A 68 year old female presented to our institution with complaints of lower right quadrant abdominal pain but an absence of nausea, vomiting, diarrhea and fever. The patient noted she thought she had a worm in her stool and submitted three stool samples for an ova and parasite exam. While the three stool samples were negative for worms and eggs, two of the three samples were positive for the organism seen below. The patient was treated with a regimen of metronidazole for ten days, and a repeat ova and parasite exam after treatment was negative for the parasite. The patient presented again three months later with similar complaints of abdominal pain, resulting in another ova and parasite exam which was again positive for the same organism.
Originally discovered in 1911, Blastocystis species are anaerobic protozoan parasites commonly found in the human gastrointestinal tract.2 In addition to being found in humans, Blastocystis spp. has also been found in a variety of animals, with at least nine different genotypes described in literature.4 While historically these organism have been referred to as Blastocystis hominis when found in humans, genotyping and molecular studies have revealed immense genetic diversity in organisms isolated from humans, resulting in a more general designation of Blastocystis spp.1 These organisms have been observed worldwide, stemming from contaminated food, water and soil and transmitted in a fecal-oral route.2 Recently, there has been much debate as to whether the parasite is truly pathogenic or commensal due to the presentation of both symptomatic and asymptomatic patients found carrying this parasite.4
Despite debate on the pathogenicity of Blastocystis spp., potential signs and symptoms from symptomatic patients have been documented. Such symptoms could include watery diarrhea, nausea, abdominal pain, bloating, excessive gas, loss of appetite, weight loss, and fatigue, among other symptoms.3 Diagnosis heavily relies on stool specimens using light microscopy to examine direct smears, with the vacuolar form of the parasite most commonly observed.1 In addition to the vacuolar form, granular, amoeboid and cyst forms have also been described in literature.4 The parasite itself lacks a cell wall, but contains mitochondria, Golgi apparatuses, and both smooth and rough ER.2 The vacuolar form, observed in Figure 1 and Figure 2, contains a large (5-40 mm) central vacuole-like body which compresses the nuclei to periphery of the cell.2
Treatment is typically not needed if the patient remains asymptomatic.3 However, for individuals who present with symptomatic infection, a regimen of metronidazole or tinidazole is recommended.3 It is important to note that due to the controversial nature of Blastocystis spp. as a pathogenic parasite, additional pathogens or causes of disease should first be ruled out prior to treatment.2
References:
1 CDC. (2019). Blastocystis sp. [online]. Accessed on October 24th, 2023 from https://www.cdc.gov/dpdx/blastocystis/index.html#:~:text=Blastocystis%20is%20a%20genetically%20diverse,been%20classified%20as%20a%20stramenopile.
2 Leder, K. (2023). Blastocystis species [website]. Accessed online on October 26th, 2023 from https://www.uptodate.com/contents/blastocystis-species
3 Mayo Clinic. (n.d.) Blastocystis Hominis [website]. Accessed online on October 24th 2023 from https://www.mayoclinic.org/diseases-conditions/blastocystis-hominis-infection/symptoms-causes/syc-20351205
4 Wawrzyniak, I., Poirier, P., Viscogliosi, E., Dionigia, M., Texier, C., Delbac, F., & Alaoui, H. E. (2013). Blastocystis, an unrecognized parasite: an overview of pathogenesis and diagnosis. Therapeutic advances in infectious disease, 1(5), 167–178. https://doi.org/10.1177/2049936113504754
-Maggie King is a Masters Student in the Department of Pathology and Laboratory Medicine at The University of Vermont Larner College of Medicine.
-Christi Wojewoda, MD, is the Director of Clinical Microbiology at the University of Vermont Medical Center and an Associate Professor at the University of Vermont.
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