Experimental echinococcus infection in the mouse model: pericystic cellular immunity reaction and effects on the lymphoid organs of immunocompetent and thymectomized mice.

Echinococcus can infect man as an accidental intermediate host causing hydatid disease. The infection persists and the growth of the cysts advances, while the patient usually remains asymptomatic for years. Experimental Echinococcus infection in mice provides a well described model for the study of the parasite-host relationship that permits the evolution of the disease despite the activation of the host's immune system. The aim of the present study was to assess the immune response to Echinococcus infection in normal and thymectomized mice. For this purpose, a total of 150 mice, divided into three equal groups (A, B and C), were infected by intraperitoneal inoculation of live protoscoleses. The mice of groups B and C underwent thymectomy, two weeks prior and after the infection, respectively. The mice of each group were further divided into three subgroups and were sacrificed at three consecutive time points: 45 days, 3 and 6 months post the infection. The hydatid cysts that subsequently developed by the metacestode-lavral stage, along with the spleen and lymph nodes were excised from each mouse and histologically studied. The results revealed a marked activation of the cell-mediated immunity against the parasite at the early stages of the disease. The initial response of the host abated with time and was minimal six months after the infection suggesting a local immunosuppression state that could account for the advancement of the disease. In addition, the thymectomized mice exhibited a higher susceptibility to the infection, which corresponded to the weak and delayed cellular immunity response observed in these groups. These results suggest that the cell-mediated immunity is crucial for the defense against Echinococcus, especially early in the course of the disease where suppression of larval growth is critical for the final outcome of the infection.