May 20, 2010 -- More than two dozen cases of locally-acquired dengue fever have hit the resort town of Key West, Fla., in the past nine months, officials from the U.S Centers for Disease Control and Prevention said.

Although not the first cases of home-grown dengue in the U.S., or even in Florida, the outbreak highlights the need for physician vigilance regarding this and other formerly exotic tropical diseases, the CDC said in the May 21 issue of Morbidity and Mortality Weekly Report.

"The re-emergence of dengue in Florida as well as the threat posed to the U.S. 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," MMWR's editors wrote in a commentary accompanying the report.

Dengue fever is a viral infection transmitted by mosquito bites. It can be debilitating, but is not usually fatal in otherwise healthy people.

It is endemic in the Western Hemisphere from Mexico southward. Most cases seen by U.S. physicians have involved travelers to such regions.

Over the past 30 years, a few cases of locally-acquired dengue have been confirmed along the Texas-Mexico border, according to the report, authored by CDC researchers, public health officials in Florida, and physicians who treated the first cases in the new Key West outbreak.

Dengue was also known in Florida in the 1930s, but no locally-acquired cases had been confirmed since then, until last August.

The first case was actually identified in Rochester, N.Y., involving a 34-year-old woman who had just returned from a week-long visit to Key West.

The day after arriving back in Rochester, she went to her doctor complaining of fever, headache, malaise, and chills. Lab analysis showed bacteria and blood in her urine.

Not surprisingly, her primary care physician and a local emergency department did not initially suspect dengue. The presumptive diagnosis was a urinary tract infection and she was treated accordingly.

Some of her symptoms resolved, but a week later she returned to her primary care doctor saying she didn't "feel right," the MMWR report said. An infectious disease specialist was called in, who inquired about travel to dengue-endemic areas.

She denied having been out of the country, but said she had received multiple mosquito bites during her Key West visit, and she was tested for dengue antibodies, which came back positive. Public health officials in the county where Key West is located were informed, and they notified local physicians and the public of the potential for locally acquired dengue.

As a result, several other recent cases of febrile illness with other symptoms such as headache, myalgia, chills, and vomiting, were discovered to be dengue.

In response, according to the MMWR report, officials in the Keys stepped up mosquito control efforts, going door-to-door to find and eradicate breeding sites. They also encouraged the public to take steps to prevent mosquito bites.

Officials conducted random serological testing in local households, finding that about 5 percent of 240 individuals carried dengue antibodies.

Key West physicians were also contacted to provide serum specimens from recent patients with symptoms consistent with dengue. Nine of 21 samples sent for testing came back positive for dengue.

The tally of confirmed dengue infections in Key West eventually grew to 28, with the most recent case diagnosed in mid-April.

Dengue is much more of a problem elsewhere in the hemisphere, with an estimated 4.6 million cases from 2000 to 2007 in Mexico, the Caribbean, and Central and South America, according to MMWR's editors.

But the Key West outbreak demonstrates the potential for significant penetration into the continental U.S., they suggested.

They noted that international travelers can pick up the virus in endemic areas, and are often still viremic when they return. In areas with mosquito species capable of transmitting the virus -- such as Aedes aegypti and Aedes albopictus, both present in the southern and southeastern U.S. -- dengue can then spread locally.

The editors also recommended that physicians consider dengue when evaluating any patient with a febrile illness who has recently been in subtropical areas of the U.S., as well as in countries to the south.

"This is particularly important when signs and symptoms such as thrombocytopenia, leukopenia, hemoconcentration, rash, or eye pain are present," they wrote.

Suspected cases should also be reported promptly to public health authorities, they indicated.

No external funding for the work was reported.

No potential conflicts of interest were reported.