Definition and epidemiology
Dysentery is defined as diarrhea containing visible blood and mucus, typically accompanied by fever, abdominal cramping, and painful straining at stool (tenesmus). The term encompasses both infectious and non-infectious causes, though it most commonly refers to acute infectious colitis.
Dysentery remains a significant cause of morbidity and mortality worldwide, with an estimated 165 million cases annually. Shigella species cause the majority of bacterial dysentery cases globally, responsible for approximately 125 million episodes per year. The disease burden is highest in South Asia and sub-Saharan Africa. Children under 5 years account for the majority of cases and deaths, with case-fatality rates reaching 5–15% in untreated severe cases.
Amoebic dysentery caused by Entamoeba histolytica affects approximately 50 million people annually, causing around 100,000 deaths. Endemic regions include Central and South America, Africa, and South Asia. Both bacterial and amoebic dysentery show seasonal peaks during warmer, rainy months when sanitation infrastructure is compromised.
- Shigella has the lowest infectious dose (10–100 organisms) of bacterial enteropathogens, explaining easy person-to-person transmission
- Fecal leukocytes indicate bacterial dysentery; trophozoites with ingested RBCs are pathognomonic for Entamoeba histolytica
- Antibiotics are contraindicated in EHEC/E. coli O157:H7 infection because they increase Shiga toxin release and HUS risk
- Amoebic dysentery requires both tissue phase (metronidazole) and luminal phase (paromomycin) treatment to prevent relapse
- Antidiarrheal agents (loperamide) are contraindicated in dysentery due to risk of toxic megacolon and prolonged pathogen shedding
- Flask-shaped colonic ulcers on colonoscopy suggest Entamoeba histolytica; diffuse friability suggests bacterial cause
- Hemolytic uremic syndrome complicates 5–10% of EHEC and S. dysenteriae type 1 infections, presenting as the triad of anemia, thrombocytopenia, and renal failure
- Azithromycin is now preferred over fluoroquinolones for shigellosis in areas with high ciprofloxacin resistance (South Asia)
Etiology and risk factors
Dysentery results from invasion of colonic mucosa by enteric pathogens. The major causative organisms include:
Bacterial causes:
- Shigella species (S. flexneri, S. sonnei, S. dysenteriae, S. boydii) — most common cause worldwide
- Campylobacter jejuni — common in developed nations
- Salmonella species (non-typhoidal)
- Enteroinvasive E. coli (EIEC) — clinically indistinguishable from shigellosis
- Enterohemorrhagic E. coli (EHEC) including O157:H7 — produces Shiga toxin
Parasitic causes:
- Entamoeba histolytica — causes amoebic dysentery
- Balantidium coli — rare cause
Major risk factors:
- Poor sanitation and inadequate water treatment
- Crowded living conditions (refugee camps, institutions)
- Young age (children 1–5 years)
- Malnutrition and immunocompromised states
- Travel to endemic areas
- Men who have sex with men (MSM) for amoebic dysentery
- Lack of handwashing facilities
- Consumption of contaminated food or water
Transmission occurs via the fecal-oral route, with person-to-person spread being most common for Shigella (infectious dose as low as 10–100 organisms). Contaminated food and water are important vehicles for all pathogens.
Pathophysiology
The pathophysiology differs between bacterial and amoebic dysentery but shares common features of mucosal invasion and inflammation.
Bacterial dysentery (Shigella)
Shigella organisms survive gastric acid and invade the colonic epithelium, particularly the distal colon and rectum. After penetrating M cells overlying Peyer's patches, bacteria multiply within the lamina propria and spread laterally into adjacent epithelial cells. Shigella produces Shiga toxin (especially S. dysenteriae type 1), which inhibits protein synthesis by cleaving ribosomal RNA, leading to cell death.
The invasion triggers intense inflammatory response with recruitment of neutrophils, forming microabscesses and ulcerations. Epithelial destruction results in loss of absorptive surface, exudation of blood and mucus, and increased intestinal motility. The inflammatory cytokines (IL-1, IL-8, TNF-α) contribute to systemic symptoms including fever.
Amoebic dysentery
Entamoeba histolytica trophozoites adhere to colonic mucin and epithelial cells via a galactose-inhibitable lectin. Trophozoites produce cysteine proteases and amoebapores (pore-forming proteins) that lyse host cells and facilitate tissue invasion. Unlike bacterial dysentery, amoebic invasion often produces flask-shaped ulcers that extend deeper into the submucosa with undermined edges and a relatively narrow neck.
