Describe cysticercosis and hydatid disease with their pathogenesis, diagnosis and management.

Cysticercosis — Pathogenesis

• Routes of infection — ingesting eggs in food or water contaminated by the faeces of a tapeworm carrier (heteroinfection, the commonest route); external autoinfection by faecal-oral transfer in a carrier with poor hygiene; and internal autoinfection by reverse peristalsis carrying proglottids into the stomach, which is debated
• The crucial epidemiological point — cysticercosis is acquired from human faeces, not from eating pork. A vegetarian can develop cysticercosis from food handled by a carrier, while eating pork causes only intestinal taeniasis. This is counterintuitive and is regularly misunderstood
• Sites — brain (neurocysticercosis, about 60 to 90% of symptomatic cases), skeletal muscle, subcutaneous tissue, and the eye

Cysticercosis — Clinical Features and Management

• Neurocysticercosis is the commonest cause of acquired epilepsy in India and worldwide, accounting for a large proportion of adult-onset seizures in endemic areas
• Presentations — seizures (much the commonest, usually focal); raised intracranial pressure and hydrocephalus, particularly with intraventricular or racemose cysts in the basal cisterns; focal deficits; and "cysticercal encephalitis", a severe reaction to many simultaneously degenerating cysts, seen chiefly in children and young women
• Ocular cysticercosis — a cyst in the vitreous or subretinal space; and subcutaneous and muscular cysts, which may be palpable and calcify to give the classical "rice grain" calcification on a soft tissue radiograph
• Diagnosis — CT and MRI are the mainstay; seeing the scolex within a cyst is pathognomonic and is best shown on MRI. Serology by enzyme-linked immunoelectrotransfer blot (EITB) is specific but insensitive with a single lesion; biopsy of a subcutaneous nodule may confirm it
• The key differential in India is tuberculoma, which also gives a ring-enhancing lesion. Points favouring cysticercosis are a visible scolex, a smaller lesion (under 20 mm), a regular thin ring and less surrounding oedema; a tuberculoma is usually larger with an irregular thick wall and marked oedema, and there may be evidence of tuberculosis elsewhere
• Management principles — (1) examine the fundus first, because killing an intraocular cyst can destroy the eye; (2) control seizures with antiepileptics, which is the main determinant of quality of life; (3) give corticosteroids before and during anthelminthic treatment to contain the inflammatory response; (4) albendazole (with praziquantel in heavy infection) for viable cysts; and (5) surgery or shunting for hydrocephalus
• Calcified lesions need NO anthelminthic — the parasite is already dead; treatment is directed at the seizures alone
• Treat the tapeworm carrier in the household, and improve sanitation and handwashing

Hydatid Disease

• Caused by the larval stage of echinococcus granulosus. The dog (and other canines) is the definitive host, harbouring the tiny adult worm; sheep, goats and cattle are the intermediate hosts; and man is an accidental dead-end intermediate host, infected by ingesting eggs from dog faeces, on vegetables, or from handling dogs
• Man does not transmit the infection, which is why human hydatid disease is epidemiologically a veterinary problem
• Sites — liver in about 60 to 70% (chiefly the right lobe, the first capillary bed encountered), lung in 20 to 25% (the second filter), and then spleen, kidney, bone, brain and heart
• Structure of the cyst — three layers: the pericyst (compressed fibrous host tissue), the ectocyst (an acellular laminated membrane, characteristic and diagnostic), and the endocyst (the germinal layer, the only living part, which produces brood capsules)
• Hydatid sand — the sediment of protoscolices and hooklets liberated into the cyst fluid; brood capsules and daughter cysts form within it
• Clinical features — cysts grow slowly over years and are often asymptomatic, discovered incidentally. Symptoms arise from pressure, from secondary bacterial infection, or from rupture, which causes anaphylaxis and seeding of daughter cysts throughout the cavity
• Echinococcus multilocularis causes alveolar echinococcosis, in which the larva grows invasively like a malignancy without a limiting cyst wall; the fox is the definitive host and the prognosis is far worse

Cysticercosis Compared with Hydatid Disease

Hydatid Disease — Diagnosis and Treatment

• Imaging is the mainstay — ultrasound shows the double-layered wall, daughter cysts giving a "water lily" sign when the membrane detaches, septations and hydatid sand; the WHO ultrasound classification (CE1 to CE5) guides management. CT and MRI define anatomy and detect extrahepatic cysts
• Serology — ELISA and indirect haemagglutination, with immunoblot for the Arc-5 antigen. Sensitivity is good for hepatic cysts but poorer for pulmonary and calcified cysts, and a negative result does not exclude the diagnosis
• The CASONI intradermal test is obsolete — poorly standardised, insensitive and non-specific, and it may itself sensitise; it should be mentioned as history rather than practice
• Eosinophilia in about a quarter, and greater after leakage; demonstration of protoscolices, hooklets or laminated membrane in sputum or a surgical specimen is definitive
• Treatment — small, inactive or calcified cysts may simply be observed; albendazole alone for small cysts and always as cover for intervention; pair (Puncture, Aspiration, Injection of a scolicidal agent, Re-aspiration) under albendazole cover for suitable cysts; and surgery (pericystectomy or, where necessary, resection) for large, complicated or superficial cysts
• Surgical precautions — the field is packed with hypertonic saline-soaked swabs and a scolicidal agent is instilled before opening, to prevent spillage and secondary hydatidosis; anaphylaxis must be anticipated
• Prevention — deworming of dogs, preventing dogs from eating raw offal from slaughtered sheep (which is the step that breaks the cycle), safe disposal of carcasses, meat inspection, handwashing after handling dogs, and washing vegetables. Hydatid disease is common in the sheep-rearing regions of India, particularly Jammu and Kashmir, Andhra Pradesh and Tamil Nadu
• Examine the fundus before treating neurocysticercosis; an intraocular cyst killed by albendazole can destroy the eye, and this step is easily forgotten in a patient presenting with seizures
• Most calcified lesions need no anthelminthic — the parasite is dead, and treatment is directed at seizure control alone
• Distinguish a cysticercus from a tuberculoma on size, wall thickness, oedema and the presence of a scolex; in India both are common and the treatments differ entirely
• Trace and treat the tapeworm carrier whenever cysticercosis is diagnosed, since the source is usually a household member or food handler and further cases are otherwise inevitable
• Hydatid cysts grow over years, so an incidental small inactive cyst may reasonably be watched rather than operated on, guided by the WHO ultrasound stage
• A vegetarian can get cysticercosis but not taeniasis — the eggs come from human faeces and the worm from pork, and stating this clearly prevents a great deal of confusion in counselling
• Anticipate anaphylaxis during hydatid surgery; the field is packed and a scolicidal agent instilled precisely because spillage is both immediately and permanently dangerous
• Serology may be negative in pulmonary and calcified hydatid cysts, so a negative result never excludes the diagnosis where the imaging is characteristic
• Symptoms come from the dying cyst, so many cysticerci are incidental and need only observation; treating every lesion found on imaging causes harm