Flesh-eating parasites are spreading across the U.S., and climate change could make this nightmare worse

As the planet warms, more and more Americans may be at risk from various forms of Leishmania, scientists warn.
Leishmania parasites inside and outside cells in tissue from a patient with cutaneous leishmaniasis. Image credit: Wikimedia, CC BY-SA 4.0

Source: Undark Author: Agostino Petroni Translator: Xu Chuchu Editor: Qi Yiyin

Three years ago, Laura Gaither from Alabama was spending her summer vacation with her family in Panama City Beach, Florida. One afternoon, while washing the sand off her feet, the 35-year-old Gaither felt something biting her legs. She found that they were small black bugs. Gaither brushed them off with her hands. Later, when she described the bites to local residents, they told Gaither that she might have been bitten by a sand fly.

Three of Gaither's five children have also been bitten by sand flies, but she's not worried. The marks on their legs and arms look like ant or mosquito bites, and while they burn and itch, they usually fade within a week.

But after about two weeks, when Gaither returned home, she noticed that the bites had turned into small open wounds. Over the next few weeks, the condition gradually worsened. But when she took the children to the pediatrician, "the doctor just blamed it on eczema," Gaither said. The younger girl's condition was the most worrisome, so Gaither took her to the emergency room at Children's Hospital of Alabama, where she was tested for fungal and bacterial infections. The final test results were negative, which meant that the antifungal and steroid topical ointments prescribed by the doctor were not effective. Meanwhile, the ulcers on the younger girl's skin grew larger and more painful.

Gaither began doing his own research and learned about a muscle-eating disease called cutaneous leishmaniasis. The skin disease is caused by more than 20 species of Leishmania and can be transmitted to humans through the bites of sand flies, which become infected when they feed on the blood of animals, usually rodents in the United States. Gaither was also told during his travels that sand flies thrive in hot, sandy and rural areas, and that in 2018, there were particularly high numbers of sand flies in Florida.

Gaither also learned that leishmaniasis is very common in tropical and subtropical countries such as Brazil, Mexico and India. While reviewing some peer-reviewed papers, she saw pictures of leishmaniasis wounds that looked very similar to her own: crater-like ulcers covered with a thick layer of yellow pus.

During visits to her pediatrician and in the emergency room, Gaither asked doctors about leishmaniasis. But they ruled out the possibility that the family had contracted a tropical disease without traveling abroad, Gaither said. “No one was even willing to consider my hypothesis.” It wasn’t until the wound on her knee started to worsen that she finally persuaded doctors to perform a biopsy for leishmaniasis, armed with the research papers. The results were still inconclusive.

Fortunately, the wounds of the Geisser children have gradually begun to heal. The ulcers finally disappeared completely three months after they appeared, and Geisser is still wondering what caused this situation. Although her family's suffering has ended, scientists say that the story of leishmaniasis in the United States has just begun.

It turns out that Americans can get infected with Leishmania without leaving the country. The parasite is currently endemic in Texas and Oklahoma, and new research suggests it may be present in other states, including Florida. While the number of reported cases of leishmaniasis infections in the United States is currently negligible, that number may soon rise. As climate change pushes the habitats of rodents and sand flies northward, more and more U.S. residents could be exposed to different species of the flesh-eating parasite in the future, scientists warn.

It is important to note that some strains of Leishmania may be life-threatening. The strain currently present in the United States is Leishmania mexicana, which causes milder symptoms that resolve on their own over time. But if doctors take it lightly or overreact, the wrong treatments and unnecessary systemic toxic drugs may cause more harm than the disease itself.

Bridget McIlwee, a dermatologist in Illinois who has treated patients with leishmaniasis in Texas, hopes her peers will pay more attention to the parasite's expansion in the U.S. "We used to think this was a disease confined to South America, but the reality is that it's quite different than that," she said. "It could be as far north as Canada in the next few decades."

Every year, leishmaniasis infects 1.5 million to 2 million people worldwide and kills about 70,000, most of them in poor rural areas. The most dangerous strains of Leishmania, such as L. infantum and L. donovani, eat away at human skin and infect the liver, spleen, and bone marrow, and can kill without treatment. The drugs used to treat these forms of leishmaniasis, such as miltefosine and amphotericin B, are either expensive or toxic, and little money has been invested in research and development of better treatments. In 2007, the World Health Organization (WHO) added leishmaniasis to its list of neglected tropical diseases (NTDs), which primarily affect the poor and receive little attention from the public.

Although Leishmania is found in approximately 90 countries, symptoms of infection depend on the specific strain. Mexican Leishmania, commonly found in Mexico and Central America, causes skin ulcers that sometimes take years to heal and can leave unsightly scars. In addition, Panamanian Leishmania (L. panamensis), found in Panama and Colombia, attacks the lining of the nose and mouth, causing permanent disfigurement.

Most cases of leishmaniasis treated in the United States today are related to international travel. However, there is evidence that more people are becoming infected in the United States, and the culprit is most likely Leishmania mexicana. From 1903 to 1996, only 27 local cases of leishmaniasis were reported in the United States; in just 10 years, from 2007 to 2017, 41 new local cases were reported.
Panama City Beach, USA. Image source: Pixabay

But those numbers may not reflect the extent of the problem, McKilvey said. Currently, only Texas requires health professionals to report cases of leishmaniasis to the state health department. In the absence of federal reporting requirements, she said, it's "difficult to say exactly" how many cases occur each year nationwide.

