Gastroenteritis aetiology and pathogenesis

Gastroenteritis aetiology and pathogenesis

Gastroenteritis is defined as an acute disease with diarrhoea and/or vomiting and a variable degree of systemicillness. 'Food poisoning' is a term used to describe a selflimiting form of gastroenteritis caused by eating contaminated food.

Aetiolog yA large variety of microorganisms can cause gastroenteritis but in over half of all cases thecausative organism cannot be identified. Salmonellae andCampylobacter species are among the most frequent causes.In children the spectrum of organisms is slightly differentand rotavirus is the most frequently recognized agent.

Epidemiology Gastroenteritis is a common infection in all parts of theworld, occurring sporadically or in epidemics. Epidemicsoccur when a common source of food or drink is contaminatedwith micro-organisms and when infection spreadsfrom person to person by the faecal-oral route. Forexample, Salmonella enteritidis frequently colonize the gutof chickens bred in battery conditions and may infect theoviduct and eggs of hens in batteries. During eviscerationin the factory the meat is contaminated and the salmonellaesurvive freezing and thawing. If such chickens are notthoroughly cooked, salmonellae are ingested and colonizethe intestine. During the last few years there have beenoutbreaks of diarrhoea due to E. coli, which inpatients of all ages can go on to cause a haemolytic uraemicsyndrome with a significant mortality. This has occurred inretail food outlets where both cooked and uncooked meatsare on sale, with the opportunity to contaminate cookedmeats with E. coli from uncooked meat.Some forms of gastroenteritis show striking seasonalvariations in frequency but these are usually significantonly in temperate areas. Rotavirus is the most commoncause of infantile diarrhoea worldwide. In Britain and theUSA its peak incidence is in 2 or 3 months during thewinter, but in tropical climates there is little seasonal variation.Outbreaks of food poisoning caused by Salmonella,Campylobacter or toxin-producing Staph. aureus are morecommon in summer.

Pathogenesis Organisms must evade the host defences and colonize the intestine before they can cause disease. Gastric acid is animportant defence, and patients with a partial gastrectomy are susceptible to intestinal infection. Some bacteria haveon their surface determinants that bind to epithelialreceptors. Examples are Vibrio cholerae and some strainsof E. coli.The known mechanisms by which microorganisms cause diarrhoea can be divided into two groups: toxin secretionand mucosal damage. These are not mutually exclusive,and for many organisms the precise mechanism is unknown.Toxin secretionThree different mechanisms are recognized by which toxinscan cause intestinal pathology. Cholera is a good example of small intestinal toxin-mediated secretory diarrhoeaand enterotoxigenic strains of E. coli secrete a heat labile enterotoxin which acts in a similar way. In staphylococcal food poisoning neurotoxin-secreting staphylococci multiply in food, and the toxin causes vomiting and diarrhoea within hours of ingestion. Staphylococci are notdetected in the stools of patients with this type of food poisoning.Bacillus cereus, which contaminates reheated rice,causes vomiting by a similar mechanism. Cytotoxins, suchas verotoxin produced by E. coli and toxin B ofClostridium difficile, cause tissue damage.Mucosal damageShigellae invade the colonic mucosa and cause localinflammation, and so the faeces contain blood and inflammatorycells. Impairment of water absorption from thecolon and exudation of protein-rich fluid through theinflamed and ulcerated mucosa are the cause of fluid stools.Salmonellae, Campylobacter and rotavirus are also invasiveand cause tissue damage, but the precise mechanismsare not understood. Damage to the brush border of the enterocytes causes temporary disaccharidase deficiency,which may prolong the diarrhoea even after the pathogenhas been eliminated.

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