Blastocystis is a genus of single-celled protozoan parasites belonging to a group of organisms known as the Stramenopiles (also called Heterokonts) that includes algae, diatoms, and water molds. Blastocystis comprises several species, living in thegastrointestinal tracts of species as diverse as humans, farm animals, birds, rodents, reptiles, amphibians, fish, and cockroaches. Blastocystis exhibits low host specificity, and many different species of Blastocystis can infect humans and by current convention, any of these species would be identified as Blastocystis hominis if they were identified in a human. These have a widespread geographic distribution and are found at a rate of 5-10% in most developed countries, and a rate of up to 50% in less developedareas. High rates of infection are also found in individuals in developed countries who work with animals. Although the role ofBlastocystis hominis in human disease is often referred to as controversial, a systematic survey of research studies conducted by 11 infectious disease specialists from nine countries, found that over 95% of papers published in the last 10 years identified it as causing illness in immunocompetent individuals. The paper attributed confusion over pathogenicity to the existence of asymptomatic carriers, a phenomenon the study noted is common to all gastrointestinal protozoa.
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