Mosquito vector diseases are expanding, but COVID-19 has halted some surveillance programs and experts are worried complacency could lead to outbreaks.
Butterflies and moths captivated Lawrence Reeves – so much so that he chased their ethereal flutters all the way to the Philippines, his mother’s homeland, as an entomology master’s student at the University of Florida.
Within days of his arrival to the oblong island of Negros, he began to feel sick, he said.
Stuck in an unairconditioned room in his grandmother’s house, the air thick with humidity, he found himself unable to move. It was difficult to breathe, to shift his hand ever so slightly. Food was out of the question: Everything tasted terrible, he said. He remained in bed, his body weakened with intermittent fever, for over a week.
He said he was reluctant to get tested at first, despite his grandmother’s urging. He was afraid of the results. But ultimately, he did – only to find another winged insect was to blame for his illness: a mosquito infected with a viral disease called dengue.
“I definitely think of it as the worst experience of my life,” said Reeves, now a research assistant scientist for the UF/IFAS Florida Medical Entomology Laboratory.
Transmission of mosquito-borne viruses is more common in tropical climates like the Philippines. But it is projected to swell around the world in the next 30 years because of climate change, according to Sadie Ryan, a UF associate professor of medical geography and lead author of a study that mapped the expansion of risk worldwide. Globally, nearly half a billion more people could be at risk for contracting these diseases.
That expansion could escalate mosquito-borne diseases in Florida in two ways: “imported” cases travelers bring to the state and locally acquired cases – those transmitted by mosquitoes here. “We do foresee that climate change will impact vector-borne disease, depending on which standpoint you’re looking at, positively,” Reeves said.
Dengue was thought to be eliminated from the United States decades ago, but Reeves now calls it the most consistent mosquito-borne disease in Florida. In the last year alone, the Florida Department of Health reported 16 locally acquired dengue cases and 397 imported, a record number within the last decade. Other similarly transmitted illnesses join its ranks in the state, including West Nile virus, eastern equine encephalitis, Zika virus and chikungunya.
The Florida Department of Health (DOH) confirmed a dengue case in the Florida Keys in March. Last week, a city of Gainesville entomologist found a species of mosquito that hasn’t been seen in the area since the 1990s: Aedes aegypti, or the yellow-fever mosquito also known as the most serious vector of exotic viruses in the state. And on Saturday, DOH’s Miami-Dade office announced a mosquito-borne illness alert after confirming a resident contracted West Nile virus locally.
Yet COVID-19 means that Florida’s routine laboratory strategies for monitoring the viruses, including testing of sentinel chickens and mosquito pools usually reported weekly, have been suspended in order to devote resources to the pandemic. A DOH spokesperson said commercial testing remains available and that the state agency and Centers for Disease Control and Prevention are working to make sure human testing is available.
These vector diseases are not to be confused with zoonotic diseases like COVID-19, which are spread from animals to humans. Instead, the “vectors” behind the name are bloodsucking insects. In Florida’s case, they’re mosquitoes. The blood-suckers have always been a menace here. But as the climate has started to shift, so have mosquitoes’ numbers, ranges and diversity – and the diseases their bites can carry.
Rising temperatures may impact species numbers
From the 1800s to the early 2000s, Reeves said only six new mosquito species were detected in Florida. But since 2002, 11 new species introductions have been recorded – two within the last year alone, he said. Ten of the 11 are tropical.
It’s difficult to gauge a single factor that caused this uptick, Reeves reasoned. But as Florida’s climate begins to more closely correlate with that found in the tropics, he said it could pave the way for new arrivals to set up household here.
“Unfortunately, there are few generalities,” Reeves said. “Each mosquito-vectored virus is going to be distinct in its transmission system.”
Warmer temperatures can also quicken what’s called the “intrinsic incubation cycle” of viruses in mosquitoes, he said. That’s the time it takes for the virus to develop in an insect’s gut after it feeds on an infected host. Then, the virus makes its way to the bug’s saliva glands to be injected into a new host with the next bite.
