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Nuneaton & Bedworth Borough Council,
Town Hall, Coton Road,
Nuneaton, Warwickshire,
CV11 5AA.

Tel: 024 7637 6376

Food Matters

Spud's Down to Earth Advice on...

SHIGELLA

Shigella infections result in bacillary dysentery (shigellosis). Humans are the primary reservoir of the organism and foods may become contaminated directly from infected people or via contaminated water. The genus Shigella comprises four species - Sh. dysenteriae (Serogroup A), Sh. flexneri (Serogroup B), Sh. boydii (Serogroup C) and Sh. sonnei (Serogroup D).

Incubation Period, Symptoms & Duration

Incubation Period
Varies from 7 hours to 7 days, although foodborne outbreaks are commonly characterised by shorter incubation periods of up to 36 hours.The organism is highly infectious, with an infective dose in the region of just 10 - 100 cells. Shigellosis can therefore spread easily in institutional settings such as schools and nurseries.

Symptoms
Symptoms are:

abdominal pain
tenesmus (clenching of gut & anus)
vomiting
fever
diarrhoea (which can range from the classic dysenteric syndrome of bloody stools containing mucus and pus in the cases of Sh. dysenteriae, Sh. flexneri and Sh. boydii, to a watery diarrhoea with Sh. sonnei).

Complications can arise from mucosal ulceration, rectal bleeding and drastic dehydration.
Sequelae (morbid conditions that occur as a result of a previous disease) following shigellosis can include reactive arthritis and haemolytic uraemic syndrome (HUS).
Milder forms of the disease are self-limiting if rehydration occurs, but Sh. dysenteriae infections may require fluid and electrolyte replacement and antibiotic therapy.

Duration
Illness can last from 3 days up to 2 weeks in some cases. Cases may also become convalescent carriers and continue to excrete the organism for 3-5 weeks or sometimes for several months.

Source & Spread

Sources/Associated Foods
As the natural habitat of Shigellae is the gut of humans and some other primates, people are usually the main source of infection. It is thought that foodborne cases of shigellosis are uncommon, although some consider the problem to be underestimated. In foodborne cases that do occur, the source of the organism is normally an asymptomatic carrier or someone recovering from the disease who is involved in food preparation. Although any food may, in principle, be contaminated via this route foods implicated in outbreaks tend to be those receiving extensive handling, such as salads (e.g. potato, tuna, shrimp, chicken), fruit and vegetables. Milk & dairy products, shellfish & seafood, poultry and egg dishes have also been implicated.

Spread
Shigellosis is primarily spread directly via the faecal-oral route, from cases with diarrhoea, in households and institutions (mainly those containing young children).
Occasionally, the infection can be spread by food and water where it arises due to:

infected/carrier food handlers with poor standards of personal hygiene contaminating food or water with their faeces, or
pests such as cockroaches, flies and rats contaminated a result of direct contact with sewage/human faeces.

Shigellosis is associated with insanitary conditions, inadequate personal hygiene facilities or practices, and poor personal hygiene environments (e.g. nurseries with children under 4 years of age).

Control

Control procedures must be directed against contamination from human sources. Measures at the food preparation and service level are the same as those necessary for the control of other more widely recognised foodborne pathogens and in particular:

preventing a carrier or someone recovering from the disease from being involved in food preparation
ensuring that all food handlers practice good personal hygiene, particularly with regard to proper hand washing after defaecation, reinforced by effective training and supervision
proper standards of sanitary hygiene and facilities
thorough cooking
adequate refrigerated storage temperatures
supervision of hand washing by children in nurseries and infant schools

Exclusion/Return to Work Parameters

Cases in Risk group 1 for should not return until 48 hours after the first normal stool.
Cases in risk group 2, 3 & 4 should be excluded until they are free from diarrhoea and are passing formed stools.

Microbiological Clearance
Should only be considered for infections with Sh. dysenteriae (2 negative faecal specimens at intervals of at least 24 hours).
Cases and contacts in Groups 1-3 should be provided with hygiene advice before return to work/school. For Group 4 each case should be considered individually.

[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

Shigella bacteria are Gram -ve, rod-shaped, non-sporeforming, non-motile, aerobic (facultatively anaerobic) organisms
In England and Wales between 1986 and 2003, there was an average of 4159 cases of Shigella dysentery, with a maximum of 17453 in 1992 (HPA website figures, January 2005).
Infections with Sh. dysenteriae almost always develop full and severe symptoms of dysentery (diarrhoea with blood, mucus and inflammatory cells), with a reported mortality (death) rate among hospitalised cases as high as 20%. Fortunately the most common species in the UK is Sh. sonnei, which causes the mildest illness (relatively mild, non-bloody diarrhoea).
Invasive strains of Sh. dysenteriae produce high levels of a toxin commonly designated Shiga toxin which is responsible for the dramatic inflammation. The toxin is entero-, neuro- and cyto- toxic.
The primary site of invasion is the colon.
The number of cases in the England and Wales has ranged from a peak of 17453 in 1992 to less than 1000 in 2003. The most commonly isolated species is Sh. sonnei.
Shigellae can ferment many carbohydrates and can therefore grow on a wide variety of food substrates.
Shigellae are poor microbial competitors, particularly in comparison with E. coli to which it is genetically similar. However, survival for more than 50 days at 25^oC in flour, milk, eggs has been recorded.

Growth Factors

Minimum A_w: >0.97
Minimum pH 4.8
Temperature range: 7^oC - 46^oC
Optimum temperature: 37^oC

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Friday 10 September 2010