TAMPA, Fla. — Not all mosquitoes feed on humans (some only bite frogs), and not all carry viruses. But the ones that do have killed more people throughout human history than any other animal.
The global number of deaths caused by mosquito-borne diseases is staggering. One statistic reported by the Centers for Disease Control and Prevention is scary. According to their data, in 2021, "619,000 people died from malaria, and over 247 million became ill across 84 countries."
Those are just the numbers for malaria and don't include dengue, West Nile virus, chikungunya, yellow fever or Zika.
According to the World Mosquito Program, "roughly 390 million people are infected yearly with dengue, and hundreds of thousands more are affected by Zika, chikungunya and yellow fever.
Floridians know how annoying mosquitoes can be and how dangerous they are. This Summer, Sarasota County reported seven locally transmitted cases of malaria. Thankfully, no fatalities.
Malaria was eradicated in the United States in the early 1950s. But that doesn't mean people traveling to highly infected areas and then coming back can't spread the disease to local populations. When that happens, mosquito management agencies react quickly.
In 2016, ABC Action News reporter Michael Paluskainterviewed a local momwhose son nearly died after contracting Eastern Equine Encephalitis. The virus causes inflammation of the brain. It begins with the sudden onset of headache, high fever, chills and vomiting. The illness may then progress into disorientation, seizures, or coma, according to the Centers for Disease Control and Prevention.
MOSQUITO MANAGEMENT
"A lot of people, especially here in Florida, they're more afraid of sharks than mosquitoes. But mosquitoes kill more people than sharks in the world," David Fiess, manager of Hillsborough County Mosquito Management Services, said.
"If you didn't exist, and this office didn't exist, and we didn't monitor them? What would it be like for citizens?" Paluska asked Fiess.
"Well, I don't think anyone would be living here because the number of mosquitoes would be like trillions," Fiess said. "People wouldn't be able to go outside, and the economy would suffer because we would have no more tourists come into our area; it would be a dramatic effect if there were no mosquito management going on in Hillsborough County and the state of Florida."
Fiess showed us the lab where workers count mosquitoes collected throughout the county, test them for disease, and determine their species. Knowing the species is critically important.
"Here in Florida, there's 80 different breeds or different types of mosquitoes that have been identified," Fiess said. "Here in Hillsborough County, we've identified 40 of those at different types. Fortunately, only about six or seven of them do carry disease. If all of them carried disease, we'd be in worse shape. But again, only six or seven do. And we are out there trying to keep an eye on them."
Mosquito management also keeps sentinel chickens in coops across the county that have their blood drawn to test for viruses.
When Fiess learned Sarasota County had confirmed malaria cases, they sounded the alarm and looked at their populations for any sign of the virus. No malaria was or has been found in Hillsborough County. That is partly because Sarasota County pinpointed where the disease was discovered and began treating the area in a radius.
"When you hear malaria in Sarasota, they don't fly that far, so it's not the whole area?" Paluska asked.
"Correct these types of mosquitoes that can carry malaria; their flight path is only about a half mile to a mile where they come off the water," Fiess said. "They are not going to fly 20 miles down the road to go maybe bite someone down that way; it's constrained to a smaller area."
Anopheles mosquitoes transmit malaria.
Hillsborough County is a massive area to treat. We went out with Steve Wilson, Inspector #1, who searched ditches and sidewalks for standing water. Using a cup attached to a long pole, he scooped up water, searching for larvae.
"If you find one, there are thousands," Wilson said. "We're making our way to the sidewalk up here because you can look at the sidewalk and see them wiggling in the water."
This is one of 14,000 sites mosquito management goes to after rain. It's a race against the clock to treat them before they get too big to kill.
"Can you kill that one?" Paluska asked.
"Yes," Wilson responded. "He's still active, so he's still feeding; tomorrow would be too late. Once they pupate, they stop eating— this insecticide treatment is giving them something they will eat."
This location was right next to a school bus stop. We watched as dozens of kids jumped out after school, walking past the flooded sidewalk. Nevertheless, thousands of larvae were wiggling beneath the soles of their shoes.
