Leprosy, a slowly progressing disease in humans causing skin lesions and nerve damage that can lead to disfigurement, disability and stigma, is caused by Mycobacterium leprae and a closely related species, Mycobacterium lepromatosis. Due to extensive reductive evolution of their genomes, they are obligate intracellular pathogens and are not capable of growth in any in vitro medium, which limited scientific study of these bacilli until the advent of cultivation in animals such as athymic nude mice and armadillos was achieved. Until recently, the only other natural host for M. leprae has been wild armadillos in the southern states in the U.S., first described in the early 1970s. Researchers in different parts of the world have suggested that M. leprae could be found resident in soil, water, in sphagnum and moss vegetation, in single celled “feral macrophages” like amoeba, in insects such as ticks or kissing bugs, and other animals such as fish, armadillos and subhuman primates such as monkeys.

Through whole genome sequencing and SNP typing, the dominant M. leprae strain type in almost 90% of wild armadillos in Texas and Louisiana and the majority of endemic human cases was type 3I-2-v1. More recently, a different strain type was found to be dominant in central Florida, 3I-2-v15, in both humans and armadillos, indicating a that second founder colonization event of M. leprae from humans into armadillos had occurred. SNP typing has also provided evidence of SNP type 3I M. leprae in red squirrels in the United Kingdom. The M. leprae strain found in the squirrels is most closely related to one that was found in human bones from a burial site dated around 730 years ago, indicating it was circulating in medieval England. Recently, investigators for the first time showed that M. leprae had infected wild chimpanzees in two separate forests in West Africa, Guinea-Bissau and Ivory Coast. SNP typing indicated that neither of the strains, 4N/O and 2F, were known to be associated with humans in these areas and that both were ancient, with an estimated divergence time during the 6th century C.E. and 2nd century C.E., respectively. This suggests that the strains in chimpanzees did not originate from humans but exist in another animal reservoir or in soil. If there are additional natural reservoirs of M. leprae in animal hosts or the environment, their origin and knowledge about how they persist and spread have important implications for epidemiology and control, not just in human populations but in other species in nature.

This Research Topic aims to identify additional niches of M. leprae and M. lepromatosis in the environment, whether in animals, plants, insects, water or soil as well as report and discussion related to the development of new One Health based research strategies and disease intervention strategies to eradicate leprosy. Due to the inability to cultivate these microorganisms in artificial media, the full array of available tests should be utilized wherever possible to support the findings in a biologically plausible manner: PCR or qPCR; identification of strains by whole genome sequencing (WGS), SNP or VNTR typing; serology; histopathology; identification of acid-fast bacilli or use of other staining techniques to identify mycobacteria in situ.

Topics include but are not limited to the following:

• Novel animal species infected with M. leprae or M. lepromatosis

• Surveys of various armadillo species for M. leprae or M. lepromatosis infection in countries or regions not yet identified.

• Case control studies of risks associated with human contact or consumption of animal reservoirs.

• Molecular identification and genetic typing of M. leprae in samples of soil or water to show an association with those strain types with human populations living in leprosy endemic areas.

• Evolutionary age or geographic origin of isolates determined by WGS or SNP typing to determine most recent common ancestor (MRCA) or phylogenetic mapping in animal and human leprosy bacilli strains.

• Paleomicrobiology in animal reservoirs and their history.

• Point-of-view and review on One Health approaches and strategies to eradicate leprosy.

We encourage scientists to contribute with the following article types: original research, review, mini-review, perspective, and opinion.

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Frontiers in Microbiology

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