Editors' ChoiceInfectious Disease

Molecular Sleuthing: Linking Leprosy and Armadillos

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Science Translational Medicine  01 Jun 2011:
Vol. 3, Issue 85, pp. 85ec82
DOI: 10.1126/scitranslmed.3002688

Mycobacterium leprae, the bacterium that causes leprosy, is somewhat of an outcast; unlike many other clinically relevant bacteria, M. leprae cannot be reliably cultivated in vitro. This deficiency hinders our complete understanding of the organism’s major modes of transmission. Exposure to respiratory droplets from patients with high-burden disease is thought to be the predominant route of transmission. However, it has been known for some time that there are persons who develop leprosy in the absence of any known exposure to infected persons. Now, Truman and colleagues report the results of a large molecular epidemiological study that links leprosy with armadillo exposure.

The nine-banded armadillo (Dasypus novemcinctus) makes its habitat across the southern United States and has the unique distinction of being the only known natural reservoir for M. leprae. There has been some support in small studies for the notion that contact with armadillos may be a risk factor for leprosy. In the current study, the authors performed a large-scale molecular analysis in which they compare the genetics of various M. leprae strains isolated from armadillos and from humans with leprosy. In doing so, they convincingly demonstrate that armadillo exposure may be a source of zoonotic (nonhuman) transmission of leprosy in the southern United States.

To illustrate this link, Truman et al. compared 50 M. leprae strains from leprosy patients, 33 strains from wild armadillos, and four reference strains (which included one armadillo strain). The authors searched among the strains first, for variations in single nucleotides [single-nucleotide polymorphisms (SNPs)] and second, for short repeating segments of DNA [variable number tandem repeats (VNTRs)]. The researchers identified a unique M. leprae strain (3I-2-v1) in armadillos that was identical to the strain that infected many U.S. patients (64%) with geographic exposure to armadillos and no history of encounters with persons suffering from leprosy. In addition, patients who were known to have contact with armadillos were four times more likely to be infected with the strain linked to armadillos compared with subjects who had no animal contact. Thus, through comparative sleuthing—balancing epidemiological methods with genetic ones—Truman and colleagues have countered the M. leprae in vitro challenge and, in doing so, provide strong evidence implicating armadillos as a zoonotic source of M. leprae infection in humans. The methods used by these investigators may be successfully applied to other fastidious bacterial conundrums.

R. W. Truman et al., Probable zoonotic leprosy in the southern United States. N. Engl. J. Med. 364, 1626–1633 (2011). [Full Text]

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