Ae. albopictus, has spread throughout the southeastern United States since its introduction in 1985 and was responsible for a dengue outbreak in Hawaii in 2001, likely after the virus was introduced by a Hawaii resident returning from Tahiti.
Cases of dengue in returning U.S. travelers have increased steadily during the past 20 years. Dengue is now the leading cause of acute febrile illness in U.S. travelers returning from the Caribbean, South America, and Asia. Many of these travelers are still viremic upon return to the United States and potentially capable of introducing dengue virus into a community with competent mosquito vectors. Because of concerns over the increasing number of travel-associated dengue infections, the risk for local transmission upon introduction of the virus, and the risk for potential transmission of the virus by blood transfusion, the Council of State and Territorial Epidemiologists (CSTE) made dengue a nationally notifiable disease in 2009.
Many dengue infections, particularly in children, cause no symptoms or a nonspecific febrile illness, but dengue infection also can cause classic dengue fever or severe life-threatening disease (e.g., dengue hemorrhagic fever or dengue shock syndrome). Laboratory confirmation of dengue infection can be obtained by viral isolation or identification of dengue virus by dengue-specific PCR in a specimen collected within the first 5 days of illness (an acute phase specimen), or seroconversion demonstrated between a paired acute phase specimen and a convalescent phase specimen (collected within 6–30 days of illness onset). Dengue NS-1 also can be detected within the first 10 days after symptom onset by an assay that is currently not approved by the Food and Drug Administration. Probable recent dengue cases are defined by identification of dengue IgM antibodies in a single specimen. The dengue case definition and additional information regarding dengue diagnosis and reporting are available at http://www.cste.org/ps2009/09-id-19.pdf .
Why dengue has reemerged in Florida at this time is unknown. Dengue might have been present in the community earlier and is only now being detected. The environmental and social conditions for dengue transmission have long been present in south Florida: the potential for introduction of virus from returning travelers and visitors, the abundant presence of a competent mosquito vector, a largely nonimmune population, and sufficient opportunity for mosquitoes to bite humans. The increased volume of international travel has been implicated in the spread of dengue globally, and the popularity of south Florida as a tourist destination enhances the likelihood of virus introduction and subsequent local transmission. The volume of domestic visitors to the area also might increase the risk for localized transmission in other parts of the United States with competent mosquito vectors. The reemergence of dengue in Florida as well as the threat posed to the United States from other emerging mosquito-borne arboviruses (e.g., chikungunya) emphasizes the necessity for strong vector-borne surveillance and mosquito control infrastructure to rapidly identify and control outbreaks of dengue or other mosquito-borne diseases.
The timely reporting of dengue in the index patient from New York illustrates that, despite an absence of compatible travel history, clinicians throughout the United States should consider appropriate laboratory testing based upon clinical presentation. Had the index patient not been evaluated promptly and reported, the cases in Key West residents likely would not have been diagnosed. Dengue should be included in the differential diagnosis of acute febrile illnesses for patients who live in or have recently traveled to subtropical areas in the United States or to the tropics. This is particularly important when signs and symptoms such as thrombocytopenia, leukopenia, hemoconcentration, rash, or eye pain are present. Prompt reporting of suspected dengue cases to public health authorities can facilitate a coordinated response resulting in detection of locally acquired cases or helping to define new areas of transmission. Additional information regarding dengue prevention, diagnosis, and management is available at http://www.cdc.gov/dengue.