title: Clostridium difficile infection seasonality: patterns across hemispheres and continents - a systematic review.
abstract: BACKGROUND
  Studies have demonstrated seasonal variability in rates of Clostridium difficile infection (CDI). Synthesising all available information on seasonality is a necessary step in identifying large-scale epidemiological patterns and elucidating underlying causes.
  
  
  METHODS
  Three medical and life sciences publication databases were searched from inception to October 2014 for longitudinal epidemiological studies written in English, Spanish or Portuguese that reported the incidence of CDI. The monthly frequency of CDI were extracted, standardized and weighted according to the number of follow-up months. Cross correlation coefficients (XCORR) were calculated to examine the correlation and lag between the year-month frequencies of reported CDI across hemispheres and continents.
  
  
  RESULTS
  The search identified 13, 5 and 2 studies from North America, Europe, and Oceania, respectively that met the inclusion criteria. CDI had a similar seasonal pattern in the Northern and Southern Hemisphere characterized by a peak in spring and lower frequencies of CDI in summer/autumn with a lag of 8 months (XCORR = 0.60) between hemispheres. There was no difference between the seasonal patterns across European and North American countries.
  
  
  CONCLUSIONS
  CDI demonstrates a distinct seasonal pattern that is consistent across North America, Europe and Oceania. Further studies are required to identify the driving factors of the observed seasonality.
authors.affiliation: Research School of Population Health, The Australian National University, Canberra, Australian Capital Territory, Australia.
authors.forename: Luis
authors.initials: L
authors.lastname: Furuya-Kanamori
authors.affiliation: School of Population Health, The University of Queensland, Herston, Queensland, Australia.
authors.forename: Samantha J
authors.initials: SJ
authors.lastname: McKenzie
authors.affiliation: London School of Hygiene and Tropical Medicine, Department of Disease Control, London, United Kingdom.
authors.forename: Laith
authors.initials: L
authors.lastname: Yakob
authors.affiliation: Drug ARM Australasia, Annerley, Queensland, Australia.
authors.forename: Justin
authors.initials: J
authors.lastname: Clark
authors.affiliation: The University of Queensland, UQ Centre for Clinical Research, Herston, Queensland, Australia.
authors.forename: David L
authors.initials: DL
authors.lastname: Paterson
authors.affiliation: Microbiology & Immunology, The University of Western Australia and Department of Microbiology PathWest Laboratory Medicine, Queen Elizabeth II Medical Centre, Nedlands, Western Australia, Australia.
authors.forename: Thomas V
authors.initials: TV
authors.lastname: Riley
authors.affiliation: Research School of Population Health, The Australian National University, Canberra, Australian Capital Territory, Australia.
authors.forename: Archie C
authors.initials: AC
authors.lastname: Clements
mesh_terms: D016360:Clostridioides difficile / Q000302:isolation & purification / Q000472:pathogenicity; D004761:Enterocolitis, Pseudomembranous / Q000453:epidemiology* / Q000382:microbiology; D005060:Europe; D006801:Humans; D009656:North America; D044349:Oceania; D012621:Seasons