A tropical parasite, passed through the bite of a sand fly, is causing skin infections in the US

Move over mosquitos. There’s another blood-sucking biter Americans need to guard against because it can spread disease: the sand fly.

Sand flies are tiny tan flies — about the quarter of the size of a mosquito — that live in warm, wet, rural and forested areas. In other parts of the world, they are known to transmit a parasite — a single celled organism — that causes an infectious disease called leishmaniasis. They’re most active at night, and they’re so tiny they can slip through ordinary mosquito nets on tents or window screens.

“Sometimes you don’t even notice that you’ve been bitten,” said Dr. Mary Kamb, a medical epidemiologist at the US Centers for Disease Control and Prevention in Atlanta, one of the few labs in this country that can distinguish the species of Leishmania parasite that has infected a person.

Kamb and her colleagues at the CDC say they’ve now detected leishmaniasis in a number of tissue samples from patients who say they have not traveled outside the United States. They conducted a genetic analysis of these samples and collected them into a new study that was presented today at the annual meeting of the American Society of Tropical Medicine and Hygiene. These patients all had leishmaniasis skin infections, which start with a small bump that erupts into ulcerous sores days to weeks after a sand fly bite.

“People could be asymptomatic and not develop anything, but when people are symptomatic, they develop ulcers on their skin and sometimes it starts like a little tiny volcano with a crater in it,” Kamb said. These sores often erupt near the site of a recent bite. The parasite disables nerves in the skin, so the sores generally are not painful but tend to scar and can be disfiguring, especially if they occur on a person’s face.

Leishmania can also infect internal organs including the liver, spleen, and bone marrow.  This more severe type of leishmaniasis, called visceral leishmaniasis, is not believed to be transmitted in the United States, but people can be exposed by traveling to tropical countries.  It’s fatal if left untreated.

Most of the skin samples in the CDC study came from Texas, which is currently the only US state that requires doctors to report leishmaniasis cases. A recent research review published in 2021 says locally acquired cases of the infection have also been reported from southeast Oklahoma.

About half a dozen cases of leishmaniasis skin infections are reported in non-travelers in the U.S. each year, said Dr. Luiz Oliveira, a staff scientist at the National Institutes of Health who has studied the transmission of leishmaniasis from sand flies but was not involved in the current research. The World Health Organization lists the disease as endemic, or regularly occurring, in this country, though most people and doctors aren’t aware of it.

“It’s not just a traveler’s disease anymore,” Oliveira said.

Sand flies capable of carrying the parasite that causes the infection can be found in a number of southern and southwestern states.

Like other types of insects that are finding new habitats as the climate warms, sand flies have been expanding their range in recent years.

Some types of these biting flies can now be found as far north as Delaware, New Jersey, Ohio and Maryland. It’s not clear whether sand flies in all these states have transmitted infections to humans, however.

“If the average temperature increases in northern states, which is expected, then sand flies will be able to establish themselves there and then you only need the parasites to have transmission,” said Dr. Pedro Cecilio, a postdoctoral researcher at the NIH, who has studied leishmaniasis transmission.

For the current study, which is being presented at the annual meeting of the American Society of Tropical Medicine and Hygiene in Chicago, the researchers analyzed more than 2,100 skin samples sent to the CDC for confirmatory testing between 2005 and 2019. The samples were submitted from all 50 states, Puerto Rico and the Virgin Islands. About half the samples, 1,222, tested positive. Most of the positive samples — 1,136 — were from people reporting a history of international travel. Eighty-six were from patients with no travel history.

The CDC found the most common species of parasite among people without any history of travel was a bug known as Leishmania mexicana.

Within this species, the CDC detected two distinct genotypes, or lineages. One of those, dubbed CCC, was present in nearly 94% of non-travelers who were infected, suggesting that this may be the particular type of parasite that’s become endemic — or locally transmitted — in the United States.

Dr. Gideon Wasserberg, a professor of disease ecology and medical entomology at the University of North Carolina at Greensboro, has studied sand flies and leishmaniasis in the United States but did not participate in the current research.

He says the new study offers solid confirmation that there’s a species of leishmania now being transmitted in the United States, something entomologists have suspected was happening within the last 5 to 10 years. He was also interested to see that in addition to mexicana, two other species of leishmania were reported from patients who had not recently traveled outside the United States.

Wasserberg says the parasite is thought to be carried by rats. When sand flies bite the infected rats, they can pick up the parasite and then pass it to humans. People can’t usually pass the infection to each other.

Wasserberg says there’s still a lot to learn about where sand flies are spreading leishmaniasis in the United States, but for now, it’s a good idea to take precautions if you find yourself in a warm, rural area.

Sand flies are repelled by bug sprays that contain DEET, and they can be killed by permethrin, so it’s a good idea to spray camping equipment and clothing for protection.

Wasserberg says it’s a smart to seek treatment for any new skin sore that erupts a few days after a bug bite and doesn’t seem to go away. It’s also a good idea to let your doctor know about any recent outdoor activities, especially if you were out at night.

Treatment for leishmaniasis involves a month-long course of treatment with medications that stymie the parasite, such as amphotericin B, an antifungal drug that is also approved by the US Food and Drug Administration to treat leishmaniasis.

Wasserberg says he’s glad to see the study because awareness of the disease in the U.S. is so low.

“Most doctors, if you ask them, ‘Is there leishmania in the U.S.?’ They’ll say ‘no way’ or ‘What is that?’ ” he said.