[Infectious cysts and pseudocysts: When parasites want to mimic tumors!]

Parasites can form true tissular cysts, with cavity(ies) delimited by a wall, covered by a specialized parasitic epithelium. In humans, these tissue cysts are caused by cestodes (segmented flatworms), which cannot complete their full cycle (e.g., in hydatidosis, alveolar echinococcosis or cysticercosis). Parasites can also form tissue pseudocysts, the wall of which consists of a resorptive granulomatous reaction to the presence of the parasite. In humans, these pseudocysts are caused by both nematodes (roundworms) and cestodes. These pathogens are rarely encountered by pathologists, and their diagnosis can be difficult, with many morphological pitfalls. This can lead to major diagnostic difficulties, resulting in misdiagnosis, and a negative impact on patient care. In case of diagnostic difficulties, a histopathological review of the slides by an infectious disease pathologist and/or confrontation with a parasitologist accustomed to diagnosis ontissue sections is necessary, and shouldbe completed, if possible, by an integrated histo-molecular diagnosis. The aim of this chapter is to detail the histopathological features of the main pathogens responsible for cysts/pseudocysts observed by the pathologist. The morphology of different infectious diseases forming cysts within tissues (hydatidosis, alveolar echinococcosis, and cysticercosis) and pseudocysts will be detailed.