PRO/EDR> Malaria – Americas (09): USA (FL, TX) autochthonous, update


A ProMED-mail post
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International Society for Infectious Diseases

Date: Fri 7 Jul 2023
Source: CNBC [abridged, edited]

The Florida Department of Health on Friday [7 Jul 2023] said 2 cases
of locally acquired malaria have been reported in Sarasota County,
bringing the total in the state to 6.

Last week, the Centers for Disease Control and Prevention issued a
nationwide advisory over the 4 initial Florida cases and one in Texas
to alert health-care providers, local health departments and the
public about the possibility of local malaria transmission.

Those 5 patients “have received treatment and are improving,”
according to the CDC. “Despite these cases, the risk of locally
acquired malaria remains extremely low in the United States,” the
agency added.

The 7 are the first known cases of “locally acquired” malaria in the
country since 2003. That means the new infections were not linked to
foreign travel and appear to have been spread by U.S. mosquitoes
carrying the parasite that causes the disease.

Malaria is a serious and potentially fatal disease typically
transmitted through the bite of an infective female anopheline
mosquito, according to the CDC. It was once endemic in the U.S.,
meaning it occurred regularly and required broad public health
interventions. The risk of the disease is higher in areas where warmer
climate conditions allow those mosquitoes to survive during most of
the year, the agency said.

The U.S. records roughly 2000 malaria cases each year, nearly all of
them in people who acquired the disease abroad, not within the

Health experts say the new locally acquired cases shouldn’t warrant
panic about widespread malaria transmission in the U.S. But they also
note that it’s important for the public to remain vigilant at a time
when climate change and a rebound in international travel increasingly
contribute to the spread of insect-borne diseases. U.S. public health
authorities and health-care providers should also be prepared to ramp
up their surveillance of malaria, experts added.

Investigations by health departments in Texas and Florida confirmed
that none of the 5 cases were directly linked to international travel,
a CDC spokesperson told CNBC. But it’s still unclear how mosquitoes in
the U.S. came to carry malaria. One possible explanation has to do
with the nature of the malaria species identified in both states: P.
, the most common form of the disease.

P. vivax isn’t the deadliest kind of malaria, but it’s more
difficult to treat than other forms, according to Daniel Parker,
associate professor of population health and disease prevention with
the UC Irvine program in public health.

The P. vivax parasite can cause symptoms — which range from fever
to difficulty breathing — soon after infection, like other forms of
malaria. But the parasite can also lie dormant in the liver for days,
months or years before popping up in the bloodstream again and causing
symptoms to reappear, Parker said. During that dormant period, P.
causes no symptoms and remains undetectable in blood tests.

It’s possible a Florida or Texas resident was infected with P. vivax
abroad and returned to the U.S. without realizing they had malaria due
to a lack of symptoms, according to Sadie Ryan, a medical geography
professor at the University of Florida and director of the Florida
Climate Institute. Local mosquitoes could have picked up malaria from
an unknowing traveler after P. vivax became active in their
bloodstream again, and those mosquitoes could have spread it to other
people in the area.

“It might be that one malaria case came to the U.S. from somewhere
else. Then local mosquitoes here picked it up and bit people locally,”
Ryan said. But without more details on the cases, experts say it’s
difficult to offer definitive explanations.

Experts told CNBC that it’s possible for locally acquired malaria
cases to spread to other parts of Florida, Texas or potentially other
states, but the probability is low. That’s largely because public
health authorities responded to the cases quickly and are mostly
equipped to contain local malaria transmission, especially in areas
known to be more suitable for mosquito-borne illnesses.

Ryan said health authorities did a “really good job” alerting the
public and health-care providers about the cases quickly. The CDC and
state-level warnings in Florida and Texas were also timely since they
were issued ahead of the Fourth of July holiday, when more people
typically expose themselves to mosquitoes outdoors, Ryan added. “They
got the message out and said, ‘Beware this is here. Here are the
things you can do to protect yourselves from it,’” she told CNBC.

Local health authorities in Florida and Texas have also carried out
aggressive “vector control” efforts in areas where the cases emerged,
she added. That involves spraying insecticide from the ground or from
a helicopter to kill off mosquitoes capable of carrying malaria.

Several factors are also making the country increasingly vulnerable to
malaria overall, regardless of whether they are local or imported
infections. Climate change is causing a shift in weather patterns that
can worsen malaria conditions, according to Dr. Rajiv Chowdhury, a
global health expert from Florida International University. He said
global warming could lead to “higher mosquito migration and abundance”
in areas of the country that were previously uninhabitable by
Anopheles mosquitoes.

Existing evidence suggests warmer temperatures can increase the growth
rate and transmissibility of the parasites responsible for malaria,
Chowdhury added. He also said climate change can lead to excess
rainfall and sea level rise in the U.S., creating more open spaces
with standing water that serve as “effective breeding grounds” for

It’s unclear whether the new local cases in Texas and Florida are
connected to rising temperatures. Ryan of the Florida Climate
Institute noted those states were already warm enough for the disease
to spread in the first place. Chowdhury agreed: “It’s really difficult
to pinpoint causation for particular cases to the broader
environmental changes that have been occurring. We need a bit more
research to make that connection in the U.S.”. A CDC spokesperson told
CNBC that “it is not clear that the recently reported cases are due to
changes in climate,” even though shifting weather conditions do
influence the distribution of diseases like malaria.

But the agency said a rebound in foreign travel levels this year
[2023] could also increase the number of imported cases of malaria in
the country. The agency last week highlighted its “concern for a
potential rise” in those cases associated with increased international
summer travel that could return to pre-COVID levels.

Parker said increased international travel could potentially lead to
more imported and local infections. “It is possible that we’ll have
more imported cases and since we already have the mosquitoes locally,
it’s possible that they’ll get some and there could be more local
transmission,” he said. But he added: “I wouldn’t say I’m not too
worried about it. As long as we remain vigilant.”

Experts noted there is more work that public health authorities,
health-care providers and people can do to manage the nation’s rising
risk of malaria. U.S. public health authorities should consider which
areas of the country are becoming more suitable for malaria
transmission and how those places can build or bolster the
infrastructure needed to deal with the disease, according to Ryan.
“That’s the sort of realm in which we need to be concerned — to think
about where people should be anticipating this and what they can do to
build that capacity needed to manage the disease with vector control,
public health messaging and other pieces of the puzzle,” she said.

Clinicians can also strengthen their surveillance of the disease by
considering malaria diagnoses in any person with a fever of unknown
origin, regardless of their travel history, according to the CDC.
“It’s possible for someone to come back with malaria and for their
physician to have never seen a malaria case before. So they aren’t
used to dealing with the disease,” Parker said. “But public health
agencies are putting out reports on local cases, so physicians should
have malaria on their radar.”

There is no malaria vaccine available to the U.S. public yet, but
travelers can prevent malaria infections during international travel
using anti-malarial medicines. Those drugs appear to be underused:
Only a quarter of travelers reported taking so-called malaria
prophylaxis in 2018.

It’s easy for people to mistake malaria for a common viral infection
since the disease often causes flu-like symptoms. But the CDC says the
“most important step” people can take is to see a doctor if they are
sick and are presently — or have recently been — in an area with

Getting a diagnosis early on can ensure that a malaria infection is
treated before it becomes serious and life-threatening, the agency
said. “Right now, we should not panic,” Chowdhury said. “But we
definitely need to keep an eye on malaria and take those preparatory

[Byline: Annika Kim Constantino]

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