blastocystis blastocystis blastocystis is a genus of single-celled heterokont parasites belonging to the stramenopiles that includes algae, diatoms, and water molds. blastocystis consists of several species, living in the gastrointestinal 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. for the disease, see blastocystosis. for the structure formed in the early embryogenesis of mammals, see blastocyst. blastocystis blastocystis sp. scientific classification domain: eukaryota clade: diaphoretickes clade: sar clade: stramenopiles phylum: bigyra subphylum: opalozoa infraphylum: placidozoa superclass: opalinata class: blastocysteazierdt et al. 1967 order: blastocystidazierdt 1978 family: blastocystidaejiang & he 1988 genus: blastocystis(alexieff 1911) brumpt 1912 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 recent 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. classification the appropriate classification of blastocystis has only recently been resolved. the original description of blastocystis was as a yeast due to its yeast-like glistening appearance in fresh wet mounts and the absence of pseudopodia and locomotion. this was then contradicted by zierdt, who reclassified it under subphylum sporozoa (and later in sarcodina), based on some distinctive protistan features of the blastocystis cell. its sensitivity to antiprotozoal drugs and its inability to grow on fungal media further indicated that it was a protozoan. however, major revisions were made to its classification. an analysis of gene sequences was performed in 1996, which placed it into the group stramenopiles. other stramenopiles include brown algae, mildew, diatoms, the organism that caused the great famine of ireland, and the organism responsible for sudden oak death disease. however, the position of blastocystis within the stramenopiles remains enigmatic. signs and symptoms