Restriction of the main lineages of Mycobacterium africanum to West Africa: Insights from host-pathogen interaction studies

Tuberculosis (TB), a disease of old, continues to plague humans after its declaration as a global health emergency in 1993. Over the years, studies have focused on understanding the causative pathogen, Mycobacterium tuberculosis complex (MTBC), and its interaction with humans from TB infection to progression to active disease. It is now known that MTBC lineage diversity impacts several disease presentations and outcomes, including disease progression and severity, virulence and antimicrobial resistance, transmissibility, and host response. Some of these lineages are highly geographically restricted, and prominent amongst them are lineages 5 and 6 of Mycobacterium africanum (Maf), mainly found in West Africa, with cases outside of this region usually prevalent in individuals of West African descent. Several hypotheses have been propounded to investigate these restrictions, ranging from the locality of an animal reservoir in certain areas to the emigration of Maf into West Africa but not spreading globally because Mycobacterium tuberculosis sensu stricto (Mtbss) outcompeted it. Another hypothesis, which states that host genetic factors can influence host susceptibility to some MTBC lineages, as well as TB progression to the severe disease state, appears more widely accepted. However, the exact mechanisms mediating this susceptibility have not been fully explored. This review seeks to highlight the advances made towards understanding the geographical restrictions of Maf and the host-pathogen interactions leading to the coevolution of Maf and humans in West Africa.