Echinococcus multilocularis
Echinococcus multilocularis, cysts
Echinococcus multilocularis is the cause of alveolar echinococcosis, one of the most severe and deadly parasitic diseases of humans. It is one of four cestodes (tapeworms) in the genus Echinococcus that cause human disease; the others are E. granulosus, E. vogeli, and E. oligarthus. Of these, E. granulosus is the cause of cystic echinococcosis, the most common form of human disease. While still rare, reports of alveolar echinococcosis cases have increased in recent years in North America, and have been associated with the more virulent invasive European haplotype. E. multilocularis (alveolar hydatid disease) is most commonly seen in northern temperate regions of Asia, Europe, Canada, and the northern United States.
Dogs and other canids such as foxes are the primary definitive host and are host to the adult tapeworm in their intestine. Infected canids shed eggs and gravid proglottids into the environment and may be eaten by an intermediate host such as a rodent. The ova then hatch to release an oncosphere which penetrates the intestinal wall and is carried by the portal circulation to the liver. Alveolar echinococcosis is characterized by rapidly growing, infiltrative, irregularly sized cysts that reproduce by budding. This is in contrast to cystic echinococcosis in which a single cyst grows slowly over many years and may eventually form daughter cysts within the parent cyst. The definitive canid host becomes infected after preying upon an infected intermediate host. Humans are aberrant, dead-end hosts that usually become infected after unintentionally eating food or fomites contaminated with dog feces containing E. multilocularis eggs. Human disease can mimic hepatocellular carcinoma or cholangiocarcinoma on imaging due to its irregular, infiltrating growth.
In tissue sections, small, irregularly-sized cysts are seen infiltrating host tissue on hematoxylin and eosin stain (WSI: https://s3.us-east-2.amazonaws.com/mayo-vm/slides/KV3SF4ykU0Oiwci6cWUj7w.html). GMS (WSI: https://s3.us-east-2.amazonaws.com/mayo-vm/slides/b4i9MKvuOEO0_A9uSePFVA.html) and PAS highlight the cyst walls. The host elicits an intense inflammatory granulomatous eosinophilic response with necrosis. The cysts have an attenuated laminated layer, but protoscoleces are rarely seen in humans.





