Effect of clinical and socio-economic factors on the rate of clustering of Mycobacterium tuberculosis clinical isolates in Elche (Spain)

Mycobacterial factors relevant for transmission of tuberculosis

BACKGROUND

Tuberculosis (TB) transmission is associated with patient-related risk factors. However, DNA fingerprint analysis has provided anecdotal evidence suggesting a role for bacteriological factors.

METHODS

To examine the importance of the bacteriological component in TB transmission, we investigated the number of tuberculin skin test-positive (TST induration, ≥ 10 mm) contacts and secondary cases observed in contact investigations around TB cases in relation to the size of the genotype cluster the patient belonged to at the time of diagnosis. We also compared the number of TST-positive contacts and secondary cases of patients with drug-resistant and drug-susceptible TB.

RESULTS

Larger clusters were independently associated with an increased number of positive contacts. The mean number of positive contacts ranged from 3.8 for clusters of 2 cases, to 4.7 for clusters of 3-10 cases, to 6.0 for cases in clusters of >10 cases (mean increase in number of positive contacts for every extra case in the cluster, 0.21; 95% confidence interval, 0.09-0.26). The mean number of positive contacts was significantly lower among index cases with isoniazid-monoresistant TB (1.6) than among index cases with pan-susceptible TB (4.6; relative number, 0.45; 95% confidence interval, 0.22-0.92).

CONCLUSION

These results suggest that spread of tuberculosis also depends on bacteriological factors.

Population-based prospective molecular and conventional epidemiological study of tuberculosis in Hong Kong

OBJECTIVE

To study the transmission of tuberculosis using conventional and molecular epidemiology in Hong Kong.

METHODS

All patients with positive sputum culture for Mycobacterium tuberculosis residing on the Island of Hong Kong were recruited from May 1999 to April 2002. The restriction fragment length polymorphism technique was used to determine DNA patterns of isolates of M. tuberculosis using the IS6110 probe, supplemented by pTBN12 as a secondary probe.

RESULTS

One thousand five hundred and fifty-three of 2337 (66%) of the patients with bacteriologically confirmed tuberculosis had restriction fragment length polymorphism analysis of their M. tuberculosis isolates. Four hundred and fifty-four (29.2%) patients belonging to 143 clusters were identified; the estimated rate of recent transmission was 20-24%. Significant predictors of clustering included young-age groups (<40 years) versus those >60 years of age (adjusted odds ratio (OR) 1.96, 95% confidence interval 1.47-2.62), permanent residency versus new or non-residents (adjusted OR 3.40, 95% 1.84-6.26) and previous default from treatment versus new cases (adjusted OR 6.12, 95% confidence interval 1.82-20.5). Alcohol and drug abuse, history of imprisonment and HIV infection were not significant risk factors for molecular clustering. Of patients belonging to clusters, 5.1% had definite, 5.5% had probable and 24.4% had possible epidemiological link, suggesting casual contact may be responsible for a high proportion of the clustered cases.

CONCLUSION

One-fifth to one quarter of the new cases of active tuberculosis in Hong Kong are due to recent transmission. In addition to early diagnosis and successful treatment of all active disease, treatment of latent disease should receive more attention in the control of tuberculosis in Hong Kong.