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Blastocystis species

Karin Leder, MBBS, FRACP, PhD, MPH, DTMH
Peter F Weller, MD, MACP
Section Editor
Edward T Ryan, MD, DTMH
Deputy Editor
Elinor L Baron, MD, DTMH


Blastocystis species (previously referred to as Blastocystis hominis) are anaerobic protozoan parasites found in the human gastrointestinal tract [1]. The organism was initially discovered in 1911 and for many years was considered to be a harmless yeast. Studies in the 1970s demonstrated that Blastocystis spp are protozoans [2,3]. Blastocystis spp are the most common eukaryotic parasites found in human stool specimens, but there is considerable controversy regarding whether they represent a commensal organism or a true pathogen. (See "Nonpathogenic enteric protozoa".)


Blastocystis spp have been observed worldwide [4]. The organism resides in the colon and cecum of children and adults. The mode of transmission is not fully understood; fecal-oral transmission has been postulated [5]. Some authors have suggested that contaminated water may also be a source of infection [6-9]. Blastocystis spp have also been found in animals including pigs, monkeys, rodents, and poultry. There seems to be poor host specificity; transmission occurs from human to human and between humans and animals [10]. Blastocystis infections are more common among individuals with occupational exposure to animals, supporting the potential for zoonotic transmission.

The prevalence of Blastocystis spp varies between countries and between communities [1]. In general, the estimated prevalence of Blastocystis spp is higher in developing countries than developed countries (30 to 50 percent versus 5 to 10 percent, respectively). This may be related to poor hygiene, animal exposure, and consumption of contaminated food or water [1]. In one study from Senegal, 100 percent of 93 fecal samples were positive for Blastocystis spp [11]. In a study from Canada, 8 percent of stool samples submitted to a reference laboratory were positive for Blastocystis spp; when Blastocystis was the sole organism identified, 76 percent of subjects continued to harbor the parasite two months after initial detection [12].

Blastocystis spp are also commonly found in the stool of returned travelers from developing countries [6,13-15]. In one study of nearly 2000 stool specimens from travelers and expatriates in Nepal, the prevalence of Blastocystis spp was 30 percent [16]. (See "Travelers' diarrhea: Microbiology, epidemiology, and prevention" and "Evaluation of fever in the returning traveler".)


Blastocystis spp demonstrate marked morphologic variability and measure between 5 and 40 mcm (picture 1). The organism lacks a cell wall but contains mitochondria, Golgi apparatus, and smooth and rough endoplasmic reticula typical of protozoa [17]. It reproduces asexually, usually by binary fission. It grows only under anaerobic conditions in culture, and it is highly susceptible to changes in temperature and in environmental tonicity.

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Literature review current through: Nov 2017. | This topic last updated: Dec 05, 2017.
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