Salmonella spp. are bacteria which cause one of the most common forms of food poisoning worldwide. There are over 2,500 different types of Salmonella spp. but most produce a similar clinical picture to other forms of infective gastroenteritis.
Salmonella typhi and Salmonella paratyphi can also cause systemic infection as described in the separate Typhoid and Paratyphoid Fever article.
Numerous serotypes of Salmonella spp. exist. Serogroups A to E are the ones that usually cause disease in humans. Serogroups B, C and D are responsible for most infections. Salmonella enteritidis is serogroup D and is the most common cause of salmonella gastroenteritis. The other epidemiologically important species is Salmonella typhimurium.
Their pathogenicity is conferred due to the ability to invade intestinal mucosa and produce toxins.
The Health Protection Agency (HPA) - now part of Public Health England - reported 14,465 cases of all salmonella in the year 2000, which had dropped by 2015 to 8451. The majority of cases are S. enteritidis. The highest rate of infection is in children, especially infants. Infection with Salmonella spp. is a possible cause of traveller's diarrhoea.
The reduction over recent years of S. enteritidis infections was mainly due to a reduction in infection with phage type PT4 and this may relate to poultry vaccination and less importing of eggs from abroad. The Lion Scheme introduced in 1998 involved poultry vaccination and marking of eggs from vaccinated hens.In 2016, an expert group, set up by the Advisory Committee on the Microbiological Safety of Food (ACMSF), to look at egg safety, found there has been a reduction in the risk from salmonella in UK shell eggs, suggesting that raw and runny eggs are now safe for even vulnerable people to eat. This report is currently in draft stage whilst undergoing consultation.
Worldwide, the World Health Organization (WHO) estimate that tens of millions of human cases occur every year and the disease results in more than hundred thousand deaths.
- Salmonella spp. are found in a great many animals - domestic, agricultural and wild. Intensive farming methods are thought to be behind its initial rise to importance.
- Contamination occurs from animal faeces, and infected foods usually look and smell normal.
- The source is usually of animal origin, such as beef, poultry, unpasteurised milk or eggs; however, all food, including vegetables, may be contaminated if it has been washed in contaminated water or been in contact with faeces from infected animals.
- Although eggs produced under the Lion Scheme in the UK are likely to be safe, eggs from abroad or eaten abroad where similar schemes do not exist, may be a risk.
- Organisms multiply rapidly in warm humid conditions and cross-contamination between surfaces and tools used in cooked and infected uncooked food areas is a potential source.
- Inadequate thawing from freezing is a common source. Heat readily kills Salmonella spp. but it can survive spit and oven roasting if not properly defrosted.
- Infection with Salmonella spp. can also be spread by the faeco-oral route if a carrier does not wash hands after using the toilet.
- Some animals can also pass the bacteria directly to people. Contact between infants/young children and pet animals (cats, dogs, but also tortoises, terrapins, turtles, snakes, lizards, etc) should be supervised.
- Gastric acidity gives some protection and thus large inoculums are required. Conversely, those with loss of acidity, including those on acid-suppressing drugs, are more at risk. Also liquids which pass through the stomach quickly, or milk and cheese that raise the pH, enable smaller inoculums to be infective.
- Those with inflammatory bowel conditions and immunocompromised states may be at increased risk.
- International travel to regions of poor sanitation increases the risk of infection.
Presentation of salmonella enteritis
- Incubation period is 6-72 hours (most commonly 12-36 hours).
- In most, symptoms are relatively mild and self-limiting.
- Diarrhoea starts with fever and abdominal cramps. The diarrhoea can be bloody. There may be nausea and/or vomiting. (Note that diarrhoea is not a feature of typhoid fever and constipation is common.)
- The illness tends to last 4-7 days and there is usually full recovery. Dehydration may occur and may lead to complications in more vulnerable individuals.
- Always enquire about recent attendance at social gatherings, anybody else with a similar illness and any recent travel.
- There is a temperature of 38-39°C for about 48 hours.
- There may be signs of dehydration.
- There is not the typical rash of typhoid.
- Other infections - for example:
- Inflammatory bowel disease.
- Pseudomembranous enterocolitis secondary to Clostridium difficile.
