Clustered tuberculosis cases: do they represent recent transmission and can they be detected earlier?

Clustered tuberculosis cases with Mycobacterium tuberculosis isolates showing identical restriction fragment length polymorphism patterns are assumed to be the result of disease transmission. In a prospective, population-based study in the province of North Holland, The Netherlands, we combined molecular methods with highly detailed epidemiologic information to determine why many clustered cases are not detected at an early stage. Of 481 patients, 138 (29%) fell into 43 clusters, suggesting recent transmission in 20%. Of 155 patients in clusters occurring within 2 years before or after the diagnosis of the disease, 21 (14%) had no epidemiologic links with other patients. Independent predictors of the absence of such links were female sex and Turkish, Moroccan, or other African ethnicity. Of 47 patients with a clear epidemiologic link, 37 (24% of 155) were identified early, e.g., by contact tracing, and 10 (6%) were missed. In 85 (55%) patients, an epidemiologic link was likely but undetected when using conventional contact tracing. Compared with clearly linked patients, only male sex was independently associated with presence in this last group. Our results indicate that 86% of clustered study patients had epidemiologic links and that opportunities for earlier identification using conventional tuberculosis control strategies are limited.