Diarrhea — Causes, Symptoms & Treatment Guide — Symptoms, Causes & Treatment | MyMedicPlus

Diarrhea is defined as the passage of three or more loose or liquid stools per day, or more frequently than is normal for the individual. It is classified as acute (less than 14 days — most commonly infectious), persistent (14-29 days), or chronic (30 or more days — usually indicates an underlying gastrointestinal, inflammatory, or systemic disorder). Diarrhea is one of the most common causes of global morbidity and mortality — particularly in children under 5 in low-income countries, where approximately 1.7 billion episodes and 525,000 deaths occur annually, primarily from dehydration and electrolyte imbalance. The most common mechanism of diarrhea is secretory (enterotoxin-mediated fluid secretion into the gut lumen), osmotic (poorly absorbed solutes drawing fluid into the lumen), or inflammatory (mucosal invasion causing bloody diarrhea — dysentery). In developed countries, the most common causes are viral gastroenteritis, food poisoning, and antibiotic-associated diarrhea.

Infectious causes of acute diarrhea: viral — norovirus (most common cause of acute gastroenteritis in adults worldwide), rotavirus (most common in children — now preventable by vaccination), adenovirus, astrovirus; bacterial — Campylobacter (most common bacterial gastroenteritis in the UK — from undercooked poultry), Salmonella, E. coli (including STEC/E. coli O157 causing haemolytic uraemic syndrome), Shigella, Clostridioides difficile (antibiotic-associated — overgrowth after antibiotic disruption of gut flora); protozoal — Giardia lamblia (camping, travel — small intestinal infection causing chronic malabsorptive diarrhea), Cryptosporidium (particularly in immunocompromised), Entamoeba histolytica (amoebiasis — tropical travel). Non-infectious causes of chronic diarrhea: irritable bowel syndrome (IBS — most common functional cause), inflammatory bowel disease (Crohn's, ulcerative colitis), coeliac disease, microscopic colitis, bile acid malabsorption (post-cholecystectomy), hyperthyroidism, pancreatic exocrine insufficiency, lactose intolerance, medications (laxatives, metformin, PPIs, SSRIs), and colorectal malignancy.

Acute infectious diarrhea: loose or watery stools (3 or more per day), abdominal cramping, nausea and vomiting, low-grade fever, and malaise. Typically self-limiting within 48-72 hours for viral gastroenteritis. Dehydration signs require urgent attention: dry mouth and reduced saliva, decreased urine output, dark urine, sunken eyes, absence of tears in children, skin turgor loss, dizziness, rapid heartbeat, and confusion. Inflammatory (dysenteric) diarrhea: bloody diarrhea with mucus, fever, severe abdominal pain, and tenesmus (recurrent urge to defecate) — suggests bacterial or amoebic dysentery or IBD flare. Chronic diarrhea features: alternating diarrhea and constipation with bloating and no rectal bleeding suggests IBS; weight loss, nocturnal diarrhea, and rectal bleeding suggest IBD or malignancy; pale, fatty, floating, malodorous stools (steatorrhoea) suggest malabsorption (coeliac disease, pancreatic insufficiency).

Acute diarrhea in healthy adults: microbiological investigation is not routinely required — most cases are self-limiting. Indications for stool culture: bloody diarrhea, severe systemic illness, immunocompromised host, suspected foodborne outbreak, recent antibiotic use (C. difficile), returning from abroad (tropical pathogens), and symptoms persisting beyond 7 days. Stool tests: stool culture and sensitivity (bacterial), microscopy for ova, cysts, and parasites (Giardia, Cryptosporidium), stool Clostridioides difficile PCR or toxin enzyme immunoassay. Faecal calprotectin: marker of intestinal inflammation — elevated in IBD, infectious enterocolitis, and colorectal cancer; helps distinguish IBD from IBS (normal calprotectin makes active IBD very unlikely). Blood tests: FBC, CRP, electrolytes (assess dehydration and electrolyte imbalance), renal function, thyroid function, and coeliac antibodies (tTG-IgA) for chronic diarrhea. Colonoscopy with biopsies: for suspected IBD, microscopic colitis, or colorectal malignancy. Breath tests: hydrogen breath test for lactose intolerance and small intestinal bacterial overgrowth (SIBO).

