South Florida has mostly dodged an outbreak of locally spread dengue fever as the Gulf Coast wrestles with the largest surge in the mosquito-borne disease in the state in decades.
So far, Hillsborough County has confirmed 223 cases, more than seven times the number of other counties combined. Broward County confirmed its first locally transmitted case this week, with eight so far documented in Miami-Dade County and one in Palm Beach County. None have been detected in the Keys.
Mosquito control experts say this year’s unusual weather conditions — extreme drought along the Gulf Coast followed by heavy rain in August — played a part in an explosion of mosquitoes.
”Once you have this daily rainfall, then all the containers pretty much fill up with rain water every day,” said Peter Jiang, head of mosquito control for Hillsborough County, said Tuesday at a press conference including officials from six county mosquito control districts. “That's why our mosquito population increased significantly for the month of August.”
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Dengue is spread by invasive Aedes aegypti mosquitoes. Unlike native marsh mosquitoes that don’t carry disease, Aedes aegypti live close to humans, laying eggs in places that collect water. That close proximity has helped spread infectious diseases around the world, including outbreaks of Zika and chikungunya in southeast Florida.
Expansion of mosquito control programs have mostly helped knock back the diseases. Florida has averaged fewer than 50 cases a year over the last decade.
But in late July, counties began reporting their first cases of locally spread infections: two in Hillsborough County, one in Miami-Dade County and one in Palm Beach County. By the first week of September, the number had surged to 152 cases in Hillsborough County alone, according to Florida Department of Health records.
That week, an 80-year-old woman also died from dengue-related complications, the only death so far connected to the outbreak.
The end of the rainy season and arrival of cooler, drier weather typically signals the end of the mosquito season. But forecasts for powerful El Niño conditions this winter have experts wary.
”Mosquito populations are driven by temperature and rainfall. If we do have an excessively wet winter, that is going to continue to allow mosquito production to occur at a higher level,” said Adriane Rogers, director of the Pasco County Mosquito Control District.
To control populations, counties regularly lend resources to neighboring counties. This year, Lee County sent three helicopters to Hillsborough to help with spraying, director Aaron LLoyd said. Workers have also been making inspections urging residents to dump and cover outdoor containers to eliminate standing water and prevent mosquitoes from laying more eggs.
But once an outbreak starts, it can be hard to stop, said Carla Fry, administrator for the Florida Department of Health in Hillsborough County.
“ We have so many folks who are walking around who have dengue. They're pretty much asymptomatic. They don't even know that they have it,” she said. “Then when they're bitten by a mosquito, that mosquito takes in a blood meal and after about seven days, when that mosquito bites another person, they are transmitting dengue to someone else.”
That also means that the disease can spread beyond where infected mosquitos are concentrated, she said.
”We have over 18 zip codes with dengue positive cases, right? So this is not just a problem in the urban core,” she said. “This is a problem throughout our county. It's beyond our county. It's in Pinellas, it's in Pasco. You know, so we're not seeing it just in one place.”
The Centers for Disease Control has said it is sending $800,000 to help fight the problem, but Fry said so far the money has not arrived.
The outbreak also arrives just as mosquito control districts face a significant loss in funding if voters approve a November ballot measure — Amendment 3 — which would significantly cut residential property tax revenue.
The Florida Mosquito Control Association has not officially taken a position on the amendment, said Keira Lucas, deputy director of the Collier County Mosquito Control District, but the current outbreak “is a pretty timely reminder of the important work that mosquito control plays in protecting public health throughout the state.”
