April 27, 2011 -- They may be built like tanks, but armadillos’ natural armor doesn’t protect them from leprosy.
Now a new study shows that some armadillos and people with leprosy in the southern United States are infected with the same bacterial strain. That suggests that the disease, which causes skin lesions and eventual nerve damage, can pass between species, although that circumstance appears to be rare.
The discovery has offered scientists a new way to think about how leprosy may persist in the environment. And it helps to explain how some people contract the disease, especially if they’ve never traveled outside the U. S.
“It’s an excellent scientific publication,” says William R. Levis, MD, a specialist in treating leprosy, which is also called Hansen’s disease, and clinical assistant professor at NYU’s Langone Medical Center in New York City.
Levis says that although the science is sound, he fears the study will misdirect attention in a disease that’s already misunderstood and underappreciated.
“While it’s a nice scientific article, they’re missing the point. The armadillo is not really a major public health issue,” says Levis, who was not involved in the research. “The fact is that leprosy in the United States is an imported disease.”
Tracking Leprosy Through DNA
The study, which is published in the New England Journal of Medicine, compared strains of Mycobacterium leprae, the bacterium that causes leprosy, from 50 patients and 33 wild armadillos captured in five Southern states: Arkansas, Alabama, Louisiana, Mississippi, and Texas.
Something that has frustrated scientists about M. leprae bacteria is that they tend not to change very much over time or between countries, making it tough to pinpoint an infection’s source.
The study used DNA fingerprinting techniques to look for changes to short, repetitive stretches of DNA in the bacterium’s genome called variable number tandem repeats (VNTRs).
Matching bacterial strains by comparing their VNTR regions is a little like playing slot machines in a casino, says study researcher Richard W. Truman, PhD, a research scientist and captain in the United States Public Health Service in Baton Rouge, La.
“In this case, you’re not playing a slot machine to get all three of your numbers to line up, you’re playing like a casino full of slot machines,” Truman says.
“If it’s like two apple, three apple, four apple, and a 7-bar or something like that, and you find that on four or five different machines simultaneously, that’s the relationship,” he says.
Researchers hit the jackpot when they found a previously unknown strain of M. Leprae in 28 of the 33 wild armadillos they captured and in half of skin samples taken from patients.
The human bacterial samples came from frozen tissue, and in some cases, researchers didn’t have access to the patient’s full history. In 15 patients who were asked about contact with armadillos, seven said they’d never had contact with the animals, while eight said that they had.
One reported frequently hunting, cooking, and eating armadillos. His case prompted researchers to caution against handling or eating the animals.
Since the 1970s, scientists have recognized that armadillos could carry the bacteria and develop full-blown leprosy infections -- they have the lower body temperatures and longer life spans that the bacteria need to thrive, and it was suspected, based on case reports, that the animals could pass the bacteria to people.
But this new study marks the first time that an animal reservoir for the disease has ever been identified, and it means that in other parts of the world, where leprosy is much more common, animal reservoirs may be important in how the disease persists in the environment.
“Never before have we had the ability to have a biological link between organisms and cases, and now we do, which should then propel us forward being able to better deduce the risk factors that are important in transmission of the infection or that may then be perpetuating leprosy not just in the United States, but also worldwide,” Truman says.
The Risk for People
People are probably not at any greater risk than they were before, experts say.
Partly that’s because about 95% of people are genetically resistant to the infection in the first place.
“Leprosy in the U.S. is rare, and not on the rise,” says study researcher Pushpendra Singh, PhD, a scientist at Ecole Polytechnique Federale de Lausanne in Switzerland.
There are about 150 to 200 cases of leprosy reported in the U.S. each year, and about two-thirds of those, Singh says, can be traced to other countries were the disease is more common, like India and Brazil.
“However, the most possible cause of infection among the people who never left the country, or never came into the contact of any other patient of leprosy, could be the infected armadillos,” Singh says.
Some studies have shown that more than 20% of wild armadillos are infected with leprosy in some U.S. locations. Although the disease is rarely fatal to people and can be treated with antibiotics, it can kill armadillos.
In people who are infected, symptoms may not show up for three to five years, Truman says, making it tough for people to remember the circumstances that might have put them in contact with the infectious agent.
The first symptoms are generally light-colored skin lesions, which may be numb to the touch. Without treatment, the bacteria can damage nerves and lead to eventual muscle weakness, but it’s rare for most people to get to that point.
Most can be managed with antibiotics.
One big hurdle to treatment, though, appears to be that most doctors never suspect leprosy could be the diagnosis.
“You need a physician who’s willing to consider the diagnosis,” says James L. Krahenbuhl, PhD, director of the National Hansen's Disease Programs at the U.S. Department of Health and Human Services’ Health Resources and Services Administration. “There’s a need for awareness in physicians that leprosy does exist.”