Question
Hydatid disease.
Answer
Hydatid disease: caused by the larval (metacestode) stage of the tapeworm Echinococcus granulosus.
Life cycle/transmission: humans are an accidental intermediate host, acquiring infection by ingesting eggs shed in the faeces of the definitive host (dog), typically via contaminated food/water or close contact with infected dogs. Ingested eggs hatch in the small intestine, releasing oncospheres that penetrate the intestinal wall and are carried via the portal circulation predominantly to the liver, and, after bypassing the liver, to the lungs and other organs, where they slowly develop into a hydatid cyst.
Structure: a fluid-filled cyst with a germinal layer (producing numerous protoscolices and daughter cysts) and an outer host-derived fibrous pericyst.
Clinical features: often asymptomatic for years due to slow cyst growth; symptoms arise from pressure effects (right upper quadrant pain/hepatomegaly for hepatic cysts; cough, chest pain, haemoptysis for pulmonary cysts); cyst rupture can cause anaphylactic reaction (due to release of highly antigenic cyst fluid) and secondary dissemination of protoscolices; secondary bacterial infection of the cyst can mimic a liver abscess.
Laboratory diagnosis:
- Imaging (ultrasound/CT/MRI) — the primary diagnostic modality, showing a characteristic cystic lesion, often with daughter cysts (“cyst within a cyst”) or a “water-lily sign” (ruptured/degenerating cyst).
- Serology — ELISA/indirect haemagglutination detecting anti-Echinococcus antibody; supportive but not always positive.
- Casoni’s skin test — a classical delayed-hypersensitivity test, historically used but largely abandoned due to poor sensitivity/specificity.
- Eosinophilia — may be present, particularly following cyst leakage.
- Direct examination of cyst fluid (during surgery) — demonstration of protoscolices/hooklets confirms diagnosis; percutaneous aspiration is generally avoided given the risk of anaphylaxis and secondary dissemination.
Prevention: deworming of dogs, controlled disposal of infected animal offal, hand hygiene, and public health education in endemic regions.