FRIDAY, Dec. 16 (HealthDay News) -- While the alarming re-emergence in 2009 and 2010 of mosquito-borne dengue fever in the continental United States seems to have subsided, that's no reason to believe the potentially deadly infection won't be back, experts warn.

The outbreak of the sometimes-excruciating viral illness centered on southern Florida. Now, researchers have issued an update on the situation for one locale in particular, Key West.

"We know now that Key West is a high-risk area for dengue and we could have ongoing dengue outbreaks again," said the report's lead author, Carina Blackmore, from the Florida Department of Health. However, if people use air conditioners and screens and stay inside during hot, muggy days there is little chance dengue will become endemic, she said.

Dengue remains a leading cause of illness and death in tropical areas but was largely thought to be absent from the United States since the 1950s.

However, in 2009, 27 people living in Key West came down with illness via locally acquired infections, and then 66 more residents contracted the illness in 2010, the researchers report. The outbreak seems to have eased since then, with no cases reported in 2011.

That doesn't mean that dengue is eliminated from the population, however, because around 75 percent of people infected never develop symptoms. Blackmore and her colleagues estimate, therefore, that about 5 percent of people living in Key West neighborhoods where cases occurred could be infected.

Because Key West has a large population of the type of mosquitoes that transmit dengue, called the "house mosquito," Blackmore's team decided to investigate the size of the outbreak there. They identified a number of cases and found that people who got dengue were less likely to use air conditioning, and they often had birdbaths or other types of containers where the mosquitoes could breed.

Blackmore noted that dengue is not transmitted person to person, but from humans to mosquitoes and then back to humans again. However, trying to eradicate house mosquitoes has never been successful, she said, because of where they tend to propagate. "House mosquitoes are lazy mosquitoes -- they breed in [even] very small containers," she said.

The report appears in the January issue of Emerging Infectious Diseases, which is published by the U.S. Centers for Disease Control and Prevention (CDC).

Dr. Hal Margolis, chief of the CDC's dengue branch, said that most dengue that appears in the United States is still brought back by people who have traveled to areas in the world where the diseases is endemic. "There are thousands of people who come back with dengue. That's really the biggest problem," he said.

There are also sporadic outbreaks along the Texas/Mexican border, Margolis said. In addition, dengue is endemic in some areas of the United States such as Puerto Rico, the U.S. Virgin Islands and Asian possessions such as Guam and American Samoa, he said.

The Key West outbreak was unusual in that it lasted for two seasons, Margolis said. "Frankly, we don't know if it is still there," he added. "How it got introduced, we don't know."

Dengue could still become endemic in Florida, Margolis said. "We won't know for several seasons. Only time will tell us; it's really had to predict," he said.

The disease can cause a high fever and people can feel sicker than they have ever felt before, Margolis said. "The danger comes in those people who get severe dengue; that usually happens with a second or third infection," he said. "Twenty-five percent of people who have first infections may go on to have severe dengue."

In severe dengue, plasma leaks out of the blood vessels, ending up around the lungs and abdomen, and sufferers can develop shock, Margolis said. About 15 percent of people have these severe signs, he said. About 1 percent may die, he added.

The biggest hope for prevention lies with a vaccine, Margolis said.

"There is a lot of effort on dengue vaccines going on, but it's going to be another three or four years before a vaccine is approved," he said. There are vaccines currently in clinical trials, he added.

Trying to control the mosquitoes to curb infections has not proven to be all that effective, he said. People who have air conditioning or screened windows may be at lower risk, since a closed house keeps the flying insects at bay.

Infectious disease expert Dr. Marc Siegel, an associate professor of medicine at New York University in New York City, agreed with the experts' warnings. "I wouldn't be surprised to see more cases," he said.

The problem is that the mosquitoes in Key West are now carrying the disease, which makes it more likely that there will be more outbreaks, Siegel said.

More information

For more information on dengue, visit the U.S. Centers for Disease Control and Prevention.