Decrease in incidence of tuberculosis in Madrid is associated to declining rates of recent transmission: an epidemiological and molecular study between 1997 and 2004

Abstract number: P1560

Iñigo J., Palenque E., Arce A., García de Viedma D., Chaves F.

Objectives: Longitudinal studies using molecular and conventional epidemiological methods are necessary to understand whether changes in incidence rates in a particular geographic area can be caused by changes in ongoing transmission. The aim of this observational study was to determine to what extent TB trends in Madrid from 1997 to 2004 were determined by changes in recent transmission of disease.

Methods: Two prospective population-based molecular and epidemiological studies of patients diagnosed with TB were conducted in three urban districts of Madrid during two different periods. The first one was performed between 1997–1999 (population: 455,050, 4% immigrants) and the second between 2002–2004 in the same geographic area (population: 488,518, 14.9% immigrants). DNA fingerprinting was performed using IS6110 and spoligotyping.

Results: In the first period the incidence of TB was on average 30.2 cases/100,000, while for the second was 25.7/100,000 (p < 0.001). The comparison of TB cases from two study periods showed an important increase of foreign-born population from 5.3% to 34% (p < 0.001), and a decrease of HIV infection from 25.7% to 12.2% (p < 0.001). In the first period, 95 of 212 patients (44.8%) were grouped in clusters, and in the second, 64 of 201 patients (31.8%). The decrease in the overall case rate was associated to a decrease in the clustered case rate, 7.0 per 100,000 in 1997–1999 to 4.4 in 2002–2004 (p < 0.001). The non-clustered case rate was 8.6 in 1997–1999 and 9.3 in 2002–2004 (p = 0.45). The overall incidence among Spain-born population decreased from 29.9 to 20.0 (p < 0.001), and this decrease occurred either in cases attributed to recent transmission (p < 0.001) and cases attributed to TB reactivation (p = 0.04). Among the foreign-born population although there was not significant changes in the overall case rate (40.0 vs 58.7), however the non-clustered case rate increased significantly (p = 0.02). Decrease in clustered case rates were also observed in women (p = 0.02) and persons younger than 35 years-old (p = 0.02).

Conclusions: The percentage of recent transmission of TB has decreased from 1997–99 to 2002–04 in Madrid and this change is associated to declining rates of recent transmission occurring mainly in the Spain-born population. The reduction in the proportion of clustered cases was also related to the increase in the number of TB cases among foreigners which could mean the introduction of new strains.