In 2014, dermatologist Bridget McIlwee encountered an unusual case in central Texas. A 3-year-old patient had developed peculiar bumps on his ear that resembled benign moles but appeared rapidly and multiplied. McIlwee sent a sample for laboratory testing, which surprisingly revealed a diagnosis of cutaneous leishmaniasis, a neglected tropical disease. Cutaneous leishmaniasis is known to affect regions such as the Americas, the Mediterranean basin, the Middle East, and Central Asia, not typically the United States.
The disease can be disfiguring but is rarely fatal. Traditionally, medical professionals associated cutaneous leishmaniasis with travelers, immigrants, military personnel returning from deployment, or individuals vacationing in tropical areas. The case of the Texas boy who hadn’t traveled anywhere led McIlwee and her colleagues to investigate whether the parasite responsible for cutaneous leishmaniasis could be endemic to North America.
In 2018, they published their findings, suggesting that the disease is indeed endemic in the United States. Recent research by the Centers for Disease Control and Prevention (CDC), presented in October at the American Society of Tropical Medicine and Hygiene’s conference, further supports the idea that a specific strain of the Leishmania parasite has likely been living in the U.S. for years.
Cutaneous leishmaniasis is transmitted by sand flies, which are tiny insects about a quarter the size of mosquitoes. These flies inhabit wooded and grassland areas and primarily bite at dawn and dusk. Leishmania parasites are often carried by rodents, and sand flies become infected when they bite these rodents, subsequently passing the parasites to humans.
Vitaliano Cama, a microbiologist at the CDC, involved in the new research, indicates that the parasite associated with locally transmitted leishmaniasis in the U.S. has been present since at least 2005. The CDC has tested clinical samples for the Leishmania parasite’s DNA, and approximately 80 cases of infection have been reported over the past 15 years, though not all cases are reported.
The predominant parasite responsible for infections in the U.S. is Leishmania mexicana, causing the cutaneous form of leishmaniasis, which affects the skin rather than internal organs. The CDC’s research shows that these parasites are genetically distinct from those found abroad, indicating their establishment in the United States.
Many people who contract cutaneous leishmaniasis may never develop symptoms. For those who do, the infection can initially manifest as a small pimple, which can progress into a chronic ulcer-like lesion over weeks and months. While not life-threatening, these lesions can be disfiguring, particularly if they develop on the face.
However, the low number of reported infections can be attributed to many people never realizing they were infected. For those with worsening symptoms, treatment can be challenging, involving several options with potential side effects and, if administered orally, a long treatment period of around 28 days. Unfortunately, there are no specific medical interventions to prevent leishmaniasis. No effective vaccine or drugs exist to protect against it.
The only recommended preventive measures are wearing clothing that covers arms and legs and using insecticide when in areas where sand flies are present. As climate change continues to impact the distribution of diseases, more parts of the world, including the U.S., become vulnerable to leishmaniasis. The research suggests that the sand fly vector, prevalent in the southern U.S., may extend its range northward in the coming decades due to climate change.
Climate models estimate that around 12 million people in the U.S. could be locally exposed to leishmaniasis, a number expected to more than double over the next 60 years. This situation places Americans at risk alongside the approximately 1 billion people worldwide who live in areas with endemic leishmaniasis, resulting in a million new infections each year.
News Reference
Npr, “A ‘tropical disease’ carried by sand flies is confirmed in a new country: the U.S.” https://www.npr.org/sections/goatsandsoda/2023/11/01/1209681147/leishmaniasis-sand-flies-tropical-disease-endemic-north-america-united-states.
