How in Tarnation Did US Armadillos Get Leprosy?
A recent review of the world status of leprosy at the Pathophilia blog produced a curious footnote: a substantial percentage of armadillos (Dasypus novemcinctus, aka 9-banded or long-nosed armadillos) in Louisiana and Texas harbor the infectious agent that causes human leprosy. This notation begs the question: How did US armadillos—which are the only known natural hosts for the leprosy pathogen Mycobacterium leprae, other than humans—acquire the infection?
The answer is provided at least partly in a 2005 review article by Richard W. Truman, PhD, of Louisiana State University and the National Hansen’s Disease (Leprosy) Program, who reports that a leprosy-like illness was first discovered in wild US armadillos as early as 1975 and later confirmed to be prevalent among these animals in the lowlands of Louisiana and Texas. The armadillo appears to be particularly susceptible to M. leprae as a consequence of its naturally low core body temperature, which runs from 32°C to 35°C (89.6°F-95°F).
Truman notes that captive armadillos were first used to propagate M. leprae experimentally in 1968, after it was recognized that the animal’s cool temperature would be favorable to the organism’s growth. However, it was observed 7 years later that some wild armadillos near New Iberia, Louisiana, already harbored M. leprae, raising the issue of how these animals became infected.
Original speculation raised the possibility that leprosy in wild US armadillos was the result of the escape or improper disposal of experimentally infected, captive animals. However, this contentious hypothesis was disproved in 1985, Truman writes, when armadillo sera from 1960-1964, collected and stored at LSU before experimental animal inoculation, demonstrated antibodies to M. leprae.
Through animal surveys, it is now known that approximately 15% of armadillos living along the coastal areas of Louisiana and Texas are infected with M. leprae; although one third or more of all adult armadillos may be infected in concentrated areas. Yet it has still not been definitively established when or where US armadillos acquired leprosy. According to Truman, single-nucleotide polymorphism (SNP) analysis indicates that leprosy was brought to North America by European and African immigrants, and that armadillos—which are indigenous to North America and likely migrated to the United States from Mexico—harbor the same M. leprae strain that originated in Europe and Africa. Truman concludes that the animals must have acquired the organism from immigrant humans some time during the last 400 years, but that the animals “could be commonly exchanging the organism with humans today.”
Surveys of leprosy rates in armadillos outside of the United States, and specifically those in Mexico, are slim to nonexistent, according to Truman; although, diagnosed cases of leprosy among Mexican immigrants in Houston and Los Angeles suggest that armadillo exposure is a significant risk factor for the illness. A study published just last year assessed the leprosy rate in 37 field-tested armadillos in the State of Espírito Santo, Brazil, where leprosy is particularly prevalent and exposure to armadillos common.* Investigators found an armadillo infection rate of approximately 30%.
In an e-mail communication, Truman indicates that several groups are beginning to survey for leprosy in Venezuelan and Colombian armadillos, as well as those in Brazil. But a substantial amount of information must be collected before any conclusions can be drawn about the relationship between US and South American infection data. To better understand the role of armadillos in the perpetuation or spread of leprosy, Truman and his colleagues are engaged in large genotyping studies of M. leprae strains that are circulating throughout the United States. He reports, “We have found that infection among armadillos appears to be more widespread than previously known.”
* The potential relationship between armadillo exposure and human leprosy in Brazil is important, because infected Brazilians make up approximately one half of all registered leprosy cases in the Americas. The study authors note that, in the State of Espírito Santo, armadillos are often killed and consumed.