Immunogenic Mycobacterium africanum Strains Associated with Ongoing Transmission in The Gambia

Tuberculosis (TB), caused by bacterial pathogens of the Mycobacterium tuberculosis complex (MTBC), is a major global health problem. Sub-Saharan Africa has the highest rate of TB per capita and the lowest case detection rate; although TB incidence is decreasing globally, incidence rates are increasing in most countries in the West Africa region (1). Moreover, almost half of all TB cases in West Africa are caused by infection with an unusual member of the MTBC, M. africanum, a lineage found exclusively in this region. Although M. africanum was initially described in Senegal in 1968 (2), and despite its importance and high prevalence in this region, relatively little is known about the bacterium (3). In general, M. africanum can be divided into 2 lineages: Afri_1, by SpolDB4 definition (4), corresponding to the green lineage 6 (5), which has the highest prevalence in Senegal, Mali, The Gambia, Guinea-Bissau, and Sierra Leone (3); and Afri_2 (4), corresponding to the brown lineage 5 (5), which is mainly found in the eastern part of West Africa, in countries such as Cote d’Ivoire, Ghana, Benin, Nigeria, and Cameroon (3). Although transmission of M. africanum from host to host is a crucial element of the spread of the disease, the underlying biological mechanisms triggering transmission are elusive. We assessed transmission dynamics and interaction between the 2 mycobacterial populations in The Gambia, a country in western West Africa, and compared the local situation with previously published data from Guinea-Bissau, another country within the region (6). In particular, considering a recent publication suggesting that conserved mycobacterial T-cell epitopes may play a role in the transmission of the mycobacteria within the host population (7), we investigated whether differences in immunogenicity between M. tuberculosis and M. africanum strains (especially of the predominant Euro-American [EA] and Afri_1 lineages) could predict the success of certain sublineages to transmit and establish themselves within the human host population.