Question
Hydatid disease
Answer
Hydatid disease (echinococcosis) is a zoonotic infection caused by the larval (cystic) stage of Echinococcus granulosus (dogs are the definitive host; sheep/cattle are the usual intermediate host; humans are an accidental intermediate host).
Transmission: humans acquire infection by ingesting eggs shed in dog faeces; ingested eggs hatch, releasing oncospheres that disseminate via the bloodstream, most commonly lodging in the liver and lungs, developing slowly into a fluid-filled hydatid cyst.
Cyst structure: outer host-derived pericyst; laminated (acellular) membrane; innermost germinal layer, producing brood capsules with numerous protoscolices (“hydatid sand”), and sometimes daughter cysts.
Clinical features: often asymptomatic for years; symptoms from mass effect; cyst rupture can cause severe anaphylactic reaction and secondary dissemination.
Laboratory diagnosis: imaging (ultrasound, CT, MRI) is the primary diagnostic modality; serology (ELISA/indirect haemagglutination); microscopy of aspirated cyst fluid for protoscolices/hooklets (direct aspiration carries risk of anaphylaxis, generally avoided outside carefully controlled procedures like PAIR); Casoni’s test — an older, largely obsolete hypersensitivity skin test.
Treatment: surgical excision (avoiding rupture/spillage) with perioperative albendazole; PAIR (Puncture-Aspiration-Injection-Reaspiration) technique for selected cysts.
Prevention: deworming of dogs, avoiding feeding dogs raw offal, hand hygiene.