Abstract: Objective To observe the ultrastructural change of intestinal mucosa in mice infected with Blastocystis hominis, and to study the pathogenic mechanism of B.hominis infection. Methods 20 Kunming mice were randomly divided into 4 groups： group A treated with immunosuppressant （dexamethasone）, group B without immunosuppressant, group C as normal control and group D as immunosuppressant control. Groups A and B were then orally infected with 20⁴ cysts of B. hominis. Groups C and D were treated as control by infusing same volume of Locke′s solution. Six days after inoculation, mice in each group were killed and mucosa of ileocecum was observed by transmission electron microscope （TEM） and scanning electron microscope （SEM）. Results Under SEM, B. hominis located in enteric cavity and on the surface of ileocecum mucosa. Individual parasites also invaded into mucosa and its fold. Partial destruction of microvilli on the mucosa was observed. TEM observation indicated a reduction of microvilli on the surface of absorptive cells. Mitochondrial edema, rough endoplasmic reticulum dilatation and degranulation were found on absorptive cells and goblet cells. Lymphocyte infiltration and eosinophilia were found in intercellular stroma. Pathological changes in group A were more serious than that of group B. No abnormal change on the mucosal ultrastructure was found in groups C and D. Conclusions B. hominis infection causes significant ultrastructural lesion on the ileocecal mucosa in mice. Immune status of the mice can affect the degree of the lesion due to infection.

Key words: Blastocystis hominis, Mice, Intestinal mucosa, Transmission electron microscope (TEM), Scanning electron microscope (SEM), Ultrastructure