A look inside one county's mosquito management efforts to control malaria spread
Hillsborough County has seen four travel-related malaria cases this year, but none acquired locally. Still, things are busy for employees working to prevent mosquito-borne diseases this rainy season.
No new locally acquired malaria infections were reported during the week ending July 8, according to the latest update from the Florida Department of Health.
Sarasota and Manatee county residents remain on mosquito-borne illness alert, along with Miami-Dade, while Orange, Polk and Walton counties are under an advisory.
Six people have acquired the disease in the north Sarasota area since May.
Nearby Hillsborough County has seen four travel-related malaria cases this year, but none acquired locally. Still, things are busy for county employees working to prevent the spread of mosquito-borne diseases this rainy season.
Tracking mosquitoes in the community
Inside Hillsborough’s mosquito management lab in the eastern outskirts of Tampa, dead mosquitoes lie in petri dishes. They’re brought in from the more than two dozen traps staff have set up around the county to track mosquito activity.
Lab workers examine the insects under microscopes to determine their species.
“We have 40 different kinds of mosquitoes here in Hillsborough County that we’ve identified, and there are only five or six that actually can carry a disease and transmit it to humans,” said David Fiess, the county’s mosquito management services manager.
In addition to lab surveillance, field workers are regularly monitoring the county for signs of disease-carrying mosquitoes, such as the anopheles, which can transmit malaria.
There are roughly 14,000 “larval production sites” the team has identified in Hillsborough, which Fiess explained are ponds, ditches or other places with stagnant water, where mosquitoes like to breed.
“We want to control them in the water. It’s a lot easier to kill mosquitoes in the water than when they’re flying around,” Fiess said.
Larvicides, which are pesticides that target immature mosquitoes, are used to kill mosquitoes before they can start biting people.
Mosquito management workers sometimes spray the treatment near stagnant water sources or can also drop briquettes containing the larvicide into ponds or ditches as they drive past using specially designed vehicles where the driver sits on the right side.
What happens if Hillsborough gets a malaria case?
Action would ramp up if the health department notified mosquito management of a suspected case of malaria.
Staff members use computer systems to pinpoint where a possible infection was reported and map out canvassing and treatment plans. Anopheles mosquitoes typically only fly within a one-mile radius of where they were born, Fiess said, so that would inform the focus area.
Mosquito management workers would then go door-to-door to alert residents about the warning and educate them about how to protect themselves with bug repellant and clear their property of standing water.
Since the anopheles flies at night, staff would drive trucks through the neighborhoods that evening to spray insecticides that kill adult mosquitoes. They use 1.5 ounces of treatment, or about enough to fill a shot glass, mixed with water per acre.
“Spraying at night, we’re only going to be hitting about 30 or 40 percent of those mosquitoes flying around because it’s just a larger area that they’re flying in,” said Fiess.
That’s why if it’s a confirmed case of malaria, staff treat the area three nights in a row.
“Let’s say that infected person isn't under treatment yet, that mosquito could go bite that person and then bite someone else and then pass it onto them,” said Fiess. “So that means we need to get in there and really hit them to try and get as many as we can.”
Workers would continue to treat nearby water sources that could breed mosquitoes with larvicides as well.
Response efforts would be similar for other mosquito-borne diseases but could alter based on the behavior of the species that carries them and other factors. For example, aedes egypti mosquitoes, which can carry dengue and West Nile virus, fly during the daytime, so workers would spray at a different time.
Other prevention tools
Hillsborough’s mosquito management team uses other tools to prevent outbreaks of mosquito-borne illnesses as well.
Outside the lab, several chickens were housed in coops covered in wire to keep out outside threats. The county stations these sentinel chickens at 10 other locations around Hillsborough to help monitor for viral threats.
Some of the mosquitoes that transmit diseases primarily feed on birds, but chickens don’t get sick from the viruses. This means they also can’t spread the virus to other mosquitoes. But their bodies still produce antibodies against these viruses, so when mosquito control staff test their blood, they can learn if certain diseases are present in a community.
Workers test the blood of sentinel chickens weekly, Fiess said, and if they find anything of concern from birds in a particular area they will treat to kill nearby mosquitoes and alert the public about risks.
Chickens are most useful in surveillance for West Nile virus, eastern equine encephalitis and St. Louis encephalitis, Fiess said.
Public education is also a critical part of the department’s mission, he said.
In addition to door-to-door canvassing after cases are detected, staff park an educational trailer at public events, visit schools and do other community outreach to help residents better understand their role in preventing the spread of mosquito-borne diseases.
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