Blastocystis species


  • Anaerobic protozoan parasites
  • Found in the human gastrointestinal tract [1]
  • Most common eukaryotic parasites found in human stool
  • Controversial if they are a commensal organism or a pathogen


  • Have been found worldwide
  • Reside in the cecum and colon of both children in adults
  • Have also been found in various animals (including pigs, monkeys, rodents, and poultry)
  • Prevalence is geographically variable, but generally higher in developing countries


  • Mode of transmission is not fully understood, but though to be fecal-oral [2]


  • Vary in size between 5 and 40 micrometers
  • Lack a cell wall
  • Have mitochondria, golgi apparatus, smooth and rough endoplasmic reticula
  • Typically reproduce by binary fission
  • Grow in anaerobic culture conditions

Clinical Features

  • Symptoms that have been associated with individuals with Blastocystis species in their stool include:
  • Generally patients are afebrile

Differential Diagnosis

Acute diarrhea



Watery Diarrhea

Traveler's Diarrhea


  • Stool examination via light microscopy of stained smears or wet mounts
  • Stool culture [4]
  • ELISA targeted at serum antibodies to Blastocystis species [5]
  • PCR [6]
  • Fecal leukocytes are usually absent



  • Do not require therapy


  • Alternative diagnoses should be considered and excluded (e.g. alternative pathogen, noninfectious cause of symptoms)
  • Initiation of treatment in symptomatic patients is controversial
  • Blastocystis species infection is often self-limiting and many mild cases will resolve quickly
  • Treatment options:
    • Metronidazole (750 mg TID for 5-10 days) [7]
    • Tinidazole (2 mg once)
    • Alternatives include: paromomycin, nitazoxanide, and trimethoprim-sulfamethoxazole


  • Most patients can be managed as an outpatient
  • Admission or observation should be considered for patients who are severely dehydrated or have significant electrolyte derangements

See Also

External Links


  1. Tan KS. New insights on classification, identification, and clinical relevance of Blastocystis spp. Clin Microbiol Rev. 2008;21(4):639-665. doi:10.1128/CMR.00022-08
  2. Blastocystis hominis: commensal or pathogen? Lancet. 1991 Mar 2;337(8740):521-2. PMID: 1671894.
  3. Marx et al. “Cholera and Gastroenteritis caused by Noncholera Vibrio Species”. Rosen’s Emergency Medicine 8th edition vol 1 pg 1245-1246.
  4. Suresh K, Smith H. Comparison of methods for detecting Blastocystis hominis. Eur J Clin Microbiol Infect Dis. 2004;23(6):509-511. doi:10.1007/s10096-004-1123-7
  5. Zierdt CH, Zierdt WS, Nagy B. Enzyme-linked immunosorbent assay for detection of serum antibody to Blastocystis hominis in symptomatic infections. J Parasitol. 1995;81(1):127-129.
  6. Jones MS 2nd, Ganac RD, Hiser G, Hudson NR, Le A, Whipps CM. Detection of Blastocystis from stool samples using real-time PCR. Parasitol Res. 2008;103(3):551-557. doi:10.1007/s00436-008-1006-4
  7. Nigro L, Larocca L, Massarelli L, et al. A placebo-controlled treatment trial of Blastocystis hominis infection with metronidazole. J Travel Med. 2003;10(2):128-130. doi:10.2310/7060.2003.31714