Leishmaniasis is a disease that thrives among the detritus of human misery and neglect: ramshackle housing, rats, overcrowded slums, garbage dumps, open sewers, feral dogs, malnutrition, addiction, lack of health care, poverty, war, and terrorism. Cutaneous leish is now running rampant in the areas of Iraq and Syria controlled by ISIS—so much so that families there are choosing to intentionally inoculate their young girls with leishmaniasis on a covered part of their body so that they will not get it on their faces, where it will leave a scar. (This type of leish is a mild variety that usually goes away on its own, leaving the person immune.)
Since 1993, the leishmania parasite has been spreading, not just because of HIV coinfection but also as people move from rural areas into cities. It is attacking people who venture into the rainforest for projects such as dam and road building, logging, and drug smuggling, as well as adventure tourism, photography, journalism, and archaeology. Strange tales abound. Almost everyone on a Costa Rican jungle yoga adventure was struck down by leish. A survival show contestant lost part of his ear to leish. A team of filmmakers shooting an adventure tourist video were stricken with leish.
Leish is now spreading in the United States. Over the course of the entire twentieth century, only twenty-nine cases of leish were reported in the United States, all of which occurred in Texas close to the Mexican border. But in 2004, a young man from a small town in southeastern Oklahoma, ten miles from the Arkansas border, visited his doctor complaining of a sore on his face that wouldn’t heal. The doctor cut it off and sent it to a pathologist in Oklahoma City, who was stymied by what it might be and stored the frozen tissue. A year later, this same pathologist, by sheer chance, got another tissue sample from another patient living in the same small town. The pathologist immediately called the Oklahoma State Department of Health and reached Dr. Kristy Bradley, the state epidemiologist. She and her staff ordered the two tissue samples sent to the Centers for Disease Control in Atlanta. The diagnosis came back: cutaneous leishmaniasis, of a mild type that can usually be cured by surgically removing the ulcer. (Both patients were, in fact, cured this way.)
At the time that Dr. Bradley was investigating the disease in Oklahoma, an outbreak of cutaneous leishmaniasis occurred in northeastern Texas and in a string of suburbs in the Dallas–Fort Worth metro area; the dozen or so victims included a little girl who had lesions on her face, and in one case a cat and a human in the same household got the disease. Doctors in the health departments of Texas and Oklahoma joined forces to track the source. They were especially worried because none of the victims had traveled: They had gotten the disease in their own backyards.
Dr. Bradley led the investigation of the two cases in Oklahoma. She assembled a team that included an entomologist and a biologist. When the team visited the patients and surveyed their properties, they noted burrows of wood rats and populations of sand flies, which they concluded must have been the host and vector. The investigators trapped a number of rats and sand flies and tested them for leish. None had the disease, but by this time the mini-outbreak had died down.
I called Bradley and asked if the leish had really died out or if it was still around. “I’m sure it hasn’t gone away,” she said. “It’s smoldering somewhere out there, quietly cycling in nature,” waiting for the right combination of circumstances to break out again. When she and her team mapped leish cases in the United States over time, they revealed an inexorable spread northeastward across Texas and Oklahoma, aiming for other states in a northeasterly direction.
Why?
Her answer was immediate: “Climate change.” As the United States becomes warmer, she said, the ranges of the sand fly and the wood rat are both creeping northward, the leish parasite tagging along. The sand fly genus known to spread this kind of leish has now been found in the United States five hundred miles northwest and two hundred miles northeast of its previously established range.
A recent study modeled the possible expansion of leishmaniasis across the United States over the next sixty-five years. Since it takes both vector and host to spread the disease, the scientists wanted to know where the sand fly/wood rat combination would migrate together. They looked at two future climate scenarios, best case and worst case. For each case, they extrapolated out to the years 2020, 2050, and 2080. Even under the best-case climate assumptions, they discovered that global warming would push leishmaniasis across the entire United States into southeastern Canada by 2080. Hundreds of millions of Americans could be exposed—and this is just by wood rats. Since many other species of mammals can host the leish parasite—including cats and dogs—we know the potential problem is far greater than what was described by this study.* A similar spread of the disease is expected in Europe and Asia.
It seems that leishmaniasis, a disease that has troubled the human race since time immemorial, has in the twenty-first century come into its own. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases at the NIH, told our team bluntly that, by going into the jungle and getting leishmaniasis, “You got a really cold jolt of what it’s like for the bottom billion people on earth.” We were, he said, confronted in a very dramatic way with what many people have to live with their entire lives. If there’s a silver lining to our ordeal, he told us, “it’s that you’ll now be telling your story, calling attention to what is a very prevalent, very serious disease.”
If leish continues to spread as predicted in the United States, by the end of the century it may no longer be confined to the “bottom billion” in faraway lands. It will be in our own backyards.
Global warming has opened the southern door of the United States not just to leish but to many other diseases. The big ones now entering our country include Zika, West Nile virus, chikungunya, and dengue fever. Even diseases like cholera, Ebola, Lyme, babesiosis, and bubonic plague will potentially infect more people as global warming accelerates.