title: Tuberculosis seasonality in the Netherlands differs between natives and non-natives: a role for vitamin D deficiency?
abstract: METHODS
  The seasonality of tuberculosis (TB) incidence suggests that the risk of infection or development of disease has a seasonal component.
  
  
  OBJECTIVE
  To investigate factors associated with seasonal patterns of TB disease in the Netherlands by splitting notifications according  to origin (natives vs. non-natives) and disease site (pulmonary TB [PTB] vs. extra-pulmonary TB [EPTB]). We focus on the presence of a seasonal peak, as much debate has centred on factors enhancing transmission vs. disease development.
  
  
  METHODS
  Monthly notifications were derived from culture  sample dates of all cases between 1993 and 2008. We fitted seasonal autoregressive integrated moving average (SARIMA) models to the time series. Seasonal decomposition revealed seasonal trends. To assess the seasonality of the peak, we repeated the analysis omitting December (trough) notifications.
  
  
  RESULTS
  TB notifications show a seasonal pattern, with a peak in spring and a trough in winter, which is present in both PTB and EPTB and in both natives and non-natives. However, when excluding December notifications, seasonality only holds in non-native EPTB and non-native TB notifications.
  
  
  CONCLUSIONS
  A seasonal peak in TB notifications (March-June) is apparent in non-natives, but is absent in natives. This peak is driven by the seasonality of EPTB notifications, which are highest in June-July. The contribution of winter crowding is discussed. Vitamin D deficiency, enhancing  disease development at the end of winter-early spring, seems the most likely factor explaining the yearly peak in EPTB.
authors.affiliation: Epidemiology and Surveillance, Tuberculosis and M/XDR-TB Programme, National Institute for Public Health and the Environment, Bilthoven, The Netherlands. hester.korthals.altes@rivm.nl
authors.forename: H
authors.initials: H
authors.lastname: Korthals Altes
authors.forename: K
authors.initials: K
authors.lastname: Kremer
authors.forename: C
authors.initials: C
authors.lastname: Erkens
authors.forename: D
authors.initials: D
authors.lastname: van Soolingen
authors.forename: J
authors.initials: J
authors.lastname: Wallinga
mesh_terms: D054242:Emigrants and Immigrants / Q000706:statistics & numerical data; D006801:Humans; D015233:Models, Statistical; D009426:Netherlands / Q000453:epidemiology; D012307:Risk Factors; D012621:Seasons; D014376:Tuberculosis / Q000453:epidemiology* / Q000208:ethnology / Q000209:etiology / Q000635:transmission; D014397:Tuberculosis, Pulmonary / Q000453:epidemiology* / Q000209:etiology / Q000635:transmission; D014808:Vitamin D Deficiency / Q000150:complications* / Q000453:epidemiology / Q000208:ethnology