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A 'tropical disease' carried by sand flies is confirmed in a new country: the U.S.

Sand flies carry the protozoan parasites that spread leishmaniasis. It was thought to be a disease of tropical climates, but leishmaniasis-causing parasites have now been found living and circulating in the United States.
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Sand flies carry the protozoan parasites that spread leishmaniasis. It was thought to be a disease of tropical climates, but leishmaniasis-causing parasites have now been found living and circulating in the United States.

Back in 2014, dermatologist Bridget McIlwee saw a 3-year-old patient in central Texas with unusual bumps on his ear.

"They looked a little bit like almost kind of a benign mole that you would see in a child, except that you wouldn't expect something like that to come up quickly and then multiply," she says.

McIlwee sent off a sample for laboratory testing, and the results came back pointing to a surprising culprit: The boy had tested positive for cutaneous leishmaniasis, a neglected tropical disease. The World Health Organization says between 600,000 and 1 million new infections happen worldwide every year, mostly in tropical regions of the Americas, the Mediterranean basin, the Middle East and Central Asia — not in Texas. These illnesses can be disfiguring, even if they are rarely fatal.

"I was shocked, because in medical school, we're taught that this is a tropical disease, something that you see in immigrants, military returning from deployment, people who went on vacation to South America or Asia or Africa," McIlwee says.

But this boy hadn't traveled anywhere, meaning he must have picked up the cutaneous leishmaniasis parasite at home in the United States. That led McIlwee and her colleagues to investigate whether the parasite could be living and spreading in North America. They published their results in 2018, arguing that cutaneous leishmaniasis is, in fact, endemic to the United States.

Now the Centers for Disease Control and Prevention is reinforcing those findings via new research presented in October at the American Society of Tropical Medicine and Hygiene's conference, suggesting that a specific strain of the Leishmania parasite has likely been living in the U.S. for years.

Sand flies spread Leishmania with a nasty bite

Mary Kamb, a medical epidemiologist at the CDC who worked on the new research, says cutaneous leishmaniasis is known to be endemic in almost 90 countries but "wasn't thought to be a problem among people living here in the United States until recently."

This disease is spread by sand flies, which Kamb says are tiny. "They're about a quarter of the size of a mosquito. They live in wooded areas and grassland areas, and they primarily bite at dawn and dusk."

McIlwee says Leishmania parasites are often harbored by rodents. Sand flies bite those rodents, get infected with the parasite and pass the parasites on to people.

Most of the infections reported are in Texas — the only state where health workers are required to report leishmaniasis to health authorities — but Kamb says, "The sand flies which have the capacity to transmit leishmaniasis exist in several U.S. states, primarily southern states." McIlwee adds, "We have data supporting that the disease vectors and reservoirs have been located as far north as Ohio."

Vitaliano Cama, a microbiologist at the CDC also involved in the new research, says we know the parasite associated with locally transmitted leishmaniasis in the U.S. has been here "at least since 2005," when the CDC started testing clinical samples for the Leishmania parasite's DNA. He says there have been roughly 80 cases of infection reported over the last 15 years, although, "of course, not everything gets reported."

The parasite responsible for most infections within the U.S., called Leishmania mexicana, causes only the cutaneous form of leishmaniasis rather than the more severe visceral leishmaniasis (the latter affects internal organs rather than the skin and is generally fatal if not treated). The CDC research shows that these parasites are genetically distinct from ones picked up abroad, making clear that they have made a home for themselves in the United States.

Most people never show symptoms

Many people who get cutaneous leishmaniasis may never develop symptoms, but for those who do, the infection "may start out as just a little pimple," according to Peter Melby, an infectious diseases physician and director of the Center for Tropical Diseases at the University of Texas Medical Branch. "But over time, weeks and months, it can grow into a chronic ulcer-type lesion. It's not life threatening, but it can be disfiguring, especially if it's on the face."

Still, Melby says most people never even know they were infected. That's one of the reasons the number of infections reported is so low. But for those who do show progressively worsening symptoms, treatment can be grueling.

"There are several [treatments], but they tend to be pretty severe and people may have side effects," Kamb says. "And if they need to take treatment by mouth, they tend to have a long treatment period of about 28 days."

Kamb says that there aren't specific medical interventions that people can take to protect themselves from leishmaniasis. "There's no effective vaccine against leishmaniasis, and there aren't any drugs that people can take to prevent it," she says. The only things that help, Kamb says, are wearing clothing that covers arms and legs and using insecticide when in an area with sand flies.

Climate change brings "tropical disease" stateside

The CDC has not yet published the new results, but the team released its preliminary findings to get the word out sooner, particularly to clinicians.

But raising awareness about leishmaniasis means challenging some long-held beliefs about this disease. "Every medical textbook, whether it's Dermatology or Principles and Practice of Infectious Disease, teaches that this is a tropical disease," McIlwee says. "So there's a huge disconnect between the clinical reality and what's being taught in medical schools."

One thing that McIlwee believes needs to change is how leishmaniasis infections are tracked. Providers in states besides Texas aren't required to report cases to health agencies.

Medical providers will need to get familiar with leishmaniasis, say experts, as climate change puts more parts of the world at risk for the disease.

"Research projections suggest that the sand fly vector, which is prevalent throughout the southern U.S., over the next number of decades is likely to be able to move further north with climate change," Melby says.

Climate models estimate that there are some 12 million people in the U.S. who could be locally exposed to leishmaniasis— a number projected to more than double over the next 60 years. These Americans join the approximately 1 billion people worldwide who live in areas with endemic leishmaniasis, causing a million infections a year.

Max Barnhart is a Ph.D. candidate and science journalist studying the evolution of heat-stress resistance in sunflowers at the University of Georgia.

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Max Barnhart
Max Barnhart is the 2022 AAAS Mass Media Science and Engineering Fellow at NPR. He is a 5th year Ph.D. candidate and science journalist studying the evolution of heat stress resistance in sunflowers at the University of Georgia.