May 20, 2010 - An "extended outbreak" of dengue fever is ongoing in Key West, Fla., where some 5% of residents were infected last fall.
The latest case of the mosquito-borne disease was in mid-April. It's not yet clear whether the April case is a continuation of the 2009 outbreak or a new outbreak from a different dengue strain.
Although only 28 cases have been definitively identified, blood tests conducted in September 2009 detected evidence of recent infection in 5.4% of 240 randomly selected residents.
"The best estimate from the survey is that about 5% of the population of Key West was infected in 2009 with dengue," dengue expert Christopher J. Gregory, MD, of the CDC's Epidemic Intelligence Service, tells WebMD.
"We had hoped we'd be able to eliminate the disease from Key West and took every effort to do so," Robert Eadie, administrator of the Monroe County Health Department, said in an April news release. "However, the [newly] confirmed case is not totally unexpected in that once dengue fever has been established in an area, it is truly almost impossible to completely eradicate it."
Once a rare disease, dengue has been spreading around the globe at an alarming rate. It's become entrenched -- endemic, as infectious disease experts say -- in Mexico and in the Caribbean, including Puerto Rico where Gregory is based.
The last U.S. "extended outbreak" was in Hawaii in 2001-2002. Except for small outbreaks along the Mexico border (linked to outbreaks in Mexican border cities), dengue has so far spared the continental U.S. That appears to be changing.
"The biggest thing people need to be aware of that it is possible to acquire dengue in the continental United States. That is widely underappreciated," Gregory says. "We have known for a while it is a possible risk, but this outbreak shows it is more than possible: It is something that did happen and could happen again."
In areas where dengue virus has become endemic, outbreaks tend to occur every six to eight months.
Despite the outbreak, neither the CDC nor the Florida Department of Health has issued travel warnings. Chris Tittel, spokesman for the Monroe County Health Department (which includes Key West), says health officials are simply stressing personal protection.
"We are just telling folks who come to visit that dengue is here, but not every mosquito carries dengue," Tittel tells WebMD. "We advise that people should not be overly concerned, but be aware and avoid mosquitoes."
The Monroe County Health Department has issued a health advisory urging residents to reduce mosquito-breeding areas, to repair windows and screens, to use air-conditioning, and to wear mosquito repellent.
Symptoms of Dengue Fever
Dengue fever ranges from asymptomatic infection (most common in school-age children) to, in rare cases, a fatal hemorrhagic disease. There is no vaccine and no specific treatment.
Illness begins three to 14 days after a bite from a mosquito carrying the virus. Symptoms may include:
- Rapid onset of high fever
- Severe frontal headache
- Bone pain
- Pain behind the eyes
- Aching muscles or joints
- Signs of bleeding (such as pinpoint red or purple spots on the skin, nosebleed, bleeding gums, blood in urine or stool, or vaginal bleeding)
- Nausea or vomiting
Many people mistake the symptoms of dengue fever for those of flu, says infectious disease specialist Mark Whiteside, MD, MPH, medical director for the Monroe County Health Department.
"It is a bad flu-type illness, and dengue's old 'breakbone fever' nickname comes from the fever and chills," Whiteside tells WebMD. "You are miserable and you might wish you were dead, but you get over it in a week or two."
Even so, fatigue and loss of appetite may linger for weeks after recovery.
There are four types of dengue virus; the Key West outbreak was type 1. A person cannot get the same type of dengue twice. However, a person who gets dengue a second time, from a different dengue strain, is at risk of severe disease. That's because antibodies to the first dengue strain fail to protect and end up enhancing infection by the second strain.
That's one of the things that worries Whiteside.
"It's often asymptomatic in kids, and I worry that many kids who may have been infected during this outbreak will get another subtype if we have a different outbreak," he says.
To keep that from happening, Key West has been conducting an extensive mosquito-control program. There's been spraying to kill adult mosquitoes, but that's not particularly effective against the Aedes aegypti and Aedes albopictus mosquitoes that carry dengue.
Both of these mosquito types live close to people's houses. They can breed in a teaspoon of water.
Experts believe that when 2% or fewer breeding sites harbor Aedes mosquitoes, disease transmission wanes. That goal is elusive. Whiteside says that a February 2010 survey of Key West breeding pools -- the time when Aedes is supposed to be at its lowest levels -- found Aedes breeding in 10% of homes.
"That is high for winter, but the same index in Old Town [the center of Key West tourism] is 15% to 20%, which is unacceptable," Whiteside says.
A beefed-up mosquito-control effort has been conducting house-to-house sweeps to bring the situation under control.
A report on the Key West dengue outbreak appears in the May 21 issue of the CDC's Morbidity and Mortality Weekly Report .