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Unexpectedly high strain diversity of Mycobacterium tuberculosis in a high-incidence community
1Department of Medical Biochemistry, University of Stellenbosch, Tygerberg, W. Cape.
Objective:
To characterise Mycobacterium tuberculosis strains present in a community experiencing an epidemic, in order to establish whether a high rate of transmission results in low strain diversity.
Design:
Sputum specimens collected for 18 months; IS6110-based DNA fingerprinting.
Setting:
The communities of Ravensmead and Uitsig, Cape Town, South Africa.
Participants:
Three hundred and thirty-four pulmonary tuberculosis patients attending the Local Authority Health Care Clinic.
Main Outcome Measure:
DNA fingerprinting.
Results:
A total of 334 M. tuberculosis isolates were characterised by IS6110-based DNA fingerprinting; 209 strains were identified, 199 having 5 or more insertions. Forty of these strains were present in 2 or more patients (clustering--126 patients in total), which indicates a recent transmission rate of 30%. The 163 unique strains suggest reactivation of latent infections. Computer analysis showed a high degree of strain diversity, and a common progenitor could only be linked to 33% of the strains. Clustering was shown in 50% of drug-resistant isolates.
Conclusions:
The low rate of transmission (30%) and the high degree of strain diversity (209 strains) was unexpected and unexplained, given the high burden of disease in this community. The clustering of drug-resistant strains suggests that transmission, rather than lack of compliance, drives the spread of antibiotic resistance in this community. Preliminary indications are that BCG vaccination, while having little effect on the incidence of tuberculosis in this community, may have altered the strain dynamics.