Floridians rarely get West Nile, so weather, spraying, air conditioning or mosquitoes' tastes must be the reason.
By LISA GREENE
Published October 13, 2003
Florida is mosquito heaven.
It's hot. It's wet. Mosquitoes - at least 68 species frolic across the state - bite here year-round.
And yet it's not Florida, but Colorado, that faces this year's big epidemic of West Nile Virus. The sometimes-deadly bug arrived in the United States in 1999 and has spread across the country with birds on the wing and mosquitoes on the prowl.
Florida has a long history of mosquito-borne diseases and past outbreaks. But even though large numbers of Florida birds became infected with West Nile two years ago, the result you might expect, a human epidemic, just hasn't happened.
"Isn't that amazing?" said Julia Gill, epidemiology program manager for the Pinellas County Health Department. "We as scientists have a lot to learn about how this disease is going."
Even in Colorado, Dr. Lyle Petersen, a leading West Nile expert, has noticed that "there's something weird" about Florida.
It's in the birds. In the horses.
Even in the alligators.
Just not the people.
Or at least, not very many of them. By last week, more than 6,600 U.S. residents had gotten the disease this year, and about 140 had died. Nebraska has more than 1,100 cases. South Dakota, close to 900. And Colorado, more than 2,100 cases.
At last count, this year Florida has seen 58 cases and five deaths - more cases than in the past, but still comparatively few. Last year, the epicenter of the disease was Illinois, with nearly 900 cases, while Florida had 28.
"This has been the most interesting part of the whole situation ... why Florida seems to have missed the thousands of cases that have occurred," said Lillian Stark, virology administrator for the Bureau of Laboratories of the state Department of Health in Tampa. "We don't know why. Hopefully if we can continue to study it, we can learn more about the virus."
Stark co-wrote a research paper documenting the paradox of Florida's 2001 West Nile cases: nearly 600 cases in horses and chickens, but only a dozen in humans.
Scientists have theories - from the habits of mosquitoes to humans to those of Mother Nature - about why Florida has escaped.
But they also say that Florida could see an epidemic in the future.
"If we have - and I think we will - an event comparable to Colorado's, we'll end up with easily 3,000 cases. And if you consider the age structure (of Florida's population), it may even be worse," said Jonathan Day, professor at the Florida Medical Entomology Laboratory of the University of Florida.
That's because older people are more likely to develop severe symptoms from West Nile. Most people infected develop no symptoms at all. About 20 percent of people develop mild, flulike symptoms, such as headache, fever, fatigue, nausea and a rash.
But about one person in 150 develops severe disease that can lead to coma, paralysis or death. Scientists are working to develop a vaccine and other treatments, but don't have one yet.
Milder outbreaks of the disease have occurred around the world since at least 1937. But when it arrived in the United States in 1999, the virus became more virulent, and more likely to affect the central nervous system, said Petersen, who is a deputy director for the vector-borne disease division of the federal Centers for Disease Control and Prevention.
In the United States, the virus has been found in at least 43 species of mosquitoes and 170 species of birds. It also been has found in at least 18 other animals, from squirrels to dogs to reindeer.
Unfortunately, now that the disease is established in American mosquitoes, it's tough to stamp out, Petersen said.
"It's here to stay," he said.
There likely will be more cases this year in Florida. But just when, and whether, it will reach epidemic level here remains a mystery.
"It may be mosquitoes; it may be weather," said Carina Blackmore, acting state public health veterinarian for the Florida Department of Health. "It may be birds. It may be the way people behave - or any combination of all of those."
One of the biggest factors: the weather. Mosquitoes need water to lay their eggs and let their larvae hatch. But it takes the right amount of rain, at the right time, to spread the virus, said Day, the entomologist.
The virus spreads primarily between mosquitoes and birds. Infected mosquitoes bite birds, which become infected, in turn infecting other mosquitoes who bite them. Scientists call this spin cycle of spiraling disease the "amplification period."
For the virus to amplify here, Day said, Florida's spring needs to be dry, so that mosquitoes and birds cluster together around shrinking water sources. Then the weather needs to become wet, so that the infected mosquitoes go out, breed, and roam the state infecting people.
The state had that weather pattern in 1977 and again in 1990, Day said, when there were epidemics of St. Louis encephalitis. But for the past two years, Mother Nature hasn't been kind to mosquitoes. 2001 and 2002 were too dry, while last spring was too wet.
"Everything hasn't come together in a way that allows a large number of infected mosquitoes," Day said.
Then there are Florida's mosquitoes.
"All mosquitoes, surprisingly enough, don't like to feed on people," Blackmore said.
It might feel as if they do, but some mosquitoes bite birds, while others prefer mammal blood. Some snack on both. The mosquito wreaking havoc in Colorado is Culex tarsalis, an "avid feeder on people" that also travels more than most species, said Colorado veterinarian Sue Montgomery, an epidemic intelligence service officer studying the disease with CDC.
Florida also may have fewer cases because it has more mosquito police than most states, Montgomery said.
"Here, mosquitoes have been nuisances," she said. "In Colorado, the focus on mosquito control has not been as intense as in Florida."
In Pinellas, for example, the county's mosquito control division seeks out tidal pools, retention ponds, ditches and other wet areas and tests them for mosquito larvae, killing them with a pesticide that's targeted to interact only with the acid level in mosquito stomachs.
Control workers also test sentinel chickens for West Nile and similar diseases, fog to kill adult mosquitoes and set traps to monitor the mosquito population. Last week's traps caught 2,209.
"Because it is so wet and warm here, we have to have mosquito control or we wouldn't be able to live here," said Nancy Page, program manager for Pinellas County Mosquito Control.
There's another possibility for the low disease numbers, Stark said. Studies haven't nailed this one down, but it sounds appealing.
"There's more air conditioning in Florida than other parts of the country, and that might play a part," Stark said.
If you buy this theory, Florida residents just spend less time in Skeeterland. While Colorado residents hike the mountains, Floridians walk the malls.
Scientists warn that, even though Florida has few human cases right now, many Florida mosquitoes are already infected. People need to take precautions to avoid being bitten, such as using repellent with DEET and avoiding the outdoors at dusk and dawn.
It's a message that Petersen, the West Nile expert, stresses. He's become known in mosquito circles for having contracted the disease himself. A quick walk to the mailbox turned into a longer chat with a neighbor and several mosquito bites. Petersen hadn't put on DEET, even though he lives in Colorado.
"It just shows how difficult it is to get a prevention message across," he joked.
In Florida, Stacy Perkins also learned the lesson too well. Perkins' grandfather, Frank Hanson, gained the dubious honor of being the first Citrus County resident to get the disease. In September, it began with sudden weakness in his legs and progressed to breathing difficulty. Hanson nearly died and is still recuperating in a rehabilitation center.
Hanson, a 67-year-old retired police officer, had been in good health, spending time mowing his Crystal River lawn and barbecuing.
"He doesn't remember getting bit," Perkins said. "I never thought anything like this would happen."
