Page 11 - Epidemiological studies on tuberculosis control and respiratory viruses

VNTR clustering in relation to the M. tuberculosis population
DNA typing is a powerful tool to trace tuberculosis (TB) transmission and outbreaks.
Clustering of Mycobacterium tuberculosis isolates based on identical DNA fingerprints is
commonly used as a proxy for recent transmission (1). However, this assumption is not
always correct and depends on many factors, such as circulation of genetically similar
strains, evolution of M. tuberculosis over time, transmission rate, DNA typing methods
applied, duration of the study period, sampling, and effectiveness of TB control (2, 2 3). Various studies have shown that not all cases in DNA fingerprint clusters have epidemiological links with other cases in the cluster (4, 5). Moreover, epidemiological
links have been found between cases caused by bacteria with slightly different DNA fingerprints (6). Clustering results among cases in the immigrant population especially should be interpreted with caution (7, 8), as isolates from these patients often belong to genetically compact strain lineages predominating in the countries of origin (9, 10, 11, 12).
In the Netherlands, more than 70% of all TB cases are found among foreign-born persons, and extensive information on each patient is stored in a national registry. We aimed to investigate the population structure of M. tuberculosis isolates among native and immigrant cases and to determine the consequences for the interpretation of recent transmission based on variable-number tandem- repeat (VNTR) typing results.
Culture-confirmed TB cases from October 2003 to December 2008 were included in this study. Patient information was obtained from the Netherlands Tuberculosis Register (NTR), held by the KNCV Tuberculosis Foundation. In total, 3,975 M. tuberculosis isolates were typed by IS6110/PGRS restriction fragment length polymorphism (RFLP) and standard 24-locus VNTR typing (13, 14) at the RIVM or by Genoscreen (Lille). Molecular data were matched with demographic data using the date of birth, sex, postal area code, and year of diagnosis, resulting in 3,793 (95%) matching cases. After exclusion of 17 foreign-born individuals because of incomplete data for several variables, 3,776 (95%) cases remained eligible.
Genotype information was uploaded to the MIRU-VNTRplus web-application (http://www.miru-vntrplus.org) (15) for phylogenetic lineage prediction, which was performed stepwise as described by Allix-Beguec et al. (16). Isolates that were part of the CAS, Beijing, EAI, Mycobacterium bovis, and Mycobacterium africanum lineages were categorized as non-Euro-American and the remaining as a Euro-American superlineage (16).
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