Food Matters

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SALMONELLA (excluding S. typhi & S. paratyphi)

Incubation Period, Symptoms & Duration

• the size of the infecting dose
• the virulence (the degree that the specific serotype (strain) can cause harm)
• the susceptibility of the host
• the nature/composition of the transmitting food

• fever, headache & general malaise
• abdominal cramps
• diarrhoea and vomiting
• dehydration

Source & Spread

• Animal and human intestines, and excreted stools. Access into the food chain is therefore via raw foods of animal origin (e.g. meat, poultry, milk & dairy products, eggs & egg products, seafood) or fruit and vegetables contaminated on the farm (e.g. by manure, litter) or later via cross-contamination with contaminated products.
• Pests (i.e. rodents, insects, birds)
• Domestic pets
• Infected food handlers

• Spread is predominantly through food, arising from the following:

• contamination of raw food materials (from direct or indirect animal faecal contamination, or cross-contamination from a contaminated source)
• inadequate processing (e.g. time/temperature combination used during cooking is insufficient to kill contaminating bacteria)
• food product is exposed to post-processing contamination through poor personal or equipment hygiene practices
• inadequate temperature control permitting bacteria to multiply

• Person-to-person spread can also occur, particularly during the acute diarrhoeal phase of the illness.
• Contact with infected animals, including domestic pets.

Control

• raw materials
• hygiene of personnel, equipment and the environment
• food manufacturing process conditions
• post-process contamination
• retail and catering practices
• consumer handling

• thorough cooking (Salmonella are readily destroyed by heat e.g. 70^oC for 2 minutes or an equivalent time/temperature combination)
• adequate temperature control/refrigeration of high risk food (most Salmonella will not grow below 7^oC)
• separation of raw and ready-to-eat foods
• prevention of cross-contamination by proper cleansing of food contact surfaces and utensils
• high standards of personal hygiene
• effective cleaning and disinfection
• effective pest control

Exclusion/Return to Work Parameters

Additional Information

• Salmonella are Gram -ve, rod-shaped, motile (except S. Gallinarum and S. Pullorum), non-sporeforming aerobic (facultatively anaerobic) organisms.
• The number of confirmed cases of Salmonellosis per year in England and Wales between 1981 - 2003 averaged 20, 907 per year. The highest annual total was 31, 480 in 1997, although the incidence has been falling more recently (HPA website figures, January 2005). It is thought that the number of confirmed cases is a gross underestimate of the actual level because the infection is generally a self-limiting gastroenteritis that may not be reported or might be misdiagnosed.
• The majority of cases are sporadic or family-linked but outbreaks, sometimes large ones, do occur in the general population and in institutions.
• The contaminating bacteria need to multiply to relatively large numbers in food in order to cause illness in healthy adults (10⁶ - 10⁹ cells). However, the infective dose can be much lower (perhaps between 10 - 100 cells), particularly for vulnerable groups such as babies & the immunocompromised, or where fatty foods (e.g. chocolate, cheese, salami) offer protection against stomach acidity.
• The organism continues to provide challenges to food safety in view of the evolution of new strains (e.g. multiple antibiotic resistant strains).
• Although there are more than 2460 serotypes (of which in the UK only about 200 may be reported in any one year), it has recently been agreed they are all designated to one species, S. enterica. This species has seven subspecies, although one (S. enterica subsp. bongori (S. bongori) is believed to represent a distinct species. Modern naming of serotypes follows this pattern: What was Salmonella enterica subsp. enterica serotype Enteritidis (previously S. enteritidis) is now shortened to Salmonella Enteritidis (S. Enteritidis).
• Salmonella serotypes can be divided into three groups:

• those that are 'host restricted' i.e. those that are almost exclusively associated with one particular host species e.g. S. Typhi - humans.
• those that are 'host adapted' i.e. those that are prevalent in one host species but that can cause disease in other hosts e.g. S. Dublin in cattle can be transmitted by food and affect humans.
• those that are 'unrestricted' i.e. can cause infection in a broad range of unrelated hosts. This group contains most of the strains responsible for foodborne illness.

• Salmonella Enteritidis accounts for the majority (about 65%) of all cases in England and Wales, with the next highest being Salmonella Typhimurium (10 - 15% of all isolations).
• The mechanism of infection is as follows:

• 1. multiplication in the small intestine
• 2. colonisation and then invasion of the intestinal tissues
• 3. production of an enterotoxin, which causes an inflammatory reaction and diarrhoea.

• In common with some other microorganisms, although Salmonella is readily destroyed by heat in foods with a high water activity (e.g. a_w > 0.98), in foods with a low water activity (e.g. high fat content) higher temperatures are needed to kill the organism.
• In frozen foods or those with a low water activity, Salmonella can survive for months, even years.
• As regards diagnosis of the illness, Salmonellosis may be confused with staphylococcal intoxication, but there are 3 important distinctions:

• Salmonella has a longer incubation period (usually 12 - 48 hours vs. 2 - 4 hours)
• Salmonella is usually accompanied by fever, which is absent in staphylococcal intoxication
• unlike Salmonella food poisoning, the acute symptoms of staphylococcal food poisoning normally disappear within 24 hours

• Minimum A_w: 0.93
• pH range: 3.8 - 9.5 (varies with the acidulant but most serotypes will not grow below pH 4.

Useful External Links

• The Bad Bug Book - US site.
• The Health Protection Agency: Infections A-Z