What is the recommended treatment for a patient with Blastocystis hominis infection?

Treatment of Blastocystis hominis Infection

For symptomatic patients with Blastocystis hominis infection after excluding other pathogens, metronidazole 500-750 mg three times daily for 10 days remains the most commonly used treatment, though Saccharomyces boulardii 250 mg twice daily for 10 days may provide superior sustained clinical response with comparable parasitological cure rates. 1, 2

When to Treat

• Treatment should be reserved for patients with persistent gastrointestinal symptoms (>14 days) only after a complete workup excludes other etiologies and no other pathogens are identified. 1
• The presence of Blastocystis in stool should prompt clinicians to search aggressively for other unrecognized co-pathogens, as 25% of Blastocystis-positive patients harbor other protozoa undetected by routine microscopy. 1, 3
• Immunocompromised patients (including those with hematological malignancies undergoing chemotherapy) warrant treatment more readily, as Blastocystis is significantly more common in this population (13% vs 1% in controls). 4

First-Line Treatment Options

Option 1: Metronidazole (Traditional First-Line)

• Dosing: 500-750 mg three times daily for 10 days 1, 5, 4
• Expected outcomes: Eradication rates are highly variable (33-80%), with clinical cure rate of 67% at day 15 1
• Major caveat: Treatment failure is common, with some patients experiencing up to 5-fold increased parasite load and worsening symptoms despite standard therapy 1
• In one study of severely infected individuals, only 4 of 12 patients (33%) achieved eradication with metronidazole 5

Option 2: Saccharomyces boulardii (Emerging Preferred Option)

• Dosing: 250 mg twice daily for 10 days 1, 2
• Expected outcomes: Clinical cure rate of 78% at day 15 and 94% at day 30; parasitological cure of 72% at day 15 and 94% at day 30 1, 2
• Comparative advantage: In a randomized trial, S. boulardii achieved 77.7% clinical cure at day 15 versus 66.6% with metronidazole and 40% with no treatment 2
• At day 30, S. boulardii demonstrated 94.4% clinical cure versus 73.3% with metronidazole, suggesting superior sustained response 2

Treatment Algorithm by Clinical Scenario

Immunocompetent Adults with Mild-Moderate Symptoms

• Consider trial of Saccharomyces boulardii 250 mg twice daily for 10 days as first-line therapy 1, 2
• If treatment fails or symptoms are severe, escalate to metronidazole 500-750 mg three times daily for 10 days 1

Immunocompromised Patients or Severe Symptoms

• Use metronidazole 500-750 mg three times daily for 10 days with close monitoring 1, 4
• All immunocompromised patients with hematological malignancy in one study became symptom-free with this regimen 4

Children with Symptomatic Infection

• Either Saccharomyces boulardii 250 mg twice daily for 10 days OR metronidazole 30 mg/kg twice daily for 10 days 2
• Both showed comparable efficacy at day 30 (94.4% vs 93.3% parasitological cure) 2

Critical Pitfalls to Avoid

Metronidazole Resistance is Real

• Different subsets of Blastocystis exhibit variable resistance to metronidazole, which explains the widely variable clinical responses 6
• Some patients experience paradoxical worsening despite treatment 1

Don't Overlook Coinfections

• Always perform comprehensive stool evaluation for other pathogens before attributing symptoms solely to Blastocystis 1, 3
• The organism's pathogenicity remains controversial, and symptoms may be due to undetected co-pathogens 6, 3

Alternative Agents Have Limited Evidence

• Trimethoprim-sulfamethoxazole (1 tablet three times daily for 10 days) achieved only 22% eradication (2 of 9 patients) in severely infected individuals 5
• Nitazoxanide has been evaluated but lacks robust comparative data 6

Natural History Without Treatment

• In untreated symptomatic children, only 40% achieved clinical cure at day 15 and 26.6% achieved parasitological cure, demonstrating that spontaneous resolution occurs but is less reliable than treatment 2