When a patient presents with diarrhoea, two broad groups of diseases must be considered—specific and non-specific inflammatory bowel disease (Table 1) 326. Although it is not the purpose of this section to describe specific infectious diseases in detail, it is important to bear them in mind when considering the management of patients with severe diarrhoea so that inappropriate surgery can be avoided. Conversely, although altered bowel function with blood in the stool in an African, Asian, or Arab patient undergoing investigations in the West is more likely to be due to bacillary or amoebic dysentery or schistosomiasis, other problems such as ulcerative colitis must be considered. With the increase in travel to tropical countries by Western patients, infectious diseases must always be ruled out first.

This disease is caused by the protozoan Entamoeba histolytica which, when ingested as a cyst, invades the large intestine, producing colonies in the colonic wall and ulceration, which may occasionally bleed briskly. Intestinal amoebiasis usually presents as a fluctuating bleeding diarrhoea, amoebic appendicitis, or an amoeboma. Rectal amoebic ulceration can mimic a carcinoma (Figs. 1, 2) 1033,1034. Barium enema may show deformity of the caecum at the site of an amoeboma (Fig. 3) 1035.

Fresh stool specimens must be examined quickly for protozoa. Treatment with metronidazole has a rapid effect, and surgery is rarely necessary.

This is often an epidemic disease and a contact history can usually be obtained. Shigella multiplies rapidly in the colon, producing endotoxins which can cause a coagulopathy or haemolytic anaemia when infection is severe. Inflammation of the colon leads to necrosis of epithelium and desquamation, with formation of a membrane and discrete ulcers. Diagnosis is usually made by stool culture. Most cases will settle with conservative and supportive measures, but antibiotics are usually required for infection with Shigella shiga.

This infection is acquired when contaminated fresh water containing the cercariae of Schistosoma mansoni, S. intercalatum, or S. japonicum come into contact with skin. Once inside the human bloodstream the adult worms pass through the liver and portal system and lodge in the wall of the large intestine. Granulomatous reactions to the parasite result in ulceration, bleeding polyps, and fibrosis. Intestinal symptoms are a late occurrence, with exacerbations of colitis occurring intermittently. Hepatic infestation and portal hypertension may also cause intestinal bleeding. Repeated stool specimens have to be examined for parasites; sigmoidoscopy may show ulceration and a rectal biopsy is helpful. Barium enema shows an immobile, irregular colon. Serological tests are valuable around 4 weeks after infection. Treatment is most commonly with niridazole.

In tropical counties, ileocaecal tuberculosis is an important and often underdiagnosed cause of intestinal disease. Colonic or anal tuberculosis alone is very rare.

Campylobacter enterocolitis and Yersinia enterocolitica may infect the small intestine, but usually cause a colitis with a characteristic infective clinical picture. Campylobacter and salmonellae are the most common cause of bloody diarrhoea. In immunocompromised patients, cytomegalovirus, herpes simplex virus, and Mycobacterium avium intestinale can all cause colitis.

Oxyuriasis caused by the threadworm or pinworm is a common infection in children and may affect whole families. Ova are ingested in contaminated food or drink or from hands contaminated with faeces. Female larvae migrate to the caecum or colon, and as their ova are shed they pass through the anus on to the perianal skin. These cause intense pruritus ani, resulting in scratching and reinfection. There may be an eosinophilia. The worms can be seen on sigmoidoscopy in the anal canal, and the ova identified in scrapings from perianal skin. Treatment, usually for the whole family, is with piperazine or mebendazole.

Rarely, Enterobius vermicularis can cause appendicitis or a perianal abscess.

Gastrointestinal symptoms develop in 10 per cent of patients receiving 50 Gy or more for abdominal or pelvic disease. Either the small bowel or large bowel can be damaged.

After 1 to 2 weeks, a diarrhoeal illness with bleeding and tenesmus results from damage to rapidly dividing cells. At this stage there may only be a mild inflammation of rectal mucosa. Delayed symptoms some weeks later result from ischaemia caused by obliteration of submucosal small vessels, and there may be ulceration or necrosis.

Six months or many years after the original radiotherapy continuing ischaemia may occur due to small vessel changes with secondary fibrosis. Ulceration, abscesses, fistulae, or strictures can occur and the mucosa looks pale with telangiectasia (Fig. 4) 1036. There is no specific treatment, but a course of local or systemic steroids may be worthwhile. Surgery is avoided if possible because of the poor healing and high incidence of complication.

A number of other conditions can mimic inflammatory bowel disease, including irritable bowel syndrome, diverticular disease, and ischaemic disorders. Eosinophilic gastroenteritis is a rare condition of unknown aetiology which may affect any part of the gastrointestinal tract, and which presents with diarrhoea, abdominal pain, and bleeding. An eosinophilia is present in biopsies and peripheral blood. Corticosteroid treatment is often effective.