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• Posted October 20, 2023

A Tropical Skin Infection Spread by Sand Flies Is Spreading in the U.S.

Climate change is bringing diseases once considered tropical afflictions to the United States, and new research warns that a parasite spread by sand flies may be the latest to join this growing list.

The Leishmania parasite causes several forms of the disease leishmaniasis, including cutaneous leishmaniasis, which causes skin sores. Cutaneous leishmaniasis infects up to 1 million people each year, mainly in the Middle East, central Asia, northern Africa and Latin America.

Another potentially life-threatening form of the disease, visceral leishmaniasis, affects the spleen, liver and bone marrow, and a new study suggests this form may be gaining a foothold in U.S. sand flies when they feed on infected dogs brought into the country by well-meaning dog rescue organizations.

The findings were presented Thursday at the annual meeting of the American Society of Tropical Medicine and Hygiene, in Chicago. Such research is considered preliminary until published in a peer-reviewed journal.

"As our weather gets more erratic and the planet as a whole gets warmer and wetter, it becomes a much better place for these bugs to live and create disease,"said study author Christine Petersen, director of the Center for Emerging Infectious Diseases at the University of Iowa.

"We need to up our game and remember these tropical diseases aren't going to be so tropical anymore due to global warming,"she added.

In addition to leishmaniasis, U.S. researchers are also seeing an uptick in other formerly tropical diseases such as malaria and yellow fever, she noted.

How to solve the problem?

As many as 1 million dogs enter the United States every year, most without receiving proper screening for infectious diseases, Petersen said.

"If the dog is infected, leishmaniasis can spread to sand flies where it will be transmitted locally, leading to human disease,"she explained. Petersen first started hearing about leishmaniasis in dogs in 2018 as rescue groups began to bring dogs into the United States from Turkey, where leishmaniasis is endemic.

"It's great that we don't want to go to puppy mills, but we have to remember that dogs can bring in parasites in from the rest of the planet,"she said. As it stands, rescue groups make sure dogs are vaccinated against rabies and often provide deworming medication, but that may be all.

There needs to be an all-hands-on-deck approach to screening these dogs at ports of entry, in addition to wider use of sand fly insecticides and repellents, she stressed.

It's important to know where any rescue pup is from and to make sure the dog is adequately tested for diseases that are endemic to that country.

"This parasite can cross the placenta in pregnant women,"Peterson said. "It is essential to know what the dog could have been exposed to."

There are no drugs to prevent this disease, but there are vaccines for dogs in Europe and Brazil, she noted.

Then there is cutaneous leishmaniasis to worry about, which has typically only been found in people who have recently traveled to endemic areas.

Not anymore.

In a second study presented Thursday at the same meeting, researchers identified a potentially new, locally acquired strain of cutaneous leishmaniasis in the United States.

For the study, researchers analyzed 1,222 samples from people with cutaneous leishmaniasis. Of these, 1,136 were from people reporting international travel and 86 came from non-travelers. Overall, a new strain called Leishmania mexicana was seen among non-travelers, primarily from Texas, while a previously known strain was seen in travelers.

Leishmania mexicana has a slightly different genetic fingerprint, which suggests that it is being spread by local sand fly populations, said study author Dr. Mary Kamb. She is a medical epidemiologist in the division of parasitic diseases and malaria at the U.S. Centers for Disease Control and Prevention's National Center for Emerging and Zoonotic Infections.

"While most of these infections were in people living in Texas, sand flies that can transmit leishmaniasis are found in many parts of the country, and especially in the southern United States,"she said.

The hope is that by identifying the new strain, researchers will be better able to stay on top of its spread. "Cutaneous leishmaniasis needs to be on doctors' radars," Kamb said.

Certain types of cutaneous leishmaniasis can cause sores in the nose, mouth or throat, and can sometimes lead to visceral leishmaniasis, the most severe form of the disease. Treatment is available, but it can be expensive and may require daily hospital visits for roughly a month.

"Some of the things driving climate change, like deforestation, are also contributing to the expansion of leishmaniasis in places like Brazil,"explained Dawn Wesson, an associate professor at Tulane University School of Public Health and Tropical Medicine, in New Orleans.

The good news? Sand flies are susceptible to insecticides, just like mosquitoes, Wesson noted.

"Some of the flea and tick topicals for dogs are also supposed to work for mosquitoes and should therefore work for sand flies, but people need to use them," Wesson said.

More information

Learn more about leishmaniasis at the U.S. Centers for Disease Control and Prevention.

SOURCES: Christine Petersen, PhD, DVM, director, Center for Emerging Infectious Diseases, University of Iowa, Iowa City; Mary Kamb, MD, MPH, medical epidemiologist, division, parasitic diseases and malaria, National Center for Emerging and Zoonotic Infections, U.S. Centers for Disease Control and Prevention, Atlanta; Dawn Wesson, PhD, associate professor, Tulane University School of Public Health and Tropical Medicine, New Orleans; presentations, American Society of Tropical Medicine and Hygiene annual meeting, Chicago, Oct. 19, 2023