Ill roadkill: Although they don't usually show outward symptoms in the wild, common U.S. armadillos are often infected with a transmissible form of leprosy. Image: Wikimedia Commons/NASA/Ken Thornsley
Leprosy was one of the last things on dermatologist John Abide's mind when a 78-year-old man walked in for a screening at the doctor's Greenville, Miss., practice. Unbeknownst to the man, two large red bumps had formed on his back. Abide diligently tested samples to make sure they did not indicate tuberculosis. When the results came back positive for atypical mycobacteria, another doctor suggested sending the samples away to have them tested for leprosy.
"I thought that was a waste of time," he says. When they came back positive, Abide was stunned. "I didn't know that they still studied it—or that it was still in existence."
In the next few years, he came across two other local patients—an 81-year-old woman with a growth above her left elbow and a 73-year-old man with a red rash on his chest—who also tested positive for leprosy, also known as Hansen's disease.
None of them had been traveling where leprosy was common, but all three reported close contact with armadillos. Did Abide ever imagine as a Mississippi dermatologist he would be treating leprosy patients? "No, indeed," he says. "It's kind of scary."
That humans would catch the disease from these armored animals might seem unlikely. The nine-banded armadillo (Dasypus novemcinctus) is common across parts of more than a dozen southern states. Leprosy is not. Although many cases might go undiagnosed, in the U.S. there are only about 150 to 200 new cases reported each year—and about 3,600 people being treated for the chronic condition. And the majority of the cases, roughly 70 percent, are presumed to be from people who picked up the bacteria while traveling abroad.
But a new study, published in the April 28 issue of The New England Journal of Medicine, found a new strain of the disease, common to both local people and armadillos, suggesting that the animals are, indeed, a wild reservoir for leprosy.
Leprosy might sound like an exotic, extinct disease, locked in the annals of history or sequestered in quarantine colonies in far-off lands. But it persists here in the U.S., and new research suggests, in addition to being spread by infected people, it is also being carried on the leathery backs of ambling armadillos.
By a fluke of genetics, armadillos and humans seem to be the only two species that are natural habitats for the bacterium Mycobacterium leprae, which causes the disease. And sometime in the past four centuries, since leprosy landed in the New World via European explorers, it made its way into these insectivores, which are also common throughout Mexico.
In addition to an unexpected genetic predisposition for leprosy, armadillos have one thing that might make them more conducive mammalian hosts: a cool body temperature. At about 32 degrees Celsius (90 degrees Fahrenheit), their core temperature is cooler than that of many mammals, which might make for a better host environment for the bacteria. In the wild, a leprous armadillo might not appear to be different from an uninfected one, Richard Truman of the U.S. Health Resources and Service Administration's National Hansen's Disease Program at Louisiana State University and a co-author of the new study explains. Sick animals will likely be picked off by predators and not survive long enough to develop obvious symptoms.
Fortunately, M. leprae is a finicky organism. It cannot survive for long outside of a living cell—a couple hours on a microscope slide, and maybe a couple weeks in moist soil, Truman notes. And most people—about 95 percent of us—are naturally immune to it. A person's likelihood of getting the disease, even with close contact, is slim and rests on variables such as his or her genetic makeup and immunologic strength.
Because the bacterium that causes leprosy cannot be grown in lab dishes, finding that the armadillo could also carry the disease proved to be a boon to researchers bent on finding new treatments.
Aside from the characteristic skin sores, in humans leprosy can cause lasting nerve damage in the extremities and sensory deterioration. Rather than shipping the afflicted away to colonies, the disease can now be treated with a long course of three drugs (rifampicin, clofazimine and dapsone), which are not without their own side effects.
The disease was only confirmed in wild armadillo populations in 1975. And the new study pinpoints Alabama, Arkansas, Louisiana, Mississippi and Texas as the states where the newly described genotype was found. As Truman points out, the genotype itself might not be so new, but rather, researchers now have more advanced genomics tools to recognize it.