- Intussusception in infants.
Diagnosis is by polymerase chain reaction (PCR) testing and confirmed by culturing the organism from the stool.A stool culture is not always necessary. It is advisable to send a stool culture for a person with diarrhoea if:
- The person is systemically unwell.
- There is blood or pus in the stool.
- The person is immunocompromised.
- There is a history of recent hospital admission and/or antibiotic treatment.
- There is a recent history of foreign travel to anywhere other than Western Europe, North America, Australia, or New Zealand.
- Diarrhoea is persistent.
- There is uncertainty about the diagnosis.
- There is a potential public health risk (eg, food handlers, health workers, or in pinpointing the source of a local outbreak).
When sending a stool sample include information about recent travel, infectious contacts, possible food sources and clinical features. Where food poisoning with salmonella is confirmed, the local health protection team should be notified.
The basis of management is rehydration. Assess for features of rehydration or shock and, where present, consider hospital admission. This is not usually required.
This can usually be achieved by the oral route but, in more severe cases, intravenous fluids may be needed. Age-specific information on advice regarding rehydration is covered in the separate Gastroenteritis in Adults and Older Children and Gastroenteritis in Children articles.
Racecadotril is an intestinal antisecretory enkephalinase inhibitor that inhibits the breakdown of endogenous enkephalins.It reduces the hypersecretion of water and electrolytes into the intestine. It is licensed for the complementary symptomatic treatment of acute diarrhoea in children (aged over 3 months) together with oral rehydration and the usual support measures (dietary advice and increased daily fluid intake), when these measures alone are insufficient to control the clinical condition. It is not recommended for use within NHS Scotland for the treatment of acute diarrhoea in children because there is insufficient evidence that it improves the recovery rate.
These should not be used routinely but may be occasionally considered for adults:
- Who need to return to work or attend a special event.
- Who have difficulty reaching the toilet quickly.
- Who need to travel.
When used, loperamide is the antimotility agent of choice. It should not be used if features suggest a possible differential diagnosis of:
- E. coli 0157
- Inflammatory bowel disease
- Pseudomembranous colitis
Antibiotics are not recommended for healthy individuals with salmonella infection. A Cochrane review showed no benefit for otherwise healthy individuals.Consider the use of antibiotics if the person:
- Is older than 50.
- Is immunocompromised.
- Has cardiac valve disease or endovascular abnormalities, including prosthetic vascular grafts.
- Is under 6 months of age.
Where an antibiotic is indicated, use ciprofloxacin 500 mg bd for one day only (assuming the stool result confirms sensitivity). Cefotaxime is an alternative.
Preventing spread of infection
For work or school the exclusion period should be 48 hours from the last episode of vomiting or diarrhoea.
Advise about other hygiene methods to help prevent spread, such as:
- Meticulous attention to hand-washing (after going to the toilet, before preparing meals or eating, after assisting a child or elderly person clean themself following diarrhoea, etc).
- Not sharing towels and flannels.
- Washing soiled bed linen and clothes at 60°C or higher.
- Cleaning and disinfecting toilet seats, flush handles, taps and bathroom door handles regularly.
In common with other causes of gastroenteritis, complications include:
- Dehydration and electrolyte disturbance may occur. Occasionally where not rectified, this can have fatal consequences. Infants, the elderly and those with immunological compromise are more likely to have more severe disease and to require admission to hospital for rehydration. Pregnant women are also more at risk of dehydration.
- Unusual complications include haemolytic uraemic syndrome and thrombotic thrombocytopenic purpura.
- Other rare complications include Guillain Barré syndrome and reactive arthritis in the form of Reiter's syndrome.
- Toxic megacolon is a rare but serious complication.
- Acute bacterial gastroenteritis has been linked with the onset of irritable bowel syndrome (IBS) symptoms in approximately 15% of patients. These cases have been called postinfectious IBS. Salmonella spp. is commonly associated with postinfectious IBS, as are E. coli O157, Campylobacter spp. and Shigella spp.
- Severe diarrhoea may interfere with absorption of regular medication required for control of chronic disease.
