A 25-year-old butcher presents with a large cyst in the right lobe of the liver identified on ultrasound. CT shows a 'double-wall' cyst with internal daughter cysts. The Casoni skin test is positive. Which parasite is responsible and which is the FIRST intermediate host in its life cycle?

• A Taenia solium — pig
• B Echinococcus granulosus — sheep/cattle ✓
• C Taenia saginata — cattle
• D Diphyllobothrium latum — copepod (Cyclops)

Explanation

Hydatid cyst disease is caused by the larval stage of Echinococcus granulosus. The definitive host is the dog; sheep and cattle serve as intermediate hosts harbouring the hydatid cyst. Humans are accidental intermediate hosts. The Casoni intradermal test detects hydatid antigen sensitisation. The cyst shows an outer pericyst (host-derived), an ectocyst (laminated), and an endocyst (germinal), with internal brood capsules and protoscolices — hence the 'daughter cyst' appearance. T. solium (pork tapeworm) causes cysticercosis, not hydatid disease.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.