Once thought to be a danger largely reserved for travelers, a
flesh-eating parasite known as Leishmania Mexicana is now likely
spreading locally through some sand flies native to the southern U.S.,
a new Centers for Disease Control and Prevention analysis has
concluded.
Skin sores caused by Leishmania mexicana typically erupt weeks to
months after people are bitten by an infected fly. The parasite can
continue to inflict damage for years, leading to long-term scarring in
its wake.
Hospitals have some options for drugs to use in trying to treat
patients suffering from this disease caused by the parasite, which
doctors call cutaneous leishmaniasis, though experts acknowledge there
is “low certainty” about their effectiveness. Many patients who
initially respond to treatment and remain at risk of leishmaniasis
relapsing.
No vaccine is available for leishmaniasis.
The immediate concern triggered by the discovery is raising awareness
of the parasite, officials say, which now appears to be “endemic” in
Texas as well as some southern border states.
“This genetic information adds credence to the idea that
leishmaniasis is occurring here in the United States, it’s endemic
here in the United States, at least in Texas and maybe the southern border
states,” Dr. Mary Kamb, of the CDC’s Division of Parasitic Diseases
and Malaria said in an interview with CBS News.
Kamb co-authored the new findings, which were released Thursday [19
Oct 2023] in an abstract at the annual meeting of the American Society
of Tropical Medicine and Hygiene.
A “very distinct” genetic fingerprint
When trying to diagnose patients, doctors typically collect samples by
taking biopsies from their infected lesions.
Those are then often shipped off to the CDC, which until 2020 ran the
only lab outside the military that could test for these parasites.
“For every case that we got, we reported this data. And after those
years, you’re just pulling those results together to do this report,
on cases collected from 2005 to 2019,” said Marcos de Almeida, who
had helmed the CDC lab at the time.
The CDC’s review found a majority of leishmaniasis patients without
travel history was infected by Leishmania mexicana.
Among these cases, 50 were infected by a strain of the parasite which
carried a slight but distinctive set of genetic changes that set them
apart.
“Those 2 snips, those 2 polymorphisms in the sequence, are very
distinct between those who reported travel history compared to those
who did not,” said the CDC’s Vitaliano Cama, a microbiologist and
veterinarian who also worked on the study.
At least one case with this genetic fingerprint has been reported each
year, Cama said.
Battling so-called “vector-borne” diseases like leishmaniasis often
focuses on stamping out bugs that transmit the parasites between
humans, as well as controlling the risk posed by other animals like
rodents that can also serve as a “reservoir.”
Controlling sandflies can pose steep challenges compared to other
larger insects. Sand flies are a fourth of the size of mosquitoes and
do not need standing water to breed.
The World Health Organization estimates that leishmania parasites
infect up to a million each year. Most cases in the Americas are
diagnosed in Brazil, though infections from these parasites have also
been reported throughout Latin America.
For U.S. doctors, leishmaniasis is not a reportable disease in most
states, limiting the CDC’s ability to track whether cases are now
accelerating.
However, Cama said the agency has been fielding a growing volume of
requests to help with diagnosing leishmaniasis cases.
“It’s a trend, it’s not a huge jump, but there’s a trend of increasing
requests,” said Cama.
The discovery also underscores the urgency of new recommendations now
being drawn up to help officials address concerns that a deadlier
cousin — Leishmania infantum — could also gain a U.S. foothold.
Leishmania infantum causes a more severe form of the disease called
visceral leishmaniasis. More than 90% of visceral leishmaniasis
patients who are not given timely treatment die, the Pan-American
Health Organization says.
Symptoms include fever, weight loss, and swelling of the liver and
spleen.
In countries battling outbreaks of visceral leishmaniasis, dogs are
considered “the main reservoir” of the parasite. Now a boom in recent
years of dog arrivals from abroad, either through travel, adoptions, or
moving, has raised concerns of this parasite’s spread.
“All of us have been fielding these calls from state public health
veterinarians, from Army veterinarians, that are trying to figure out
what to do,” said Christine Petersen, director of the University of
Iowa’s Center for Emerging Infectious Diseases.
Sporadic cases of Leishmania infantum have already been reported
spreading into dogs who have not traveled abroad. In addition to
giving the parasite to flies, dogs can also spread the parasite
directly through contact or bites.
Humans can also be unwitting carriers. Blood tests suggest as many as
1 in 5 American soldiers who were deployed to Iraq may have developed
asymptomatic infections.
Working with scientists from the military and the CDC, Petersen
co-authored a new “risk assessment tool” they hope will help
veterinarians and public health officials figure out how to handle
infected dogs if they catch the parasite. A draft of the tool was
previewed at the ASTMH’s meeting.
Decisions on whether to euthanize the dog can be made guided by
weighing factors like where the dog is from and the likelihood of
local sand flies which might transmit the parasite.
“That’s what this whole risk assessment thing was about. Aside from
rabies, maybe we should start talking about the other things
that dogs carry that can kill us,” said Petersen.
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