CDC: Tropical diseases are now endemic in the United States. In a more deadly form, it comes via dogs.
The tropical disease once seen almost exclusively among Americans returning from travel abroad is now a uniquely American strain.
Health officials warn that a more deadly parasite seen in other countries could flourish in the United States due to improved climate conditions for the disease.
The parasite known as Leishmania is spread when sandflies, historically found in tropical climates, bite people. Sand flies that carry the parasite also infect other mammals, such as wood mice, allowing them to be more mobile. Some researchers say climate change may expand the sandfly's geographic range and thus spread the disease.
The related parasite goes undetected by the million dogs entering the country annually. The United States does not have adequate testing for the parasite, something researchers hope to address.
Previous infections came to the United States when people traveling from warmer regions returned the disease. The United States has no federal reporting on the disease, making it difficult to understand its sudden spread in recent years. But new findings from the Centers for Disease Control and Prevention point to cases of a milder form of the disease, cutaneous leishmaniasis, derived from a slightly different American parasite strain.
"This is a disease that we don't think about in the United States," said Dr. Mary Camp, a medical epidemiologist in the Division of Parasitic Diseases and Malaria at the Centers for Disease Control and Prevention. "It is a disease that belongs to other countries."
The World Health Organization estimates that up to 1 million people develop cutaneous leishmaniasis annually. Most affected populations are found in regions with warmer climates, such as the Mediterranean, the Middle East, and Latin America. However, health officials suspect warmer southern US states, including Texas, have more favorable conditions for sand flies to thrive and transmit the disease.
The disease can disfigure people's skin with sores that sometimes take weeks or months to appear after a person is bitten. It can leave recognizable scars that researchers say bring social stigma in low-income countries. They said that cutaneous leishmaniasis does not cause death or severe disability.
On Thursday, Camp and other CDC researchers presented an analysis at the American Society of Tropical Medicine and Hygiene annual conference in Chicago that looks at cases sent to CDC laboratories for testing from 2005 to 2019. The CDC findings are based on the fact that its prevention has led to more than 2,000 cases across the United States, Puerto Rico, and the US Virgin Islands. Eighty-six people participating in the research had yet to travel abroad before contracting leishmaniasis.
While cases found in the United States typically have genetic strains from abroad, an analysis by CDC researchers suggests that the parasitic strain circulating in the United States for years is little different from the parasitic strain of Leishmania mexicana that is commonly found in Mexico and Central America.
Camp said the findings suggest the local strain has been circulating for some time, and the study authors recommend the United States develop better screening for the disease.
In addition to the study presented this week, other research has previously found that leishmaniasis occurs within the United States.
While training in Texas, Dr. Bridget McElwee, a dermatologist now based in Springfield, Illinois, worked with a patient with no history of international travel who developed leishmaniasis. The man had small bumps on his ears. McElwee noted that the signs do not look like school pictures of the cases, so she said they could be confused with another benign health condition. After a biopsy on the bumps, the tissue samples were consistent with leishmaniasis.
In a 2018 study, McElwee examined additional cases, most reported to the Texas Department of State Health Services, including leishmaniasis as a reportable condition from 2007 to 2017. About 59% of the cases involved patients who had not traveled abroad for ten years.
McElwee added that even for the cases where doctors diagnosed leishmaniasis, few were reported to public health officials.
"If it weren't required to be reported, we wouldn't be able to track it nationally," McElwee said. "This is another interesting puzzle piece because monitoring it without a national reporting mechanism would be difficult."
As temperatures rise due to climate change, sandfly habitats are expected to expand northward, increasing the vectors and reservoirs of leishmaniasis, said McElwee, who has upcoming research on the outlook for the parasite's spread.
Growing evidence of cutaneous leishmaniasis among American sandfly populations also increases the risk that a more severe form of tropical disease will emerge in the United States.
The most deadly disease the researchers studied is visceral leishmaniasis, and it sometimes spreads among local insect populations after spreading from imported dogs carrying the pathogen into communities, according to researchers from the University of Iowa and the Army Veterinary Services.
Johns Hopkins University and the CDC, who planned the risk briefing on Thursday. This strain is transmitted in the same way as the skin-associated disease via sandfly bites, but visceral leishmaniasis contains a related parasite, Leishmania infantum, which affects organs and kills up to 20,000 people a year in areas where the parasite thrives.
There is no human vaccine against leishmaniasis, but vaccines are available for dogs in Europe and Brazil. Researchers on Thursday planned to demonstrate a new tool to enhance screening processes at ports of entry.
With both mild and severe forms of leishmaniasis, the United States needs to work with other countries to combat infectious diseases, according to Dr. Daniel Bausch, president of the American Society of Tropical Medicine and Hygiene.
"A global approach is particularly important because climate change allows insects that carry pathogens such as Leishmania, dengue virus, and malaria to expand their range," he said.
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