A novel approach - the propensity to propagate (PTP) method for controlling for host factors in studying the transmission of Mycobacterium tuberculosis.
BACKGROUND
Understanding the genetic variations among Mycobacterium tuberculosis (MTB) strains with differential ability to transmit would be a major step forward in preventing transmission.


OBJECTIVE
To describe a method to extend conventional proxy measures of transmissibility by adjusting for patient-related factors, thus strengthening the causal association found with bacterial factors.


METHODS
Clinical, demographic and molecular fingerprinting data were obtained during routine surveillance of verified MTB cases reported in the Netherlands between 1993 and 2011, and the phylogenetic lineages of the isolates were inferred. Odds ratios for host risk factors for clustering were used to obtain a measure of each patient's and cluster's propensity to propagate (CPP). Mean and median cluster sizes across different categories of CPP were compared amongst four different phylogenetic lineages.


RESULTS
Both mean and median cluster size grew with increasing CPP category. On average, CPP values from Euro-American lineage strains were higher than Beijing and EAI strains. There were no significant differences between the mean and median cluster sizes among the four phylogenetic lineages within each CPP category.


CONCLUSIONS
Our finding that the distribution of CPP scores was unequal across four different phylogenetic lineages supports the notion that host-related factors should be controlled for to attain comparability in measuring the different phylogenetic lineages' ability to propagate. Although Euro-American strains were more likely to be in clusters in an unadjusted analysis, no significant differences among the four lineages persisted after we controlled for host factors.
National Institute for Public Health and the Environment (RIVM), Bilthoven, The Netherlands; Life and Health Sciences Research Institute (ICVS), School of Health Sciences, University of Minho, Braga, Portugal; ICVS/3B's, PT Government Associate Laboratory, Braga/Guimarães, Portugal.
Hanna
H
Nebenzahl-Guimaraes
Public Health Service, Amsterdam, Netherlands; Department of Clinical Epidemiology, Academic Medical Centre, University of Amsterdam, Amsterdam, Netherlands.
Martien W
MW
Borgdorff
Department of Global Health and Social Medicine, Harvard Medical School, Boston, United States of America.
Megan B
MB
Murray
National Institute for Public Health and the Environment (RIVM), Bilthoven, The Netherlands; Department of Medical Microbiology, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands.
Dick
D
van Soolingen
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D016000:Cluster Analysis; D004812:Epidemiologic Methods; D005260:Female; D014644:Genetic Variation; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D008297:Male; D008875:Middle Aged; D009169:Mycobacterium tuberculosis / Q000235:genetics / Q000502:physiology*; D010802:Phylogeny; D014376:Tuberculosis / Q000453:epidemiology* / Q000635:transmission*; D055815:Young Adult