Diarrhoea — Causes, Dehydration Risk & When to See a Doctor — Causes, Diagnosis & When to See a Doctor | MyMedicPlus

Diarrhoea is defined as 3 or more loose or watery bowel movements per day, representing a change from the individual's normal pattern. It is classified by duration: acute diarrhoea lasts fewer than 14 days (most commonly infectious in origin), persistent diarrhoea continues for 14–29 days, and chronic diarrhoea lasts 4 weeks or more — typically signalling an underlying structural, inflammatory, or functional gastrointestinal condition. Diarrhoea is one of the world's most significant public health problems: globally, diarrhoeal disease is the second leading cause of death in children under 5, killing approximately 525,000 children annually, largely due to preventable dehydration. In higher-income settings, diarrhoea is more often associated with discomfort and lost productivity rather than mortality, but the dehydration risk remains important — particularly in elderly patients, those with diabetes, people taking diuretics, and individuals with underlying kidney disease or heart failure. The most urgent immediate danger in acute diarrhoea is fluid and electrolyte depletion: watery diarrhoea can result in the loss of 5–10 litres of fluid per day in severe cases, leading to hypovolaemic shock if untreated. Accurate assessment of hydration status is the first clinical priority in anyone presenting with diarrhoea.

Viral gastroenteritis is the most common cause of acute diarrhoea worldwide: norovirus (the winter vomiting bug) affects all ages in explosive outbreaks; rotavirus causes severe diarrhoea in young children and is largely preventable by vaccination; adenovirus and astrovirus are additional viral causes. Bacterial food poisoning from Salmonella (undercooked poultry and eggs), Campylobacter (raw or undercooked poultry), Shiga toxin-producing Escherichia coli (O157:H7 — undercooked beef, raw vegetables), Staphylococcus aureus (contaminated dairy and cooked meats), and Bacillus cereus (reheated rice) cause diarrhoea with onset 1–72 hours after consumption. Clostridium difficile (C. diff) causes antibiotic-associated diarrhoea — particularly after broad-spectrum antibiotics — and can cause severe life-threatening colitis. Parasitic infections including Giardia lamblia and Cryptosporidium cause persistent diarrhoea after travel or contaminated water exposure. Traveller's diarrhoea affects 30–70% of travellers to tropical and subtropical regions, most commonly caused by enterotoxigenic E. coli (ETEC). Chronic diarrhoea causes include IBS-diarrhoea predominant (IBS-D) — functional, stress-related, no inflammation; Crohn's disease and ulcerative colitis (IBD) — inflammation and ulceration of the bowel; coeliac disease — immune-mediated gluten intolerance causing small intestinal damage; lactose intolerance — inability to digest lactose from dairy; microscopic colitis; and medication side effects including metformin, antibiotics, and magnesium-containing antacids.

Recognise and respond to signs of dehydration — the most dangerous complication of diarrhoea — without delay. Mild to moderate dehydration presents with thirst, dry mouth, reduced urine output (darker than usual urine), fatigue, and mild dizziness. Severe dehydration — which constitutes a medical emergency — is indicated by very dark or absent urine output, sunken eyes, extreme dizziness or inability to stand, rapid weak pulse, cold clammy extremities, confusion or altered consciousness, or marked skin tenting. Seek emergency care immediately for any of these severe dehydration features. Seek urgent medical assessment for diarrhoea with blood or mucus in the stool — this may indicate invasive bacterial infection (Shigella, Campylobacter, C. diff), ulcerative colitis, or colorectal cancer. A high fever above 38.5°C with diarrhoea suggests bacterial infection requiring evaluation. Diarrhoea lasting more than 2 days without improvement in adults, or more than 24 hours in young children or infants, warrants medical review. Onset of diarrhoea in an immunocompromised person (HIV, transplant recipient, chemotherapy patient) is always serious. Suspected foodborne outbreak affecting multiple people who ate the same meal requires public health reporting.

Clinical history is the cornerstone: the physician establishes the onset, frequency, consistency, colour, and presence of blood, mucus, or pus in the stool; associated symptoms including nausea, vomiting, and abdominal cramping; recent dietary history and food exposures; travel history; sick contacts; antibiotic or other medication use; and the patient's usual bowel pattern (to distinguish an acute change from baseline chronic symptoms). Physical examination assesses hydration status through mucous membranes, skin turgor, capillary refill, blood pressure, and pulse. Stool culture identifies bacterial pathogens — Salmonella, Shigella, Campylobacter, E. coli O157, and Yersinia — and is sent in all cases of bloody diarrhoea, diarrhoea in immunocompromised patients, and traveller's diarrhoea. Stool PCR panels detect a broad range of bacterial, viral, and parasitic pathogens simultaneously with high sensitivity. Stool microscopy identifies ova, cysts, and parasites. C. difficile stool antigen (GDH) and toxin testing is indicated after recent antibiotic use. Blood tests including full blood count (anaemia, eosinophilia for parasites), urea and electrolytes (dehydration severity), CRP (inflammation), and calprotectin (bowel inflammation marker) help assess severity and guide investigation. Coeliac antibodies (tissue transglutaminase IgA) are tested in chronic diarrhoea. Colonoscopy or flexible sigmoidoscopy with biopsies is the definitive investigation for suspected IBD, microscopic colitis, or colorectal cancer.

