oPatientPlus articles are written by UK doctors and are based on research evidence, UK and European Guidelines. They are designed for health professionals to use, so you may find the language more technical than the condition leaflets.
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 they all 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 article Typhoid and Paratyphoid Fever.
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 and there has been a downward trend in the subsequent years (figures for 2010 show a total of 9,133 cases). The majority of cases are S. enteritidis. The highest rate of infection is in those aged over 70 and under 20, especially infants. Infection with Salmonella spp. is a possible cause of traveller's diarrhoea.
The number of S. enteritidis infections reported to the HPA for England and Wales for 2000 (all phage types) was 8,618 and the corresponding figure for 2010 was 2,444. The reduction in infections was mainly due to phage type PT4 and this may relate to poultry vaccination and less importing of eggs from abroad.
In the UK in 2012 there was an unexplained increase of S. typhimurium phage type U323 with resistance to several antibiotics, mainly in women in the southern part of England.
- 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.
- Many cases arise from outbreaks - eg, at weddings.
- The source is usually of animal origin, such as beef, poultry, unpasteurised milk or eggs; however, all food, including vegetables, may be contaminated.
- Eggs continue to be a source of infection from time to time. In 2006, the Foods Standards Agency examined eggs for Salmonella spp. and estimated that contamination was present in 1 box out of 30. Of 1,744 boxes sampled, there were positive results from the shells in 157 cases and from within the egg in 10 instances. Spain and France were the most common source of contaminated eggs but only 10% of eggs used in the UK came from abroad and most of these were used in the catering industry. A further outbreak in 2011 was traced to a specific shed on one farm in Spain.
- 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.
- 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.
Presentation of salmonella enteritis
- Incubation period is 12-72 hours.
- Diarrhoea starts with fever and abdominal cramps. The diarrhoea can be bloody. (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 recovery.
- 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.
- Campylobacter spp.
- Cryptosporidium spp.
- Escherichia coli O157 (can cause bloody diarrhoea)
- Listeria monocytogenes
- Cyclospora spp.
- Shigella spp.
- Yersinia spp.
- Diagnosis is by polymerase chain reaction (PCR) testing and confirmed by culturing the organism from the stool.
- FBC will probably show an elevated white cell count; however, in most cases in primary care, stool culture is the only necessary investigation.
- Agglutination tests such as the Widal test are not recommended, as there are often false positives.
- Attention to dehydration, usually just oral rehydration fluids.
- Attention to hand washing to prevent spread to others.
- Admission to hospital may be required in infants younger than 3 months or younger than 12 months with a temperature in excess of 39°C.
- Other indications for admission may include immunosuppression, chronic gastrointestinal illness and haemoglobinopathies.
- Antibiotics do not shorten the illness but may prolong the carrier stage. There is also a problem of multiple antibiotic resistance.
- They may be used in those with or at high risk of bacteraemia. This includes infants under the age of 3 months (the British National Formulary recommends 6 months), the immunocompromised, those with prosthetic valves or vascular grafts and those with prosthetic joints. Antibiotics licensed for UK use are ciprofloxacin and cefotaxime.
- Sometimes antidiarrhoeal or antispasmodic drugs may be required. Their use is controversial, as prolongation of the transit time may prolong the disease.
Return to work
When diarrhoea has settled, the vast majority are not a risk to others and may return to work with no further testing. Children and adults should stay away from nursery, school or work for 48 hours after their symptoms have settled. Food handlers and those who work with vulnerable groups such as children, the elderly and those in poor health, should advise their employers.
Food poisoning is a notifiable disease and doctors must notify the proper officer of their Local Authority using the appropriate form.
- Infants, the elderly and those with immunological compromise are more likely to have more severe disease and to require admission to hospital for rehydration.
- Seeding of bacteria outside the gut is rare but raises mortality rates. Sites include endocarditis and arterial infections, cholecystitis, hepatic and splenic abscesses, urinary tract infections (if stones are present), pneumonia or empyema, meningitis, septic arthritis and osteomyelitis.
Infection with Salmonella spp. is responsible for about 80 deaths occurring in the UK each year. Those most at risk are the elderly and infants. Most people recover uneventfully.
- Poultry and meat, including burgers, should be well cooked, not pink in the middle. Vegetables and salads should be thoroughly washed before eating. The Chief Medical Officer advises against recipes with uncooked or lightly cooked eggs. Adequate cooking of eggs, until the yolk is set, kills Salmonella spp. Take care that food does not become contaminated after cooking.
- To avoid cross-contamination, uncooked meats should be kept separate from cooked and ready-to-eat food.
- Hands, chopping boards, knives and other utensils should be washed thoroughly in hot soapy water immediately after raw meat and poultry have been handled. Strict separation of surfaces, tools, clothing and staff for cooked food preparation areas and uncooked food preparation areas must be enforced (contaminated aprons/knives can easily transmit infection between areas).
- Hands should be washed before handling different food items, before eating or drinking, after going to the toilet and also after contact with animals - particularly pets and their bedding.
- Those who are ill should not prepare or handle food.
- Carrier states - some people may be asymptomatic carriers of Salmonella spp. This is important especially in those who handle food or work in nurseries and the health profession. The use of quinolone antibiotics in these cases - eg, norfloxacin and ciprofloxacin - has been successful in eradicating carriage of Salmonella spp.
Further reading & references
- Lee MB, Greig JD; A review of nosocomial Salmonella outbreaks: infection control interventions found effective. Public Health. 2013 Mar;127(3):199-206. doi: 10.1016/j.puhe.2012.12.013. Epub 2013 Feb 22.
- Daniel Elmer Salmon; Brief biography, whonamedit.com
- Baker S, Dougan G; The genome of Salmonella enterica serovar Typhi. Clin Infect Dis. 2007 Jul 15;45 Suppl 1:S29-33.
- Hallstrom K, McCormick BA; Salmonella Interaction with and Passage through the Intestinal Mucosa: Through the Lens of the Organism. Front Microbiol. 2011;2:88. doi: 10.3389/fmicb.2011.00088. Epub 2011 Apr 29.
- Salmonella by Serotype, England and Wales 2000-2010; Public Health England
- Salmonella Enteritidis Phage types (PT) in humans; Health Protection Agency
- National increase in Salmonella Typhimurium PT U323; Health Protection Report, Autumn 2012
- FSA surveys non-UK eggs for salmonella; Food Standards Agency, 2006
- Outbreak of salmonella in eggs now over; Food Standards Agency, 2011
- 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 Mar;12(2):220-5. doi: 10.2353/jmoldx.2010.090036. Epub 2010 Jan 28.
- Crum-Cianflone NF; Salmonellosis and the gastrointestinal tract: more than just peanut butter. Curr Gastroenterol Rep. 2008 Aug;10(4):424-31.
- Casadesus J; Salmonella: from basic science to clinical issues. Future Microbiol. 2011 Feb;6(2):133-5. doi: 10.2217/fmb.10.160.
- British National Formulary; 65th Edition (Mar 2013) British Medical Association and Royal Pharmaceutical Society of Great Britain, London
- Salmonella; Public Health England
- Wrigley P; The infection biology of Salmonella, University of Liverpool.
- Preventing Foodborne Disease; National Disease Surveillance Centre, 2004
Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.
Dr Gurvinder Rull
Dr Laurence Knott
Dr Adrian Bonsall