Tropical disease now endemic in U.S., CDC says. In deadlier form, it's coming via dogs.

Eduardo Cuevas, USA TODAY
Updated Thu, October 19, 2023

A tropical disease once seen almost exclusively among Americans returning from travel abroad now has a unique U.S. strain.

Health officials warn that a related, deadlier parasite seen in other countries could thrive in the U.S. due to these improved climatological conditions for the disease.

The parasite known as leishmania spreads when sandflies, historically found in tropical climes, bite people. Sandflies carrying the parasite also infect other mammals such as woodrats which further allow its movement. Climate change, some researchers say, may be expanding the geographical reach of sandflies and, consequently, the reach of the disease.

A related parasite also comes in undetected by way of one million dogs entering the country annually. The U.S. doesn't have adequate screening in place for the parasite, which is something researchers hope to address.

Previous infections came to the U.S. when people traveling from warmer areas brought the disease back. The U.S. doesn’t have federal reporting on the disease, making it difficult to understand its sudden prevalence in recent years. But new findings from the Centers for Disease Control and Prevention indicate cases of the milder form of the disease, cutaneous leishmaniasis, derived from a slightly different American parasitic strain.

“This is a disease that we in the United States don’t really think about,” said Dr. Mary Kamb, a medical epidemiologist at the Centers for Disease Control and Prevention’s Division of Parasitic Diseases and Malaria. “It’s really a disease that belongs to other countries.”

The World Health Organization estimates as many as one million people get cutaneous leishmaniasis annually. The populations infected are mostly in areas with warmer climates such as the Mediterranean, the Middle East and Latin America. However, health officials suspect warmer southern American states, including Texas, have more suitable conditions for sandflies to thrive and pass on the disease.

The disease can disfigure people’s skin with ulcers that sometimes take weeks or months to show after a person has been bitten. It can leave scarring that researchers say is recognizable and brings a social stigma in low-income countries. Cutaneous leishmaniasis doesn’t cause death or severe disability, they said.

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On Thursday, Kamb and other CDC researchers presented an analysis at the American Society of Tropical Medicine and Hygiene’s annual conference in Chicago looking at cases sent to CDC laboratories for testing from 2005 to 2019. The CDC findings are based on more than 2,000 cases across the U.S., Puerto Rico and the U.S. Virgin Islands. Eighty-six of the people involved in the research had not traveled abroad prior to developing leishmaniasis.

While cases found in the U.S. typically have genetic strains from abroad, the CDC researchers’ analysis indicates that a parasitic strain circulating in the U.S. for years is slightly different from the parasitic strain of leishmania mexicana that’s typically found in Mexico and Central America.

The findings suggest the local strain has circulated for some time, and study authors recommend the U.S. develop better screening of the disease, Kamb said.
Hard to track without national reporting

Other research, in addition to the study presented this week, has previously found leishmaniasis occurring within the U.S.

While training in Texas, Dr. Bridget McIlwee, a dermatologist now based in Springfield, Illinois, worked with a patient with no history of international travel who contracted leishmaniasis. The man had small bumps on his ears. The marks didn’t resemble textbook images of cases, McIlwee noted, so she said it could have been confused for another benign health condition. After performing a biopsy on the bumps, tissue samples matched leishmaniasis.

In a 2018 study, McIlwee examined additional cases, mostly reported to the Texas Department of State Health Services, which includes leishmaniasis as a reportable condition, from 2007 to 2017. About 59% of the cases involved patients who had not traveled abroad for 10 years.

Even for cases that doctors diagnosed leishmaniasis, few were actually reported to public health officials, McIlwee added.

“If it's not required to be reported, then we can't really track it nationally,” McIlwee said. “That is another interesting piece of the puzzle because it's going to be hard for us to keep an eye on it without a mechanism for national reporting.”

Dogs could bring more deadly disease, officials warn

As conditions warm due to climate change, habitats for sandflies are projected to expand northward, increasing vectors and reservoirs for leishmaniasis, said McIlwee, who has upcoming research on projections about the spread of the parasite.

The increased evidence of cutaneous leishmaniasis among American sandfly populations also increases the risk of a more severe form of the tropical disease surfacing in the U.S.

The focus of this week's presentation introduces another element for transmission: Dogs.

Known as visceral leishmaniasis, the deadlier disease researchers studied sometimes circulates among local insect populations after spreading from imported dogs carrying the pathogen into communities, according to researchers from the University of Iowa, U.S. Army Veterinary Services, Johns Hopkins University and the CDC who planned to present on the risks on Thursday. This strain is transmitted the same way as the skin-related disease, via sandfly bites, but visceral leishmaniasis contains a related parasite, leishmania infantum, that affects organs and kills upwards of 20,000 people annually in regions where the parasite thrives.

There isn’t a human vaccine for leishmaniasis, but vaccines are available for dogs in Europe and Brazil. Researchers on Thursday planned to show a new tool to promote screenings at ports of entry.

With both mild and severe forms of leishmaniasis, the U.S. needs to work with other countries to fight infectious diseases, according to Dr. Daniel Bausch, the president of the American Society of Tropical Medicine and Hygiene.

“A global approach is especially important," he said in a statement, "since climate change allows insects that carry pathogens like Leishmania, dengue virus and malaria to expand their range.”

Eduardo Cuevas covers health and breaking news for USA TODAY. He can be reached at EMCuevas1@usatoday.com.

This article originally appeared on USA TODAY: Tropical disease endemic, CDC says. Coming in deadlier form via dogs.