“That was the winter when the H1N1 pandemic flu was circulating, and whatever dengue cases we had would have been masked by that and went unrecognized,” says Danielle Stanek of the Florida Department of Health. “When the flu settled down and we realized there were still dengue cases, that was a wake-up call for us.”
Local spread of dengue is still happening in Florida. On Key West, 5 percent of people show immunologic evidence of having had a dengue infection, and the disease is found farther north as well. This year, four residents caught “locally acquired” dengue, two in Miami and two near Orlando, Fla. Another 112 were diagnosed with dengue they had caught somewhere else and brought there.
The CDC’s experts assume there are more cases that haven’t been counted, and not just in Florida. “When you’re seeing a patient early on, dengue looks like a lot of other acute (fever-causing) illnesses,” says Kay Tomashek, chief of epidemiology in the agency’s dengue branch. “If you are a physician in New York and you see a patient with fever, headache, and muscle pain, you might not be thinking about that.”
Detecting imported cases is important because the more frequently the disease comes across the border, the more risks from it increase. And not just the risk of catching the disease. There are four types of dengue, distributed unevenly across the tropics and subtropical zone. Becoming infected with any one causes the classic breakbone fever. But if you acquire and recover from one type and then contract a different type even years later, you are more likely to develop the disease’s worst version, dengue hemorrhagic fever. DHF disrupts the circulation, sends patients into shock, and kills up to 1 in 5.
The U.S. outbreaks to date, as well as the locally adapted Key West strain, are all caused by the first type, known as DEN-1. But 10 of the imported cases in Florida this year were in tourists from Central and South America, where DEN-2, DEN-3, and DEN-4 circulate as well.
Could more dengue outbreaks happen? To spark one, you need three things. First, imported virus: check. Second, a population with no immunity. The United States has that, since dengue was last widespread in the 1940s. And third, mosquitoes that can transmit it. Those are already widespread.
The spraying campaigns that ended U.S. epidemics of malaria and dengue in the 1940s turned out to be only a temporary solution. National eradication programs petered out in 1972, and the main dengue vector, Aedes aegypti, quickly returned; it is now in 23 states and ranges as far north as New York City. In 1985, a second species that can spread dengue—Aedes albopictus, better known as the Asian tiger mosquito—arrived in Texas in a shipment of used tires from the Pacific Rim that had been stored outdoors and held puddles of rainwater. It is now in 26 states and has been found as far north as Chicago. A. albopictus is what entomologists call a “less competent” vector; it doesn’t spread the disease as efficiently as A. aegypti does. But it has other abilities that have huge significance for disease transmission: It bites all day long, not just at dawn and dusk, and it can survive both winter temperatures and drought.
Because there is no vaccine for dengue, the best hope of stopping its advance relies on individual action, such as getting people to wear repellent and persuading them to scour their homes and properties for small puddles—underneath a planter, inside a tiki torch—after every rain. Or convincing them to stay inside. Researchers theorize the 2005 Brownsville outbreak was smaller than the 2009 Key West because of the “Texas lifestyle” of sealed, air-conditioned houses—so different from the patio culture of Hawaii and Key West.
It’s impossible to say, at this point, if climate change will move the risks of dengue farther north. Researchers disagree on whether higher temperatures automatically mean bigger mosquito populations, since the insects are also affected by unpredictable changes in rainfall, humidity, and wind. But barring some other factor that no one can foresee, the experts agree: Dengue is coming.
“It may not swamp the entire U.S.,” Adalja acknowledged. “But the entire South already harbors those mosquitoes, and that is bad enough. Dengue shouldn’t have to swamp the entire country for us to make it a priority.”