Acute diarrhoea (Box 22.1) is usually due to intestinal infection or an adverse effect of medication. Clostridium difficile followed by norovirus infection are the main causes of fatal illness and should always be considered in hospitalized patients. Norovirus is the commonest cause overall (around 20% of all cases). The commonest bacterial causes are Salmonella and Campylobacter. Inflammatory bowel disease should be excluded if there is bloody diarrhoea (see Chapters 74 and

Establish the differential diagnosis from the history and examination (Table 22.1). Strict infection control protocols must be followed (including barrier nursing and hand-washing). Investigations needed urgently are given in Table 22.2.

• Features of community-acquired infective diarrhoea are given in Table 22.3 and hospital-acquired diarrhoea in Table 22.4.
• Particular causes to consider in the returning traveller are summarized in Table 22.5, and those in patients with HIV/ADS or immunosuppression in Table 22.6.

If the patient is severely ill (impaired consciousness level, severe volume depletion, marked abdominal distension or tenderness):

• Start vigorous fluid resuscitation with crystalloid, initially via a peripheral IV line, and correct electrolyte abnormalities.
• Start antibiotic therapy to cover the likely pathogens: ciprofloxacin 400 mg 12-hourly IV and metronidazole 500 mg 8-hourly IV (immunocompetent patients) or gentamicin 5 mg/kg IV (immunosuppressed patients).
• Obtain an abdominal X-ray to check for segmental or total colonic distension indicative of toxic megacolon.
• Seek urgent help from a microbiologist.
• Nurse the patient with standard isolation technique in a single room until the diagnosis is established.

All other patients:

• Further management will be determined by the results of microscopy and culture of the stool, and other investigations.
• If the patient has a fever, is dehydrated or is immunocompromised, start azithromycin 500 mg OD PO or ciprofloxacin PO or IV whilst awaiting the stool test results. The choice will depend upon your local antibiotic guidelines. (Campylobacter is currently the commonest bacterial cause and most strains are resistant to ciprofloxacin. Moreover azithromycin-resistent Shigella infections are common in certain high-risk patients.)
• Anti-motility drugs such as loperamide are best avoided but can be given for short-term symptomatic relief. They are absolutely contraindicated in patients with shigellosis or dysentery (bloody stools and fever) due to the risk of a toxic megacolon.

This is directed by the working diagnosis.

Always consider the possibility of C. difficile infection and norovirus – involve the Infectious Diseases team early.

A flexible sigmoidoscopy and biopsies are not routinely needed, but if a new presentation of IBD is a possibility, a gastroenterology opinion should be sought (see below).