Bloody Diarrhea with Hemolytic Uremic Syndrome: Primary Causes
The primary cause of bloody diarrhea with hemolytic uremic syndrome (HUS) is Shiga toxin-producing Escherichia coli (STEC), particularly E. coli O157:H7, which accounts for the majority of cases in the United States. 1
Primary Pathogen: STEC
E. coli O157:H7 is the most common culprit, causing approximately 73,000 cases annually in the United States. Approximately 8% of patients with O157 STEC infection develop HUS, a life-threatening condition characterized by thrombocytopenia, hemolytic anemia, and renal failure. 1
Non-O157 STEC Serotypes
Six non-O157 serogroups are also significant causes:
- O26, O45, O103, O111, O121, and O145 account for the majority of non-O157 STEC infections 1
- These strains producing Shiga toxin 2 (Stx2) are more virulent and more frequently associated with HUS than those producing only Stx1 or both toxins 1, 2
Transmission Routes
STEC has a remarkably low infectious dose (<100 organisms for O157 and O111), transmitted through: 1
- Contaminated food: undercooked ground beef, unpasteurized juice, raw milk, raw produce (lettuce, spinach, alfalfa sprouts)
- Contaminated water
- Animal contact or their environment
- Person-to-person transmission (especially in childcare settings)
Other Infectious Causes
While STEC dominates, other pathogens can cause bloody diarrhea with HUS:
Shigella dysenteriae type 1
- Produces Shiga toxin similar to STEC
- Should be considered especially with international travel history or contact with travelers 2
Less Common Bacterial Causes
- Salmonella and Campylobacter jejuni have been reported to cause HUS through a different mechanism involving bacterial neuraminidase, which exposes Thomsen-Friedenreich cryptantigen on cell surfaces, triggering an inflammatory response 3
- Streptococcus pneumoniae can cause HUS through a similar neuraminidase-mediated mechanism
Critical Clinical Features
High-Risk Populations
- Children under 5 years have the highest incidence and greatest risk for HUS 1
- Peak transmission occurs during summer months, though cases occur year-round
Diagnostic Red Flags
When evaluating for STEC infection, look for: 2
- Bloody diarrhea (though ~10% of HUS cases lack visible blood)
- Abdominal tenderness
- Absence of fever at initial presentation
- Elevated white blood cell count (>10,000 cells/µL in ~65% of E. coli O157 cases)
- Decreasing platelet count trend during days 1-14 of illness indicates higher HUS risk
Diagnostic Approach
When STEC is suspected, laboratories must test for both Shiga toxin (or genes encoding them) AND culture for E. coli O157:H7. 2 This dual approach is essential because:
- Shiga toxin detection identifies all STEC serotypes
- Culture specifically identifies O157:H7, the most consistently virulent strain
- Distinguishing between Stx1 and Stx2 helps predict severity (Stx2 is more potent)
Important Caveat
Approximately 10% of laboratory-confirmed STEC patients who develop HUS do not have bloody diarrhea, so maintain high suspicion even without visible blood. 2
Pathophysiology
The Shiga toxins bind to endothelial cells, particularly in the kidneys, causing direct endothelial injury and platelet activation. This leads to the characteristic thrombotic microangiopathy with microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. 4