An Epidemiological Study of Aeromonas spp. Infections in Norway

Research Article

Austin J Public Health Epidemiol. 2023; 10(3): 1150.

An Epidemiological Study of Aeromonas spp. Infections in Norway

Rohringer A1,4; Syre H2; Hyllestad S3; Amato E3*

1Department of infection control and vaccine, Norwegian Institute of Public Health, Norway

2Department of Medical Microbiology, Stavanger University Hospital, Norway

3Department of infection control and preparedness, Norwegian Institute of Public Health, Norway

4European Programme for Public Health Microbiology Training (EUPHEM), European Centre for Disease Prevention and Control (ECDC), Stockholm, Sweden

*Corresponding author: Amato E Department of Infection Control and Preparedness, Norwegian Institute of Public Health, 0456, Oslo, Norway. Email: [email protected]

Received: June 12, 2023 Accepted: July 12, 2023 Published: July 19, 2023


Aeromonas spp. is ubiquitous in aquatic habitats causing a wide range of infections in humans after exposure to contaminated water or food. We conducted an epidemiological study of Aeromonas infections detected in Norway, using laboratory-based surveillance data during 2014-2018, in order to identify risk factors associated with developing a severe infection. We identified 503 Aeromonas cases over a 5-year period with an average incidence of 1.9 per 100,000 inhabitants per year. Aeromonas mostly caused gastrointestinal infections (69.8%, n=351), followed by wound (8.6%, n=43) and blood infections (7.4%, n=37). Gastrointestinal and wound infections peaked in the summer months. Major isolated species were A. hydrophila (15.3%), A. veronii (10.7 %), and A. caviae (10.7%). Hospitalisation was reported for 81.1% of blood infections (n=30), 51.2% of wound infections (n=22) and 23.1% of gastrointestinal infections (n=81). Risk factors for gastrointestinal infections associated with hospitalisation were (i) age group 65-79 years old (adjOR=3.10; 95% CI: 1.39-6.93) and >80 years old (adjOR=17.66; 95% CI: 5.05-61.79) and (ii) infections caused by A. caviae (adjOR=3.26; 95% CI: 1.3-8.1). This study showed that Aeromonas infections are common throughout the years suggesting a diverse and continuous source of exposure. Future research on environmental sources and preventive measures particularly for severe Aeromonas infections is recommended.

Keywords: Aeromonas spp.; Epidemiology; Risk factors; Waterborne infections; Norway


The genus Aeromonas belongs to the Aeromonadaceae family and comprises a group of Gram-negative bacteria widely distributed in aquatic environments, with some species able to cause disease in humans, fish, and other aquatic and terrestrial animals [1-3]. Among the 36 species described in the genus Aeromonas, four species (Aeromonas caviae, Aeromonas dhakensis, Aeromonas veronii and Aeromonas hydrophila) have more frequently been reported in clinical cases than other Aeromonas spp. [4].

Aeromonas spp. are considered opportunistic pathogens that can cause a wide range of mild to severe infections, including gastrointestinal (GI), wound and blood infections, in both immunocompromised and immunocompetent hosts [5-7]. The source of infection is usually contaminated food or water [8,9]. Particularly, Aeromonas spp. have been isolated from a variety of food sources including meat [10,11], milk [12,13], vegetables [11], shellfish [11,14] and fish [3,11,13,15]. Moreover, it can be present in drinking water systems, where Aeromonas spp. readily form biofilms with other heterotrophic bacteria tolerating chlorination and even pipe cleaning [16,17].

The epidemiology of Aeromonas infections in not well studied, cases are sporadic rather than associated with large outbreaks and data are therefore derived from a limited number of studies [18]. Only a few studies reporting the incidence on the national level have been published. In those studies, the overall reported or estimated incidence range from 0.2 in France to 9.9 cases per 100,000 inhabitants in Australia [5,19,20].

