The threat is significant enough that the Keys district is hoping to test a genetically modified mosquito designed to breed with and kill aegypti, a controversial treatment option that has not yet received state and federal approval.

So far this year, flocks of “sentinel chickens’’ monitored by the Florida Department of Health have shown no early indicators of mosquito-borne viruses.

But the risk of an outbreak dictates that agencies stay ahead of the swarms with aerial spraying and trucks that target concentrated hot spots, said Joseph Marhefka, manager of Broward County’s mosquito control section.

“The best protection from mosquito-transmitted diseases is avoiding exposure to mosquito bites,’’ he said.

The weather is what largely shapes South Florida’s annual mosquito season - both where and when skeeters strike.

“We’re constantly watching it,’’ said Vasquez. “We’re playing with two things here, rainfall and the direction of the winds.’’

Rains hatch dormant eggs laid in moist soil and just about anything that holds water, from planters to bottle caps. One day, winds can blow hungry salt marsh mosquitoes from far-off locations, like Flamingo in Everglades National Park or from Elliott Key in Biscayne National, into suburbs. They next day, winds can shift and blow them back out. That makes border areas like the Redland hotspots in most seasons, said Vasquez.

While there have been pockets of heavy infestation, the mosquito season so far is notable mainly for its early start. Judging by landing rates tests, traps and resident complaints, the numbers are up a bit in some spots, down in others.

Miami-Dade fielded about 300 calls last week for treatment. Last July, Vasquez said, there were 6,000 calls.

In the Keys, the hot spot ran from Key Largo down to Upper Matecumbe, where the landing rate ran 7.69 mosquitoes, Doyle said. Last year, the highs ran around 20, he said.

The Keys district, with an annual operating budget of $11 million, spent more on control than any other county in the state. Miami-Dade, in contrast, spends about $1.5 million. Much of that goes into year-round larvicide treatments that dramatically knock back the adult population in inhabited areas, Doyle said.

The Keys routinely run tests at 250 “landing rate” sites from Key West to Key Largo, with employees selecting 15 to 20 sites to visit every morning. At protected areas where spraying is banned, landing rates in a minute might run into the hundreds, he said. Traps in places like Everglades National Park can produce a pound of mosquitoes overnight, hundreds of thousands or more.

“Our efforts suppress them by the billions,’’ he said. “The ones we’re seeing are the ones that got away.’’

Whatever the landing rates tests indicate, there are plenty of anecdotal reports of a heavy mosquito crop this year.

Shelby Moneysmith, a biologist at Biscayne National, said swarms have chased her while jogging near her home in Homestead’s Keys Gate community.

“It makes you run a lot faster,’’ she said.