Objective:Blastocystis sp. is the most common enteric protozoan in human, but its pathogenesis is still unclear. To compare the infectious effects of various Blastocystis sp. on tissue damage in immunocompetent and immunocompromised status.Methods:We set up immunocompromised model by adding dexamethasone (DEX) to drinking water in half of experimental Sprague Dawley (SD) rats, and then orally challenged rats with different isolates of Blastocystis subtype 1 (ST1), ST3 and ST7, and then examined the histological changes. Results:We found that the lamina propria was invaded by Blastocystis sp in the vacuolar form along with the mucus membrane slough and inflammatory cell infiltration, and the infections of strains isolated from symptomatic patients resulted in more severe histopathology changes than those from asymptomatic carriers in the same immune function status. In immunocompetent rats, ST1 and ST3 infection induced more severe histological changes than ST7 infection. The histopathological changes of immunocompromised rats were more severe than those of immunocompetent rats. Conclusion:the pathogenicity and augmentation of Blastocystis is enhanced when the strains were isolated from symptomatic patients and the host immunity was suppressed. ST1 and ST3 stains isolated from symptomatic patients have higher levels of virulence than other strains. The findings indicate the pathogenesis of Blastocystis sp depends on virulence of the strains and the immune status of the host. Objective: Blastocystis sp. Is the most common enteric protozoan in human, but its pathogenesis is still unclear. To compare the infectious effects of various Blastocystis sp. On tissue damage in immunocompetent and immunocompromised status. Methods: We set up immunocompromised model by adding dexamethasone (DEX) to drinking water in half of experimental Sprague Dawley (SD) rats, and then orally challenged rats with different isolates of Blastocystis subtype 1 (ST1), ST3 and ST7, and then examined the histological changes. : We found that the lamina propria was invaded by Blastocystis sp in the vacuolar form along with the mucus membrane slough and inflammatory cell infiltration, and the infections of strains isolated from symptomatic patients resulted in more severe histopathology changes than from asymptomatic carriers in the same immune function status. In immunocompetent rats, ST1 and ST3 infection induced more severe histological changes than ST7 infe Conclusion: the pathogenicity and augmentation of Blastocystis is enhanced when the strains were isolated from symptomatic patients and the host immunity was suppressed. ST1 and ST3 stains isolated from symptomatic patients have higher levels of virulence than other strains. The findings indicate the pathogenesis of Blastocystis sp depends on virulence of the strains and the immune status of the host.