[Leprosy: a paradigm for the study of human genetic susceptibility to infectious diseases]

Pauci- and Multibacillary Leprosy: Two Distinct, Genetically Neglected Diseases

After sustained exposure to Mycobacterium leprae, only a subset of exposed individuals develops clinical leprosy. Moreover, leprosy patients show a wide spectrum of clinical manifestations that extend from the paucibacillary (PB) to the multibacillary (MB) form of the disease. This "polarization" of leprosy has long been a major focus of investigation for immunologists because of the different immune response in these two forms. But while leprosy per se has been shown to be under tight human genetic control, few epidemiological or genetic studies have focused on leprosy subtypes. Using PubMed, we collected available data in English on the epidemiology of leprosy polarization and the possible role of human genetics in its pathophysiology until September 2015. At the genetic level, we assembled a list of 28 genes from the literature that are associated with leprosy subtypes or implicated in the polarization process. Our bibliographical search revealed that improved study designs are needed to identify genes associated with leprosy polarization. Future investigations should not be restricted to a subanalysis of leprosy per se studies but should instead contrast MB to PB individuals. We show the latter approach to be the most powerful design for the identification of genetic polarization determinants. Finally, we bring to light the important resource represented by the nine-banded armadillo model, a unique animal model for leprosy.

[Leprosy is definitely not a disease of the past]

In this review we present a synthesis of the current knowledge of leprosy based on the epidemiological studies and the latest data obtained in basic research and the strategies undertaken to fight out this pathology. The significant progress in the genomic study of the leprosy bacillus, and in understanding the basic mechanisms governing the individual susceptibility to the disease, together with the stagnation in the incidence of the new cases observed for ten years, represent focus of interest. If the efficiency of the recommended treatments is actually confirmed by a low relapse rate, however the drying up of new cases, which could be expected by reducing the infectiousness of the treated cases, is nonetheless slow in coming. Recent studies support the effectiveness of prophylaxis based on a single dose of rifampicin and/or a BCG vaccination of patient contacts. This could allow an increased reduction of the incidence of the disease. However, no official position validates such strategies. Because of an ambiguity in the expression of the objectives for managing the disease, i.e. « elimination of a disease considered as a public health problem », which should not to be confused with « eradication of the disease », which, in fact, has no relevance today, demobilization in fighting against leprosy is to be feared.