title: Heredity versus environment in tuberculosis in twins: the 1950s United Kingdom Prophit Survey Simonds and Comstock revisited.
abstract: BACKGROUND
  In his 1978 article on tuberculosis (TB) in twins, Comstock concluded that the 2.5-fold higher concordance rate for TB among monozygotic versus dizygotic twins in the Prophit survey of the 1950s implicated inherited susceptibility as a major risk factor for TB in humans. His analysis did not take into account strong imbalance of variables within subgroups, underestimating possible confounding effects of environmental factors.
  
  
  OBJECTIVE
  To reconsider the role of environmental versus hereditary factors in determining the concordance rate of TB among twin pairs.
  
  
  METHODS
  Reanalysis of the Prophit Survey.
  
  
  RESULTS
  A known Mycobacterium tuberculosis-positive or M. tuberculosis-negative sputum in the index TB case markedly influenced the odds ratio (OR) of concordance in the twin pairs. In 87 pairs with co-twins exposed to a sputum-negative index case, monozygotic and dizygotic twins did not differ in concordance for TB (OR, 1.1; 95% confidence interval [95% CI], 0.4-2.8). A higher concordance rate for TB among monozygotic versus dizygotic twins was confined to 106 pairs with the co-twins exposed to a sputum-positive index case (OR, 3.4; 95% CI, 1.6-7.2), and was highest in adolescent twins living together. ORs of TB concordance were proportional to intensity of exposure (sputum smear positivity, physical proximity between twin pairs, contagiousness of disease, and living together) rather than to zygosity.
  
  
  CONCLUSIONS
  In the Prophit survey of susceptibility to TB among twins, environmental factors (i.e., intensity of exposure to tubercle bacilli) outweigh the importance of hereditary factors. Environmental factors and the context of transmission should be given more emphasis when studying interindividual and population differences in susceptibility to infectious diseases such as TB.
authors.affiliation: Department of Infectious Diseases, C5-P, Leiden University Medical Center, P.O. Box 9600, 2300 RC Leiden, The Netherlands.
authors.forename: Ellen A
authors.initials: EA
authors.lastname: van der Eijk
authors.forename: Esther
authors.initials: E
authors.lastname: van de Vosse
authors.forename: Jan P
authors.initials: JP
authors.lastname: Vandenbroucke
authors.forename: Jaap T
authors.initials: JT
authors.lastname: van Dissel
mesh_terms: D015982:Bias; D015986:Confounding Factors, Epidemiologic; D005260:Female; D020022:Genetic Predisposition to Disease; D006801:Humans; D019570:Inhalation Exposure / Q000706:statistics & numerical data*; D008297:Male; D015999:Multivariate Analysis; D009169:Mycobacterium tuberculosis; D016017:Odds Ratio; D015203:Reproducibility of Results; D014376:Tuberculosis / Q000453:epidemiology / Q000235:genetics* / Q000635:transmission*; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic; D006113:United Kingdom / Q000453:epidemiology