Biting Sand Flies Are Spreading a Skin-Scarring Parasite in U.S.

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Oct. 20, 2023 – A parasitic skin disease spread by sand flies that was once only linked to international travel is no longer considered an “imported” disease in the U.S. 

Called cutaneous leishmaniasis, the disfiguring condition resulting from sand fly bites has now evolved into a form unique to the American South. The disease can be difficult to diagnose. At one time, it was so rare that it was not often considered possible when making a diagnosis. It can also mimic other diseases like leprosy, skin cancer, and fungal skin conditions, according to the CDC, which discussed the topic at the annual meeting of the American Society of Tropical Medicine and Hygiene (ASTMH) this week in Chicago.

Cutaneous leishmaniasis has been diagnosed in 86 people in the U.S. in the past decade who had not traveled internationally, the CDC researchers reported. They used genetic sequencing tools to analyze tissue samples from infected people to determine that the non-travelers had a unique version of the disease, meaning it was being spread by local fly populations.

“There have been previous indications of local transmission based on a small number of case reports, but now, for the first time, we have a distinct genetic fingerprint from a relatively large cluster, providing further evidence that leishmaniasis may be well-established in some parts of the United States,” Mary Kamb, MD, MPH, with the Division of Parasitic Diseases and Malaria at CDC’s National Center for Emerging and Zoonotic Infections, said in a statement. “While most of these infections were in people living in Texas, sand flies that can transmit leishmaniasis are found in many parts of the country and especially in the southern United States.”

The risk for sand fly bites is greatest at twilight and at night, when the insects are feeding. The disease can cause a variety of skin problems, including large nodules or plaques that usually show up weeks or months after infection. Sores from the disease can change in appearance and size over time, becoming open sores with a raised border and central crater or ulcer that can be covered with crust or scales, the CDC says. Even without treatment, most sores eventually heal, even after lasting for months or years. The resulting scarring is disfiguring.

Scientists are concerned that a more lethal form of the disease called visceral leishmaniasis, spread also by sand flies, could gain a foothold in the U.S. The flies feed on imported dogs that carry a parasite called Leishmania infantum. The disease attacks internal organs and kills between 20,000 and 30,000 people globally per year, according to a news release from the ASTMH.

A veterinarian and infectious disease expert said 1 million dogs are imported to the U.S. annually, and many don’t receive proper screening for infectious diseases.

“Dogs are the primary host for this disease, and there are dogs now regularly coming into the U.S. that have lived in areas where leishmania parasites circulate in animals and people,” said Christine Petersen, DVM, PhD, who is director of the Center for Emerging Infectious Diseases at the University of Iowa, according to the ASTMH. “That’s why we need a better system in the United States for guarding against the risk of introducing Leishmania infantum, one of the world’s deadliest tropical parasites, into U.S. sand fly populations.”