Risk factors for clustering of tuberculosis cases: a systematic review of population-based molecular epidemiology studies.

BACKGROUND

Many molecular epidemiology studies have been conducted to identify risk factors for clustering of tuberculosis (TB) cases in the population.

OBJECTIVE

To estimate the impact of commonly investigated risk factors on TB clustering.

METHODS

Ten electronic databases were searched up to January 2006 along with a hand search of the International Journal of Tuberculosis and Lung Disease and bibliographies of review articles. Meta-analyses of odds ratios (ORs) for various risk factors were conducted using random effect models, stratified by TB incidence. Meta-regressions were employed to account for the heterogeneity in clustering proportions and the magnitudes of risk.

FINDINGS

The TB clustering proportion varied greatly (7.0-72.3%) among 36 studies in 17 countries. In multiple meta-regression analyses, high TB incidence, mean cluster size and conventional contact tracing were significantly associated with higher clustering. The pooled ORs (95%CIs) for low and high/intermediate TB incidence studies, using a cut off of 25/100000 per year, were 3.4 (2.7- 4.2) and 1.6 (1.3-2.1) for local-born status, 1.6 (1.5-1.7) and 1.7 (1.3-2.2) for pulmonary TB and 1.2 (1.1-1.3) and 1.3 (1.1-1.7) for smear-positive cases, respectively. Male sex, local birth, alcohol abuse and injection drug use were significantly higher risks in low TB incidence studies than in the high/intermediate ones.

INTERPRETATION

Meta-analyses yielded significant estimates of ORs for several risk factors across both levels of TB incidence. Alcohol abuse, injection drug use and homelessness--all characteristics of marginalized populations--were found to be consistently significant in populations of low TB incidence. More research is needed to better understand TB transmission dynamics in high-burden countries.