Echinococcus metacestode: in search of viability markers

Abstract

Epidemiological studies have demonstrated that most humans infected with Echinococcus spp. exhibit resistance to disease. When infection leads to disease, the parasite is partially controlled by host immunity: in case of immunocompetence, the normal alveolar echinococcosis (AE) or cystic echinococcosis (CE) situation, the metacestode grows slowly, and first clinical signs appear years after infection; in case of impaired immunity (AIDS; other immunodeficiencies), uncontrolled proliferation of the metacestode leads to rapidly progressing disease. Assessing Echinococcus multilocularis viability in vivo following therapeutic interventions in AE patients may be of tremendous benefit when compared with the invasive procedures used to perform biopsies. Current options are F18-fluorodeoxyglucose-positron emission tomography (FDG-PET), which visualizes periparasitic inflammation due to the metabolic activity of the metacestode, and measurement of antibodies against recEm18, a viability-associated protein, that rapidly regresses upon metacestode inactivation. For Echinococcus granulosus, similar prognosis-associated follow-up parameters are still lacking but a few candidates may be listed. Other possible markers include functional and diffusion-weighted Magnetic Resonance Imaging (MRI), and measurement of products from the parasite (circulating antigens or DNA), and from the host (inflammation markers, cytokines, or chemokines). Even though some of them have been promising in pilot studies, none has been properly validated in an appropriate number of patients until now to be recommended for further use in clinical settings. There is therefore still a need to develop reliable tools for improved viability assessment to provide the sufficient information needed to reliably withdraw anti-parasite benzimidazole chemotherapy, and a basis for the development of new alternative therapeutic tools.

Figure 1.

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).