Major Impact of Massive Migration on Spread of Mycobacterium tuberculosis Strains

Dissemination of epidemic Mycobacterium tuberculosis strains is a global concern, and any world region may be at risk of their introduction. This is a well-known cliché, but the reality is more nuanced. Based on the analysis of East-Asian and Euro-American lineages of M. tuberculosis, three conclusions can be drawn. First, an ordinary human exchange is not sufficient for wide dissemination of an M. tuberculosis strain in a new location. In contrast, a massive influx of migrants dramatically changes the population structure (both human and the pathogen’s). Second, an emerging strain can emerge in its area of origin, but it will not necessarily be successful elsewhere. Third, a strain should be sufficiently prevalent in its country of origin to be efficiently imported to a new location. Summing up, the virulence of M. tuberculosis strains is conditional, but it is not an absolute feature, and the role of human migration in their spread is crucial.