The incidence of tuberculosis transmission among family members and outside households

The risk of Mycobacterium tuberculosis complex (MTBC) infection is correlated with the concentration of infectious particles and exposure time. In closed populations, healthy people staying in very frequent, close and prolonged contact with a smear-positive person, become infected and represent another link in the chain of transmission of the disease. Therefore, in the fight against tuberculosis, an important element is quick identification of the patient and potentially infected people from his environment. In epidemiological investigation of tuberculosis (TB), family members are brought under special control as they are particularly exposed to transmission of infectious diseases.

The study included 150 patients with bacteriologically confirmed tuberculosis who were members of 59 families. In the years 2003-2013 this population represented all TB cases detected in Poland in a family environment.Three PCR-based genotyping methods: spoligotyping, IS6110-Mtb1-Mtb2 PCR and MIRU-VNTR typing were used.

Of 150 patients, 138 could be assigned to intra-household transmission on the basis of identical DNA fingerprints upon a combined typing approach. For 12 patients in 6 households, the M. tuberculosis isolates were clearly distinct in individual analysis - IS6110-Mtb1-Mtb2 PCR, spoligotyping or MIRU-VNTR typing or in three genotyping methods, suggesting that these patients were infected by the sources in the community.

The analysis confirmed the transmission of tuberculosis among members of 53 families. In the remaining 6 families the source of infection were people outside the households. In all families with young children, strains isolated from them have identical DNA patterns as strains obtained from their adult caregivers. To confirm the transmission of TB in the study population of patients, epidemiological analysis required the addition of a genotyping methods characterised by high discriminatory power.