Macroscopic and microscopic aspects of Echinococcus multilocularis and Echinococcus granulosus in humans – a guideline for pathologists

Background: Echinococcosis is a parasitic disease, caused by the larval stage of the genus Echinococcus. Echincoccus multilocularis and Echinococcus granulosus cause alveolar (AE) and cystic echinococcosis (CE) in humans. Pathologists may be confronted with the larval stage of both species. Diagnosis may be difficult due to the low incidence in daily routine. However, right diagnosis of the pathologist is still the gold standard and crucial for further treatment of the patient. Summary: Macroscopically, specimen of AE show multiple small cysts and a bread-like appearance. Instead, CE manifests as solitary or multiple grape-like cysts. In this review we give a short algorithm for the differential diagnosis on histological grounds. Haematoxylin and eosin staining is supported by an additional PAS staining, highlighting the laminated layer (LL) as key microscopic structure. Lymphocytes and fibrosis are present at the interface with the adjoining liver tissue. The main difference lies in the morphology of the LL. In the PAS staining, this deeply violet structure is fragmented and thin in AE, while in CE, it is broad and striated. Further important characteristics are the size of the cysts and the pericystic fibrosis. The different types of CT lesions correspond to different histological features. Treatment with Benzimidazoles may influence morphology. Immunohistochemistry (IHC) with the monoclonal antibodies (mAb) EmG3 and Em2G11 shows differences in staining patterns regarding AE and CE. IHC is decisive to settle the diagnosis in unclear situations and when only small particles of E. multilocularis (SPEMS) or E. granulosus (SPEGS) are present, e.g. in lymph nodes.