Also rarely with salmonella, systemic invasion occurs with bacterial seeding elsewhere causing infection of:
- Endovascular lining
- Cardiac valves
Most people recover uneventfully. Death following salmonella infection is uncommon and results from complications such as dehydration or systemic invasion. Those most at risk are the elderly and infants. Globally, it has been estimated that non-typhoidal salmonella causes 150,000 deaths each year, of which 1600 occur in Western Europe.
Prevention requires measures at all stages of the food chain, from agricultural production to domestic preparation of food, as well as advice to travellers. A national surveillance scheme oversees salmonella infection rates and patterns.
For the general public at home and when travelling abroad, advise them to:
- Ensure food is properly cooked and still hot when served.
- Drink only pasteurised or boiled milk.
- Avoid uncooked or lightly cooked eggs, unless certified to come from hens vaccinated against salmonella (as per the UK Lion Code Scheme). Adequate cooking of eggs, until the yolk is set, kills Salmonella spp. The Food Standards Agency is currently consulting in the UK about whether advice for vulnerable individuals to continue to avoid undercooked eggs can be withdrawn in the light of evidence about low contamination rates.
- Wash hands thoroughly and frequently using soap, in particular after contact with pets or farm animals, or after having been to the toilet.
- Keep uncooked meats separate from cooked and ready-to-eat food to avoid cross-contamination.
- Hands, chopping boards, knives and other utensils should be washed thoroughly in hot soapy water immediately after raw meat and poultry have been handled.
- Not prepare or handle food if ill with salmonella.
- Wash fruits and vegetables carefully, particularly if they are eaten raw. If possible when travelling to areas of high risk, vegetables and fruits should be peeled.
- Avoid ice unless it is made from safe water.
- When the safety of drinking water is questionable, boil and/or disinfect it.
Further reading and references
Lee MB, Greig JD; A review of nosocomial Salmonella outbreaks: infection control interventions found effective. Public Health. 2013 Mar127(3):199-206. doi: 10.1016/j.puhe.2012.12.013. Epub 2013 Feb 22.
Daniel Elmer Salmon; whonamedit.com
Foodborne Disease Strategy; Food Standards Agency (FSA) May 2011
Salmonella: guidance, data and analysis; Public Health England
Hallstrom K, McCormick BA; Salmonella Interaction with and Passage through the Intestinal Mucosa: Through the Lens of the Organism. Front Microbiol. 20112:88. doi: 10.3389/fmicb.2011.00088. Epub 2011 Apr 29.
Eggs and salmonella; Egg Info
Food Standards Agency launches consultation on eggs; Advisory Committee on the Microbiological Safety of Food (ACMSF). February 2016
Salmonella Fact Sheet; World Health Organization (WHO) August 2013
Munoz N, Diaz-Osorio M, Moreno J, et al; Development and evaluation of a multiplex real-time polymerase chain reaction procedure to clinically type prevalent Salmonella enterica serovars. J Mol Diagn. 2010 Mar12(2):220-5. doi: 10.2353/jmoldx.2010.090036. Epub 2010 Jan 28.
UK Standards for Microbiology Investigations: Gastroenteritis and Diarrhoea; Public Health England, 2013
List of notifiable diseases (England); Public Health England
Gastroenteritis; NICE CKS, July 2015 (UK access only)
Acute diarrhoea in children: racecadotril as an adjunct to oral rehydration; NICE Evidence Summary, March 2013
British National Formulary (BNF); NICE Evidence Services (UK access only)
Onwuezobe IA, Oshun PO, Odigwe CC; Antimicrobials for treating symptomatic non-typhoidal Salmonella infection. Cochrane Database Syst Rev. 2012 Nov 1411:CD001167. doi: 10.1002/14651858.CD001167.pub2.
Guidance on infection control in schools and other childcare settings; Public Health England (September 2017)
Smith JL, Bayles D; Postinfectious irritable bowel syndrome: a long-term consequence of bacterial gastroenteritis. J Food Prot. 2007 Jul70(7):1762-9.
Majowicz SE, Musto J, Scallan E, et al; The global burden of nontyphoidal Salmonella gastroenteritis. Clin Infect Dis. 2010 Mar 1550(6):882-9. doi: 10.1086/650733.
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