Wild armadillos and patients infected with the same unique strain of M. leprae confirms suspicions that armadillos are a natural reservoir of the disease.
Humans at low risk for leprosy, armadillos are easy targets
Leprosy isn’t a common human disease: fewer than 250,000 cases were reported globally in 2008 and only 150 of these were in the U.S.; nor is Mycobacterium leprae, the bacterium that causes it, particularly aggressive. In fact, M. leprae is a microbial wimp, taking 12-14 days to multiply and unable to spread without long-term direct contact. Even then, M. leprae is unlikely to infect a new human because vast majority of us are naturally immune.
In contrast, armadillos are easily infected with M. leprae and in some places in the southern U.S. more than 20% of them have the disease. Scientists speculate that their low body temperature coupled with an innate susceptibility to infection with M. leprae puts armadillos at particular risk of leprosy, noting that M. leprae also prefer cooler areas of infected humans such as the skin and nasal mucosa.
The disease is not native to the new world, and because leprosy was well established among settlers around New Orleans by the 1750s, it’s probable that armadillos originally caught it from humans.
Human-to-human transmission of leprosy the exception rather than rule in the southern U.S.
It has been widely believed that the leprosy bacterium, a close relative of Mycobacterium tuberculosis, requires human contact to spread. This has led to the tradition of segregating people with the disease from the healthy population, causing the affected individuals to not only suffer mutilation, but also rejection and exclusion from the rest of society. However, the statistics argue otherwise, at least in the southern U.S. where the transmission of M. leprae through close contact with infected humans appears to be the exception rather than the rule.
Genotyping of home-grown M. leprae reveals common armadillo and human strain
Armadillos have long been suspected as a possible source of M. leprae. Now Richard W. Truman, at Louisiana State University in Baton Rouge, and colleagues around the world have proof that this is an issue. “A high percentage of unrelated patients in the southern U.S. were infected with a unique strain of M. leprae that occurs naturally among wild armadillos,” says Truman. “The genome sequences of the predominate armadillo and human strains in this region are essentially identical and the M. leprae of this genotype has not been reported anywhere else in the world,” he adds. Transfer of the 3I-2-v1 strain appears highly efficient among armadillos, since it is now found across five southern states.
Initially, a naturally infected wild armadillos from Louisiana was captured, its M. leprae isolated and whole genome sequencing was used to compare the bacilli from this animal with those from the skin of three patients with leprosy. They matched.
Then Truman, working with a team of leprosy experts from around the world, tested M. leprae DNA from 33 wild armadillos and 39 patients being treated at a Louisiana clinic. They compared these leprosy bacteria with those from 64 Venezuelan patients and also against four foreign reference strains. The M. leprae from patients with foreign exposure were different from those infecting the U.S. patients. “The 3I-2-v1 strain was significantly associated from patients living in areas where M. leprae– infected armadillos are found,” according to Truman. Furthermore, eight of the Louisiana patients recalled having direct contact with the animals and one reported that he frequently hunted, cooked and ate them -obviously not a good idea.
Avoiding armadillos, not killing them, advised and early diagnosis and treatment urged
“Frequent direct contact with armadillos, especially cooking and eating them should be discouraged,” cautions Truman. However, “The last thing we want is to induce panic in the population and incite the slaughter of armadillos,” says Stewart T. Cole, from the Global Health Institute in Lausanne, Switzerland who participated in this study. “The best way to combat further infection is though education and prudence,” he adds. Moreover, since effective treatments for leprosy is now available, early diagnosis and prompt initiation of anti-mycobacterial antibiotics is critical. Truman and colleagues alert physicians to “consider leprosy in their differential diagnosis of chronic skin lesions, especially those not responsive to common treatments.”
Truman and Cole emphasize that their findings “should decrease the already low risk of humans acquiring leprosy from armadillos even futher as the public will be more careful about interacting with these animals.” People should take special care in Alabama, Arkansas, Louisiana, Mississippi and Texas where infected animals have been found.
Note: The expert leprosy team involved in this study includes scientists from the Global Health Institute, École Polytechnique Fédérale de Lausanne and the Swiss Institute of Bioinformatics, both in Lausanne, Switzerland; the Instituto de Biomedicine, Caracas, Venezuela; and Institut Pasteur in Paris, France. Whole-genome sequencing, single-nucleotide polymorphism (SNP) typing, and variable-number tandem-repeat (VNTR) analysis were used to identify and compare the infecting M. leprae from both patients and armadillos.
The study was primarily supported by the Health Resources and Services Administration, and the National Institutes of Allergy and Infectious Diseases.
Truman, Richard W., Singh, Pushpendra, Sharma, Rahul, et al. Probable Zoonotic Leprosy in the Southern United States. (2011); N Engl J Med; 364: 1626 -1633.
Truman, Richard W. Environmental Sources of M. leprae: Issues and Evidence. (2010); Lepr Rev; 81:89-95.
Blake, Leslie A., West, Burton C., Lary, Cynthia H., Todd, John R. IV. Environmental Nonhuman Sources of Leprosy. (1987); Rev Infect Dis; 9:562-577.
The World Health Organization: Leprosy. Accessed May 16, 2011.
EFPL School of Life Sciences. Stewart T. Cole Lab. Accessed May 16, 2011.
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