By Marie Rosenthal, MS
Finding a domestic case of malaria puts everyone in high gear, according to Andrea Morrison, PhD, a vector-borne disease surveillance coordinator, Florida Department of Health, Division of Disease Control and Health Protection, Bureau of Epidemiology, in Tallahassee.
Even as late as the early 1900s, vector-borne diseases like malaria and dengue were common in states such as Florida, which had perfect breeding grounds for mosquitoes. The state eliminated malaria in the late 1940s, but about 70 to 80 travel-related cases still occur every year. “We’ve had malaria transmission in all 67 counties in Florida at one point or another,” Dr. Morrison explained at ASM Microbe 2024, held in Atlanta. “And we’ve had both Plasmodium falciparum and Plasmodium vivax.”
There are eight different species of Anopheles mosquitoes, primarily a nighttime feeder, which can cause malaria transmission in Florida, so the fact the state does not have at least that many domestic cases annually is a testament to its mosquito-control efforts. Still, they do occur, and it is her job to coordinate everyone to ensure the cases do not become large outbreaks. Dr. Morrison watches four key areas regarding mosquito-borne diseases:
- What symptoms did the person report? Were they consistent with that illness?
- Have they traveled?
- Has there been confirmatory testing by state public health laboratory or the CDC?
- Has mosquito control been notified?
“Your goal is to try to identify this malaria very quickly and to speciate it quickly as well,” she said.
Unfortunately, not every county in the United States or even in Florida has mosquito control, she reminded. Because finding an infected mosquito in the state during routine surveillance is challenging, they need to monitor potential illnesses, she said. So, an outbreak investigation can begin with one sick person turning up in the physician’s office, the emergency department or clinic.
Once there, the healthcare provider must include the disease in their differential and order the right test for that disease, which is not always done, she admitted; but if it is, malaria is a reportable disease, and both providers and laboratories are required by law to report it to the Florida Department of Health.
“I would say that the vast majority of reports we get are from commercial laboratories more than anywhere else,” Dr. Morrison said.
That report puts her in high gear. She requests samples be forwarded to the state health department laboratory for confirmation. They try to obtain medical records from the healthcare provider, so they can interview the patient.
“We are collecting those pieces of information that are key for us to understand that person’s illness,” she said.
Considered the “subject matter expert” for the state, Dr. Morrison coordinates and collates all that data, working with the CDC, which does national surveillance; county health departments; mosquito control; healthcare providers; and informing the public.
In May 2023, a 64-year-old homeless person was diagnosed with malaria in Sarasota County with a two-week history of fever, nausea, vomiting and abdominal pain.
Because symptoms can develop long after a trip, it can be challenging to determine whether a case is travel related, a relapse from a previous case or a new domestic case, she admitted. Even without a history of travel, often, cases occur after a friend or relative visited from an endemic country.
In this case, the person had never traveled outside the area, didn’t see any overseas visitors, and never received a transplant or transfusion (a rare but possible source of malaria).
“Now this hospital does manual reviews of their CBC [complete blood count] blood smear differentials, which is fantastic,” she said, because an astute laboratory technologist immediately recognized the parasite. Also, the hospital had already onboarded a rapid diagnostic test for malaria, which can be positive for P. falciparum.
“Now, we’re leaning a little more in the malaria direction,” she said. “The hospital reached out to the county health department, reached out to me, and we talked about this. At this point we’re not excited. We’re like, ‘OK, it’s a regular old malaria case. We get 70 to 80 a year.’
“When the county health department got the medical records and did an initial interview with the person, we found out he had not traveled in the two years before his symptoms had started, which is already kind of like a red flag right [pointing to a domestic case],” she said.
On follow-up, the person said he’d never been out of the country, never had a transfusion or transplant, and had no other types of bloodborne exposures. In addition, he was homeless and, therefore, was at higher risk because he slept outside at night, when the vector was most likely to take a blood meal.
As it points to being locally acquired, the problems increase, because now public health officials are left with determining the magnitude of the problem. This person waited two weeks before seeking medical care, and all that time mosquitoes were biting him.
