Site of infection: The small adult tape-worms attach to the mucosa of the small intestines in dogs, sometimes in their thousands. The larval stages (hydatids) most commonly infect visceral tissues and organs, especially the liver, in their mammalian intermediate hosts, although cysts may be found in many other locations, including the brain and long bones.

Pathogenesis: The adult stages are considered benign and do not cause disease in dogs, as the worms do not invade or feed on host tissues. Encysted larval stages generally do not cause clinical disease in domestic livestock as they are often confined to visceral tissues. However, significant pathological changes occur in humans when the slowly-growing cysts put pressure on surrounding tissues and produce chronic space-occupying lesions.Cysts may grow around 1 mm per month and can become extremely large, up to 30cm in diameter with litres of fluid containing thousands of protoscoleces. Organ enlargement may be accompanied by a variety of clinical signs depending on the size and location of the cysts. Compression of liver may result in jaundice, portal hypertension and abdominal distention. Cysts in the lung may cause haemoptysis (coughing up blood), dyspnoea (difficulty breathing) and chest pain. Cysts in the brain or spinal cord can provoke acute inflammatory responses and numerous neurological sequalae, including epilepsy and blindness. Cyst rupture has been associated with acute clinical signs (such as peritonitis and pneumothorax), and the sudden release of hydatid fluid may cause severe allergic reactions (such as asthma and anaphylactic shock). Protoscoleces released from ruptured cysts can regress and form new hydatid cysts throughout the body.

Mode of transmission: Tape-worms have an indirect life-cycle involving predator-prey transmission between definitive (canid) and intermediate (mammalian) hosts. Mature tape-worms release numerous thick-shelled eggs which are excreted with dog faeces. The eggs are very resistant to external environmental conditions and can survive for months on pasture. Herbivores and omnivores become infected by ingesting eggs; either on herbage, in water, or by hand-to-mouth transfer. Following ingestion, the eggs hatch releasing the oncosphere which uses its three pairs of hooks to penetrate the gut, enter the circulation and settle in various organs and tissues (frequently in the liver after being filtered out by portal capillaries). They form hydatid cysts over many months and eventually produce multiple infective protoscoleces. When mature cysts in offal or carcases are eaten by canids, the cyst wall is digested away freeing the protoscoleces, which evaginate and attach to the small intestinal mucosa. They mature to adult worms in about 8 weeks and may live for 5-20 months. Various strains of E. granulosus have been recognized based on differences in parasite morphology, development, biochemistry, genetics and host specificity. Strains are often adapted to particular intermediate host species and do not develop well in other species. Infections are well adapted to pastoral cycles involving farm dogs and domestic livestock (notably sheep and cattle), as well as sylvatic cycles involving wild carnivores (wolves, coyotes, dingoes) and free-ranging herbivores (such as deer, moose and wallabies). Infections in human populations occur more frequently in rural areas, particularly where local traditions are conducive to transmission; e.g. feeding dogs offal, eating dog intestines, not burying the dead, and even using dog faeces to tan hides.

Differential diagnosis: Infections in dogs may be diagnosed by the detection of eggs, and occasionally worms, in faecal samples. Immuno-coprological tests have also been developed to detect parasite antigens in faecal samples. Infections in intermediate hosts are generally diagnosed well after the larvae have encysted. Clinical symptoms of a slow-growing tumour accompanied by eosinophilia are suggestive. Cysts may be visualized by various medical imaging techniques (computerized axial tomography (CAT) scans, X-rays, ultrasound). Several immunoserological tests have been developed to detect host antibodies against crude and purified parasite antigens, and an intradermal (Casoni) test using hydatid fluid has been used in surveys.

Treatment and control: Despite some promising indications, the treatment of hydatid disease with conventional anthelmintic drugs has not proven wholly effective, being complicated by the large size and inaccessible location of cysts and their thick, possibly impenetrable, walls. Variable results have been obtained using praziquantel and mebendazole, while albendazole and niclosamide have been less effective. The only remaining treatment option is for the surgical removal of cysts, provided they are in favourable sites. Surgeons must take care not to rupture cysts as protoscoleces may spread to new sites to form more cysts. