To assess seasonal variations in Gram-negative and healthcare-associated infections (HCAIs), a literature search was performed with combinations of the keywords ‘seasonality’, ‘seasonal variations’, ‘Gram-negative bacilli’, ‘infections’, ‘nosocomial infections’, and ‘health care associated infections’, to retrieve articles published in English in peer-reviewed journals from 1 January 1970 to 29 February 2012. Seasonality was demonstrated for infections, mostly bloodstream infections (BSIs), caused by Acinetobacter spp., Escherichia coli, Enterobacter cloacae, Klebsiella spp., and Pseudomonas aeruginosa, with higher rates of infection during the summer months in North America, Europe, the Middle East, Australia, and Asia. Correlations were observed between temperature increase and rates of BSI for Acinetobacter spp., P. aeruginosa, E. coli, Klebsiella pneumoniae, and extended-spectrum β-lactamase-producing Enterobacteriaceae. A significant correlation between lower urinary tract infections and higher temperature and decreased relative humidity could explain the seasonality of some BSIs. Regarding HCAI, seasonality is intrinsically present in most viral respiratory and gastrointestinal infections, because viruses are introduced into hospitals during seasonal community outbreaks. Other HCAIs subject to seasonal variations include surgical wound infections, with winter peaks in the USA and summer peaks in Finland, central-line-associated BSIs in haematology/oncology paediatric outpatients, and dialysis-associated peritonitis. In summary, seasonal variations have been shown for infections caused by many Gram-negative bacilli, as well as for a few HCAIs, but many studies remain to be performed in order to better understand the mechanisms of these variations.