FIGHTING MOSQUITOES WITH FUNGI AND BACTERIA
"In my laboratory, we're interested in understanding novel forms of mosquito control, so alternatives to insecticides, and we're also interested in understanding interactions between mosquitoes and arboviruses, like dengue virus and microorganisms that naturally associate with the mosquitoes," Eric Caragata, assistant professor in the Department of Entomology and Nematology at the University of Florida said. "And the reason is that these microorganisms can alter the relationship between a mosquito and an arbovirus. They can make it more or less likely that a particular mosquito can transmit an arbovirus or become infected."
Caragata gave Paluska a tour of their insectary — a small room inside the Florida Medical Entomology Laboratory in Vero Beach. There are full-grown mosquitoes and larvae. All will be used for testing to find new ways to control mosquitoes without insecticides.
"We are interested in microorganisms that have natural mosquitocidal properties, which means they naturally kill mosquitoes," Caragata said.
One bacteria getting tested on lab-grown mosquitoes is Wolbachia.
"Wolbachia is a naturally occurring bacterium which lives inside the cells of insects and other arthropods. Wolbachia has natural anti-viral properties, making mosquitoes more resistant to a viral infection. So there are some very large scale projects ongoing with Wolbachia all around the world, where people are releasing Wolbachia infected mosquitoes to replace wild populations of mosquitoes," Caragata said. "So, in effect, you take this virus-susceptible population of mosquitoes and replace it with a virus-resistant mosquito population harboring Wolbachia."
"That's fascinating," Paluska said. "Because it's almost as though you're giving a vaccine to all these tiny little mosquitoes and then having them spread the vaccine to the wild population?"
"Exactly," Caragata responded. "The other use of Wolbachia is to suppress mosquito populations. So, it also causes this reproductive manipulation called cytoplasmic incompatibility. And if you release millions of male Wolbachia-infected mosquitoes, they'll go and mate with the female mosquitoes in the same species. And then, because of the Wolbachia, their eggs won't hatch. So over time, the population size decreases and then eventually crashes."
With insecticide resistance on the rise, Caragata said society must do everything possible to fight them.
"Quite concerning because insecticides are the key tool in our toolbox for controlling mosquito populations. So, for many of these mosquito-transmitted viruses, we have no vaccines. That means we rely on mosquito control methods to keep transmission and cases of these diseases low. And, if our major tool is becoming less effective, then we have to seek out alternative strategies for controlling these mosquito populations."
"How do they build up a resistance? Is it just nature is helping them reengineer even though we're giving them heavy doses of chemicals?" Paluska asked.
"That's right. So, when you treat with an insecticide, not all mosquitoes that are treated will necessarily die. And those that survive may have a genetic resistance to the insecticide and then pass that resistance on to their progeny, their offspring," Caragata said.
SEX-LETHAL GENE
Only Anopheles mosquitoes can transmit malaria. Only female mosquitoes bite humans. They drink blood to produce eggs. Male mosquitoes feed off of nectar. So genetically modified mosquitoes with a sex-lethal gene are also being studied to control the female populations.
In the Florida Keys, a pilot program is underway doing just that.
"What does the genetically modified mosquito do to help you guys control an outbreak before it happens?" Paluska asked Chad Huff, the public education information officer for the Florida Keys Mosquito Control District.
"The Oxitec method of doing this uses a genetically adjusted or modified male mosquito and introduces that into the environment in the form of an egg," Huff said. "All the eggs that are produced by that female from that union are on only males, so all the females die in the larval stage, and then only the males emerge. And what you have is, if you have a lot of males and not a lot of females, then in a small area, you see that population start to collapse."
Huff said the mosquitoes used in their program target are Aedyes Egypt, which carry dengue, yellow fever, chikungunya and Zika.
"When you trap mosquitoes, do you see more males since you started doing the genetically modified ones? Can you quantify it visually?" Paluska asked.
"I cannot quantify it; you got to think about it. They're doing the GMO on only six locations in the Middle Keys. And then we have hundreds of traps. So, I couldn't anecdotally say that we see more males. We see overall a lesser population of Aedes Egypti, and that's the important thing: that's what we want. Suppose those females never make it out of the larval phase, and only the males emerge, even for that one generation. In that case, you see a knockdown of the biting mosquito population almost immediately. And that's what we're after."
Here's one final thought to leave you with — throughout human existence, an estimated 108 billion people have lived on Earth, and the pesky mosquito is estimated to have killed nearly half of us— 52 billion people.
