Campylobacter, Salmonella, Shigella, Escherichia coli

 • Common bacterial causes of GI infection.

• Mucosal biopsies usually show an acute colitis, with neutrophils present in the lamina propria and within crypts.

 • enterotoxigenic E. coli (ETEC) is a common cause of diarrhoea in travellers. ETEC possesses fimbriae which allow the bacteria to adhere to small bowel epithelial cells and produce toxins, causing massive fluid loss.

• enterohaemorrhagic E. coli produces a cytotoxin, leading to haemorrhagic necrosis of the colonic mucosa and bloody diarrhoea. Susceptible individuals, particularly children, are at risk of developing thrombotic microangiopathy, leading to haemolysis and ARF (haemolytic uraemic syndrome, HUS).

Clostridium difficile

• An important cause of colitis, often associated with broad- spectrum antibiotic use in hospitalized patients.

• the clinical picture is highly varied, ranging from mild diarrhoea to fulminant colitis with a risk of perforation and death.

 • Macroscopically, the colitis leads to the formation of cream- coloured pseudomembranes on the mucosal surface of the colon (fig. 1).

 • Microscopically, crypts distended with neutrophils and mucin are covered by pseudomembranes composed of fibrin and neutrophils.

Mycobacterium avium

 • Significant opportunistic pathogen in the immunosuppressed especially HIV positive patients with low CD4 counts).

 • Disseminated infection throughout the small and large bowel causes chronic diarrhoea.

• Mucosal biopsy shows extensive infiltration of the lamina propria by macrophages filled with acid- fast bacilli.

 Rotavirus

• Most common cause of severe diarrhoea in infants and young children.

 • faecal–oral transmission.

• Immunity develops during childhood, such that adult infection is rare.

 Norovirus

 • Common cause of epidemic outbreaks of gastroenteritis.

 • highly infectious with transmission through contaminated food or water, person- to- person contact, and contamination of surfaces.

 • Often seen in close communities such as institutions, hospitals, and cruise ships.

Cytomegalovirus

• Usually associated with immunocompromise.

 • CMV infection is an important cause of a sudden clinical deterioration in immunosuppressed patients with inflammatory bowel disease.

• Microscopically, the changes vary from mild inflammation to deep ulceration. CMV inclusions are found in endothelial and stromal cells.

 • Immunohistochemical staining is very helpful in confirming the diagnosis.

FIG1. Clostridium difficile colitis. this is a freshly opened colon from a patient with profuse diarrhoea, following broad- spectrum antibiotic treatment. Note the large number of cream- coloured plaques studded across the mucosal surface, representing collections of neutrophils, fibrin, and cell debris. reproduced with permission from Clinical Pathology (Oxford Core texts), Carton, James, Daly, richard, and ramani, Pramila, Oxford University Press (2006), p. 163, figure 8.14.

Giardia lamblia

 • Protozoan transmitted by drinking water contaminated with cysts of the organism.

• the mature pathogen attaches to the brush border of the epithelial cells of the upper small bowel and may be diagnosed on duodenal biopsy.

• the inflammatory reaction causes a mild diarrhoeal illness which lasts ~1 week and then resolves.

• Immunocompromised individuals may develop chronic infection.

 Entamoeba histolytica

 • Common protozoal infection, affecting ~10% of people worldwide.

 • Symptoms range from mild diarrhoea and abdominal pain to severe fulminant colitis.

 • the infection can disseminate to other sites such as the liver, and rarely large inflammatory masses can form (amoebomas).

• Mild cases show neutrophilic infiltration only, but more severe cases are associated with deep ulceration of the bowel.

 • the organisms are round structures with a bean- shaped nucleus and foamy cytoplasm containing ingested red blood cells (RBCs).

Enterobius vermicularis

 • Nematode pinworm transmitted by hand- to- mouth transfer of eggs.

 • Larvae mature into adult organisms, residing mainly in the caecum.

 • At night, female organisms migrate to the anus to deposit eggs which cause marked perianal itching.

Necator americanus and Ancylostoma duodenale

• Nematode hookworms which attach themselves to the jejunal mucosa.

 • A pump mechanism is used to ingest blood and interstitial fluid from the host. high worm loads can lead to significant cumulative blood loss.

• hookworm infestation is the most common cause of iron deficiency anaemia worldwide.

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