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Vol. LXIII, No. 11
May 27, 2011

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Genetic Clues Help Explain Mysterious Leprosy Infections

A nine-banded armadillo

A nine-banded armadillo

Photo: Smithsonian National Museum of Natural History

A recent genetic study by NIAID-funded scientists at the National Hansen’s Disease Program (NHDP) found that wild armadillos in the southern U.S. can and do transmit leprosy to humans. “Leprosy is thought of as an ancient disease from biblical times, but it’s still a problem in many parts of the world,” said NIAID project officer Dr. Tina Parker.

Each year, about 250,000 new cases of leprosy are reported worldwide, including 150 or so in the United States. Most U.S. cases are traced to origins overseas, but about one-third of new infections seem to arise within the states. The source of those new infections, mostly appearing in Texas and Louisiana, remained a mystery until recently.

A chronic infectious disease caused by Mycobacterium leprae, leprosy (also called Hansen’s disease) is found mostly in tropical and semi-tropical areas of the world. People with mild cases experience skin discoloration. More severe cases can result in skin lesions and peripheral nerve damage.

From earlier epidemiological studies, NHDP scientists knew that M. leprae had been found among wild armadillos in Texas and Louisiana, suggesting that human contact with infected armadillos might lead to human infection. Led by Dr. Richard W. Truman, the researchers used genetic analysis to test this hypothesis and discovered they were right.

Other than humans, armadillos are the only animal known to be susceptible to leprosy. Because of this, colonies of armadillos have been important in research to model the disease since the 1970s. In the new study, NHDP researchers compared the gene sequences of M. leprae samples taken from humans and from infected wild armadillos and found that 64 percent of human samples had a particular genotype that had never been seen before. What’s more, 85 percent of armadillo samples shared that genotype.

These results showed that the two strains were related and that wild armadillos are a likely source of some human infections. But more importantly, it established leprosy as a zoonosis: a disease that can be transmitted back and forth between animals and humans. “There is transmission of disease at the human-animal interface,” explained Dr. Christine Sizemore, chief of NIAID’s tuberculosis and other mycobacterial diseases section. “In fact, 65 to 70 percent of emerging infectious diseases in humans are zoonotic.”

Experts caution that the chance of catching leprosy from an armadillo remains very low; armadillo transmission accounts for only about 30-40 known leprosy cases among people.

Now that genetics and genomics have become important strategies for studying how diseases behave in natural populations, these methods can be used as an example in other areas of the world. Future research steps include replicating the study in other countries and in areas where armadillos are common.—Nalini Padmanabhan NIHRecord Icon

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