A faster incubation period could spell trouble in a warming world since that means some viruses can replicate quicker, Reeves said, potentially infecting more hosts than under cooler temperatures.
Piecing together the vector puzzle
A disease’s success as the climate changes depends on its specific set of thermal boundaries, said Ryan, who holds a joint appointment in UF’s Emerging Pathogens Institute. Some diseases may actually decline in warmer weather, like the endemic Saint Louis encephalitis virus. It has rapidly disappeared from Florida since its outbreak in the 1990s.
Just as heat isn’t ideal for every virus, not every mosquito is an ideal vector for every disease. Rather, relationships are unique between the type of pathogen and the several predominant species in Florida:
- Aedes aegypti, or the yellow-fever mosquitoes like the one just found in Gainesville, are “sippers”: “They go from one person to another, and they don’t take an entire blood meal from a given person,” Ryan said. “And so they’re sort of bopping around doing a bit more transmission per blood meal.” Not only do they prefer humans over other hosts, but they also are the most serious vectors of exotic viruses like dengue, Zika virus and chikungunya.
- Aedes albopictus, or the Asian tiger mosquito, also carries these diseases but isn’t as good at spreading them. While the yellow-fever mosquito thrives in the hotter and denser regions in South Florida, the Asian tiger mosquito has a slightly cooler temperature profile. They’re more inclined to temperate regions, Ryan said, thus their spread further up into North Florida.
- Culex mosquitoes can host endemic mosquito-borne illnesses like eastern equine encephalitis or West Nile Virus, said Ryan. And Culex are “pretty much everywhere.”
Matching a mosquito species with the diseases it could possibly carry and where it can typically be found in Florida can help piece together the puzzle of infection ranges statewide. But when climate change comes into play, the pieces get scrambled. Disease distributions will change along with the length of their transmission periods, Ryan said.
“As you get a warming Florida, things become more suitable for the hotter mosquito, as in Aedes aegypti, and so for more of the year, Aedes aegypti can do its thing,” she said. She also noted that Aedes albopictus, the Asian tiger mosquito, could move further north as temperatures rise above normal ranges.
Statewide average temperatures have been higher than normal for 57 out of the last 60 months, according to state climatologist David Zierden at the Florida Climate Center in Tallahassee. Florida shattered heat records this spring when statewide average temperatures measured 7.1 degrees above normal for March, he said.
Fears of complacency
Along with many medical entomologists, Roxanne Connelly said she’s hesitant to point to an exact connection between climate change and vector-borne disease outbreaks. As the chief entomologist in the Division of Vector-Borne Diseases within the Centers for Disease Control and Prevention’s National Center for Emerging and Zoonotic Infectious Diseases, she said she knows climate has a direct effect on mosquitoes and their viruses.
While climate changes influence how quickly pathogens replicate, the lifecycle of vectors and disease distributions, she said, several other factors could skew the correlation. People could move around, bringing infections with them. Increased testing in a particular area could reveal greater incidence.
“Across the nation, looking at where these vector-borne diseases occur, I think what we’re still trying to figure out is: Is this really due directly to climate change factors? Or in some cases, is it just that we’re starting to see an increase in reporting?” Connelly said.
But Connelly said she has seen evidence that the species distribution changes are now expanding beyond Florida, possibly connected to warming temperatures.
Since future outbreaks are difficult to predict, especially in light of the already erratic climate projections, Connelly said it’s crucial for Florida to continue education and testing programs.
“They’re just always going to be at risk for some mosquito-borne disease, and having that network in place where you always have surveillance – it’s something that’s justified,” she said.
Amid the COVID-19 emergency, Ryan said she worries mosquito protection may not be where it needs to be at this point in the year as attention and resources are directed elsewhere. “I’m actually concerned that there’s a potential to see novel infection happening in unexpected ways, or at least more mosquitos emerging because people are becoming complacent,” she said.
“It’s not like mosquitoes went on hold because of COVID,” she said. “The message needs to stay alive.”