A tropical disease once seen exclusively among Americans returning from overseas travel now has a uniquely American strain.
Health officials warn that a related, deadly parasite found in other countries may thrive in the United States because of these improved climates for disease.
A parasite called Leishmania, historically found in tropical climates, is spread when sand flies bite people. Sandflies carrying the parasite also infect other mammals, such as woodrats, which further allow its movement. Some researchers suggest that climate change is expanding the geographic range of sand flies and, as a result, the range of the disease.
A million dogs enter the country annually and the associated parasite goes undiagnosed. There is insufficient screening in the United States for the parasite, which the researchers hope to address.
Earlier infections came to the Americas when people traveling from warmer regions brought back the disease. There is no federal reporting of the disease in the United States, making it difficult to understand its sudden spread in recent years. But new findings from the Centers for Disease Control and Prevention indicate cases of a milder form of the disease, cutaneous leishmaniasis, are derived from a slightly different strain of the American parasite.
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“It’s a disease that we don’t really think about in the United States,” said Dr. Mary Kamp, a clinical 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 that one million people are infected with cutaneous leishmaniasis annually. Affected populations are mostly in warm climates such as the Mediterranean, the Middle East and Latin America. However, health officials suspect that in warmer South American states, including Texas, conditions are more suitable for sand flies to thrive and spread disease.
The disease can disfigure people’s skin with sores that last weeks or months after biting a person. It can leave a scar that researchers say is recognizable and carries a social stigma in low-income countries. Cutaneous leishmaniasis does not cause death or severe disability, they said.
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On Thursday, Kamp and other CDC researchers analyzed cases sent to CDC labs for testing from 2005 to 2019 at the American Society of Tropical Medicine and Hygiene annual conference in Chicago. The CDC findings are based on more than 2,000 cases. US, Puerto Rico and US Virgin Islands. Eighty-six of the study participants had not traveled abroad before contracting leishmaniasis.
Although cases found in the U.S. typically involve genetic strains from abroad, the CDC researchers’ analysis indicates that the parasite strain that has been circulating in the U.S. for years is slightly different from the parasite strain of Leishmania mexicana commonly found in Mexico and Central America.
The findings suggest the local strain has been circulating for some time, and the study authors suggest the U.S. develop better screening for the disease, Kamp said.
Monitoring is difficult without national reporting
Other research, in addition to the study presented this week, has previously identified leishmaniasis in the United States
While practicing in Texas, Dr. Bridget McElwee, now a dermatologist in Springfield, Illinois, worked with a patient with no history of international travel who developed leishmaniasis. The man had small bumps on his ear. The marks didn’t resemble the textbook pictures of the cases, McElwee noted, so it could have been mistaken for another benign health condition, he said. After a biopsy on the bumps, tissue samples were consistent with leishmaniasis.
A 2018 studyMcIlwee analyzed additional cases reported to the Texas Department of State Health Services from 2007 to 2017 that included leishmaniasis as a reportable condition. About 59% of cases involved patients who had not traveled abroad for 10 years.
Even in cases where doctors diagnosed leishmaniasis, few were actually reported to public health officials, McIlwee added.
“If it’s not required to be reported, we can’t track it nationally,” McIlwee said. “That’s another interesting piece of the puzzle, because without a national reporting mechanism it would be difficult for us to track it.”
Officials warn that dogs can carry even more deadly diseases
As conditions warm due to climate change, sand fly habitats are expected to expand northward, increasing vectors and reservoirs for leishmaniasis, said McElwee, who is involved in upcoming research on projections of the parasite’s spread.
Increased evidence of cutaneous leishmaniasis among American sandfly populations also increases the risk of a severe form of the tropical disease in the United States.
The focus of this week’s presentation introduces another element to the exchange: dogs.
The deadly disease, known as visceral leishmaniasis, is sometimes spread from imported dogs that carry the pathogen to communities, which can sometimes spread among local pest populations, according to researchers from the University of Iowa, the US Army Veterinary Services, Johns Hopkins University and the CDC. Risks on Thursday. The strain, like the skin-related disease, is transmitted by sand fly bites, but visceral leishmaniasis has a related parasite, Leishmania infantum, which infects organs and kills more than 20,000 people annually in areas where the parasite thrives.
There is no human vaccine for leishmaniasis, but dog vaccines are available in Europe and Brazil. Researchers plan Thursday to demonstrate a new tool to promote screening at ports of entry.
In both mild and severe forms of leishmaniasis, the United States must work with other countries to combat the epidemic, says Dr. Daniel Bosch, president of the American Society of Tropical Medicine and Hygiene.
“A global approach is critical,” he said in a statement, “as 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 approached [email protected].