- Karin Leder, MBBS, FRACP, PhD, MPH, DTMH
Karin Leder, MBBS, FRACP, PhD, MPH, DTMH
- Section Editor — Travel Medicine
- Head of Infectious Diseases Unit
- Monash University, Australia
- Peter F Weller, MD, MACP
Peter F Weller, MD, MACP
- Editor-in-Chief — Infectious Diseases
- Section Editor — Tropical Medicine
- William Bosworth Castle Professor of Medicine
- Harvard Medical School
- Professor of Immunology and Infectious Diseases
- Harvard T. H. Chan School of Public Health
Blastocystis species (previously referred to as Blastocystis hominis) are anaerobic protozoan parasites found in the human gastrointestinal tract . 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 among the most common organisms to be detected in 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 . 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 . 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 . 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 . 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 . In one study from Senegal, 100 percent of 93 fecal samples were positive for Blastocystis spp . 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 .
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 . (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 . 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.To continue reading this article, you must log in with your personal, hospital, or group practice subscription. For more information on subscription options, click below on the option that best describes you:
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