Emma Hitt, PhD

July 16, 2010

July 16, 2010 (Atlanta, Georgia) — Nearly 5% of the population of Key West, Florida, showed evidence of recent exposure to dengue virus in 2009, according to a study from the Centers for Disease Control and Prevention (CDC).

Glen R. Gallagher, from the CDC's National Center for Zoonotic, Vector-Borne, and Enteric Diseases (NCVED), in Puerto Rico, and colleagues from the Florida Department of Health presented the findings in a poster session here at the International Conference on Emerging Infectious Diseases 2010.

First Dengue Fever Reports Surfaced in September 2009

"After a 75-year absence, dengue virus reemerged in Florida in 2009," the authors note in their abstract. Three cases of dengue fever were transmitted within Key West and were reported in early September 2009. The initial case was recorded by a New York state physician who suspected dengue in a patient whose only recent travel had been to Key West. Within the subsequent 2 weeks, 2 dengue infections in Key West residents without recent travel were reported and confirmed. By the end of 2009, 27 cases had been identified.

As a result of these reports, the Florida Department of Health conducted a seroprevalence study in which 240 participants completed a questionnaire to ascertain their risk for previous dengue exposure and provided a blood sample.

Blood samples were tested for the presence of virus or evidence of a previous dengue infection. In addition, enzyme-linked immunosorbent assay was used to detect antidengue immunoglobulin (Ig)M and IgG antibodies, indicative of recent infection. Serum from participants who reported febrile symptoms within the previous 7 days also underwent molecular testing for dengue nonstructural protein 1 to identify acute infections. The infecting dengue serotype was determined for all IgM- and IgG-positive samples.

Of the survey participants, 41% showed evidence of a previous infection, and 13 participants showed evidence of either an acute, recent, or presumptive recent dengue infection.

Nearly 5% Incidence of Acute or Recent Dengue Infection

On the basis of these findings, the weighted estimate of acute and recent infections for the study was 4.9% (95% confidence interval, 1.8 - 7.9). Dengue virus serotype 1 was determined to be the predominant serotype, which was genetically similar to a previously identified strain from Mexico.

"The message for clinicians is to put dengue in your differential diagnoses if a patient presents with acute febrile illness, rash, and retroorbital pain," Harold Margolis, MD, chief of the dengue branch, NCVED, at CDC, told Medscape Medical News in a phone interview. "Consider also where your patient has traveled. The most common cause of febrile illness in patients returning from Asia and Latin America is dengue, not malaria."

He added that patients should be monitored for hemorrhagic shock, which typically occurs as the fever is disappearing. At that point, fluid management is indicated.

According to Dr. Margolis, it is unclear if or how the dengue virus will spread. "There are places in the world, such as Southern Taiwan and Queensland in Northern Australia, where the dengue virus gets reintroduced and dies out annually," he said. "We don't know what has happened in the Keys — whether this was an isolated introduction or whether it has been there for a while and has gone unnoticed. Dengue is a highly underrecognized illness, and it could possibly have been there for some period of time," Dr. Margolis said.

Independent commentator Michael S. Diamond, MD, PhD, professor of internal medicine and dengue researcher at Washington University, in St. Louis, Missouri, pointed out that the findings are not too surprising, since dengue was "previously endemic in parts of the United States and has intermittently surfaced in Florida and Texas."

Most Dengue Symptoms Are Subclinical

He told Medscape Medical News that many dengue infections are either subclinical or mild, and difficult to distinguish from common "flu-like" illnesses. "In the field, we think of it as a pyramid, with many of the mild cases on the bottom (higher percentage) and only a smaller number of severe cases (lower percentage) on the top," he said.

Dr. Diamond added that dengue is transmitted from mosquito to human to mosquito, so it has the potential for rapid outbreaks if enough mosquitoes get infected. "It is not uncommon in outbreak settings to have an incidence of 5% or more, with a significant fraction not aware they have been infected," he said.

Not Possible to Predict Spread

According to Dr. Diamond, it is not possible to predict the spread of dengue since the humans and mosquitoes are present, but the ecological factors are complicated. Clinicians should be aware that if febrile illnesses occur during peak mosquito season, dengue may occur." As of now there is no approved treatment or vaccine," he said.

He also pointed out that the greatest risk for severe disease occurs after the second infection with dengue. Thus, if "dengue is present in the community, for now the prudent message should be to initiate behaviors that minimize mosquito bites," such as draining standing water, using repellants, and wearing long-sleeved clothing.

According to background information from the CDC, dengue is the most common virus transmitted worldwide by mosquitoes and causes an estimated 50 million to 100 million infections and 25,000 deaths each year. No cases of dengue were reported from 1946 to 1980 in the continental United States. The last outbreak in Florida was in 1934.

The researchers and commentator have disclosed no relevant financial relationships.

International Conference on Emerging Infectious Diseases (ICEID) 2010. Presented July 13, 2010.