In an otherwise healthy adult with acute gastroenteritis due to Entamoeba histolytica presenting with blood‑streaked loose stools, is there a single‑dose metronidazole treatment protocol?

Single-Dose Metronidazole for Amoebiasis: Not Recommended

There is no validated single-dose metronidazole protocol for intestinal amoebiasis with blood-streaked stools; the standard regimen is metronidazole 750 mg three times daily for 5–10 days, and single-dose therapy has proven ineffective. 1

Why Single-Dose Fails

• Single-dose metronidazole (even at 2 g) achieves only 53% cure rates for symptomatic intestinal amoebiasis, compared to 90% with tinidazole, because metronidazole's rapid absorption and short half-life cannot sustain adequate luminal concentrations. 2, 3

• Asymptomatic carriers treated with short courses show 37% recurrence of E. histolytica cysts within 2 weeks, demonstrating that brief exposure fails to eradicate the parasite from the intestinal lumen. 4

• Blood-streaked stools indicate invasive disease requiring tissue-active therapy for 5–10 days, not the brief exposure provided by single-dose regimens. 1

Recommended Treatment Protocol

Standard Regimen (First-Line)

• Metronidazole 750 mg orally three times daily for 5–10 days is the guideline-endorsed dose for adults with symptomatic intestinal amoebiasis. 1

• For children, use 30 mg/kg/day divided into three doses for 5–10 days. 1

When to Suspect Amoebiasis

• Do not empirically treat for amoebiasis unless microscopy demonstrates E. histolytica trophozoites in fresh stool, or two different antibiotics for shigellosis have failed. 1

• Blood-streaked diarrhea in acute gastroenteritis is far more commonly bacterial (especially Shigella, Campylobacter, or enteroinvasive E. coli) than amoebic in most settings. 1

• Fluoroquinolones (e.g., ciprofloxacin 500 mg twice daily for 3 days) are first-line empiric therapy for dysentery when the pathogen is unknown, because bacterial causes predominate. 1

Alternative: Tinidazole (If Available)

• Tinidazole 2 g orally once daily for 3 days achieves 90–95% cure rates and is better tolerated than metronidazole, but still requires multi-day dosing. 2, 5, 6

• A true single-dose tinidazole regimen (2 g × 1) cures only 62% of cases, making it inadequate for symptomatic disease. 4

Critical Pitfalls

• Never use metronidazole empirically for blood-streaked diarrhea without confirming amoebiasis by microscopy, as you will miss bacterial pathogens that require different antibiotics and may worsen outcomes. 1

• Avoid repeated or prolonged metronidazole courses beyond 14 days due to cumulative, potentially irreversible peripheral neuropathy and cerebellar toxicity. 7

• Single-dose or 3-day metronidazole regimens fail because the drug does not achieve sustained luminal concentrations needed to kill trophozoites in the colonic mucosa and lumen. 2, 4, 3

Practical Algorithm

1. Obtain fresh stool microscopy to identify E. histolytica trophozoites (not cysts alone, which may represent non-pathogenic E. dispar). 1

2. If microscopy is unavailable or negative and the patient has not responded to two courses of antibiotics for shigellosis, then treat empirically for amoebiasis with metronidazole 750 mg three times daily for 5–10 days. 1

3. If microscopy confirms E. histolytica, give metronidazole 750 mg three times daily for 5–10 days (or tinidazole 2 g daily for 3 days if available). 1, 2

4. Follow with a luminal agent (e.g., paromomycin 25–35 mg/kg/day in three divided doses for 7 days) to eradicate cysts and prevent relapse, though this step is often omitted in resource-limited settings. 1