When European settlers first arrived in North America, they kindly introduced a number of diseases endemic to the Old World but new to the New. Small pox would be the best known and most dramatic example. Introduced by the Conquistadors on the island of Hispaniola in 1507, epidemic small pox devastated native populations and was a major factor in the fall of the Aztecs and Incas to the Spaniards. Another passenger in the explorer’s Noah’s Ark of human pathogens was Mycobacterium leprae, an obligate intracellular bacterium (lives only inside the cells of infected animals), the cause of leprosy (also known as Hansen’s Disease in medicine’s “Great Dead White Men” nomenclature).
For reasons that are not clear, M. leprae is able to infect the cells of the nine-banded armadillo, a native species of Texas, Louisiana, and the Southeastern states. There is no evidence to suggest that the armadillo carried M. leprae before exposure to affected humans and other animal species do not appear to be susceptible to infection. Armadillos have been used as the primary laboratory means of growing M. leprae since the 1970s. Although experimentally infected armadillos can show evidence of the disease after extended periods of infection, it is felt that armadillos do not live long enough in the wild to develop leprosy. Fortunately for them, armadillos do not seem to be much bothered by the bacteria to which they have the bad luck of serving as natural reservoir.
Leprosy is a fascinating example of a disease that occurs due to the interplay of an infectious agent and a particularly susceptible individual. It is estimated that 95% of humans infected with M. leprae do not developed the clinical symptoms of leprosy. This phenomenon, different individuals respond differently to the same infection, is key to the mystery of many diseases. The factors that lead the immune system of some M. leprae-infected people to develop inflammation (of a special type – granulomas) in the skin, respiratory mucosa, and peripheral nerves are not understood (tough disease to study, but this would make a very interesting area for investigation). One form of the disease presents with ring-shaped lesions with diminished sensation due to involvement of the peripheral nerves (check out a clinical image that ran with the new study). Leprosy is rare in the U.S. (about 150 cases a year) and is unusual (250,000 cases a year) in endemic areas such as India, Brazil, Africa, and the Philippines. There are still more than 1,000 leper colonies in India. The leper colonies established by King Kamehameha V of Hawai’i on Moloka’i in 1865 must be among the most famous and beautiful (Jack London’s 1908 story “The Lepers of Molokai” from the “South Sea Stories” is a great read). By the way, the widespread belief that leprosy causes body parts to fall off is myth. The nerve damage in leprosy lesions can lead to tissue loss, but the notion of digits falling off is over-dramatized. There is effective treatment for leprosy with a drug combination similar to that used for tuberculosis. The main challenges in combating leprosy worldwide are socioeconomic and diagnostic.
Most cases of leprosy in the US occur in individuals who have been in close contact with affected patients or in people who have a history of travel to endemic areas. About a third of patients do not have a history of either risk factor. How these folks came to contract leprosy has been an epidemiological mystery. Many infectious disease doctors have long-suspected what you now do: people can contract leprosy from hunting, handling, and possibly eating armadillos. Infectious disease doctors specialize in asking patients about their travels and exposure to odd animals and foods. Although this questioning has suggested that armadillos may be the missing link of the leprosy epidemiology mystery, the smoking gun has not come until now – a study in the April 28th New England Journal of Medicine. The key piece of evidence is that sequencing of DNA from M. leprae revealed a particular strain (or genotype) that was found in 28 of 33 infected wild armadillos and in 25 of 39 human cases of leprosy in patients who live within the armadillo’s geographic range. This same strain of M. leprae has not been found anywhere else in the world, making another type of exposure in these patients very unlikely.
A few months back, I saw an armadillo (alive, not road kill) for the first time. It was late afternoon on a blustery and overcast March day in old East Dallas. I was sitting at a table in the courtyard of the research institute where I work, nestled between a large urban hospital and a transitional residential community. I saw something grey, too big to be a squirrel, definitely not a cat, working busily, nose down in the yard. On closer inspection, I was delighted to see something resembling a toy-sized stegosaurus. What surprised me most was how nimbly the armadillo moved on its dainty feet and the intelligent dexterity of its raccoon-like hands. When I became too close for comfort, the armadillo took off with a bounding stride much faster than one would expect for a critter with body armor. The encounter was as charming as it was unexpected.
In transmitting leprosy to people made susceptible by their genetics and animal-handling behaviors, it seems that armadillos are returning a centuries-old favor. As if another reminder was needed, we change and are changed by the natural environments that we are a part of.