Definition and epidemiology
Diarrhea is defined by the World Health Organization as the passage of three or more loose or liquid stools per day, or more frequently than is normal for the individual. An increase in stool volume to greater than 200 grams per day is the quantitative definition. Diarrhea is classified by duration: acute (less than 14 days), persistent (14-29 days), and chronic (30 days or longer).
Globally, diarrheal diseases are the second leading cause of death in children under five years old, causing approximately 525,000 deaths annually. The burden is highest in South Asia and sub-Saharan Africa. In developed countries, diarrhea remains common with adults experiencing one to two episodes per year on average.
Acute infectious diarrhea is most commonly seen in children, travelers to endemic areas, immunocompromised patients, and those with recent antibiotic use. Chronic diarrhea affects approximately 5% of the population in Western countries, with functional disorders like irritable bowel syndrome being the most common cause.
- Diarrhea is defined as ≥3 loose stools per day; oral rehydration solution is first-line treatment regardless of etiology
- Stool osmotic gap <50 mOsm/kg indicates secretory diarrhea; >125 mOsm/kg indicates osmotic diarrhea
- Avoid antibiotics in suspected E. coli O157:H7 infection due to increased risk of hemolytic uremic syndrome
- Fecal leukocytes or lactoferrin suggest inflammatory diarrhea from invasive bacteria or inflammatory bowel disease
- C. difficile colitis should be suspected in anyone with diarrhea within 3 months of antibiotic use; treat with oral vancomycin
- Loperamide is contraindicated in bloody diarrhea, high fever, or suspected C. difficile due to toxic megacolon risk
- Reactive arthritis can follow Campylobacter, Salmonella, Shigella, or Yersinia gastroenteritis by several weeks
- Zinc supplementation in children reduces duration and severity of acute infectious diarrhea
Etiology and risk factors
Infectious causes
Infectious diarrhea accounts for the majority of acute cases worldwide. Pathogens include:
Viral: Rotavirus (most common in children globally), norovirus (outbreaks in closed settings), adenovirus, astrovirus
Bacterial: Escherichia coli (enterotoxigenic, enteropathogenic, enterohemorrhagic, enteroinvasive), Campylobacter jejuni, Salmonella species, Shigella species, Vibrio cholerae, Clostridium difficile, Yersinia enterocolitica, Staphylococcus aureus (toxin-mediated)
Parasitic: Giardia lamblia, Entamoeba histolytica, Cryptosporidium, Cyclospora
Non-infectious causes
- Medications: Antibiotics, magnesium-containing antacids, metformin, NSAIDs, chemotherapy agents, colchicine
- Malabsorption syndromes: Celiac disease, lactose intolerance, pancreatic insufficiency, short bowel syndrome
- Inflammatory bowel disease: Crohn's disease, ulcerative colitis
- Functional disorders: Irritable bowel syndrome (IBS-D)
- Endocrine: Hyperthyroidism, diabetic autonomic neuropathy, carcinoid syndrome
- Neoplastic: Colorectal cancer, villous adenoma, VIPoma, gastrinoma
- Ischemic: Mesenteric ischemia
- Food-related: Food allergies, food poisoning, excessive caffeine or artificial sweeteners
Risk factors
- Poor sanitation and contaminated water supply
- Recent travel to developing countries
- Consumption of undercooked food or unpasteurized dairy
- Recent antibiotic use (risk for C. difficile)
- Immunosuppression (HIV/AIDS, chemotherapy, organ transplant)
- Achlorhydria or proton pump inhibitor use
- Institutional living (nursing homes, daycare centers)
- Age extremes (young children and elderly)
Pathophysiology
Diarrhea results from disruption of normal intestinal water and electrolyte transport. Four main mechanisms exist:
Secretory diarrhea
Active secretion of electrolytes and water into the intestinal lumen without mucosal damage. Caused by bacterial toxins (V. cholerae, enterotoxigenic E. coli), hormones (VIPoma, carcinoid), bile acids, and laxatives. Characterized by large volume watery stools that persist with fasting. The stool osmotic gap is less than 50 mOsm/kg.
Osmotic diarrhea
Presence of poorly absorbed osmotically active solutes in the intestinal lumen draws water into the bowel. Causes include lactose intolerance, magnesium-containing laxatives, and malabsorption syndromes. Diarrhea improves with fasting. The stool osmotic gap exceeds 125 mOsm/kg.
Inflammatory diarrhea
Mucosal invasion and damage by pathogens (Shigella, Salmonella, Campylobacter, E. coli O157:H7, Entamoeba) or inflammatory bowel disease causes exudation of mucus, blood, and protein. Characterized by frequent small-volume stools with blood and pus, fever, and systemic symptoms. Fecal leukocytes and lactoferrin are positive.
