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At first, Hannah Heath thought she probably just had a bad case of food poisoning. The Sarasota, Fla. resident was vomiting and had chills and a fever. But four days later, she was still really sick.

"Finally I called my husband and I was like, 'You have to take me to the ER, I think I'm dehydrated; I think I need an IV,'" said Heath, 39.

This was in late June, and Heath hadn't yet heard that malaria cases were cropping up in Sarasota county. When doctors at Sarasota Memorial Hospital told her she had the disease, she was in disbelief.

"I was like, 'You're kidding me, right?'" Heath said. "Because I haven't been outside the country, so it was just surreal."

Heath spends time outside with her family, so mosquito bites are not uncommon. She has no idea which bite might have infected her, or when. Doctors told her it probably happened weeks before her symptoms first developed.

First local transmission in U.S. in 20 years

Heath is one of eight known people in recent months who have contracted malaria in the U.S., after being bitten by a local mosquito, rather than while traveling abroad. The cases comprise the nation's first locally transmitted outbreak in 20 years. The last time this occurred was in 2003, when eight people tested positive for malaria in Palm Beach, Fla.

One of the eight cases is in Texas; the rest occurred in the northern part of Sarasota County.

The Florida Department of Health recorded the most recent case in its weekly arbovirus report for July 9-15.

For the past month, health officials have issued a mosquito-borne illness alert for residents in Sarasota and neighboring Manatee County. Mosquito management teams are working to suppress the population of the type of mosquito that carries malaria, Anopheles.

Sarasota Memorial Hospital has treated five of the county's seven malaria patients, according to Dr. Manuel Gordillo, director of infection control.

"The cases that are coming in are classic malaria, you know they come in with fever, body aches, headaches, nausea, vomiting, diarrhea," Gordillo said, explaining that his hospital usually treats just one or two patients a year who acquire malaria while traveling abroad in Central or South America, or Africa.

She felt 'miserable' at first

Malaria is mostly found in tropical countries and is caused by a blood-borne parasite that spreads to humans through some species of mosquito. It can take weeks for someone to develop symptoms after they've been bitten by an infected mosquito.

The diagnostic challenge in Sarasota, initially, was that those symptoms are common in many diseases. The first patient was admitted in late May, but had no recent travel history, so staff at Sarasota Memorial weren't expecting malaria. It took lab workers noticing parasites in a blood sample for doctors to even consider that as a diagnosis, said Gordillo.

But by the time Heath arrived in the emergency room on June 27, they were on alert.

"They knew what to do pretty quickly, so I could start treatment pretty quickly," Heath said.

Heath was hospitalized for five days. She considers herself to be an active person who is normally in great health, but the malaria left her exhausted and "miserable."

In addition to suffering from dehydration, her platelet count had dropped. That increases the risk of internal bleeding, which meant her movements had to be closely monitored.

"I couldn't get out of bed without somebody there to make sure I didn't fall, and they wrapped padding around the edges of the bed, the side rails, because I could have hurt myself if I would have hit my arm, there could have been internal bleeding," Heath said.

A slow but steady recovery

During this acute phase, Heath's doctors prescribed Coartem, an oral antimalarial drug, to treat the parasites infecting her red blood cells.

The nausea and headaches she suffered are common symptoms of malaria as well as common side effects of the medication, so Heath couldn't pinpoint the cause. Regardless, she described her first few days in the hospital as particularly rough. Medical teams gave her other medications to relieve her symptoms and discomfort.

Some other malaria patients developed severe symptoms as well, Gordillo said, but in each case, staff were able to manage them.

"This has been around for years," he said. "There are good treatments, there's straight-forward diagnosis" — once it's suspected in an area.

All the locally acquired cases were of Plasmodium vivax malaria, a strain that typically produces milder symptoms or can even be asymptomatic, according to the Centers for Disease Control and Prevention. But the strain can still cause death, and pregnant people and children are particularly vulnerable.

So far, the patients Sarasota Memorial has seen have all responded well to treatment, Gordillo said.

After a few days, Heath started to feel the benefits of her malaria treatment and was discharged after five days. There was some lingering fatigue, but Heath reported she was doing yoga again a few days after coming home.

