Question
Hydatid disease.
Answer
Hydatid disease (echinococcosis) is a zoonotic infection caused by the larval (cystic) stage of the tapeworm Echinococcus granulosus (dogs are the definitive host, harbouring the adult worm; sheep/cattle are the usual intermediate host; humans are an accidental intermediate host).
Life cycle and transmission: humans acquire infection by ingesting eggs shed in dog faeces (via contaminated food/water/hands, or close contact with infected dogs); ingested eggs hatch in the human intestine, releasing oncospheres that penetrate the gut wall and disseminate via the bloodstream, most commonly lodging in the liver (~65%) and lungs (~25%), where they develop slowly (over years) into a fluid-filled hydatid cyst.
Cyst structure: an outer host-derived fibrous capsule, an inner laminated (acellular) membrane, and an innermost germinal layer, which produces brood capsules containing numerous protoscolices (“hydatid sand”); daughter cysts may also form.
Clinical features: often asymptomatic for years, growing slowly; symptoms arise from mass effect on the affected organ (hepatomegaly, biliary obstruction; cough, chest pain, haemoptysis for pulmonary cysts); cyst rupture (spontaneous, traumatic, or during surgical/percutaneous manipulation) is a serious complication, releasing antigenic fluid that can cause severe anaphylactic reaction and, if protoscolices spill into surrounding tissue, secondary dissemination (secondary hydatidosis).
Laboratory diagnosis:
- Imaging (ultrasound, CT, MRI) — the primary diagnostic modality, showing characteristic cystic lesions with daughter cysts.
- Serology — ELISA/indirect haemagglutination for anti-echinococcal antibody.
- Casoni’s test — an older intradermal hypersensitivity skin test, now largely obsolete due to poor sensitivity/specificity.
- Microscopy — demonstration of protoscolices/hooklets (“hydatid sand”) in aspirated cyst fluid or surgical specimen, though direct aspiration carries a risk of anaphylaxis/dissemination and is generally avoided unless under carefully controlled conditions (e.g., PAIR technique with concurrent scolicidal agent and precautions).
Treatment: surgical excision (with care to avoid spillage) remains the definitive treatment for accessible cysts; albendazole (pre- and post-operatively, or as primary medical therapy for inoperable/multiple small cysts); PAIR (Puncture-Aspiration-Injection-Reaspiration) technique for selected cysts, combined with albendazole cover.
Prevention: deworming of dogs, avoiding feeding dogs raw offal, hand hygiene, and health education in endemic pastoral/farming communities.