Blastocystis

Blastocystis is a genus of single-celled parasites belonging to the Stramenopiles that includes algae, diatoms, and water molds. There are several species, living in the gastrointestinal tracts of species as diverse as humans, farm animals, birds, rodents, reptiles, amphibians, fish, and cockroaches. Blastocystis has 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.

Blastocystis is one of the most common human parasites in the world and has a global distribution. It is the most common parasitic infection in the United States, where it infected approximately 23% of the total population during year 2000. In less developed areas, infection rates as high as 100% have been observed. High rates of infection are found in individuals in developed countries who work with animals. Although the role of Blastocystis 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 10 years prior 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. However, Blastocystis has never fulfilled Koch's postulate that infection of a healthy individual with Blastocystis leads to disease. The fact that Blastocystis' infection route is oral-anal indicates that carriers have been in contact with faecal contaminated matter which might have included other intestinal pathogens that explain the observed symptoms. A more likely explanation is the presence of virulent and non-virulent strains since there exists an enormous genetic variation between different strains (or genotypes). See the genotype paper by Rune Stensvold and the 2017 Blastocystis genome paper expanding on this diversity. An alternative theory that Blastocystis is not a pathogen at all has recently been strengthened based on its biochemistry.