Multidrug-resistant tuberculosis (MDRTB) is an important public health problem. Two papers in the October issue of Nature Medicine analyze the emergence of MDRTB and discuss the suitability of the epidemiological measures that aim to contain it.
Sally Blower and Tom Chou developed a mathematical model that tracks the emergence of ‘hot zones’ — areas with high prevalence of MDRTB. They reconstructed possible evolutionary trajectories that generated hot zones over the past three decades and identified the causal factors. They found that areas where drug-sensitive strains had been successfully reduced frequently evolved into hot zones. Some hot zones emerged even when MDRTB strains were substantially less transmissible than drug-sensitive strains. As levels of MDRTB depended not only on detection and cure rates, but also on the amplification probability of the resistant strains, the authors propose that it is essential to minimize this probability to control MDRTB in hot zones.

Ted Cohen and Megan Murray used a mathematical model to predict the future burden of MDRTB, and challenged the idea that the threat of resistance to tuberculosis control depends on the relative ‘fitness’ of MDRTB strains to compete against drug-resistant strains. It was thought that if MDRTB strains were less fit than drug-sensitive strains, the emergence of resistance would not jeopardize tuberculosis control efforts. Here the authors show that even a small subpopulation of a relatively fit MDRTB strain may outcompete drug-sensitive and less fit MDRTB strains. These results imply that current epidemiological measures might not be sufficient to contain MDRTB strains in there are no specific efforts to limit the transmission of these strains.