Trophozoites phagocytose erythrocytes (a diagnostic feature) and can penetrate blood vessels, potentially causing extraintestinal complications. Host immune response is often limited, allowing chronic infection. The inflammatory response is typically less acute than bacterial dysentery, with fewer polymorphonuclear leukocytes.
Clinical features
The clinical presentation varies by causative organism but shares core features.
Bacterial dysentery
Onset: Abrupt, typically 1–3 days after exposure
Cardinal symptoms:
- Bloody diarrhea with mucus and pus — multiple small-volume stools (10–30 per day)
- High fever (39–40°C) — prominent in bacterial causes
- Severe abdominal cramping — colicky, lower quadrants
- Tenesmus — painful rectal straining with sensation of incomplete evacuation
- Nausea and vomiting (less prominent than in upper GI infections)
Physical examination:
- Diffuse abdominal tenderness, especially lower quadrants
- Hyperactive bowel sounds
- Dehydration signs (tachycardia, dry mucous membranes, decreased skin turgor)
- Fever
- Toxic appearance in severe cases
Complications of S. dysenteriae type 1:
- Hemolytic uremic syndrome (HUS) — especially in children
- Seizures — may occur before diarrhea onset in young children
- Toxic megacolon (rare)
Amoebic dysentery
Onset: Gradual, over weeks
Distinguishing features:
- Subacute course — symptoms develop over days to weeks
- Lower-grade fever or afebrile
- Bloody diarrhea with mucus, but typically fewer stools than bacterial (3–8 per day)
- Alternating diarrhea and constipation
- Weight loss in chronic cases
- Right upper quadrant pain if hepatic involvement
Physical examination:
- Tender hepatomegaly (if liver abscess present)
- Less severe systemic toxicity than bacterial dysentery
- Chronic cases may show cachexia
Key distinguishing features
| Feature | Bacterial | Amoebic |
|---|---|---|
| Onset | Acute (hours-days) | Gradual (days-weeks) |
| Fever | High (>39°C) | Low-grade or absent |
| Stool frequency | Very high (>10/day) | Moderate (3–8/day) |
| Systemic toxicity | Marked | Mild to moderate |
| Chronicity | Self-limited (days) | Can persist weeks-months |
Diagnosis
Diagnosis requires clinical assessment combined with laboratory confirmation of the specific pathogen.
Clinical diagnosis
Dysentery should be suspected in any patient presenting with:
- Diarrhea containing visible blood
- Fever and abdominal pain
- History of travel to or residence in endemic area
- Recent contact with confirmed case
Laboratory investigations
Stool examination — essential for pathogen identification:
- Stool microscopy:
- Bacterial: numerous fecal leukocytes (polymorphonuclear cells)
- Amoebic: trophozoites containing ingested red blood cells (pathognomonic for E. histolytica)
- Fresh, warm stool required for amoebic trophozoites (within 30 minutes)
- Cysts may be seen in formed stool
- Stool culture:
- Gold standard for bacterial dysentery
- Shigella, Salmonella, Campylobacter identified
- MacConkey agar, Hektoen enteric agar, or XLD agar used
- Requires 48–72 hours
- Stool antigen tests:
- E. histolytica antigen detection (ELISA) — distinguishes from non-pathogenic E. dispar
- Shiga toxin detection for EHEC
- Molecular testing:
- PCR-based multiplex panels — increasingly available, rapid results
- High sensitivity and specificity
- Can detect multiple pathogens simultaneously
Blood tests:
- Complete blood count: leukocytosis with left shift in bacterial causes
- Electrolytes: assess dehydration, hyponatremia, hypokalemia
- Renal function: monitor for HUS development
- Blood cultures: positive in 5–10% of Shigella cases
- Serologic tests: anti-amoebic antibodies (useful for extraintestinal amoebiasis, less so for acute dysentery)
Imaging:
- Usually not required for uncomplicated cases
- Ultrasound or CT abdomen: if amoebic liver abscess suspected (right lobe, "anchovy paste" appearance)
- Plain abdominal radiograph: if toxic megacolon suspected
Endoscopy:
- Colonoscopy/sigmoidoscopy: reserved for unclear diagnosis or suspected complications
- Bacterial: diffuse mucosal inflammation, friability, superficial ulcerations
