Evolutionary history of Mycobacterium leprae in the Pacific Islands

As one of the oldest known human diseases, leprosy or Hansen's disease remains a public health concern around the world with over 200 000 new cases in 2018. Most human leprosy cases are caused by Mycobacterium leprae, but a small number of cases are now known to be caused by Mycobacterium lepromatosis, a sister taxon of M. leprae. The global pattern of genomic variation in M. leprae is not well defined. Particularly, in the Pacific Islands, the origins of leprosy are disputed. Historically, it has been argued that leprosy arrived on the islands during nineteenth century colonialism, but some oral traditions and palaeopathological evidence suggest an older introduction. To address this, as well as investigate patterns of pathogen exchange across the Pacific Islands, we extracted DNA from 39 formalin-fixed paraffin-embedded biopsy blocks dating to 1992-2016. Using whole-genome enrichment and next-generation sequencing, we produced nine M. leprae genomes dating to 1998-2015 and ranging from 4-63x depth of coverage. Phylogenetic analyses indicate that these strains belong to basal lineages within the M. leprae phylogeny, specifically falling in branches 0 and 5. The phylogeographical patterning and evolutionary dating analysis of these strains support a pre-modern introduction of M. leprae into the Pacific Islands. This article is part of the theme issue 'Insights into health and disease from ancient biomolecules'.