Characterising transmission of a tuberculosis genotype in Scotland: a qualitative approach to social network enquiry

SETTING: Contact investigation resulting from specimens sent to the Scottish Mycobacteria Reference Laboratory.OBJECTIVE: To characterise patients and types of exposures associated with transmission of a prevalent Mycobacterium tuberculosis genotype in Scotland.DESIGN: A combined approach using molecular epidemiology and semi-structured patient interviews for social network enquiry.RESULTS: We investigated social connections between 64 patients diagnosed between 1994 and 2004. Fifty-five per cent had ≥1 identifiable contact. One third (n = 14, 32.6%) of the 43 epidemiological links detected were discerned as a result of patient interviews and were not previously recorded on surveillance reports, nor recognised by nurse specialists (all were non-household contacts). Sixteen putative sites of exposure were identified, 11 were public houses. Rather than a single-source outbreak, eight pockets of transmission were identified, the largest involving UK-born alcohol-misusing males frequenting several public houses.CONCLUSIONS: Using a standardised approach to explore themes around which individuals may have been exposed to TB resulted in the detection of previously unrecognised epidemiological links. Epidemiological data obtained from cluster investigations, e.g., risk and social behaviours that increase the risk of infection and sites of putative exposure, can enhance the development of more appropriate questions for the contact tracing interview.