Mosquito season has officially arrived in Florida, although many would argue it never left.
That perception may soon become reality, according to new studies that show the higher temperatures brought on by climate change are already increasing the range and biting season for many mosquitoes, including the Aedes aegypti — the infamous carriers of viruses like dengue and Zika, which hit Miami hard enough in 2016 to scare off many tourists.
Researchers believe the climate shifts will also raise the risks that other mosquito-borne diseases considered largely eliminated as public health threats in the mainland United States could return. Yellow fever tops that list.
Nationally, illnesses from insects like mosquitoes, ticks and fleas have already tripled from 2004 to 2016, according to a report released Tuesday by the Centers for Disease Control. In that time, nine new germs spread by mosquitoes and ticks were found or introduced into the U.S. The report noted an "accelerating trend of mosquito-borne diseases introduced from other parts of the world."
Sign Up and Save
Get six months of free digital access to the Miami Herald
That trend will be felt most acutely in South Florida, where experts say the region's warm and wet climate — as well as its reputation as an international travel hub — make it the perfect spot for mosquito-borne viruses to flourish. Most travelers to the Magic City come from countries in South America and the Caribbean where diseases spread by the insects are prevalent.
Dengue, Zika and chikungunya aren't unusual in South Florida, but a new study suggests climate change and an increase in international travel could revive long dormant threats.
Almost 3 million travelers from countries with an active outbreak of yellow fever came to the U.S. in 2016, according to a report issued last month from the World Health Organization. The paper named Miami as a U.S. city at risk for a deadly outbreak, mostly because the weather already suits the mosquito that spreads the disease, but also because the U.S. doesn't require travelers from infected areas to provide proof they were vaccinated. Yellow fever gets its name from the jaundice it can cause in patients, but serious cases can cause internal bleeding and organ failure.
“When you put these together, the risks — from an infectious disease standpoint — start to increase,” said Dr. Kamran Khan, an infectious disease physician and scientist at St. Michael’s Hospital in Toronto and a contributor to the study.
To most scientists, the Zika outbreak that began in Miami's artsy — and tourist packed — Wynwood district wasn't exactly unexpected.
“Wynwood wasn’t a surprise, but it was a surprise,” said Sadie Ryan, an associate professor in medical geography at the University of Florida. “Could we have pinpointed that this is exactly what would happen, no. Could we have predicted that something like this would happen, of course.”
But the first major Zika outbreak in America was a shock for many residents — particularly pregnant woman, worried the virus could leave babies with severe physical and mental disabilities. Tourism also took a hit as authorities initially struggled to slow the spread.
South Florida's mosquito season typically begins around May, generally linked to the arrival of the rainy season. But as climate change causes average temperatures to creep up, Ryan said mosquitoes will have more months of the year to spread disease. It also gives the insects more time for the viruses to spread from the blood in their belly to their saliva glands, which turns the bugs from backyard pests to carriers of potentially deadly diseases. A new (and yet-to-be peer reviewed) study Ryan contributed to even suggests that process could happen faster in warmer temperatures.
Heat isn't the only climate change symptom that could influence the spread of mosquito-borne diseases.
More intense hurricanes, which are linked to climate change, mean more damaged houses, more debris on the ground and more people spending more time outside fixing their homes. That's all more opportunities for mosquitoes to bite or proliferate. All the Aedes aegypti mosquito needs is a teaspoon of water to lay eggs and start the next generation of disease carriers.
“That’s lots and lots of little teaspoons of habitat,” Ryan said.
Since 2016, Florida has seen more than 1,500 cases of the Zika virus — both locally contracted and travel related, although they've taken a significant dip in recent years. The last locally transmitted case in Miami-Dade was in 2017, and so far this year the 30 cases of Zika are all travel-related.
The outbreak forced Miami-Dade County to dramatically expand what proved to be an overwhelmed and underfunded Mosquito Control Division.
In the two years since, the county nearly quadrupled the staff and increased the budget from around $2 million to about $16 million, said the department's new head, Bill Petrie.
Petrie, who used to head the mosquito control agency in the Cayman Islands, said he's on the lookout for a change to Florida's mosquito season, like he witnessed in his former job. He said over the years the season grew less predictable, and he worries he could see the same change here. A longer season means more employees out spraying for mosquitoes more frequently and less down time to repair the equipment.
In Cayman he also oversaw the release of genetically modified mosquitoes from British biotech company Oxitec. The ‘Oxitec Friendly mosquitoes” are males modified with a gene that dramatically shortens their life span. That gene is passed on when they mate with female mosquitoes, the company says, so the offspring don’t reach adulthood.
A campaign in the Keys to bring the "GMO mosquitoes" to the island chain has been repeatedly delayed by activist opposition. The Environmental Protection Agency has until July to decide on whether to allow Oxitec to experiment with the insects in the Keys.
During the 2016 Zika outbreak, the county considered working with Oxitec to stem the tide of infected mosquitoes but didn't move forward with any agreement.
Instead, Miami-Dade took part in a $4.1 million field trial in South Miami (the largest of its kind so far) to release millions of mosquitoes sterilized another way — by infecting the males with a bacteria that alters mosquito sperm so that it doesn’t fertilize eggs.
Petrie said his department is better prepared than it was when Zika first hit. With the start of the season underway, his team is busy getting equipment in order, checking their traps and keeping a close eye on outbreaks of mosquito-borne diseases around the world.
“It’s all there waiting to happen,” he said. “It’s why we have to be watching and stay on top of it.”
“We’ll keep our fingers crossed."