The normal epidemiology tools are available, doing medical record reviews to find similar cases, notifying hospitals, clinics and other healthcare providers. Because the index case was homeless, they also notified shelters and organizations that worked with the homeless. “We recommended that individuals that were homeless with fever or chills of unknown cause and patients with fever or chills with thrombocytopenia or anemia of unknown cause be screened for malaria,” she said.
Ironically, they were in the middle of validating their malaria polymerase chain reaction (PCR) testing in the state public health laboratory, so this outbreak quickened that process.
“As we identified more cases, we’re doing in-depth case interviews; we’re mapping possible exposure locations. Where do you live? Where do you work? Where are you spending time outside, especially during those nighttime hours, and then rapidly notifying mosquito control,” she said.
For syndromic surveillance, they get de-identified patient data from many different data sources looking at discharge codes, symptoms and the word “malaria” trying to find the other cases.
Although these are just snapshots of a patient’s medical presentation, rather than a detailed medical record review, they can do this quickly, and then follow up if people filled the case definition. Even though all the cases had fever, chills, thrombocytopenia and some had anemia, those terms were not always in the snapshot data, so they added splenomegaly and abdominal pain.
“Outreach is huge when you’re doing things like this,” she said.
In all, they found seven cases; three were among homeless people. All the people lived within 4 miles of each other, and they all had the same strain of Plasmodium.
“The flight range of the mosquito is about a mile, so, someone is moving it around. It’s not necessarily just the mosquito that’s being impacted,” Dr. Morrison said.
Mosquito Control
The other side of the effort is mosquito control. They must capture mosquitos, both the larval and adult stages for testing. Despite a huge effort done in Sarasota and Manatee Counties, because the initial case lived near the county border, they only captured 600 mosquitoes. They were sent to the CDC where they were dissected to look for the parasites.
She got a phone call at 4 p.m. on a Friday from the CDC to report they found three mosquitoes positive for Plasmodium DNA in the midgut. “Do ‘things’ only happen at 4 p.m. on Friday?” she joked.
They started spraying. Unfortunately, only Sarasota had aerial spraying capabilities, so spraying from trucks and by hand were also done. It is important to rotate products, she said, so that the vector does not develop resistance to any one insecticide.
They used every tool they had—even fish—to reduce the mosquito population focusing particularly on old cypress and pine swamps, and non-flowing ditches and canals.
One of the ironies of the outbreak response was that they had rapid tests to help hospitals identify cases, but they did not want them, which surprised her. “Why are you in the middle of an outbreak not taking these? And the answer was because they didn’t have the time and resources to validate the test and to put it in their electronic health records, so they could document those results,” she said.
By the time everything would have been up and running, the outbreak would have been over.
In addition, they could not even use the tests in the field because of the complexity of the tests. They would have needed waivers that could not be sorted fast enough.
“Not every hospital—most hospitals, in fact—don’t have the capacity to do PCR internally. And so when they’re ordering PCR, it’s a send-out test usually to a commercial lab or other reference lab. And so that might not be timely enough for treatment decisions in that case either. And so we certainly don’t recommend relying on PCR alone because the blood smear again is that gold standard,” she said.
And none of their cases had severe malaria. Different drugs were used because the cases occurred over two hospitals: one that had the capacity for testing and one that did not. Some cases require primaquine, which required another test, which often is not back until the person’s already left the hospital, and they need to be brought back in. That can be difficult, especially if the patient has no known address.
And one individual suffered a malaria relapse early in 2024. “So, even when they take the primaquine, it’s not necessarily a done deal. We are keeping an eye on the area,” she said.
The Florida outbreak “highlights the importance of prompt diagnosis, treatment and reporting, and highlights that need for hospitals to have the ability to do blood smears and perform rapid diagnostic tests in-house,” she said.
In addition, it is crucial to work with partners “to better reach our impacted population,” she said.
In all, four states—Florida, Texas, Maryland and Arkansas—saw locally reported cases of malaria in 2023.
