What are hydatid cysts?

• A. Fluid-filled cyst containing tapeworm larvae
• B. Nodules of scar tissue often found in abdominal organs
• C. Sac-like structure excreted in feces of trematode-infected animals
• D. Skin abscesses around hair shafts

Answer: (A)

Hydatid cysts are fluid-filled sacs formed by the larval stage of certain tapeworms (most commonly Echinococcus species) within the tissues of intermediate hosts. Infection starts when eggs shed in the feces of a definitive host are ingested, eggs hatch, and the larvae migrate to organs such as the liver or lungs, where they develop into cysts. The cyst has a laminated outer wall with an inner germinal layer that can produce brood capsules and protoscolices—the infectious forms for the definitive host if ingested. These cysts can grow slowly and cause organ dysfunction, and rupture can release fluid and protoscolices, potentially triggering anaphylaxis or secondary infection. Hydatid disease is a zoonosis, linking dogs and other canids to humans and livestock. The correct description is a fluid-filled cyst containing tapeworm larvae, rather than scar tissue nodules, eggs shed in feces, or skin abscesses around hair shafts.