Can parasites and bacteria cause occult blood in stool?

Parasites and Bacteria as Causes of Occult Blood in Stool

Yes, both parasites and bacteria can cause occult blood in stool, with bacterial pathogens being more frequently associated with positive fecal occult blood tests than parasitic infections.

Bacterial Causes of Occult Blood

Bacterial infections are significant causes of occult blood in stool, particularly those that cause invasive disease:

• Campylobacter jejuni: Causes occult blood in 52% of infected patients 1
• Salmonella: Associated with occult blood in 43% of cases 1
• Shigella: Results in occult blood in 59% of cases 1
• Shiga toxin-producing E. coli (STEC): Has the highest association with occult blood at 83% of cases 1

These bacteria typically cause occult blood through direct invasion of the intestinal mucosa, resulting in inflammation and microscopic bleeding that may not be visible to the naked eye.

Parasitic Causes of Occult Blood

Parasitic infections have a more variable relationship with occult blood:

• Hookworm infections: Can cause occult bleeding, particularly with moderate to heavy infections. Studies show positive occult blood tests in 57% of moderate infections and 67% of heavy infections 2
• Roundworm (Ascaris): May cause acute gastrointestinal bleeding, though this is less common 3
• Trichuris (whipworm): Limited evidence for causing occult blood in the absence of dysentery syndrome 4

A key study from Saudi Arabia found no significant association between general parasitic infections and positive guaiac fecal occult blood tests 5, suggesting that many parasitic infections may not routinely cause occult bleeding.

Diagnostic Considerations

When evaluating occult blood in stool:

1. Consider bacterial causes first in patients with diarrheal illness, especially if accompanied by fever

2. Test specifically for bacterial pathogens using:

   • PCR-based multiplex panels (preferred) 6
   • Stool cultures for Campylobacter, Salmonella, and Shigella 6

3. For parasitic evaluation:

   • Stool examination for ova and parasites is recommended when clinically indicated 1
   • Consider enteroscopy in cases of obscure GI bleeding with negative upper endoscopy, as some parasites (especially hookworms) may be found in the jejunum 3

Clinical Implications

• Occult blood should not be automatically attributed to hemorrhoids until the colon is adequately evaluated 1
• In patients with diarrhea and occult blood, bacterial pathogens should be high on the differential diagnosis 1
• Heavy hookworm infections should be considered in endemic areas when anemia and occult blood are present 2
• The presence of occult blood may help distinguish invasive from non-invasive diarrheal illnesses 1

Management Approach

1. For bacterial causes:

   • Azithromycin is first-line for many enteric bacterial infections 6
   • Avoid antimotility agents in patients with high fever or bloody diarrhea 6

2. For parasitic causes:

   • Mebendazole (100 mg twice daily for 3 days) for hookworm and roundworm infections 3
   • Iron supplementation for associated anemia 3

Remember that a positive fecal occult blood test should prompt appropriate investigation for both infectious and non-infectious causes, as the test alone cannot distinguish between them.