- Amoebic: discrete flask-shaped ulcers with normal intervening mucosa
- Biopsy may show trophozoites in amoebic disease
Differential diagnosis
Several conditions present with bloody diarrhea and must be distinguished from infectious dysentery:
Inflammatory bowel disease (IBD):
- Ulcerative colitis — chronic, relapsing bloody diarrhea
- Crohn's disease — may involve colon
- Distinguished by: longer duration (weeks-months before presentation), extraintestinal manifestations, lack of fever (unless fulminant), negative stool cultures, characteristic endoscopic and histologic findings
Ischemic colitis:
- Older patients with vascular risk factors
- Abrupt onset of crampy abdominal pain followed by bloody diarrhea
- Affects "watershed" areas (splenic flexure, rectosigmoid)
- CT shows colonic wall thickening, thumbprinting
Pseudomembranous colitis (Clostridioides difficile):
- History of recent antibiotic use or hospitalization
- Diarrhea typically non-bloody, but may have blood
- Stool C. difficile toxin assay positive
- Colonoscopy shows pseudomembranes
Colorectal cancer:
- Gradual onset, chronic course
- Change in bowel habits, weight loss
- Older age group (usually >50 years)
- Colonoscopy diagnostic
Hemorrhoids:
- Bright red blood on toilet paper or coating stool surface
- No diarrhea, no systemic symptoms
- Blood unmixed with stool
Diverticulitis:
- Left lower quadrant pain predominant
- Usually painless bright red bleeding (diverticular hemorrhage) or no bleeding
- CT shows diverticular inflammation
Intussusception:
- Infants and young children
- Intermittent colicky pain, "currant jelly" stools
- Palpable abdominal mass
- Ultrasound shows "target sign"
Other infectious causes of bloody diarrhea:
- Yersinia enterocolitica — may mimic appendicitis
- Cytomegalovirus colitis — immunocompromised patients
- Schistosomiasis — endemic areas, eosinophilia
Management
Management depends on the causative organism, severity, and patient factors. The approach includes supportive care and antimicrobial therapy.
Supportive care (all cases)
Rehydration:
- Oral rehydration solution (ORS) — first-line for mild-moderate dehydration
- WHO-recommended low-osmolarity ORS
- Intravenous fluids — severe dehydration, shock, inability to tolerate oral intake
- Ringer's lactate or normal saline with potassium supplementation
Nutritional support:
- Continue feeding, especially in children (reduces duration, improves nutrition)
- Zinc supplementation in children reduces severity and duration
Symptomatic treatment:
- Avoid antidiarrheal agents (loperamide, diphenoxylate) — may prolong illness, increase risk of complications including toxic megacolon and HUS
- Antipyretics for fever if needed
Antimicrobial therapy
Bacterial dysentery (Shigella, EIEC):
Antibiotics shorten duration, reduce transmission, and prevent complications. Increasing resistance requires local susceptibility data.
First-line options:
- Ciprofloxacin — adults; resistance increasing in South Asia
- Azithromycin — preferred where fluoroquinolone resistance high; safe in children
- Ceftriaxone — third-generation cephalosporin, parenteral option for severe cases
Alternative agents:
- Cefixime (oral third-generation cephalosporin)
- Pivmecillinam (where available)
Duration: typically 3 days for fluoroquinolones, 3–5 days for others
Campylobacter:
- Azithromycin or erythromycin
- Often self-limited; reserve antibiotics for severe cases
Salmonella:
- Antibiotics generally not indicated for uncomplicated gastroenteritis (may prolong carriage)
- Treat if: age <3 months, >50 years, immunocompromised, severe disease, bacteremia
- Options: ciprofloxacin, ceftriaxone, azithromycin
EHEC (E. coli O157:H7):
- Antibiotics contraindicated — increase risk of HUS by promoting toxin release
- Supportive care only
Amoebic dysentery
Luminal and tissue amebicide required:
Tissue phase:
- Metronidazole — first-line for invasive disease
- Tinidazole — alternative, better tolerated, single daily dose
Luminal phase (to eradicate cysts):
- Paromomycin — poorly absorbed aminoglycoside
- Iodoquinol or diloxanide furoate (where available)
Treatment must include both phases to prevent relapse and ongoing transmission.
