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Can climate change-related heat bring on more mosquito-borne diseases?

Robert Taylor, of Orange County Mosquito Control, sprays a synthetic pyrethroid made from chrysanthemum — safe for humans, deadly for mosquitoes. He's creating a treatment barrier in an Orlando neighborhood where a resident contracted a travel-associated case of dengue.
Joe Mario Pedersen
Robert Taylor, of Orange County Mosquito Control, sprays a synthetic pyrethroid made from chrysanthemum — safe for humans, deadly for mosquitoes. He's creating a treatment barrier in an Orlando neighborhood where a resident contracted a travel-associated case of dengue.

Scientists are watching mosquito breeding levels after a record hot summer amid concern more tropical heat could mean more illness.

A team dressed head-to-toe, donning breathing masks, and armed with a motorized spray pack of insecticide spread out around a west Orlando neighborhood setting up an aerosol treatment barrier and knocking down any blood-sucking, disease-carrying insects. The Orange County Mosquito Control was on scene for a routine call after a resident tested positive for dengue — a mosquito-borne illness.

The disease has caused quite a buzz in Florida as experts are concerned climate change-related events may be creating an ideal environment for mosquito breeding and disease.

"We don't know for sure whether it's a locally acquired case, but we like to always assume that it's not and we'll do everything we can to prevent it from becoming local," said Austin Hoover a mosquito control technician at the scene. The patient had a travel-associated case after visiting a dengue-endemic country.

The team is working to prevent dengue from being locally acquired in the county. A possibility Mosquito Control takes seriously since Orange faces a higher level of risk due to its status as a popular tourist destination.

"We get notified of what's going on. We get boots on the ground. We have the spray and put the barrier treatment up. The night guys come out here and we'll just nuke the whole area and kill off the adults," Hoover said, armed with a spray pack carrying mosquito poison — a synthetic pyrethroid made from chrysanthemum and safe for humans. "The barrier treatment it's like a forcefield around these homes. So any (mosquitoes) that do come and touch the barrier treatment die off."
The work is hard, but Hoover said the only big challenge for him has been working through this summer's intense heat. Experts are concerned tropical heat may fuel mosquito populations.

Among its many effects, research shows climate change-related events are expected to increase mosquito breeding, meaning a larger increase in mosquito-borne illnesses, as well.

"You'll see higher mosquito densities when you have warmer temperatures, especially paired with high humidity, and so you'll have mosquitoes surviving longer," said Michael Von Fricken a researcher of vector-borne diseases at the University of Florida. "There's a saying that nothing's deadlier than an old mosquito. That's because they've had more bites and more opportunities to pick up something new and nasty."

Dengue cases heat up

Dengue is a viral infection commonly carried by the aedes aegypti species of mosquito. Most cases of dengue are not lethal, but the fever and body aches it brings can be quite painful. Dengue is also known as "bone break fever."

Dengue does not spread commonly in the U.S. but that hasn't stopped the virus from infecting Florida residents.

Earlier this year, the World Health Organization released research explaining climate change-related events would lead to more mosquito breeding, and by proxy more dengue.

Tedros Adhanom Ghebreyesus, the director of the WHO said that rates of dengue have grown dramatically around the world in recent decades, especially in the Americas, which reported 2.8 million cases and 1,280 deaths last year. In 2021, the WHO found that the number of climate-change-related disasters has increased by a factor of five over the 50-year period — with water hazards accounting for 50% of all disasters.

According to the Environmental Protection Agency, climate change greatly increases the risk of human exposure to mosquito-borne illnesses. Illustrating that, in 2019 the world observed the fourth largest amount of billion-dollar disasters: flooding being among the most common causes. The same year also reported the greatest spread of dengue ever noted globally.

According to the WHO, dengue cases reported this year were more than double those reported in 2022 and more than four times higher than the average of the last five years.

Mosquito-borne illnesses in Florida

The Florida Department of Healthhas observed 274 cases of travel-associated dengue, meaning the virus was picked up from a country where dengue is endemic. Last year, 1,249 travel-associated cases were identified throughout the country, meaning Florida's current cases would make up 22% of the national total from last year. The national total was less than 400 in both 2020 and 2021.

More concerning to scientists is the observance of locally acquired dengue. Florida has seen 19 cases this year.

"We're being surprised by the introduction of diseases that might not have been endemic here before gaining a foothold. We're not sure if they are going to be a spark in the pan or is this going to be something that is going to be endemic for the long run," Von Friken said.

The FDOH has issued a dengue advisory in Broward and Miami-Dade counties. But with the rise in cases, experts like Von Fricken are asking: has dengue become endemic (occurring regularly) in Florida?

