Salmonella gastroenteritis

Salmonella

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.¹

Salmonella typhi and Salmonella paratyphi can also cause systemic infection as described in the separate Typhoid and Paratyphoid Fever article.

What causes salmonella gastroenteritis?

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.²

See also separate Traveller's Diarrhoea, Gastroenteritis in Adults and Older Children and Gastroenteritis in Children articles.

How common is salmonella gastroenteritis?¹

The UK Health Security Agency (formerly Public Health England) reported 8,398 cases of all salmonella in the year 2019 (the last year's data available), which had dropped by 440 cases from 2018. The number of cases has been around 8,000 for the last decade. The majority of cases are S. enteritidis. The highest rate of infection is in children under 10 years and October sees the peak number of cases.

Where cases have been notified in the UK, the food-borne sources reported included rice, eggs, chicken, pork and lamb. Infection with Salmonella spp. is also a possible cause of traveller's diarrhoea.

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.³

Risk factors⁴

• 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.

Symptoms of salmonella gastroenteritis³

Symptoms

• 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.

Signs

• 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.

Assessment for dehydration is covered in the separate Gastroenteritis in Adults and Older Children and Gastroenteritis in Children articles.

Differential diagnosis⁴

• Other infections - for example:
  

  • Escherichia coli O157 (can also cause bloody diarrhoea).

  • Campylobacter spp.

  • Shigella spp.

  • Amoebic dysentery.

  • Listeria monocytogenes.

• Appendicitis.

• Inflammatory bowel disease.

• Pseudomembranous enterocolitis secondary to Clostridioides difficile (Clostridium difficile).

• Intussusception in infants.

Investigations

Diagnosis is by polymerase chain reaction (PCR) testing and confirmed by culturing the organism from the stool.⁵ Some laboratories only offer microscopy and culture and this may be less sensitive. Multiple samples may be necessary.

A stool culture is not necessary in every case of diarrhoea. It is advisable to send a stool culture for a person with diarrhoea if: ⁶

• 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.⁷

Salmonella gastroenteritis treatment⁸

The basis of management of salmonella enteritis is rehydration. Assess for features of rehydration or shock and, where present, consider hospital admission. This is not usually required.

Rehydration

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.

Antimotility medication

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:

• Dysentery.

• E. coli 0157.

• Shigella.

• Inflammatory bowel disease.

• Pseudomembranous colitis.

Antibiotics

Antibiotics are not recommended for healthy individuals with salmonella infection. A Cochrane review showed no benefit for otherwise healthy individuals.⁹ 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.¹⁰

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.¹¹

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.

Complications⁸

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.

• 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.¹² These cases have been called postinfectious IBS. A 2019 systematic review found the risk was similar for all the common pathogens.¹³

• 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.

• Bones.

• Joints.

• Meninges.

• Gallbladder.

Prognosis

Most people recover uneventfully in the UK. 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 however, it has been estimated that in 2017 non-typhoidal salmonella caused 535,000 cases and 77,500 deaths.¹⁴ The average all-age case fatality was 14.5%.

Salmonella gastroenteritis prevention^{1 3}

Prevention of salmonella gastroenteritis 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.