In the lifecycle of Echinococcus granulosus, which hosts are the definitive and intermediate hosts, and where do hydatid cysts typically form in humans?

• A. Canids are definitive, livestock intermediate; cysts form in liver and lungs
• B. Felines definitive, rodents intermediate; cysts form in brain
• C. Bovines definitive, equines intermediate; cysts form in heart
• D. Canids intermediate, livestock definitive; cysts form in spleen

Answer: (B)

In the lifecycle of Echinococcus granulosus, dogs and other canids serve as the definitive hosts, where the adult tapeworm lives in the intestine and releases eggs in the feces. Livestock such as sheep, goats, or cattle act as intermediate hosts, ingesting those eggs and developing hydatid cysts in their organs. Humans are accidental intermediate hosts, and in people the hydatid cysts most commonly form in the liver, with the lungs coming next as a frequent site. Brain involvement can occur but is much less common, and involvement of the heart or spleen is not typical. So the pairing that reflects the real lifecycle and site of cyst formation is canids as definitive hosts, livestock as intermediate hosts, with cysts in the liver and lungs in humans. The option mentioning felines as definitive hosts and rodents as intermediate, with brain cysts, doesn’t fit the usual lifecycle or common organ involvement.