Salmonella Gastroenteritis

Salmonella is a bacterium which causes one of the commonest forms of food poisoning worldwide. There are over 2,000 different types of salmonella,¹ but they produce a similar clinical picture to other forms of infective gastroenteritis. Salmonella typhi and S. paratyphi also can cause systemic infection as described in typhoid fever. It affects only humans and is spread through consumption of contaminated food and drink handled by people who shed the organism from stool or, less commonly urine, or from water from sewage contaminated with S. typhi. Shellfish from water polluted by raw sewage and canned meat production with poor technique have caused outbreaks.

Numerous serotypes of salmonella exist. Serogroups A to E are the ones that usually cause disease in humans. Serogroups B, C, and D are responsible for most infections. S. enteritidis is serogroup D and is the most common cause of gastroenteritis. The other important species is S. typhimurium.

Their pathogenicity is conferred due to the ability to invade intestinal mucosa,² and produce toxins. Salmonella are widespread in a variety of animals, from shellfish to elephants, but poultry and bovines are the usual source of human infection.

There are many serotypes of salmonella.

Salmonella, in all forms, is a notifiable disease. In 1995 there were 80,000 cases notified in the UK, rising to just over 90,000 in 1997 but falling to about 68,000 in 2000. Some people think this represents a lull rather than a downward trend.³ The majority of cases are S. enteritidis. The highest rate of infection is in those over 70 and under 20, especially infants. Salmonella infection is possibly a cause of traveller's diarrhoea.

S. enteritides are classified as either PT4 or others. The number reported to the HPA for England and Wales for 2004 were 2,226 and 6,163 respectively and provisional figures for 2005 were 1,776 and 4,901 respectively.⁴

• Salmonella 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, but all food, including vegetables may be contaminated.
• Eggs continue to be a source of infection. In 2006, the Foods Standards Agency examined eggs for salmonella and found them present in about 1 box in 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 commonest source of contaminated eggs but only 10% of eggs used in the UK come from abroad and most of these are used in the catering industry.⁵
• 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, but it can survive spit and oven roasting if not properly defrosted.
• Salmonella infection 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.

Symptoms

• Incubation period is 12 to 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 to 7 days and there is recovery.

Signs

• There is a temperature of 38 to 39°C for about 48 hours.
• There may be signs of dehydration.
• There is not the typical rash of typhoid.

• Campylobacter
• Cryptosporosis
• E coli O157 (can cause bloody diarrhoea)
• Listeria monocytogenes
• Cyclospora
• Shigella
• Yersinia.

• Diagnosis is by culturing the organism from the stool.
• FBC will probably show an elevated white cell count but 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.

Non-drug

• 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 GI illness and haemoglobinopathies.

Drugs

• Antibiotics do not shorten the illness but may prolong the carrier stage.⁶ There is also a problem of multiple antibiotic resistance.⁷
• They may be used in the severely ill, especially the immuno-compromised.⁸
• Sometimes anti-diarrhoea or anti-spasmodic 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. The following need advice from Environmental Health officers or a Consultant in Communicable Disease Control (CCDC):

• Food handlers who touch unwrapped food to be consumed raw or without further cooking.
• Health-care, nursery or other staff who have direct contact with people who are susceptible to infection or for whom a salmonella infection would have very serious consequences. This includes those simply serving food to them.
• Children under 5 years attending nurseries, play groups, nursery schools etc.
• Older children or adults with poor standards of personal hygiene like the mentally ill, handicapped or the elderly infirm.

Surgery

Chronic carriers with gall bladder disease may benefit from cholecystectomy.

• 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 present), pneumonia or empyema, meningitis, septic arthritis, and osteomyelitis.

Mortality is about 0.4% but about 70 times higher in patients from residential homes. 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.
• In Scotland when it became illegal to sell unpasteurised milk the incidence of salmonella fell.
• Uncooked meats kept 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 handling raw meat and poultry. 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 and before eating or drinking and after going to the toilet.
• Those who are ill should not prepare or handle food.
• Everyone must wash their hands after contact with animals particularly pets and their bedding. Wash hands thoroughly with soap in warm running water and dry with a clean towel or disposable paper hand towel.
• Salmonella infections usually result from a combination of contaminated foods, poor kitchen hygiene and inadequate cooking.
• Good kitchen practices including thorough cooking of potentially contaminated foods, especially chicken, should be applied in the home and by the chef. The Chief Medical Officer advises against recipes with uncooked or lightly cooked eggs. Adequate cooking of eggs, until the yolk is set, kills salmonellas. Take care that food does not become contaminated after cooking.

Salmonella was named by the French bacteriologist Joseph Lignieres in honour of Daniel Salmon, the American veterinary pathologist. Daniel Salmon was born in 1850 and died in 1914. The organism was discovered by Salmon's assistant, Theobald Smith, but they are best remembered for the discovery that dead, heat killed organisms could immunise animals against living organisms. From this work the first Typhus vaccine and Salk's polio vaccine were developed.