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Puerto Rico Declares Dengue Fever Epidemic

By Amesh A. Adalja, MD, March 22, 2010

On January 26, 2010, an epidemic of dengue fever was declared in Puerto Rico after 210 cases were recorded during the month—triple the number of cases in January 2007. Of the cases reported, only 3 were of the hemorrhagic variety. No fatalities have been reported.1

Virus that Causes Dengue Fever Is Spread by Mosquitoes

Dengue fever is caused by infection with the dengue fever virus, an RNA virus that is a member of the flavivirus family of viruses. It is most commonly spread by the bite of the Aedes aegypti mosquito (also a vector for the yellow fever virus), but other Aedes mosquitoes can transmit the virus as well.2 Figure 1 illustrates the distribution of the mosquito.3 There are 4 serotypes of the virus, and infection with one type does not confer immunity to the other serotypes.2

Clinical Manifestations

Infection with the dengue fever virus can cause 1 of 3 clinical presentations: classic dengue fever, dengue hemorrhagic fever (DHF), or dengue shock syndrome.2

Classic dengue feverFever, muscle aches, headaches, and rash; can be asymptomatic
Dengue hemorrhagic fever/dengue shock syndrome (DHF/DSS)Classic symptoms plus hypotension, thrombocytopenia, and hemorrhagic symptoms

The usual course of illness is that a first encounter with the virus will present with classic dengue fever. Subsequent exposures to other serotypes of the virus could result in DHF/DSS by means of a phenomenon known as antibody-dependent immune enhancement—allowing increased viral uptake/infection by cells.2 More recent evidence suggests that the antibodies also facilitate immune evasion by the virus.4

Diagnosis Is Usually Clinical

In areas where dengue fever is common, clinical diagnosis is the norm; however, because of the importance of identifying the serotype responsible for infection, PCR testing is recommended. Serologic testing for antibodies is also used. The tourniquet test—in which a blood pressure cuff is inflated and the resultant number of petechiae generated on the forearm is quantified—is a criterion for diagnosing DHF, but this method suffers from poor specificity.2

Treatment Is Supportive

No antiviral treatment is currently available to treat dengue virus infections. Treatment chiefly consists of antipyretics and rehydration.2

No Vaccine Exists

No vaccine exists to prevent dengue virus infection, so prevention is centered on mosquito control.2 Vaccine development has been hampered by the fact that the vaccine must be tetravalent to avoid mediating DHF by stimulating the production of antibodies that would enhance the pathogenicity of non-included serotypes.5

Other Caribbean Islands also Harbor the Virus

The situation in Puerto Rico is not unique. The population of the vector mosquito has rebounded since the 1970s, allowing the virus to become endemic in Latin America and the Caribbean in countries including Cuba, the Dominican Republic, and Martinique. In response to the rising incidence of dengue fever, in 1997 the Pan American Health Organization (PAHO) drafted a hemispheric eradication plan.3,6

U.S. Risks

The majority of cases of dengue fever in the United States occur among returning travelers. Between 2001 and 2007, 796 cases of dengue were reported in the United States. Indigenous cases have occurred in Hawaii, Texas, and New Mexico, areas in which A. aegypti is present.3

While the relatively small number of cases that occur in the US is manageable, sustained transmission of the virus in Puerto Rico is concerning given the travel that occurs between the island and the mainland of the US. Viremic travelers to areas where Aedes mosquitoes are present could provide an enhanced opportunity for the virus to establish residence on the mainland.


  1. Puerto Rico declares epidemic of dengue fever. Associated Press February 26, 2010. Accessed March 13, 2010.

  2. Vaughn DW, Barrett A, Solomon T. Flaviviruses (yellow fever, dengue, dengue hemorrhagic fever, Japanese encephalitis, West Nile encephalitis, St. Louis encephalitis, tick-borne encephalitis). In: Mandell GL, Bennett JE, Dolin R, eds. Mandell, Douglas, and Bennett’s Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, PA: Churchill Livingstone; 2010:2133-2156.

  3. WHO. Dengue: Guidelines for Treatment, Prevention and Control. 2009. Accessed March 13, 2010.

  4. Ubol S, Phuklia W, Kalayanarooj S, Modhiran N. Mechanisms of immune evasion induced by a complex of dengue virus and preexisting enhancing antibodies. J Infect Dis 2010;201:923-935. Accessed March 13, 2010.

  5. PAHO. Dengue. Accessed March 13, 2010.

  6. Cardosa MJ. Dengue vaccine design: issues and challenges. Br Med Bull 1998;54:395-405. Accessed March 15, 2010.

Figure 1: A. aegypti distribution
A. aegypti distribution image