How should an adult patient with diarrhea and a positive Blastocystis hominis test be treated?

Treatment of Diarrhea with Positive Blastocystis hominis

Most patients with diarrhea and positive B. hominis should receive supportive care with oral rehydration therapy rather than antimicrobial treatment, as the organism's pathogenicity remains controversial and many patients improve without specific therapy. 1

Initial Assessment and Supportive Care

The cornerstone of management is rehydration, regardless of whether you ultimately decide to treat the organism:

• Administer reduced osmolarity oral rehydration solution (ORS) as first-line therapy for mild to moderate dehydration 1, 2
• Reserve IV fluids (lactated Ringer's or normal saline) only for severe dehydration, shock, altered mental status, or failure of ORS therapy 1, 2
• Resume age-appropriate diet immediately during or after rehydration—do not withhold food 1, 2
• Replace ongoing stool losses with ORS until diarrhea resolves 1, 2

When to Consider Antimicrobial Treatment

The 2017 IDSA guidelines do not specifically address B. hominis, but provide clear guidance that empiric antimicrobial therapy is NOT recommended for most patients with acute watery diarrhea without recent international travel 1, 2. However, treatment may be warranted in specific circumstances:

Consider treatment if:

• Symptoms persist despite supportive care and no other pathogen is identified 3, 4
• Patient is immunocompromised 4, 5
• Heavy parasite burden is documented on stool examination 6
• Symptoms are severe (persistent diarrhea, significant abdominal pain, nausea, vomiting) 7, 6

Search for co-pathogens first: The presence of B. hominis should prompt you to look for other unrecognized pathogens, as co-infection is common 3. Many patients attributed to B. hominis may actually have another unidentified cause of diarrhea 7.

Antimicrobial Options When Treatment Is Indicated

If you decide treatment is necessary after ruling out other pathogens:

First-line option: Metronidazole 4, 6

• Dose: 1-2 grams/day orally in divided doses for 7-10 days 6
• Most commonly used and appears most effective based on clinical responses 4
• Note: Variable cure rates exist, and some B. hominis subtypes show resistance 4

Alternative options:

• Nitazoxanide: 500 mg twice daily for 3 days in adults (≥12 years); 200 mg twice daily for 3 days in children 4-11 years; 100 mg twice daily for 3 days in children 1-3 years 7
  • Demonstrated 86% symptom resolution vs 38% with placebo in one high-quality study 7
  • Also achieved 86% parasite eradication vs 12% with placebo 7
• Trimethoprim-sulfamethoxazole: Alternative when metronidazole fails or is contraindicated 3, 4, 5

Adjunctive Therapy

• Loperamide may be given to immunocompetent adults ONLY after adequate hydration 1, 2
• Never give loperamide to children <18 years with acute diarrhea 1, 2
• Avoid antimotility agents if fever or bloody diarrhea present (risk of toxic megacolon) 1, 2
• Probiotics may be offered to reduce symptom severity and duration 1, 2

Common Pitfalls to Avoid

• Do not automatically treat every positive B. hominis test—many patients are asymptomatic carriers or have self-limiting infection 4, 6
• Do not withhold fluids or food—this worsens outcomes 1, 2
• Do not use empiric antimicrobials without first ensuring adequate hydration and ruling out other pathogens 1, 3
• Do not assume treatment failure means the organism is resistant—consider reinfection or an unidentified co-pathogen 4

Clinical Algorithm Summary

1. Assess hydration status and initiate ORS 1, 2
2. Resume normal diet immediately 1, 2
3. Search for other pathogens (repeat stool studies if needed) 3
4. If no other pathogen found and symptoms persist beyond 7-10 days with supportive care alone, consider antimicrobial therapy 4, 7
5. Choose metronidazole or nitazoxanide based on availability and patient factors 4, 7, 6
6. Modify or discontinue antimicrobials if another clinically plausible organism is identified 1