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. each year. Now, they have proof. A genetic study published today in The New England Journal of Medicine shows that U.S. armadillos and human patients share what appears to be a unique strain of the bacterium that causes leprosy.
It’s a tricky illness to research: The bacteria grows naturally only in humans and armadillos, and in experiments will grow on the footpads of mice that are genetically engineered.
In most areas around the world where leprosy shows up, the disease is thought to pass from person to person. And in some areas, 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 That’s housed at Louisiana State University in Baton Rouge. Scientists believe their low body temperature gives a fantastic environment for Mycobacterium leprae, the leprosy germs; in humans, too, M. leprae prefers cooler areas, such as nostrils, fingers, and feet.
Whether armadillos are connected to human infections in the United States has been”very hard to address,” Truman says. The number of U.S. instances is minuscule–only 150 people are diagnosed with leprosy each year, and only 30 to 50 of these are believed 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 these were subjected to armadillos. “She might have inhaled fecal material.” And two male patients had murdered armadillos near their homes. 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, along with other scientists. They captured wild armadillos in five southern states, conducted whole-genome sequencing of M. leprae found in among them, and compared it to the whole genome of bacteria isolated from the skin of three patients. All four strains were basically the same, and, interestingly, didn’t match leprosy strains reported in other parts of the world, suggesting this one was unique to the USA.
Twenty-eight of those critters and 25 of the patients had the new breed. The others harbored previously reported strains the researchers speculate may circulate at a low level in the United States.
“I wouldn’t dig in soil which has a lot of armadillo excrement.” And when 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 nevertheless, he says, he recommends steering clear of these animals.