Hidden Threat: Unveiling the Rising Epidemic of Skin Infections Caused by Tropical Parasites in the US

Forget about mosquitos. Another blood-sucking creature that poses a disease-spreading risk to Americans is the sand fly. These minuscule tan flies, about a quarter of the size of mosquitos, thrive in warm, wet, rural, and forested regions. In various parts of the globe, they are notorious for transmitting leishmaniasis, an infectious disease caused by a parasite consisting of a single celled organism. These pests are predominantly active during the night and are so small that they easily infiltrate regular mosquito nets on tents or window screens.

Dr. Mary Kamb, a medical epidemiologist at the US Centers for Disease Control and Prevention in Atlanta, mentioned that people may not even realize when they have been bitten. Her team at the CDC has identified cases of leishmaniasis in patients who have not traveled outside the United States. Through genetic analysis of tissue samples, they have gathered evidence for a new study presented at the annual meeting of the American Society of Tropical Medicine and Hygiene. The patients in question have all developed leishmaniasis skin infections, which manifest as small bumps that later become ulcerous sores after being bitten by a sand fly.

"People who are asymptomatic may not experience any symptoms, but when symptoms do occur, they typically manifest as ulcers on the skin. These ulcers can resemble small volcanoes with craters and often appear near the site of a recent insect bite. The parasite responsible for this condition weakens the nerves in the skin, resulting in non-painful sores that tend to leave scars. If these ulcers develop on a person's face, they can be disfiguring.

A child with leishmaniasis on her hands in Syria.

Muhammed Abdullah/Anadolu Agency via Getty Images

Leishmania can infect internal organs such as the liver, spleen, and bone marrow, causing a more severe form of leishmaniasis known as visceral leishmaniasis. This type of leishmaniasis is not believed to be transmitted within the United States, but individuals can be exposed by traveling to tropical countries. If left untreated, visceral leishmaniasis can be fatal.

The majority of skin samples analyzed in the CDC study were obtained from Texas, which is currently the only state in the US that requires doctors to report leishmaniasis cases. However, a recent research review published in 2021 also reveals locally acquired cases of the infection in southeast Oklahoma.

Dr. Luiz Oliveira, a staff scientist at the National Institutes of Health who has conducted research on leishmaniasis transmission from sand flies but was not involved in the current research, stated that there are approximately six cases of leishmaniasis skin infections reported in non-travelers in the U.S. each year. Although the World Health Organization classifies the disease as endemic in the country, it is not widely recognized by the general public or medical professionals. According to Oliveira, leishmaniasis is no longer exclusive to travelers.

Jim Gathany / CDC

Locally acquired malaria case reported in Arkansas

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

Sand flies, like other insects adapting to a warmer climate, have been extending their geographic area in recent years. There are now sightings of certain species of these biting insects as far north as Delaware, New Jersey, Ohio, and Maryland. It remains uncertain if sand flies in these states have transmitted any infections to humans.

Dr. Pedro Cecilio, a postdoctoral researcher at the NIH, explains that if the average temperature increases in northern states as expected, sand flies will be able to establish themselves there, leading to the transmission of parasites. The researchers, who are presenting their findings at the annual meeting of the American Society of Tropical Medicine and Hygiene in Chicago, analyzed over 2,100 skin samples sent to the CDC for confirmatory testing between 2005 and 2019. These samples were obtained from all 50 states, Puerto Rico, and the Virgin Islands. Among the samples, 1,222 tested positive for leishmaniasis, with the majority coming from individuals with a history of international travel (1,136 samples) and a smaller number from patients with no travel history (86 samples).

The CDC discovered that the prevailing parasite species among individuals with no travel history was Leishmania mexicana, commonly referred to as a bug.

Among this species, the CDC identified two separate genotypes or lineages. Out of these, one called CCC was found in almost 94% of non-travelers affected, indicating that this may be the specific parasite type that has become established or locally transmitted within the United States.

The National Environmental Agency's mosquito production facility in Singapore showcases a visual representation of Wolbachia-Aedes aegypti mosquitoes on August 19, 2020. The accompanying photograph captures the scene accurately. (Photo: REUTERS/Edgar Su; August 19, 2020)

Edgar Su/Reuters

Rare mosquito-borne virus reported in Alabama and New York, with one death

Dr. Gideon Wasserberg, a professor at the University of North Carolina at Greensboro specializing in disease ecology and medical entomology, did not take part in the present research but has previously conducted studies on sand flies and leishmaniasis in the United States. According to him, the recent study provides solid evidence for the presence of a leishmania species being transmitted within the United States, a suspicion that entomologists have had for the past 5 to 10 years. Additionally, he found it intriguing that, aside from mexicana, two other species of leishmania were detected in patients who had not recently traveled outside the United States.

Wasserberg explains that the parasite is believed to be transmitted by rats. When sand flies bite these infected rats, they can acquire the parasite and subsequently transmit it to humans. It is important to note that humans cannot typically transmit the infection amongst one another.

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Subscribe now to receive The Results Are In with Dr. Sanjay Gupta every Tuesday, brought to you by the CNN Health team. Stay informed about the spreading of leishmaniasis in the United States and take necessary precautions when in warm rural areas, as there is still much to discover about the distribution of sand flies.

Bug sprays containing DEET are effective in repelling sand flies, while permethrin can effectively kill them. To ensure protection, it is advisable to spray camping equipment and clothing.

If a new skin sore develops a few days after being bitten by a bug and does not heal, seeking treatment is recommended, as advised by Wasserberg. Additionally, it is important to inform your doctor about any recent outdoor activities, particularly if they occurred at night.

The treatment of leishmaniasis requires a month-long medication course that inhibits the parasite. One such medication is amphotericin B, an antifungal drug that has also been authorized by the US Food and Drug Administration for treating leishmaniasis.

Wasserberg expresses his satisfaction with the study, as awareness of the disease in the U.S. is alarmingly low.

"Most doctors, if you ask them, Is there leishmania in the U.S.? Theyll say no way or What is that? " he said.