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Use whole genome sequencing (WGS) in combination with social network analysis to trace the previously unidentified source of a local outbreak of tuberculosis (TB) in a Canadian town. 

WGS was used to analyse 32 samples of Mycobacterium tuberculosis from the outbreak and four samples from the same area before the outbreak. Genomic and epidemiological data were combined with mapping of social contacts based on interviews with TB patients. 

Two genetically distinct forms of the same strain of M. tuberculosis were identified, both already present in the community prior to the outbreak. Multiple transmission events were identified including a small number of ‘superspreaders’, patients who each passed TB on to 3-6 new cases. 

Social or environmental triggers, as opposed to genetic or biological factors, are mostly likely to have caused the TB outbreak. Epidemiological data showed that the outbreak coincided with a surge in the use of crack cocaine, and this is the probable cause due to factors such as lower immunity, decreased use of health services, and crowded / poorly ventilated environments shared by cocaine users, who comprised more than 60% of TB cases.

Using WGS revealed that the apparent outbreak was in fact two simultaneous outbreaks of slightly different M. tuberculosis strains, which made it much simpler to map and analyse social contacts to trace the sources. This is an example of how the application of modern sequencing technologies in microbial genomics is becoming an element of standard public health practice, and can have a significant impact on mapping outbreaks of infectious disease.