Blastocystis hominis, although previously regarded as a yeast, has the
morphological and biological characteristics of a protozoon. Very little is known of
the basic biology of this organism. Several morphological forms have been
recognised: ameboid, vacuolar, avacuolar, multivacuolar, granular, cyst). The life
cycle is not well known. Which of the forms is responsable for transmission is not
known. Molecular typing has revealed extensive genetic diversity in morphological
identical strains. Until about 1930 it was regarded as a cause of diarrhoea, but
thereafter it was generally considered to be an apathogenic commensal. The
parasite colonises chiefly the caecum and to a lesser extent the distal colon. Since
1975, Blastocystis hominis has once more been regarded as responsible for long-
term but less specific cases of diarrhoea. The pathogenicity appears to depend on
the parasitic load (more than 5 Blastocystis per 40x field). The pathogenicity of
Blastocystis nevertheless remains controversial. For some clinicians it is only an
indication of the presence of one or other parasite or microorganism which is
responsible for the symptoms of disease. If considered necessary, imidazoles such
as metronidazol or otherwise clioquinol are used for treatment