The photograph shows a large bile-stained Hydatid cyst in the liver of a 24 y/o male. Echinococcosis occurs in 4 forms: cystic echinococcosis (Hydatid cyst) caused by Echinococcus granulosus, alveolar echinococcosis caused by E. multilocularis, polycystic echinococcosis caused by E. vogeli, and unicystic echinococcosis caused by Echinococcus oligarthrus. A number of herbivorous and omnivorous animals can act as intermediate hosts of echinococcus. They get infected by ingesting the parasite eggs and develop parasitic larva in their viscera. Humans are accidental intermediate hosts and are unable to transmit the disease. Dogs and other canids are the definitive hosts. They get infected by ingesting the viscera of infected herbivores. Image copyright: pathorama.ch.