What are the causes of blood in diarrhea?

Causes of Blood in Diarrhea

The most common causes of bloody diarrhea are infectious pathogens, particularly STEC (Shiga toxin-producing E. coli), Shigella, Salmonella, Campylobacter, and Entamoeba histolytica, which should be suspected in any patient presenting with visible blood in stool. 1

Infectious Causes

Bacterial Pathogens

• STEC (Shiga toxin-producing E. coli) - Particularly concerning as it's associated with hemolytic uremic syndrome (HUS). Notably, STEC O157 is found in 63% of visibly bloody stool specimens and patients typically present with severe abdominal pain, grossly bloody stools, and minimal or no fever. 1
• Shigella - Causes bacillary dysentery with fever, abdominal cramps, tenesmus, and bloody diarrhea. Present in 15% of bloody stool specimens. 1
• Salmonella - Often presents with fever and abdominal pain alongside bloody diarrhea. Present in 5% of bloody stool specimens. 1
• Campylobacter - Causes inflammatory diarrhea with fever and abdominal pain. Present in 8% of bloody stool specimens. 1, 2
• Yersinia - Can cause persistent abdominal pain and fever, sometimes mimicking appendicitis. 1
• Non-cholera Vibrio species - Associated with seafood consumption or seacoast exposure. 1
• Clostridium difficile - Important consideration in patients with recent antibiotic exposure (within 8-12 weeks). 1
• Plesiomonas - Less common bacterial cause. 1

Parasitic Causes

• Entamoeba histolytica - Causes amoebic colitis with more indolent onset compared to bacterial dysentery. 1
• Balantidium coli - Rare parasitic cause of bloody diarrhea. 1

Non-Infectious Causes

Inflammatory Conditions

• Inflammatory Bowel Disease (IBD) - Should be considered in patients with persistent symptoms (>14 days) without identified infectious cause. 1
• Radiation colitis - History of radiation therapy to abdomen/pelvis. 1
• Ischemic colitis - More common in elderly patients. 1

Vascular Abnormalities

• Angiodysplasia - More common in elderly patients, can cause intermittent bleeding. 1

Neoplastic Causes

• Colorectal cancer/polyps - Should be considered especially in older adults with new-onset bloody diarrhea. 1, 3

Anorectal Causes

• Hemorrhoids - Common cause of bright red blood, typically on toilet paper or coating stool rather than mixed with stool. 1
• Anal fissures - Painful defecation with bright red blood. 1

Diagnostic Approach

When evaluating bloody diarrhea, consider:

1. Severity assessment:

   • Presence of dehydration
   • Hemodynamic stability
   • Frequency and volume of bloody stools
   • Presence of fever, severe abdominal pain, or tenesmus

2. Risk factors:

   • Recent travel
   • Antibiotic use
   • Immunocompromised status
   • Food exposures (raw/undercooked meat, unpasteurized products)
   • Contact with animals or contaminated water

3. Testing recommendations:

   • Stool testing should be performed for Salmonella, Shigella, Campylobacter, Yersinia, C. difficile, and STEC in patients with bloody diarrhea. 1
   • STEC O157 should be assessed by culture and non-O157 STEC by Shiga toxin detection. 1
   • For patients with persistent symptoms or recent travel, consider parasitic testing. 1
   • Blood counts should be monitored in patients with suspected STEC infection to detect early signs of HUS. 1

Special Considerations

• Children and elderly are at higher risk for complications from infectious bloody diarrhea, particularly STEC infections. 4
• Immunocompromised patients require broader diagnostic evaluation including testing for opportunistic pathogens. 1
• Travelers with bloody diarrhea should be evaluated for destination-specific pathogens, including Entamoeba histolytica if returning from endemic areas. 1
• Antibiotic use should be avoided in suspected STEC infections as it may increase the risk of HUS. 1

Pitfalls to Avoid

• Misdiagnosing infectious colitis as IBD - Up to 38% of patients initially suspected of having IBD actually have infectious colitis. 5
• Failing to consider C. difficile in patients with recent antibiotic exposure, even in community settings. 1
• Premature antibiotic treatment in undiagnosed bloody diarrhea, which can increase risk of HUS in STEC infections. 1
• Inadequate monitoring for complications like HUS in patients with STEC infection. 1
• Overlooking non-infectious causes in patients with persistent symptoms despite negative infectious workup. 3

Bloody diarrhea should always be considered a medical emergency requiring prompt evaluation, especially in vulnerable populations like children, elderly, and immunocompromised individuals. 4