"Dengue may already be endemic in Miami-Dade County," said Jonathan Day, a professor of entomology at the University of Florida. Day wrote about the possibility in his Disease Activity newsletter. "Dengue is extremely abundant throughout the Caribbean basin as it is transmitted via a high effect vector, aedes aegypti."

Florida has easy access to the Caribbean as well as Central and South America, where dengue has also been proliferating. As a result, Florida has become not only an easy access point for the disease to enter the country, but dengue may have just the right environment to make a home and grow in Florida, Day said, after observing locally acquired infection cases reported from January to July.

"This suggests the possibility of ongoing low-level dengue transmission, or that the viruses are now endemic to Miami-Dade County," he said.

Dengue hasn't been the only mosquito-borne illness to create buzz in Florida. An outbreak of malaria took place in Sarasota County earlier this summer. The county reported seven cases of locally acquired malaria. The outbreak made headlines as it was the first time a locally acquired outbreak had been observed in Florida in the last 20 years. In 2003, malaria was found in Palm Beach County when eight people tested positive, according to the CDC.

Day doesn't think malaria has reached a foothold in the same manner dengue may have.

"Malaria will have an extremely difficult time in becoming established in Florida," he said.

Anopheles mosquitoes are known for carrying malaria. They're nighttime flyers and don't have wide access to humans due to window screens and air conditioning.

Heat hurricanes and mosquitoes

It's been an odd summer for mosquitoes in Central Florida. Heat is good for mosquito breeding... but only so much. Temperatures were record-breaking throughout Florida partly due to a lack of rain — the second ingredient necessary for a booming mosquito population.

However, along with the heat, tropical storms, and hurricanes have given mosquitoes a leg up here in Central Florida when it comes to breeding, said Steve Harrison the director of Orange County Mosquito Control.

"After Hurricane Ian, we saw a huge spike in certain areas, and we're like five times as many mosquitoes if not more," Harrison said.

Harrison said Ian dumped 12 inches of rain locally. This year after Hurricane Idalia, Orange County saw a 25% increase in its mosquito population. While it's a big increase Harrison said the rainfall helped bring mosquito levels to normal, which had previously been down due to the exacerbating heat.

Orange County Mosquito Control is keeping its eyes open for mosquito populations growing and the possibility of illness spreading. To help monitor for viral activity, mosquito control has strategically placed sentinel chicken coops – they’re chickens that act like a mosquito-virus alert system around the county.

The chickens are routinely tested by the Department of Health for mosquito-borne illnesses.

Orange County hasn't seen any locally acquired dengue or malaria activity. However, travel-associated cases have been reported, including at least two dengue cases in Orlando in August. However, experts are seeing activity from different viruses including West Nile virus and eastern equine encephalitis, which is rare and is typically carried by birds but can be deadly in humans.

Mosquitoes-borne illness coming to roost?

It's not malaria or even dengue that keeps Harrison up at night. It's the Eastern Equine Encephalitis Virus.

"I'm just envisioning, you know, all these wild birds flying around and could potentially be bitten by mosquitoes and transmit that virus to humans," he said.

Earlier this summer, the department detected Eastern Equine Encephalitis Virus, or "EEEV" in Orange County. This summer marked the third EEEV advisory in four years.

"I’ve had employees here for decades and they’ve said we’ve never had so many advisories this close together," Harrison said.

There have been 18 Orange County chickens that tested positive for EEEV this year, including one chicken that tested positive in August. Statewide, 114 sentinel chickens, 11 horses, one sparrow, one laughing gull, and two humans have been reported from 21 counties. Neither of the human cases were reported in Central Florida counties.

Harrison isn’t sure if it’s related to climate change, but he is sure that climate change will influence mosquito breeding and give EEEV more of a chance to potentially spread. EEEV lingers on his mind because it’s hard to detect even with the chickens.

"It's kind of like fishing. I have my chicken coops out there, and we're here fishing for this virus," Harrison said. You know there's all these 'fish' all throughout the county and then I see the activity in my flocks, which means I know it's and that there is an increased risk to the citizens because of the wild birds that are out there.

In 2021, theNational Library of Medicine published a study projecting that as climate-change effects worsen, the disease burden of EEEV will likely increase.

"It’s a bad virus. to me, that would be the biggest impact to have on a wider range of people. And a lethal one as well," Harrison said.

The EEEV advisory in Orange and the dengue advisories in Broward and Dade will likely last through the end of the season, in December.

Mosquito Control technician Austin Hoover said that he and his fellow techs cover up their limbs with PPE; long shirts, hats, and masks, to make sure they don't leave any openings for mosquitoes. However, the protection comes at the cost of feeling the heat. He and the team take frequent breaks during the hotter days and make sure to hydrate. They also check up on each other.

"We ask each other, do you need any help? We cover the bases and keep each other in check to make sure no one's dropping like flies... Or in this case, mosquitoes," he said.

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