This patient, who recently travelled to the Caribbean, has symptoms characteristic of dengue fever (DF), a flavivirus transmitted by the Aedes mosquito. DF is the most common arthropod-borne viral infection in humans. Endemic areas include Southeast Asia, the South Pacific, South and Central America, and the Caribbean. Most infections encountered in the United States are diagnosed in persons visiting or returning from endemic geographic areas. Recently, the incidence of infection from the Caribbean islands has increased. The hallmarks of DF are sudden high fever, frontal headache and retro-orbital pain, myalgia and arthralgia, severe lower back pain (“break-bone fever”), and a maculopapular rash that often appears as the fever abates. Mild hemorrhagic symptoms and the development of petechiae, elicited by a tourniquet test, can occur. A more severe and rare form of infection (dengue hemorrhagic fever) can be seen in patients previously infected by one of the four dengue virus serotypes who subsequently become infected with another serotype. Typical laboratory findings in DF include leukopenia with relative lymphocytosis, hemoconcentration, thrombocytopenia, and elevation of hepatocellular enzymes. Diagnosis confirmation generally relies on specific serologic assays.

Human granulocytic anaplasmosis is one of the forms of ehrlichiosis transmitted by ticks. This febrile illness is accompanied by laboratory abnormalities similar to those seen with DF; however, rash with human granulocytic anaplasmosis is rare. This obligate, intracellular, gram-negative organism can be seen in the cytoplasm of neutrophils in 20% to 80% of infected patients. Although ehrlichiosis can occur worldwide, the Ixodes tick vector is infrequently found in the Caribbean. The diagnosis usually is made using serologic assays.

Chikungunya virus is also transmitted by the bite of an Aedes mosquito and recently has become an epidemic in several Caribbean islands. Although it shares many clinical features with DF, an important distinction is its severe joint pain and stiffness. Other distinguishing features include high fever, which often recurs after a brief afebrile period (“saddle-back fever”); more significant polyarticular and migratory joint pains involving the small joints of the hands, wrists, and ankles; and much less thrombocytopenia.

Leptospirosis may be confused with DF but typically involves more organ systems, including the pulmonary, renal, hepatic, and central nervous systems. Characteristically, conjunctival suffusion occurs but a generalized rash is rare. Aseptic meningitis is a common feature of the immune or delayed phase of this spirochetal infection. A more severe form of infection, known as icteric leptospirosis (Weil syndrome), presents with profound jaundice, hepatic necrosis, kidney disease, and pulmonary manifestations.