Oral rehydration is the most important treatment principle in acute diarrhoea and saves millions of lives annually. WHO oral rehydration solution (ORS) — a precisely balanced sodium-glucose formulation — corrects dehydration 5 times more effectively than plain water alone, as the sodium-glucose cotransporter in the intestinal wall actively transports both sodium and water even during active secretory diarrhoea. Commercial ORS sachets (Dioralyte, Electrolade, WHO-ORS) are preferable to homemade solutions. Intravenous fluids are required when diarrhoea is so severe or vomiting so persistent that oral rehydration is not possible. A bland, easily digestible diet (rice, boiled potatoes, dry toast, bananas) helps firm stools without prolonging illness; the outdated BRAT diet restriction is no longer recommended — normal food should be resumed as tolerated. Evidence-based probiotics including Lactobacillus rhamnosus GG and Saccharomyces boulardii shorten the duration of acute infectious diarrhoea by approximately 1 day. Loperamide (Imodium) reduces stool frequency and is safe in adults with non-bloody, non-febrile diarrhoea for symptomatic relief, but should not be used with suspected invasive bacterial infection. Antibiotics are indicated for confirmed bacterial infections in severe cases (ciprofloxacin or azithromycin for Campylobacter, Salmonella, and traveller's diarrhoea), C. difficile (vancomycin or fidaxomicin), and in immunocompromised patients. Chronic diarrhoea from IBD requires disease-specific therapy including mesalazine for ulcerative colitis, corticosteroids, and biologics (infliximab, vedolizumab) for IBD. Coeliac disease is managed with a strict lifelong gluten-free diet.

Thorough and frequent handwashing with soap and water remains the single most effective intervention to prevent infectious diarrhoea — wash hands before food preparation and consumption, after using the toilet, and after contact with animals or sick individuals. Alcohol-based hand gel is insufficient against norovirus and C. difficile, where soap and water is essential. Practice safe food handling: cook meat and poultry to appropriate internal temperatures (at least 74°C for poultry); refrigerate perishable foods within 2 hours of cooking; avoid cross-contamination between raw meat and ready-to-eat foods by using separate chopping boards; and respect expiry dates and storage instructions. When travelling to high-risk regions (South and Southeast Asia, sub-Saharan Africa, Central America), apply the principle — boil it, cook it, peel it, or leave it. Drink only bottled or boiled water and avoid ice. Rotavirus vaccination for infants (given at 6–12 weeks and 10–16 weeks of age) dramatically reduces severe rotavirus diarrhoea and related hospitalisations. Typhoid vaccination is recommended before travel to endemic regions. Antibiotic use should be limited to clearly indicated cases — unnecessary antibiotics disrupt the gut microbiome, increase C. difficile risk, and contribute to antimicrobial resistance. Maintain a diverse, high-fibre diet to support a healthy gut microbiome, which provides colonisation resistance against enteric pathogens.

Call emergency services or proceed immediately to the nearest emergency department for signs of severe dehydration — extreme dizziness, inability to stand, absent urine output, rapid weak pulse, cold extremities, or confusion — particularly in children, elderly individuals, or those with chronic illness. Seek emergency care for diarrhoea with blood and significant abdominal pain (possible ischaemic colitis or severe IBD), diarrhoea with rigors and high fever suggesting septicaemia, or suspected foodborne outbreak in a healthcare setting or elderly care home. Seek urgent same-day medical assessment for diarrhoea lasting more than 2 days without improvement in adults, diarrhoea more than 24 hours in children under 5 or infants, bloody or mucus-containing diarrhoea, fever above 38.5°C with diarrhoea, inability to maintain oral hydration due to persistent vomiting, onset after travel to a high-risk region (particularly if bloody or febrile), or diarrhoea in an immunocompromised person. Schedule a routine GP appointment for chronic diarrhoea lasting more than 4 weeks, recurrent episodes without a clear cause, diarrhoea associated with weight loss, or diarrhoea with bloating and abdominal pain that may suggest IBS, coeliac disease, or IBD. Never give infants plain water or diluted formula to treat diarrhoea — always use WHO-standard ORS.