A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Is it true that armadillos carry leprosy?

February 19, 1999

Dear Cecil:

I have heard that armadillos carry leprosy. Is this true? How about any other nasty diseases?

Cecil replies:

A query to the Straight Dope Science Advisory Board drew the following response: "Q. Why don't lepers play hockey? A. Too many face-offs." You see why scientific progress is slow. I have, however, established that the answer to your question is yes — armadillos do carry leprosy.

I know, I know. Armadillos?

Hey, don't look at me. Nobody else is quite sure what to make of it either. Leprosy, one of history's most dread diseases, has been around since ancient times. But it has never been easy to study because the bacillus that causes it, Mycobacterium leprae, can't be grown in the lab. We're still not sure of such basic facts as how you catch it. (Apparently nasal discharges are one form of transmission, so I guess you don't want to borrow a hanky in a leper colony.)

It was long thought only humans could get leprosy. Then in the late 1960s researchers speculated that armadillos might be a good test bed for leprosy research because (a) M. leprae thrives in cooler parts of the body (feet, nose, ears, etc.); (b) armadillos have a relatively low body temperature as mammals go, 30 to 35 degrees Celsius compared to 37 degrees in humans (98.6 Fahrenheit for you retro types); (c) armadillos live long enough, 12 to 15 years, for this slow-acting disease to emerge; and (d) armadillo litters almost invariably consist of identical quadruplets, which was useful for genetic experiments.

Aspects of this conjecture might seem far-fetched (I'm thinking of the low body temp part), but it panned out. Several nine-banded armadillos, the type found in the U.S., were inoculated with leprosy germs and came down with full-blown cases of the disease.

Later the researchers discovered something odd: some armadillos already had leprosy. At first they thought the animals had escaped from the leprosy-inoculation experiment or become infected through contact with the lab's waste. But eventually these possibilities were ruled out. Nine-banded armadillos, of which there are 30 to 50 million in the southeastern U.S., are believed to be the only significant natural reservoir of leprosy apart from humans. (A few cases have been found in chimps and mangabey monkeys in Africa.) How the armadillos got leprosy in the first place nobody knows. But there you are.

Should you live in fear that you'll be infected by an armadillo? Well, if you're going to worry about AIDS and hepatitis C, you might as well round out your paranoia by worrying about leprosy. Realistically, though, the chances are slim.

While suspected instances of 'dillo-to-human transmission have been reported, leprosy remains uncommon in the U.S. and Canada (6,000 U.S. cases) and is in long-term decline worldwide — an estimated 2.4 million cases as of 1994. Fewer than 5 percent of wild armadillos have it, though I grant you that 5 percent of 30 to 50 million is a lot of armadillos.

The disease is not especially contagious; researchers think that 95 percent of humans are naturally immune. Leprosy is treatable, and a vaccine (not totally effective) is currently available. While one doesn't wish to minimize the consequences of this disease, it's not the certain nightmare it used to be. Equally important, there's no need for people who have it to be treated like, you know, lepers.

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