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AEROMONAS SPECIES (AEROMONADS)

Aeromonads are ubiquitous in water and in many foods. Although their role in foodborne illness has not yet been firmly established they have attracted particular attention because they can grow at chill temperatures, prompting concern that any threat that might be posed will increase with the rising consumption of chilled food.
The two species most commonly associated with diarrhoea are Aeromonas hydrophila and Aeromonas caviae.

Incubation Period, Symptoms & Duration

Incubation Period
Unknown at present.

Symptoms
As its role as an agent of foodborne illness has not been definitely established, the following descriptions should be treated with some care.Two distinct types of gastroenteritis have been associated with A. hydrophila:

a cholera-like illness with a watery diarrhoea with mild or absent fever, possibly accompanied by abdominal pain and nausea (perhaps accounting for more than 75% of cases)
a dysentery-like illness characterized by loose stools containing blood and mucus.

Gastroenteritis associated with Aeromonads occurs most commonly in children aged 6 months to 2 years and the immunocompromised, including the elderly. Vomiting is not normally reported.

There have also been instances when individuals with underlying illness (i.e. the immunocompromised) have developed septicaemia (a general infection in which the organism spreads throughout the body) and meningitis.

Duration
Mild infections are self-limiting, recovery taking 1 - 7 days. In more severe cases the dysentery-like symptoms are prolonged and may last for several weeks.

Source & Spread

Sources/Associated Foods
Aeromonads are bacteria commonly found in all freshwater environments and in estuarine/brackish (slightly salty) water.
A. hydrophila has frequently been found in fish and shellfish (e.g. oysters, prawns, shrimps). It has also been found in red meats (beef, pork, lamb), poultry, raw milk and fresh vegetables (e.g. watercress, lettuce, chicory).

Spread
The organism is thought to be spread primarily via contaminated water, and foods. Person-to-person spread is thought to be rare.

Control

Preventing contamination of foods or water supplies/pipework
Thermal or other processing for safety (the organism is not heat resistant)
Prevention of recontamination
Use of a salad wash should be considered for salads and vegetables eaten raw
Growth cannot be totally prevented by refrigeration, so storage periods and temperatures are important control factors
Avoidance of relevant high risk foods (outlined above) by the immunocompromised.

Exclusion/Return to Work Parameters

Cases in risk groups 1- 4 not to return until 48 hours after the first normal stool.

[Group 1 - food handlers; Group 2 - health care workers; Group 3 - children under 5 in nurseries etc.; Group 4 - older children and adults who may find it difficult to implement good standards of personal hygiene]

Additional Information

Aeromonads are Gram -ve, rod-shaped, motile (via single polar flagellum), and facultatively anaerobic bacteria. There are mesophilic and psychrotrophic strains.
Aeromonads can cause illness in fish and amphibians as well as in humans who may acquire infections through open wounds or by ingestion of a sufficient (probably large) number of the organisms in food or water.
Figures for England and Wales for 1981 - 2003 show an average of 352 confirmed Aeromonas infections per year, with a maximum of 659 in 1994 (HPA website figures, January 2005).
The method by which Aeromonads cause illness is probably linked to a number of 'virulence factors' they possess which have been associated with human gastroenteritis (e.g. haemolysins, endotoxin (lipopolysaccharide - LPS) and S-layers). The latter are particular arrangements of protein or glycoprotein subunits on the bacterial cell wall that increase the ability of the cell to adhere to and colonise the intestinal epithelium.
Given our current knowledge about how A. hydrophila may cause illness, it is presumed that not all strains are harmful, given the wide distribution of the organism.
The infective dose may be as high as 5 x 10¹⁰ or above.

Growth Factors

Minimum pH: 5.0
Temperature range (varies between strains): 0^oC - 45^oC.
Optimum temperature: 15^oC - 20^oC for psychrotrophic (cold-loving) strains.

View/Download Additional Information

Foodborne Illness Guidance Notes pdf (170 KB)

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Thursday 9 September 2010