After rains in South Florida, mosquito boom raises health concerns

The storms of the last few weeks promise a boom in mosquitoes — South Florida’s most persistent, painful pests and a growing public health concern. Some places are already abuzz.

“It’s definitely been the most active season we’ve seen in recent years,’’ said Andrea Leal, operations director for the Florida Keys Mosquito Control District.

In the Keys, bordered by mangroves and tidal marshes that are prime breeding grounds, the bite has come early and often. A district worker’s “landing rate” count of 50 — the number lighting on a bare arm in a minute — in Key Largo early this month launched spray trucks and planes. The goal is no more than three in an island chain that depends on tourists seeking tans not welts. Breezes keeping skeeters in the Everglades have helped spare Miami-Dade and Broward — so far.

For mosquito agencies and public health authorities across the state, the surge is shaping up as another season of increasing concern about mosquito-borne diseases like West Nile virus, malaria, Eastern equine encephalitis and dengue fever, which has become a rising risk in South Florida.

The flu-like virus, sometimes called “break-bone fever” because of the excruciating joint pain it can cause, emerged in Key West in 2009 with 27 cases from mosquito bites, the first in Florida since 1934. Another 66 cases were recorded in 2010; none since. But a handful have since cropped up in Miami-Dade, Broward and Palm Beach counties.

With dengue spreading globally and running at epidemic levels in many Latin and Caribbean countries, public health experts expect the disease to slowly spread in Florida, with its large populations of immigrants and international travelers. The virus is carried in the human bloodstream but transmitted by the bite of a common and difficult-to-control mosquito called Aedes aegypti.

“Miami-Dade is an entry gate,’’ said Pedro Noya-Chaveco, a biological scientist for the Miami-Dade County Health Department. “We have a lot of people traveling to endemic areas. People come here for a visit and go to other places.’’

Though there have only been a handful of infections traced directly to mosquitoes since Key West’s two small outbreaks, the state’s “imported cases” have continued to creep up each year. Through June 1, the Florida Department of Health has recorded 38, with Miami-Dade leading with 12 cases, followed by Orange with eight, Palm Beach with six and Broward with three.

Dengue effects vary widely. A mild headache for some can be a high fever, rash and severe pain in the eyes, joints and bones for others. A small percentage of cases can develop life-threatening internal bleeding called hemorrhagic fever. A first infection is rarely serious but it can be dangerous for infants or the elderly.

Some experts fear the next outbreak in Florida could be larger and more severe — particularly if it happens again in Key West.

In a recent edition of BuzzWords, the quarterly newsletter of the Florida Mosquito Control Association, University of Florida entomologist Walter Tabachnick warned that Key West was facing “a potentially catastrophic epidemic.’’

“It is almost certain that failure to aggressively combat dengue will result in many sick people and some may die as a result,’’ Tabachnick wrote. Such an event, he said, would be a double economic whammy, discouraging tourism and running up healthcare costs.

Tabachnick, director of UF’s medical entomology laboratory in Vero Beach, isn’t predicting an imminent outbreak but he argues the risk factors for Key West have been greatly increased because of an after-effect of an initial infection.

There are four dengue strains. Antibodies left in the blood of victims can make them more susceptible to a new strain. In Key West, victims were exposed to one strain but all three others have since been detected in the state.

Though fewer than 100 Key West residents were officially classified with dengue in 2009 and 2010, blood tests conducted by the U.S. Centers for Disease Control and Prevention showed much higher exposure. By Tabachnick’s calculations, as many as 4,000 people could be vulnerable to a second, more serious bout.

“I don’t think Key West as a community is prepared for that,’’ said Tabachnick, who has called for fining Key West residents who don’t drain pots and other household containers where Aedes aegypti commonly lay eggs. “I think Key West as a community believes they have dealt with it.’’

Florida Keys mosquito control and public health managers share Tabachnick’s concerns — to an extent — but say the community has mounted a public awareness campaign that has reduced the risks.

Mark Whiteside, medical director of the Monroe County Health Department, called Tabachnick’s warnings “a bit of a scare tactic.’’

Every victim recovered and the city is entering a third summer without a documented case, he said. Residents need to stay alert, he said, but the risk of another outbreak “doesn’t keep me up at night.’’

“At least we know what to do,’’ he said. “We have more experience with it than anybody in the United States.’’

Leal, the Keys district’s operations director, called the potential of another strain making its way to Key West “a huge concern for us’’ and said her agency had expanded operations to target dengue-carrying mosquitoes.

The district’s chief weapon against the most common pest, the black salt marsh mosquito, is larvicide. But insecticides sprayed from trucks and planes to control adults aren’t as effective on aegypti, which often dodges fogging in sheltered areas and bites during the day.

Leal said district workers try to visit every home in the city once a month to check for undrained containers but the district also is testing larvicide use in the city and awaiting federal approval to test a genetically modified mosquito designed to breed with and kill aegypti, a controversial option.

Dengue is just one of the diseases mosquitoes can carry. This year, the Florida DOH has recorded two people infected with eastern equine encephalitis in Levy and Hillsborough counties, which are both under mosquito-borne illness advisories, as well as seven sick horses in other Central Florida counties. Dozens of “sentinel chickens” monitored by the department also have tested positive for exposures to other diseases across North and Central Florida. South Florida has recorded no reports so far but mosquito season typically doesn’t peak until at least July.

For mosquito control agencies, the recent storms compound problems. Rains can hatch dormant eggs and provide additional breeding areas. Mosquitoes grow into biting adults in three to five days.

Storms can also restrict the operations. Agencies can’t spray in the rain and storms can ground planes.

“I would have had aircraft up if the weather wasn’t so bad,’’ Leal said late last week as Tropical Storm Andrea’s feeder bands swept across South Florida.

In Miami-Dade and Broward counties, calls from residents are rising but not at an alarming rate. They are expected to pick up, particularly if winds shifts and Everglades mosquitoes drift into suburbs.

“Wind direction helps us tremendously,’’ said Manuel Garcia, director of Miami-Dade’s mosquito control division. “It also hurts us tremendously when the winds come out of the west.’’