A new study analysed how various strains of Tuberculosis migrate across mixed populations in cosmopolitan cities, the chances of an exposed person becoming infected with TB vary depending on whether the human and the bacterium share a hometown. The study was led by researchers from the Harvard Medical School. The study provides evidence that pathogen, place, and human host interact in a unique way that affects infection risk and susceptibility.

The findings of this study can also help in prevention and new treatment approaches for TB which affects more than 10 million people and causes more than a million deaths worldwide, according to the World Health Organization (WHO).

For the study, the researchers custom-built a study cohort by combining case files from patients with TB in New York City, Amsterdam, and Hamburg. This gave them enough data to power their models.

The analysis showed that close household contacts of people diagnosed with a strain of TB from a geographically restricted lineage had a 14 per cent lower rate of infection and a 45 per cent lower rate of developing active TB disease compared with those exposed to a strain belonging to a widespread lineage.

The study also showed that strains with narrow geographic ranges are much more likely to infect people with roots in the bacteria's native geographic region than people from outside the region.

The researchers found that the odds of infection dropped by 38 per cent when a contact is exposed to a restricted pathogen from a geographic region that doesn’t match the person’s background, compared with when a person is exposed to a geographically restricted microbe from a region that does match their home country.

The researchers said that this pathogen-host affinity points to a shared evolution between humans and microbes with certain biological features rendering both more compatible and fueling the risk for infection.

Maha Farhat, Gilbert S. Omenn, MD '65, PhD Associate Professor of Biomedical Informatics in the Blavatnik Institute at HMS said, “The size of the effect is surprisingly large. That’s a good indicator that the impact on public health is substantial.”

The new study highlights that for geographically restricted strains, whether a person has ancestors who lived where the strain is common was an even bigger predictor of infection risk than bacterial load in the sputum. In the cases analyzed in the study, this risk of common ancestry even outweighed the risk stemming from having diabetes and other chronic diseases previously shown to render people more susceptible to infection.

The findings add to a growing body of evidence of the importance of paying attention to the wide variation between different lineages of tuberculosis and to the details of how different lineages of tuberculosis interact with different host populations.

Matthias Groeschel, research fellow in biomedical informatics in Farhat’s lab at HMS; resident physician at Charite, a university hospital in Berlin and the study’s first author said, “These findings emphasize how important it is to understand what makes different strains of TB behave so differently from one another, and why some strains have such a close affinity for specific, related groups of people.”

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