Blastocystis hominis is the most common intestinal protozoan infecting humans in the United States, but it is probably the least understood. The parasite is also the most frequently diagnosed intestinal protozoan in Oregon. Records from the Good Samaritan Regional Medical Center, Corvallis, diagnostic laboratory show 40 cases of Blastocystis in 2006 vs. 10 for Giardia lamblia, a usual cause of gastrointestinal symptoms. Indeed, the prevalence of the infection might be much higher as routine methods used to detect other intestinal protozoans often miss Blastocystis.1 2

Blastocystis has been a medical mystery for years. Many people who carry the organism show no symptoms, while others inexplicably become very ill. Recent research has identified multiple genetically diverse sub-types of Blastocystis which can infect humans,3 and suggested only a specific sub-group is responsible for the disease.4 5 This model would be consistent with research suggesting a phylogenetic link between Blastocystis and Entamoeba.8 If Blastocystis follows Entamoeba's pattern, some subtypes would be harmless while others would cause mild to severe, possibly disabling infection. The development of tests specific to pathogenic Blastocystis would be permit physicians to make more informed decisions concerning treatment.

We have assembled a group of experts in parasitology, pathology, and molecular pathogenesis at Oregon State University that would address some of the important issues. Contact information

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1. Develop a fecal PCR test capable of identifying the presence of the pathogenic variant. In addition, develop a serologic diagnostic (ELISA) to enable doctors to determine if the patient is experiencing an extreme immune response to Blastocystis which is unique to the pathogenic variants.6 7
2. Investigate the mechanisms by which Blastocystis infects the host and causes symptoms.
3. Develop a laboratory test to examine the susceptibility of Blastocystis to antibiotics. Ultimately, the plan is to develop a therapeutic regimen that would reliably treat the infection. Current antibiotic treatments fail in many cases, and physicians have no guidance for second line treatments.

For More Information:

Patients: The OSU project studies human isolates of the parasite with the help of a select group of physicians who work with Blastocystis patients. If your physician would like to participate, please have him or her contact the department at the number below.
Physicians: Physicians who would like to participate in the research of this parasite can contact the department at 541-737-6532.

External Links:

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For more information:

Contact the Department of Biomedical Sciences at 541-737-6532.
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Last updated December 5, 2007


1. Suresh K, Smith H. Comparison of Methods for Detecting Blastocystis minis. European Journal of Clinical Microbiology and Infectious Diseases. 2004 Jun;23(6):509-11. Epub 2004 May 28. {Pubmed 15168139}

2. Leelayoova S, Taamasri P, Rangsin R, Naaglor T, Thathaisong U, Mungthin M. In-vitro Cultivation: A Sensitive Method for Detecting Blastocystis hominis. Annals of Tropical Medicine and Parasitology. 2002 Dec;96(8):803-7. {Pubmed 12625935}

3. Noel C, Dufernez F, Gerbod D, Edgcomb VP, Delgado-Viscogliosi P, Ho LC, Singh M, Wintjens R, Sogin ML, Capron M, Pierce R, Zenner L, Viscogliosi E. Molecular Phylogenies of Blastocystis Isolates from Different Hosts: Implications for Genetic Diversity, Identification of Species, and Zoonosis. Journal of Clinical Microbiology. 2005 Jan;43(1):348-55. {Pubmed ID 15634993}

4. Tan TC, Suresh KG, Thong KL, Smith HV. PCR fingerprinting of Blastocystis isolated from symptomatic and asymptomatic human hosts. Parasitol Res. 2006 Apr 21; {Pubmed ID 16628457}

5. Tan TC, Suresh KG. Predominance of amoeboid forms of Blastocystis hominis in isolates from symptomatic patients. Parasitol Res. 2006 Feb;98(3):189-93. Epub 2005 Dec 2. {Pubmed ID 16323025}

6. Zierdt CH, Nagy B. Antibody response to Blastocystis hominis infections. Ann Intern Med. 1993 Jun 15;118(12):985-6. {Pubmed ID 8489119}

7. Mahmoud MS, Saleh WA. Secretory and humoral antibody responses to Blastocystis hominis in symptomatic and asymptomatic human infections. Journal of the Egyptian Society of Parasitology. 2003 Apr;33(1):13-30. {Pubmed ID: 12739797}

8. Ho LC, Jeyaseelan K, Singh M. Use of the elongation factor-1 alpha gene in a polymerase chain reaction-based restriction-fragment-length polymorphism analysis of genetic heterogeneity among Blastocystis species. Mol Biochem Parasitol. 2001 Feb;112(2):287-91. {Pubmed ID 11223135}