While the actual number of cases in the U.S. is certainly lower than the data from tropical regions, a 2010 study sounded the alarm. Scientists at the University of Texas at Austin and the National Autonomous University of Mexico spent a lot of time doing fieldwork, capturing sand flies and rodents in Texas and northern Mexico to determine the distribution ranges of these species. They then integrated this data into computer models to map their ecological niches, the highly specific environmental conditions that allow sand flies to sustain a population, and also considered how temperatures across North America would be affected by climate change. This work allowed the international research team to predict the geographic expansion of sand flies and Leishmania-infected rodents.

Model predictions at the time showed that by 2020, this rodent-sand fly-Leishmania habitat was expected to spread to Oklahoma, Kansas, Arkansas and Missouri; by 2080, the habitat would extend to southern Canada, leaving nearly 27 million people in North America at risk of the disease.

“There is a strong link between climate change and the emergence of zoonotic diseases,” said Víctor Sánchez-Cordero, a professor of ecology at the National Autonomous University of Mexico and one of the authors of the study. “It is possible that human cases of leishmaniasis will soon appear in the United States (except in Texas and Oklahoma) where they did not exist before.” In fact, at least one case has already been reported in North Dakota.

Sahotra Sarkar, another author of the study and a professor of integrative biology at the University of Texas at Austin, said the team needs a few more years to collect data to verify the accuracy of the modeling, but based on unpublished field data and citizen science reports, he believes the study's predictions for 2020 are accurate.

Sarkar said climate change may not be the only factor driving the expansion of these species' habitats; human development could also play a role. When wild areas such as forests or savannahs are destroyed, species that live there migrate. This can bring migratory species into closer contact with humans, increasing the risk of disease transmission to humans.

In other countries, climate change is similarly expanding the range of animals that carry Leishmania.

“People have underestimated how transmissible this disease really is,” said Camila González Rosas, a biology professor at the University of the Andes in Colombia. Her research has shown that a warming climate is pushing these vectors to higher altitudes in Colombia.

Rojelio Mejia, an infectious disease physician at Baylor College of Medicine in Houston, Texas, said he treated a patient who had traveled to Mexico's Yucatán Peninsula a few years ago. The patient contracted leishmaniasis there, but it was not the strain of Leishmania common there but the strain of L. braziliensis that was rampant further south. According to Mejia, this strain of Leishmania, which is more aggressive and destructive than L. mexicanus, should not have been present in Mexico.

“The question then becomes, if we continue to allow climate change to continue, will this strain of Leishmania braziliensis continue to expand?” Mejia lamented. If it does continue to spread northward, the public health problem it causes will be far more serious than the coronavirus pandemic that the United States is currently dealing with.

In 2018, McKilvey co-authored a study that found 41 human cases of leishmaniasis in the U.S. since 2007, most of which occurred in Texas. The paper noted that most physicians were unaware that the disease could be contracted in the U.S. and would only diagnose it if the patient had a history of international travel.
“When doctors see skin lesions, they don’t think it could be leishmaniasis,” McKilvey said. Researchers already know that physicians can mistake lesions for symptoms of bacterial infection, a misdiagnosis that can lead to inappropriate treatments, such as prescribing antibiotics that may suppress the body’s immune system and allow the parasite to multiply.

Overtreatment can also be a problem.

“When most medical students learn about leishmaniasis in medical textbooks, what they see are these impressive ulcerated, twisted wounds,” McKilvey noted. These cases sometimes require treatments that can cause serious side effects, but if caught early, Leishmania mexicana infections can actually be treated with relatively mild treatments.

“The cures I saw were very subtle,” she says, recalling her days as a dermatology resident at the University of North Texas Health Science Center. “The disease wasn’t very advanced, and there wasn’t a lot of damage to the surrounding skin or anything like that. All of those were treated locally.” There, she successfully treated a patient with ear lesions using liquid nitrogen. And she’s not the only one to opt for this treatment: Dustin Wilkes, a dermatologist in Weatherford, Texas, recently used the same method to successfully treat an elderly patient with three leishmaniasis lesions on his left shoulder. Before meeting Wilkes, the 65-year-old had refused a prescription for irritant medication from another doctor.

For those battling more aggressive strains of Leishmania in other countries, both ancient remedies and modern approaches are worth a try. Mayan healers in Mexico have been treating the disease, known locally as the úlcera de los chicleros, for thousands of years, and they may have found a less invasive way to treat it by applying an herbal poultice to the sores for one to two weeks. In a 2018 study published in the Journal of Ethnopharmacology, researchers found that the plant, Cleoserrata serrata, which grows primarily in southern Mexico, significantly inhibited the growth of Leishmania.

In addition, Abhay Satoskar, a professor of pathology at Ohio State University, is working on a vaccine that he calls "very promising." Satoskar said clinical trials of the vaccine are scheduled to begin next year, and Indian manufacturers are developing plans for its commercial production.

As doctors and researchers begin to understand these flesh-eating parasites, scientists say new challenges are coming. “Leishmaniasis is just one of many diseases we’re going to face,” McKilvey said, as climate change pushes disease vectors northward.

Original link: https://undark.org/2021/06/21/climate-change-could-fuel-spread-flesh-eating-parasite/

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