Rehydration is the cornerstone of diarrhea management — preventing dehydration is more important than stopping diarrhea. Oral rehydration solution (ORS — WHO formula: 2.6g NaCl, 2.9g trisodium citrate, 1.5g KCl, 13.5g glucose per litre of clean water) is the gold standard for mild-moderate dehydration in both children and adults; it is as effective as IV fluids for most cases. Intravenous fluids (normal saline, Hartmann's solution) for severe dehydration, inability to tolerate oral fluids, or compromised absorption. Zinc supplementation (10-20mg daily for 10-14 days) reduces diarrhea duration and severity in children in developing countries. Antimotility agents: loperamide (adults only, not for bloody or febrile diarrhea — risk of prolonging illness with STEC) reduces stool frequency and duration for non-specific diarrhea; bismuth subsalicylate as alternative. Antibiotics are indicated for: Shigella (azithromycin or ciprofloxacin), Campylobacter in severe or high-risk patients (azithromycin), Giardia (metronidazole or tinidazole), Clostridioides difficile (vancomycin oral or fidaxomicin — metronidazole no longer first-line). Do not use antibiotics for norovirus, rotavirus, or STEC (E. coli O157) — may worsen outcomes. Chronic diarrhea: treat the underlying cause — strict gluten-free diet for coeliac disease; mesalazine or steroids for IBD; antispasmodics and low-FODMAP diet for IBS; bile acid sequestrants (cholestyramine) for bile acid diarrhea.

Dehydration is the most common and life-threatening complication of acute diarrhea, particularly in children under 5 and the elderly — severe dehydration causes acute kidney injury, electrolyte imbalances (hypokalaemia causing cardiac arrhythmias; hyponatraemia causing seizures), circulatory collapse, and death. Globally, dehydration from diarrhea kills over 500,000 children annually. Haemolytic uraemic syndrome (HUS): a severe complication of Shiga toxin-producing E. coli (STEC, particularly E. coli O157:H7) — the triad of microangiopathic haemolytic anaemia, thrombocytopaenia, and acute kidney failure; affects predominantly children; loperamide use worsens the risk by prolonging toxin contact with the intestinal mucosa; 5-10% of affected children develop end-stage renal disease. Reactive arthritis (formerly Reiter's syndrome): joint inflammation occurring weeks after enteric infection with Salmonella, Shigella, Campylobacter, or Yersinia — more common in HLA-B27 positive individuals; may cause uveitis and urethritis. Clostridioides difficile colitis complications: toxic megacolon (life-threatening colonic dilatation above 6 cm), colonic perforation, septicaemia, and multi-organ failure — requiring emergency colectomy. Malabsorption and malnutrition: chronic or persistent diarrhea in children causes protein-energy malnutrition, micronutrient deficiencies (iron, zinc, vitamin A), impaired growth, cognitive development delays, and increased susceptibility to further infections — a vicious cycle in low-income settings.

Hand hygiene is the single most effective preventive measure: wash hands with soap and water for at least 20 seconds after using the toilet and before handling food — alcohol hand gels are not effective against norovirus or C. difficile. Food safety: cook poultry thoroughly (internal temperature 75°C/165°F), refrigerate food promptly (below 5°C), separate raw meat from ready-to-eat foods, and avoid unpasteurised dairy products. Safe water: use bottled or boiled water in areas with limited sanitation; water purification tablets for travel. Rotavirus vaccination: oral live attenuated rotavirus vaccines (Rotarix, RotaTeq) are recommended in infancy by WHO — dramatically reduce rotavirus gastroenteritis deaths; now part of routine childhood vaccination schedules in many countries. C. difficile prevention: judicious antibiotic prescribing (avoid unnecessary antibiotics), probiotics (Lactobacillus rhamnosus GG or Saccharomyces boulardii) during and after antibiotics reduce antibiotic-associated diarrhea risk. Breastfeeding exclusively for 6 months significantly reduces infant infectious diarrhea morbidity.

Seek immediate medical care for: signs of severe dehydration (no urine output for 8 hours, sunken eyes, very dry mouth, confusion, rapid heartbeat), bloody diarrhea (dysentery), high fever (above 39°C) with diarrhea, suspected meningitis or septicaemia, and diarrhea in infants under 3 months or the elderly and immunocompromised. See a GP promptly for: diarrhea lasting more than 7 days, diarrhea in a returning traveller from a tropical country, recent antibiotic use with diarrhea (C. difficile), and first episode of bloody diarrhea. Seek urgent investigation for: new diarrhea in a person over 50 (requires exclusion of colorectal malignancy), diarrhea with unintentional weight loss, nocturnal diarrhea, or diarrhea associated with a mass or anaemia.