Dysmotility
Altered intestinal transit time reduces contact time for absorption. Seen in IBS, diabetic neuropathy, hyperthyroidism, and post-surgical states. The mechanism is functional rather than structural.
In infectious diarrhea, fluid losses can reach several liters per day, leading to severe dehydration, electrolyte imbalances (hyponatremia, hypokalemia), metabolic acidosis, and hypovolemic shock if untreated. Children and elderly are at highest risk for complications.
Clinical features
History
Key features to elicit include:
- Stool characteristics: Frequency, volume, consistency, presence of blood, mucus, pus, or undigested food
- Duration: Acute versus chronic
- Associated symptoms: Fever, abdominal pain and cramping, nausea, vomiting, tenesmus
- Recent exposures: Travel history, food consumption (especially seafood, undercooked meat, unpasteurized products), sick contacts, antibiotic use
- Medical history: Immunosuppression, previous bowel surgery, chronic diseases
- Medication review: Recent antibiotics, new medications
Physical examination
Assessment of dehydration is critical, especially in children:
- Mild (3-5% loss): Slightly dry mucous membranes, normal vital signs, minimal clinical signs
- Moderate (6-9% loss): Dry mucous membranes, decreased skin turgor, sunken eyes, decreased urine output, mild tachycardia
- Severe (≥10% loss): Very dry mucous membranes, tenting skin, sunken fontanelle (infants), weak pulse, hypotension, altered mental status, oliguria or anuria
Abdominal examination may reveal diffuse tenderness, hyperactive bowel sounds, or signs of peritonitis if complications have developed. Digital rectal examination may show blood or be needed for stool sampling.
Red flag symptoms requiring urgent evaluation
- Signs of severe dehydration
- High fever (>38.5°C)
- Bloody diarrhea (especially with severe abdominal pain)
- Profuse watery diarrhea with dehydration
- Age >70 years or <6 months
- Immunocompromised state
- Recent antibiotic use
- Severe abdominal pain
- Symptoms lasting >7 days without improvement
Diagnosis
Diagnosis is primarily clinical for acute uncomplicated diarrhea. Investigations are reserved for severe cases, bloody diarrhea, immunocompromised patients, persistent symptoms, or suspected outbreaks.
Laboratory investigations
Stool studies:
- Stool culture: For bacterial pathogens (Salmonella, Shigella, Campylobacter). Indicated in bloody diarrhea, severe disease, or suspected outbreak
- Ova and parasites: Three specimens for parasitic infections, especially with travel history or chronic symptoms
- Fecal leukocytes or lactoferrin: Suggests inflammatory diarrhea
- Clostridium difficile toxin assay: In patients with antibiotic use in preceding 3 months or healthcare-associated diarrhea
- Stool osmotic gap: Helps differentiate secretory from osmotic diarrhea. Gap = 290 - 2([Na] + [K])
Blood tests:
- Complete blood count (leukocytosis in infection, anemia in chronic blood loss)
- Electrolytes (hyponatremia, hypokalemia, metabolic acidosis)
- Renal function (acute kidney injury from dehydration)
- Inflammatory markers (CRP, ESR in inflammatory bowel disease)
Imaging
Abdominal X-ray or CT scan if concern for toxic megacolon, perforation, or bowel obstruction.
Endoscopy
Colonoscopy with biopsy for chronic diarrhea, suspected inflammatory bowel disease, or when malignancy needs exclusion. Upper endoscopy with small bowel biopsy for suspected celiac disease or small intestinal pathology.
Specialized tests for chronic diarrhea
- Celiac serologies (tissue transglutaminase antibodies)
- Thyroid function tests
- 24-hour stool collection (for volume, fat, electrolytes)
- Breath tests (hydrogen breath test for lactose intolerance, SIBO)
- Chromogranin A (carcinoid syndrome)
- Fecal elastase (pancreatic insufficiency)
- Colonoscopy with random biopsies (microscopic colitis)
Differential diagnosis
The differential diagnosis depends on clinical presentation:
Acute watery diarrhea
- Viral gastroenteritis (most common)
- Enterotoxigenic E. coli and other bacterial toxins
- Food poisoning (S. aureus, Bacillus cereus)
- Giardiasis
- Medication-related
- Early inflammatory bowel disease
Acute bloody diarrhea (dysentery)
- Shigella (most common cause of bloody diarrhea in children)
- Campylobacter jejuni
- Non-typhoidal Salmonella
- Enterohemorrhagic E. coli (O157:H7)
- Entamoeba histolytica
- Inflammatory bowel disease flare
- Ischemic colitis (elderly)
Chronic diarrhea (>4 weeks)
- Watery: IBS, lactose intolerance, celiac disease, microscopic colitis, hyperthyroidism, laxative abuse
- Fatty (steatorrhea): Pancreatic insufficiency, celiac disease, small bowel bacterial overgrowth, Crohn's disease
- Inflammatory: Crohn's disease, ulcerative colitis, chronic infections (C. difficile, Entamoeba)
Important mimics
- Fecal impaction with overflow diarrhea (elderly, immobile patients)
- Pseudodiarrhea (frequent passage of small amounts of stool in anorectal disorders)
- Malabsorption syndromes presenting with diarrhea
Management
Management focuses on rehydration, symptomatic relief, and treating underlying causes when appropriate.
