Post-encystment/established immunity in cystic echinococcosis: is it really that simple?

In 1982, Rickard & Williams stated that immunity to larval taeniid cestodes could be divided into two different phases: the establishment phase, operating against oncospheres, and the established phase, operating against established metacestodes in the tissues. These concepts were largely based on experimental and veterinary data. As a result of subsequent research combined with clinical data, we can consider that, for metacestodes of Echinococcus spp, a single established phase is probably too simple a concept. These parasites show developmental changes in different hosts over time, the most significant being growth, fertility and degeneration. Recent clinical cyst classifications (WHO/IWGE) have highlighted that all hydatid cysts are morphologically not the same. This is important for treatment but fails to completely address variation in the host/parasite response and the understanding of parasite survival. This review attempts to relate the immunological and developmental data obtained from experimental, veterinary and medical studies to update our understanding of established immunity against E. granulosus. It proposes that a single 'Established' phase for E. granulosus could be subdivided into four additional phases: maturing; stable; unstable and degenerative. Combining this type of information with existing cyst classification systems could significantly benefit our understanding of the parasites immunobiology.