Amoebic liver abscess:
- Metronidazole or tinidazole
- Aspiration if large (>5 cm), left lobe, or imminent rupture
- Always follow with luminal agent
Infection control
- Isolate hospitalized patients (contact precautions)
- Hand hygiene emphasis
- Exclude food handlers and healthcare workers until culture-negative
- Report cases to public health authorities
- Screen and treat household contacts in outbreak settings
Management of complications
Hemolytic uremic syndrome:
- Supportive care, dialysis if needed
- No proven specific therapy
- Avoid antibiotics if EHEC suspected
Severe dehydration/shock:
- Aggressive IV fluid resuscitation
- ICU monitoring
Toxic megacolon:
- Bowel rest, IV fluids, broad-spectrum antibiotics
- Surgical consultation — may require colectomy
Complications and prognosis
Acute complications
Severe dehydration and electrolyte imbalance:
- Most common cause of death, especially in young children and elderly
- Hypovolemic shock if untreated
- Hyponatremia, hypokalemia, metabolic acidosis
Hemolytic uremic syndrome (HUS):
- Occurs in 5–10% of EHEC infections, 5–15% of S. dysenteriae type 1
- Triad: microangiopathic hemolytic anemia, thrombocytopenia, acute renal failure
- Most common cause of acute renal failure in children
- Mortality 3–5% in developed countries, higher in resource-limited settings
Bacteremia and sepsis:
- Occurs in 5–10% of Shigella cases
- Higher risk in malnourished children, infants, immunocompromised
- S. flexneri and S. sonnei most common
Intestinal complications:
- Toxic megacolon — rare but life-threatening, may require emergency colectomy
- Intestinal perforation — especially with S. dysenteriae type 1
- Rectal prolapse — from severe tenesmus, especially in children
- Intestinal obstruction — from pseudomembranes or intussusception
Seizures:
- Occur in 10–45% of children with shigellosis
- May precede diarrhea
- Mechanisms: neurotoxin effect, fever, hyponatremia, hypoglycemia
Extraintestinal complications (amoebic)
Amoebic liver abscess:
- Most common extraintestinal complication
- Occurs in <10% of cases, typically weeks-months after initial infection
- Right lobe predominance
- May rupture into peritoneum, pleura, or pericardium
Other rare sites:
- Pleuropulmonary (extension from liver)
- Brain abscess
- Genitourinary involvement
Chronic complications
Post-infectious irritable bowel syndrome:
- Develops in 5–10% after acute infectious gastroenteritis
- Persistent abdominal pain, altered bowel habits
Reactive arthritis:
- Part of reactive arthritis triad (formerly Reiter syndrome)
- Follows Shigella, Salmonella, Campylobacter infections
- Asymmetric oligoarthritis, urethritis, conjunctivitis
- HLA-B27 association
Malnutrition:
- Particularly in children with repeated episodes
- Contributes to stunting in endemic areas
Guillain-Barré syndrome:
- Rare complication, primarily after Campylobacter
Prognosis
Bacterial dysentery:
- Most cases self-limited, resolving in 5–7 days even without antibiotics
- With appropriate treatment: symptoms improve within 48 hours, complete recovery within 7 days
- Mortality <1% in developed countries with access to rehydration and antibiotics
- Mortality 5–15% in resource-limited settings without treatment, especially in children
Amoebic dysentery:
- Untreated: may become chronic or develop extraintestinal complications
- With treatment: >95% cure rate
- Relapse in 5–10% if luminal phase not treated
- Mortality <1% with appropriate therapy
- Higher mortality with liver abscess complications (rupture)
Factors associated with worse prognosis:
- Age <5 years or >65 years
- Severe malnutrition
- Immunocompromised state
- Delay in appropriate treatment
- S. dysenteriae type 1 infection
- Development of HUS or sepsis
Long-term outlook:
- Most patients recover completely without sequelae
- Repeated infections in endemic areas contribute to malnutrition and developmental delays in children
- Post-infectious complications (reactive arthritis, IBS) affect quality of life but are generally not life-threatening
SHIGELLA for features of bacterial dysentery: Small-volume stools, High fever, Invasive (fecal leukocytes), Gastrointestinal cramping, Epidemic potential, Low infectious dose, Left shift (leukocytosis), Abrupt onset
Practice questions
1. A 4-year-old boy is brought to the emergency department with 2 days of high fever, severe abdominal cramping, and frequent bloody stools. He recently returned from a refugee camp where several children had similar symptoms. On examination, his temperature is 39.5°C, heart rate 130/min, and he appears moderately dehydrated. Stool microscopy shows numerous polymorphonuclear leukocytes. Which of the following is the most appropriate antimicrobial therapy?
