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In addition to conventional AE found in human patients, so-called abortive (resistant, R) cases, where the parasite metacestode has spontaneously died out, are more and more frequently found. R1: by imaging techniques (e.g. CT), an abortive metacestode lesion (arrow) appears as a small, fully calcified structure. A surgically resected liver lobe containing such a lesion (arrows) is shown in R2 and following opening in R3. Histologically, an abortive lesion appears as an acellular structure, centrally composed of a collagenous and fibrous mass (cfm) void of any metacestode structures such as the germinal or laminated layers (R4, HE-stain; ×400); this mass is surrounded by a layer of fibrous connective tissue that still contains some laminated layer fibers and some parasite DNA (PCR-positivity of the material), but no remaining live parasite cells; there is still some inflammatory reaction around this died-out lesion, putatively responsible for the maintenance of a positive humoral immune response such as anti-Em2-seropositivity. In contrast, the classical viable AE metacestode lesion (V1, HE-stain; ×200) is composed of fluid-filled microvesicles that may rarely contain protoscolices (p); the actual living metacestode structure is the very thin germinal layer (arrow), which is closely adjacent to the PAS-positive outer laminated layer (V2, PAS-stain; ×200), indicated by the arrow. Before dying-out, the metacestode undergoes a transitional stage where it becomes more tightly encapsulated within a fibrous-collagenous mass, and where the germinal layers start to disappear, while the PAS-positive laminated layer still remains prominently present as vesiculated structures (T1, PAS-stain, ×100).
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