But her treatment is not over. Last week, Heath started a second round of medication from home called Primaquine, which targets any remaining malaria parasites that may be in her liver, which can cause relapse. She will complete that course of treatment this weekend.

She expressed gratitude for hospital staff for taking care of her during a difficult time.

"I do appreciate that they knew what it was. They knew how to take care of it and I'm feeling great now," Heath said.

Tackling the mosquitoes that spread malaria

Malaria does not spread from human-to-human contact; a mosquito carrying the disease has to bite someone to transmit the parasites.

Workers with Sarasota County Mosquito Management Services have been especially busy since May 26, when the first local case was confirmed.

Like similar departments across Florida, the team is experienced in responding to small outbreaks of mosquito-borne illnesses such as West Nile virus or dengue. They have protocols for addressing travel-related cases of malaria as well, but have ramped up their efforts now that they have confirmation that transmission is occurring locally between mosquitoes and humans.

Staff have increased surveillance for Anopheles mosquitoes in the northern Sarasota area, where all the malaria cases so far have been. Workers regularly check traps set throughout the county for adult mosquitoes, and examine water sources for signs of larvae.

Their primary goal is to eliminate mosquitoes before they mature and can start flying around biting people. Part of the process involves spraying chemicals called larvicides near ponds, ditches and other places containing pools of stagnant water, where mosquitoes like to lay their eggs.

When necessary, the team also uses targeted spraying of insecticides to kill adult mosquitoes in the air. They spray at night, when Anopheles are most active. This is particularly important in the days immediately following a confirmed malaria case.

Now that a seventh case has been reported, workers are scouring woods, ponds and other places to see if they've missed anything, according to manager Wade Brennan.

"This is what our crews are focused on, but when it comes to private property we need everybody's help," said Brennan, speaking at a press conference on July 18.

They are asking residents to check their yards for standing water that attracts mosquitoes. Those with large wooded areas or ponds on their property can contact mosquito management for assistance.

The county sent 140 mosquito specimens to the CDC for testing. Three came back positive for malaria, but none since early June.

While organizations like the World Health Organization have cautioned climate change could lead to more global cases and deaths from malaria and other mosquito-borne diseases, experts say it's too soon to tell if the local transmission seen these past two months has any connection to extreme heat or flooding.

"We don't have any reason to think that climate change has contributed to these particular cases," said Ben Beard, deputy director of the CDC's division of vector-borne diseases and deputy incident manager for this year's local malaria response.

"In a more general sense though, milder winters, earlier springs, warmer, longer summers – all of those things sort of translate into mosquitoes coming out earlier, getting their replication cycles sooner, going through those cycles faster and being out longer," he said. And so we are concerned about the impact of climate change and environmental change in general on what we call vector-borne diseases.".

Beard co-authored a 2019 report that highlights a significant increase in diseases spread by ticks and mosquitoes in recent decades. Lyme disease and West Nile virus were among the top five most prevalent.

"In the big picture it's a very significant concern that we have," he said.

Lessons learned about local risks — and prevention

For now, officials say the best protection against malaria for residents is to use insect repellant and cover up with long-sleeved clothing, especially before sunrise and after sunset, when Anopheles mosquitoes are most active.

"It's just so important, if we can stop those mosquito bites we can stop this from going any further," said Brennan, of Sarasota's mosquito control unit.

Hannah Heath is definitely on board. Since she's come home, she makes sure she, her husband and 6 year-old son have bug spray when they go outside.

"I don't want anybody to go through that, but I'm just thinking like, I don't want to see my son go through what I went through, so I'm more aware of it," she said.

Heath says her neighbors in the Sarasota area should seek treatment quickly if they have malaria symptoms.

For most Americans the risk of contracting this disease is extremely low, according to the CDC. But awareness is still important, Beard notes.

"This is mosquito season and people need to wear the repellants," he said. "Malaria is a risk in the Sarasota area but there are a lot of other mosquito-borne illnesses as well."

This story comes from NPR's health reporting partnership with WUSF and KFF Health News.