For years, scientists have speculated that armadillos can pass on leprosy to humans, and that they are behind the few dozen cases of the disease that occur in the U.S. every year. Now, they have evidence. A genetic study published today in The New England Journal of Medicine shows that U.S. armadillos and human patients share what seems to be a unique strain of the bacterium that causes leprosy.

Leprosy, also known as Hansen's disease after the physician who first described it, attacks the skin and the nerves. It's a difficult illness to study: The bacteria grows naturally only in people and armadillos, and in experiments will grow on the footpads of genetically engineered mice.

In most places around the world where leprosy shows up, the disease is thought to pass from person to person. But in Central America and parts of the U.S. South and Southwest, armadillos are common, showing up in backyards, under porches, and by the side of the road. And in some places, more than 20% of armadillos are infected with leprosy. "It's always been a curiosity," says Richard Truman, a microbiologist at the National Hansen's Disease Program which is housed at Louisiana State University in Baton Rouge. Scientists think their low body temperature provides a good environment for Mycobacterium leprae, the leprosy bacteria; in humans, too, M. leprae prefers cooler areas, such as nostrils, fingers, and toes.

Whether armadillos are linked to human infections in the United States has been "very difficult to address," Truman says. The number of U.S. cases is minuscule—just 150 people are diagnosed with leprosy each year, and only 30 to 50 of those are thought to have contracted the disease locally. There have been several reports of leprosy patients who came into contact with armadillos. John Abide, a dermatologist in Greenville, Mississippi, runs a solo practice and in recent years has seen three patients with the disease; further questioning revealed that all three of them had been exposed to armadillos. One woman often worked in her garden, where there were armadillos "everywhere," Abide says. "She could have inhaled fecal material." And two male patients had killed armadillos near their houses. Abide published these case studies in 2008.

To learn more about the home-grown U.S. cases, Truman collaborated with Stewart Cole at the Global Health Institute at the École Polytechnique Fédérale de Lausanne in Switzerland, and other scientists. They captured wild armadillos in five southern states, performed whole-genome sequencing of M. leprae found in one of them, and compared it to the whole genome of bacteria isolated from the skin of three patients. All four strains were essentially the same, and, interestingly, did not match leprosy strains reported in other parts of the world, suggesting this one was unique to the United States.

Then they tested the DNA of M. leprae from 33 wild armadillos and 39 U.S. patients. Twenty-eight of the animals and 25 of the patients had the new strain. The others harbored previously reported strains that the researchers speculate may circulate at a low level in the United States. But the new strain, which they dubbed 3I-2-v1, was the only one found in more than one person.

Abide, who was not involved in the study, says the findings confirm his suspicions that armadillos are bad news. "I would not dig in soil that has a lot of armadillo excrement." And if an armadillo's blood "got on my tires of my car from running [the animal] over, I would wash it down." Abide's patients recovered--leprosy is easily treated with a cocktail of three antibiotics—but still, he says, he recommends steering clear of the animals.