Epidemiology Degree

Gastroenteritis is defined as an acute disease withdiarrhoea and/or vomiting and a variable degree of systemicillness. ‘Foodpoisoning’ is a term used to describe a selflimiting form of gastroenteritiscaused by eating contaminated food.

Aetiolog yA large variety of microorganisms can causegastroenteritis but in over half of all cases thecausative organism cannot beidentified. Salmonellae andCampylobacter species are among the most frequentcauses.In children the spectrum of organisms is slightly differentand rotavirusis the most frequently recognized agent.

Epidemiology Gastroenteritis is a common infection in allparts of theworld, occurring sporadically or in epidemics. Epidemicsoccur whena common source of food or drink is contaminatedwith micro-organisms and wheninfection spreadsfrom person to person by the faecal-oral route. Forexample,Salmonella enteritidis frequently colonize the gutof chickens bred in batteryconditions and may infect theoviduct and eggs of hens in batteries. Duringeviscerationin the factory the meat is contaminated and the salmonellaesurvivefreezing and thawing. If such chickens are notthoroughly cooked, salmonellaeare ingested and colonizethe intestine. During the last few years there havebeenoutbreaks of diarrhoea due to E. coli, which inpatients of all ages can goon to cause a haemolytic uraemicsyndrome with a significant mortality. This hasoccurred inretail food outlets where both cooked and uncooked meatsare on sale,with the opportunity to contaminate cookedmeats with E. coli  fromuncooked meat.Some forms of gastroenteritis show striking seasonalvariations infrequency but these are usually significantonly in temperate areas. Rotavirusis the most common cause of infantile diarrhoea worldwide. In Britain andtheUSA its peak incidence is in 2 or 3 months during thewinter, but in tropicalclimates there is little seasonal variation.Outbreaks of food poisoning causedby Salmonella,Campylobacter or toxin-producing Staph. aureus are morecommon insummer.

Pathogenesis Organisms must evade the host defences andcolonize the intestine before they can cause disease. Gastric acid isanimportant defence, and patients with a partial gastrectomy are susceptible tointestinal infection. Some bacteria haveon their surface determinants that bindto epithelialreceptors. Examples are Vibrio cholerae and some strainsof E.coli.The known mechanisms by which microorganisms cause diarrhoea can bedivided into two groups: toxin secretionand mucosal damage. These are notmutually exclusive,and for many organisms the precise mechanism isunknown.Toxin secretionThree different mechanisms are recognized by whichtoxinscan cause intestinal pathology. Cholera is a good example of smallintestinal toxin-mediated secretory diarrhoea and enterotoxigenic strainsof E. coli secrete a heat labile enterotoxin which acts in a similar way. Instaphylococcal food poisoning neurotoxin-secreting staphylococci multiply infood, and the toxin causes vomiting and diarrhoea within hours of ingestion.Staphylococci are notdetected in the stools of patients with this type of foodpoisoning.Bacillus cereus, which contaminates reheated rice,causes vomiting bya similar mechanism. Cytotoxins, suchas verotoxin produced by E. coli  andtoxin B ofClostridium difficile, cause tissue damage.Mucosal damageShigellaeinvade the colonic mucosa and cause localinflammation, and so the faecescontain blood and inflammatorycells. Impairment of water absorption fromthecolon and exudation of protein-rich fluid through the inflamed andulcerated mucosa are the cause of fluid stools.Salmonellae, Campylobacter androtavirus are also invasiveand cause tissue damage, but the precisemechanismsare not understood. Damage to the brush border of the enterocytescauses temporary disaccharidase deficiency,which may prolong the diarrhoea evenafter the pathogenhas been eliminated.