What is the differential diagnosis and treatment approach for a patient presenting with symptoms of diarrhea, to distinguish between amoebic (Entamoeba histolytica) dysentery and bacterial diarrhea?

Distinguishing Amoebic Dysentery from Bacterial Diarrhea

When a patient presents with bloody diarrhea, treat empirically for bacterial dysentery (specifically Shigella) unless microscopy confirms Entamoeba histolytica trophozoites, and only switch to amebicidal therapy if two different antibiotics for shigellosis fail. 1

Critical Clinical Distinctions

Fever Pattern (Most Important Differentiator)

• Amoebic dysentery: Minimal or absent fever (only 8% of patients present with fever), which is the single most critical distinguishing feature 2
• Bacterial dysentery: High fever is expected, particularly with Shigella, Salmonella, and Campylobacter 1
• The absence of fever does NOT rule out serious invasive disease in amebiasis—this is the most dangerous clinical trap 2

Temporal Pattern

• Amoebic dysentery: Persistent or chronic (weeks to months), with gradual onset 2, 3
• Bacterial dysentery: Acute onset (days), with rapid progression 3

Stool Characteristics

• Amoebic dysentery: Bloody mucoid stools with severe abdominal pain but minimal fever; fecal leukocytes present in only 28% of cases 2, 4
• Bacterial dysentery: Bloody stools with high fever, abdominal cramping, and tenesmus; fecal leukocytes typically abundant 1, 3

Diagnostic Algorithm

Step 1: Immediate Microscopy (Within 15-30 Minutes)

• Examine fresh stool for motile E. histolytica trophozoites with ingested red blood cells 5, 2
• Critical pitfall: Distinguish trophozoites from white blood cells—amebic dysentery is frequently misdiagnosed 1
• If microscopy unavailable or trophozoites not definitively seen, proceed to Step 2 1

Step 2: Empiric Treatment for Bacterial Dysentery

• First-line for Shigella (if no local resistance data): 1
  • Adults: TMP-SMX 160/800 mg twice daily for 5 days
  • Children: TMP-SMX 10/50 mg/kg/day in 2 divided doses for 5 days
• Alternative if resistance suspected: 1
  • Nalidixic acid 55 mg/kg/day in 4 divided doses for 5 days
  • Note: Quinolone resistance increasingly reported in Campylobacter from Asia; consider macrolide instead 5

Step 3: Reassess at 48 Hours

• If no clinical improvement after 2 days: Switch to alternative antibiotic for Shigella 1
• If still no improvement after additional 2 days: Refer for stool microscopy and reconsider diagnosis of amebiasis 1, 5
• At this stage, resistant shigellosis remains more likely than amebiasis 1

Step 4: Confirmatory Testing for Amebiasis (If Available)

• Coproantigen ELISA for E. histolytica adhesin (high sensitivity/specificity) 5, 6, 7
• PCR (BD Max Enteric Parasite Panel or nested multiplex PCR) for definitive species identification 6, 7
• Avoid microscopy alone: Studies show 75% of microscopically "suspicious" cases are negative by ELISA/PCR, indicating massive overdiagnosis 6

Treatment of Confirmed Amebiasis

Two-Phase Regimen (Both Phases Mandatory)

Phase 1: Tissue Amebicide 5, 8, 9

• Metronidazole 750 mg PO three times daily for 5-10 days (adults) 5, 8
• Metronidazole 30 mg/kg/day for 5-10 days (children) 5, 8
• Alternative: Tinidazole (same dosing regimen) 5, 9

Phase 2: Luminal Amebicide (Essential to Prevent Relapse) 5

• Paromomycin 30 mg/kg/day PO in 3 divided doses for 10 days, OR
• Diloxanide furoate 500 mg PO three times daily for 10 days
• Failure to give luminal agent results in relapse from intestinal cysts 5

High-Risk Populations Requiring Special Consideration

Immunocompromised Patients

• Broader differential diagnosis required, including Cryptosporidium, Cyclospora, Cystoisospora, microsporidia 1
• Consider empiric antibiotics for severe illness with bloody diarrhea 1

Men Who Have Sex With Men (MSM)

• Higher risk for both amebiasis and bacterial dysentery 10
• All 13 HIV-positive patients in one European cohort were MSM 10

Inflammatory Bowel Disease (IBD) Patients

• Critical pitfall: Active search for stool parasites mandatory before diagnosing IBD flare to avoid misdiagnosis 10
• 10% of amoebic colitis cases in one series had underlying IBD 10

Travelers and Immigrants

• Indian subcontinent, South/Central America, and sub-Saharan Africa are highest-risk regions 10
• Consider amebiasis in travelers with diarrhea lasting ≥14 days 1

Contraindications and Critical Warnings

NEVER Treat STEC with Antibiotics

• Avoid all antibiotics (fluoroquinolones, β-lactams, TMP-SMX, metronidazole) for STEC O157 and Shiga toxin 2-producing STEC due to increased risk of hemolytic uremic syndrome 1
• STEC typically presents with severe abdominal pain, grossly bloody stools, and minimal or no fever—mimicking amebiasis 1

Asymptomatic Contacts

• Do not offer empiric treatment to asymptomatic contacts of patients with bloody diarrhea 1

Key Epidemiological Reality

Amebiasis is vastly overdiagnosed: In one pediatric study of children with "amebic dysentery," only 28% actually had E. histolytica, while 50% had invasive bacteria (primarily Shigella) 4. Another study found that 75% of microscopically "suspicious" samples were negative by ELISA/PCR 6. This overdiagnosis leads to inappropriate treatment and missed bacterial infections.