Season, weather and predictors of healthcare-associated Gram-negative bloodstream infections: a case-only study
Introduction
Seasonality of healthcare-associated infections (HCAI) caused by Gram-negative bacilli (GNB) has been reported recently [1]. It is usually characterized by ‘summer peaks’ in incidence [2], [3], [4], [5] or association with environmental temperature [6], [7]. Coincidentally, a multi-centre study reported increased incidence of GNB bloodstream infections (GNB-BSI) in hospitals located closer to the Equator [8].
Factors underlying this phenomenon have not been elucidated. Increased environmental reservoirs (inside or outside healthcare settings), understaffing associated with summer holidays, and expression of virulence factors in GNB have been suggested as contributing causes [1], [9]. Ecological studies [3], [6], [7], although valuable for identifying seasonality, have not contributed to reinforcing either of these hypotheses.
Seasonality in the incidence of healthcare-associated pathogens could reflect seasonal variations in their epidemiological determinants. For some years, traditional risk factors for healthcare-associated GNB (especially those that are multi-drug-resistant) have been described repeatedly [10]. They include severity of illness, comorbidities, time of exposure to health care (‘time at risk’), invasive procedures and use of antimicrobials [11], [12]. It is therefore worth investigating if those factors vary between seasons, thus contributing to seasonality.
This was the rationale of this study. Briefly, an observational, individual-based study was conducted to identify the interplay of season, weather and usual predictors of healthcare-associated GNB-BSI.
Section snippets
Setting
This study was conducted in a 450-bed teaching hospital in Botucatu, São Paulo State, Brazil. The hospital is a centre for tertiary care for several cities in an area comprising one million inhabitants [7].
Study design
A case-only study was conducted, enrolling subjects with healthcare-associated GNB-BSI diagnosed from July 2012 to June 2016.
Inclusion criteria
All adult patients (aged ≥18 years) admitted to the hospital who acquired healthcare-associated GNB-BSI caused by Escherichia coli, Enterobacter spp., Klebsiella spp.,
Ethical issues
This study was approved by the local committee for ethics in research.
Results
This study enrolled 446 subjects with GNB-BSI caused by the following micro-organisms: E. coli, 75; Enterobacter spp., 65; Klebsiella spp., 126; P. aeruginosa, 64; and A. baumannii, 116. Central-line-associated BSI (CLABSI) accounted for 68.8% of cases. Overall, cases were more frequent in summer (27.4%) and spring (28.9%) than in winter (19.5%) and autumn (24.2%) (P = 0.01). Table I presents a description of meteorological data from the days of collection of positive blood cultures, with
Discussion
Seasonality of infectious diseases is a complex phenomenon, the determinants of which are not fully understood [18]. The impact of season and climate on parasite–host systems is relatively easy to understand for vector-borne diseases, but not as clear for human-to-human transmission, or for acquisition of pathogens from endogenous or environmental sources (such as occurs in HCAI) [19]. Even so, understanding the mechanisms and pattern of seasonality may provide clues regarding how and when
Conflict of interest statement
None declared.
Funding source
FACL was a medical student for part of this study, and received a student grant from the National Council of Scientific and Technological Development of Brazil (CNPq).
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