Rehydration therapy
Oral rehydration solution (ORS) is the cornerstone of treatment for mild to moderate dehydration. WHO-recommended ORS contains glucose and electrolytes in specific concentrations that optimize intestinal absorption. Administer small, frequent amounts and gradually increase as tolerated. Zinc supplementation reduces duration and severity in children.
Intravenous fluids are indicated for:
- Severe dehydration
- Persistent vomiting preventing oral intake
- Altered mental status
- Hemodynamic instability
Use isotonic crystalloids (normal saline or Ringer's lactate) with potassium supplementation once urine output is established.
Dietary management
Continued feeding during diarrheal illness is recommended, especially in children. The BRAT diet (bananas, rice, applesauce, toast) is commonly used but nutritionally incomplete; normal diet resumption is preferred. Avoid lactose temporarily if intolerance suspected. Probiotics may reduce duration by approximately one day.
Antimotility agents
Loperamide reduces stool frequency and is useful in non-bloody, non-febrile diarrhea. Contraindicated in bloody diarrhea, high fever, or suspected C. difficile colitis due to risk of toxic megacolon. Not recommended in children under 2 years.
Bismuth subsalicylate has mild antimotility and antisecretory effects.
Antibiotic therapy
Empirical antibiotics are indicated for:
- Moderate to severe traveler's diarrhea
- Bloody diarrhea with fever (dysentery)
- Immunocompromised patients
- Suspected cholera
- Documented bacterial infection with systemic symptoms
Avoid antibiotics in suspected E. coli O157:H7 (increases hemolytic uremic syndrome risk) and uncomplicated viral gastroenteritis.
Common regimens:
- Fluoroquinolones (ciprofloxacin) or azithromycin for traveler's diarrhea and bacterial gastroenteritis
- Metronidazole or vancomycin for C. difficile
- Nitazoxanide for Giardia and Cryptosporidium
- Metronidazole or tinidazole for Entamoeba histolytica
Specific causes
- C. difficile colitis: Discontinue offending antibiotic if possible; treat with oral vancomycin or fidaxomicin. Metronidazole is now second-line.
- Cholera: Aggressive rehydration plus doxycycline or azithromycin
- Chronic diarrhea: Treat underlying cause (gluten-free diet for celiac disease, pancreatic enzyme replacement, inflammatory bowel disease medications)
Infection control
Hand hygiene is critical. Isolation precautions for hospitalized patients with infectious diarrhea. Food handlers should not work until symptom-free.
Complications and prognosis
Complications
Dehydration and electrolyte disturbances are the most common and dangerous complications, particularly in young children and elderly. Severe dehydration can lead to hypovolemic shock, acute kidney injury, and death.
Metabolic acidosis results from bicarbonate loss in stool and lactic acidosis from hypoperfusion.
Hypokalemia from stool potassium losses can cause cardiac arrhythmias and muscle weakness.
Malnutrition develops with persistent or chronic diarrhea, especially in children, leading to growth faltering and micronutrient deficiencies.
Hemolytic uremic syndrome (HUS) is a life-threatening complication of E. coli O157:H7 infection, characterized by microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. It occurs in approximately 10% of cases, especially in children. Antibiotic use increases HUS risk.
Reactive arthritis (formerly Reiter syndrome) can follow infection with Campylobacter, Salmonella, Shigella, or Yersinia, presenting weeks after diarrhea resolves.
Guillain-Barré syndrome is a rare complication, most commonly associated with Campylobacter jejuni infection.
Toxic megacolon is a severe complication of C. difficile colitis or inflammatory bowel disease, with colonic dilatation, systemic toxicity, and risk of perforation.
Irritable bowel syndrome develops in 10-15% of patients following acute bacterial gastroenteritis (post-infectious IBS).
Lactose intolerance can develop temporarily after acute gastroenteritis due to brush border damage.