2. A 35-year-old man presents with a 3-week history of gradually worsening diarrhea with blood and mucus, mild abdominal discomfort, and intermittent low-grade fever. He recently returned from a 6-month work assignment in rural India. Stool microscopy reveals trophozoites containing ingested red blood cells. Which of the following treatment regimens is most appropriate?
3. A 6-year-old girl develops bloody diarrhea and abdominal pain after eating at a fast-food restaurant. Five days later, she develops pallor, decreased urine output, and petechiae. Laboratory studies show hemoglobin 7.5 g/dL, platelets 45,000/mm³, creatinine 3.2 mg/dL, and schistocytes on peripheral smear. Stool culture is pending. Which of the following is the most likely pathogen?
Frequently asked questions
What causes dysentery?
Dysentery is caused by bacterial or parasitic invasion of the colon. Shigella species are the most common bacterial cause worldwide, followed by Campylobacter, Salmonella, and enteroinvasive E. coli. Entamoeba histolytica is the primary parasitic cause. These pathogens are transmitted through contaminated food, water, or direct fecal-oral contact. Poor sanitation, crowding, and lack of clean water increase transmission risk. Shigella requires only 10–100 organisms to cause infection, making person-to-person spread very efficient.
What are the first symptoms of dysentery?
The first symptoms typically include sudden onset of frequent diarrhea with visible blood and mucus, accompanied by severe abdominal cramping and high fever in bacterial cases. Patients experience tenesmus—painful straining during bowel movements with a feeling of incomplete evacuation. Bacterial dysentery (Shigella) usually begins abruptly within 1–3 days of exposure with high fever and many small-volume bloody stools. Amoebic dysentery develops more gradually over days to weeks, with lower-grade fever and fewer bowel movements. Nausea and vomiting may occur but are less prominent than in upper gastrointestinal infections.
How is dysentery diagnosed?
Dysentery is diagnosed through stool examination and culture. Microscopy of fresh stool reveals fecal leukocytes in bacterial dysentery or motile trophozoites containing ingested red blood cells in amoebic dysentery. Stool culture on selective media identifies bacterial pathogens like Shigella, Salmonella, and Campylobacter. E. histolytica can be detected through antigen tests or PCR, which distinguish it from non-pathogenic species. Blood tests show leukocytosis in bacterial causes. Colonoscopy is reserved for unclear cases and shows diffuse inflammation in bacterial dysentery or characteristic flask-shaped ulcers in amoebic dysentery.
Is dysentery curable?
Yes, dysentery is curable with appropriate treatment. Bacterial dysentery responds to antibiotics such as azithromycin, ciprofloxacin, or ceftriaxone, with symptoms improving within 48 hours and complete recovery in about one week. Amoebic dysentery requires metronidazole or tinidazole to treat tissue infection, followed by a luminal agent like paromomycin to eliminate intestinal cysts and prevent relapse. The cure rate exceeds 95% with proper treatment. Supportive care with oral or intravenous rehydration is essential for all cases. Even without antibiotics, most bacterial dysentery resolves in 5–7 days, though treatment shortens illness and reduces transmission.
Bacterial vs amoebic dysentery — what is the difference?
Bacterial dysentery (usually Shigella) has abrupt onset with high fever over 39°C, very frequent small-volume bloody stools (10–30 per day), and marked systemic toxicity. Stool microscopy shows numerous fecal leukocytes. Amoebic dysentery caused by Entamoeba histolytica develops gradually over days to weeks, with lower-grade or absent fever, fewer bowel movements (3–8 per day), and less systemic illness. Stool shows trophozoites with ingested red blood cells. Bacterial dysentery is self-limited within a week; amoebic can become chronic and cause liver abscesses. Treatment differs: antibiotics for bacterial, metronidazole plus paromomycin for amoebic.
Key drugs
Related conditions and differentials
Sources and further reading
StatPearls (NCBI Bookshelf)MedlinePlusWHO ICD-10: A03.9
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.