The aim of this study was to describe the epidemiology of Aeromonas cases to determine the burden of these infections in Norway and identify risk factors associated with developing severe infection for this opportunistic waterborne pathogen.

Materials and Methods

Study Design and Case Definition

We conducted a retrospective study to analyse the epidemiology of Aeromonas spp. infections in Norway during the period 2014-2018. A case of Aeromonas infection was defined as a laboratory confirmed Aeromonas spp. from a clinical sample.

Data Source and Collection

We collected data on Aeromonas infections through a nation-wide survey sent to Norwegian medical microbiology laboratories. Twenty-one laboratories representing the whole country were invited to participate in this study, of which 16 participated by submitting the number of cases and related epidemiological information. The following variables were collected: patients’ sex, age group, year and month of infection, region of residence, identified Aeromonas species, type of sample, infection type, underlying conditions, coinfections, and hospitalisation status. Species identification was performed through Matrix Assisted Laser Desorption Ionization-Time of Flight mass spectrometry (MALDI-TOF) following their routine clinical procedures.

Epidemiological Investigation

We described the epidemiology of Aeromonas infections by sex, age group, geographic distribution by county and region, time of infection, identified species, seasonality, and severity of infection. Severity of infection was inferred from data on hospital admission.

Aeromonas infections were classified into 8 groups based on their site of isolation: (i) blood infections including blood samples; (ii) GI infections including faecal samples; (iii) wound infections including purulent exudate in surgical wound and wound swabs, (iv) ear infections including ear swab samples, (v) skin infections including skin swabs, (vi) urinary tract infections including urine samples and urinary tract/bladder biopsies, (vii) pulmonary infection including sputum, tracheobronchial aspirate, or lung biopsy (pneumonia), (viii) other infections including cholangitis, peritoneal exudate or intraabdominal abscess pus (peritonitis) and unknown infections and sample types.

Seasons were defined according to the northern hemisphere seasons (spring: March – May; summer: June – August; autumn: September – November; winter: December – February). Population data per year were publicly available from national statistics [21].

Data and Statistical Analysis

For each independent variable (sex, age group, season, region, and species), we estimated crude Odds Ratios (OR) and 95% CI of hospitalisation by univariate logistic regression analysis. Adjusted ORs (adjOR) with 95% CI were estimated in a Multivariate Analysis (MVA). Binary outcome was hospitalised/non hospitalised Aeromonas infection. Observations with missing values for variables under comparison were excluded from the respective analysis. We used an alpha level of 0.05 for all statistical tests (Stata version 16.0, 2019. Stata Statistical Software: Release 11. College Station, TX: StataCorp LP. USA). STATA outputs of p-values p<0.000 are reported as p<0.001.

Ethical Considerations

Ethics committee approvals was obtained in line with internal procedures and the General Data Protection Regulation (ethical approval 2019/123/REK sør-øst C on 13.03.19 and its revision on 21.11.2019 and 01.03.2023). Only aggregated data were analysed in this study.


Epidemiology of Aeromonas Infections in Norway

Sixteen out of the 21 invited medical microbiology laboratories replied to the national survey on total number of Aeromonas cases during the 5-years study period, reaching a response rate of 76% and 503 unique Aeromonas cases.

Our data showed that Aeromonas infections were reported with an average of 101 cases per year (range: 88-117) and an overall incidence of 1.9 per 100,000 inhabitants per year. The number of cases followed a seasonal distribution, peaking each year during the summer months (n=178, 35.4%) (Figure 1). Aeromonas infections were more frequently reported from females (n=266, 52.9, %, male to female ratio 0.9), and in the age group 45-64 (n=140, 27.8%). While the highest incidence was reported in the elderly more than 65 years old, and in children below 4 years old (Figure 2). The most common type of infection reported was GI infection (n=351, 69.8%) followed by wound (n=43, 8.6%) and blood (n=37, 7.4%) (Table 1).