Connecting the dots: understanding how human mobility shapes TB epidemics

Abstract

Tuberculosis (TB) remains a leading infectious cause of death worldwide. Reducing TB infections and TB-related deaths rests ultimately on stopping forward transmission from infectious to susceptible individuals. Critical to this effort is understanding how human host mobility shapes the transmission and dispersal of new or existing strains of Mycobacterium tuberculosis (Mtb). Important questions remain unanswered. What kinds of mobility, over what temporal and spatial scales, facilitate TB transmission? How do human mobility patterns influence the dispersal of novel Mtb strains, including emergent drug-resistant strains? This review summarizes the current state of knowledge on mobility and TB epidemic dynamics, using examples from three topic areas, including inference of genetic and spatial clustering of infections, delineating source-sink dynamics, and mapping the dispersal of novel TB strains, to examine scientific questions and methodological issues within this topic. We also review new data sources for measuring human mobility, including mobile phone-associated movement data, and discuss important limitations on their use in TB epidemiology.

Figure 1.. Geographic distance, effective connectivity, and genetic clustering of Mycobacterium tuberculosis cases.

Genetically clustered cases may not exhibit spatial clustering if geographic distance is not consistently correlated with effective connectivity between locations (left panel). Considering effective network distances (for example, connectivity via human mobility between locations) may identify clustered infections (right panel).

Figure 2.. Range expansion of a Mycobacterium tuberculosis strain.

During geographic range expansion of a strain, a population spreads via serial founder events that result in distinct population genetic signatures, including decreasing genetic diversity and increasing genetic divergence when compared with samples from the origin population. A ➔ B: short-distance founder events between adjacent territories. B ➔ C: long-distance founder event between noncontiguous territories. Orange territories indicate geographic areas in which a new TB strain is present at a given time.