Prognosis
Most acute diarrheal illnesses are self-limited, resolving within 3-7 days with appropriate supportive care. Mortality is rare in developed countries with access to rehydration therapy.
In developing countries, diarrhea remains a major cause of childhood mortality, responsible for approximately 525,000 deaths annually in children under five. Most deaths are preventable with adequate hydration and zinc supplementation.
Chronic diarrhea prognosis depends on underlying etiology. Functional disorders like IBS have excellent prognosis with no mortality but can significantly impact quality of life. Inflammatory bowel disease requires lifelong management. Diarrhea from malignancy carries a prognosis related to the underlying cancer.
Factors associated with poorer outcomes include extremes of age, immunosuppression, delayed presentation, inadequate rehydration, and specific pathogens like E. coli O157:H7.
Red flag features requiring stool studies - ABCDEF: Antibiotics (recent use), Blood in stool, Chronic (>7 days), Dehydration (severe), Elderly/immunocompromised, Fever (high-grade)
Practice questions
1. A 4-year-old boy is brought to the emergency department with 2 days of watery diarrhea, vomiting, and low-grade fever. He attends daycare where several children are sick with similar symptoms. On examination, he has dry mucous membranes, decreased skin turgor, and sunken eyes. Vital signs show heart rate 130/min and blood pressure 85/50 mmHg. Capillary refill is 3 seconds. What is the most appropriate initial management?
2. A 28-year-old woman returns from a trip to Mexico and develops severe watery diarrhea, abdominal cramps, and nausea. She reports 8-10 bowel movements daily for 3 days. Stool is non-bloody. She is afebrile and has normal vital signs with mild diffuse abdominal tenderness. What is the most appropriate treatment?
3. A 72-year-old man hospitalized for pneumonia and treated with ceftriaxone for 5 days develops watery diarrhea with lower abdominal cramping. He has 6-8 bowel movements daily. Temperature is 38.3°C (101°F). White blood cell count is 16,000/mm³. Stool shows no blood but is positive for leukocytes. What is the most likely diagnosis and appropriate next step?
Frequently asked questions
What causes diarrhea?
Diarrhea is most commonly caused by viral infections (rotavirus, norovirus), followed by bacterial infections (E. coli, Campylobacter, Salmonella) and parasites (Giardia). Non-infectious causes include medications (especially antibiotics), food intolerances (lactose), inflammatory bowel disease, irritable bowel syndrome, malabsorption syndromes, and endocrine disorders like hyperthyroidism. In developing countries, contaminated water and poor sanitation are major contributors.
What are the first symptoms of infectious diarrhea?
The first symptoms typically include abdominal cramping and urgency, followed by loose or watery stools. Nausea and vomiting often precede or accompany diarrhea. Fever may develop with bacterial infections. Symptoms usually begin 12-72 hours after exposure depending on the pathogen. Viral gastroenteritis typically starts with nausea and vomiting, while bacterial causes may present with fever and bloody stools if invasive.
How is the cause of diarrhea diagnosed?
Acute uncomplicated diarrhea is diagnosed clinically without testing. Stool studies (culture, ova and parasites, C. difficile toxin) are ordered for bloody diarrhea, severe disease, recent antibiotic use, immunocompromised patients, or symptoms lasting over 7 days. Fecal leukocytes suggest inflammatory causes. Blood tests assess dehydration and electrolyte abnormalities. For chronic diarrhea, additional testing includes celiac serology, thyroid function, colonoscopy with biopsy, and stool studies for fat and volume.
Is diarrhea curable?
Acute infectious diarrhea is self-limited and resolves within 3-7 days with supportive care in most cases. Treatment focuses on preventing dehydration with oral rehydration solutions. Antibiotics cure specific bacterial and parasitic infections. Chronic diarrhea cure depends on the underlying cause—celiac disease improves with gluten-free diet, bacterial overgrowth responds to antibiotics, but inflammatory bowel disease requires long-term management. Most cases are manageable with appropriate treatment.
Secretory diarrhea vs osmotic diarrhea — what is the difference?
Secretory diarrhea results from active electrolyte secretion into the intestinal lumen, continues during fasting, and has a stool osmotic gap under 50 mOsm/kg. Causes include cholera toxin, hormones, and bile acids. Osmotic diarrhea occurs when poorly absorbed solutes draw water into the bowel, stops with fasting, and has an osmotic gap over 125 mOsm/kg. Causes include lactose intolerance, magnesium laxatives, and malabsorption syndromes.
Key drugs
Related conditions and differentials
Sources and further reading
StatPearls (NCBI Bookshelf)MedlinePlusWHO ICD-10: K59.1
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.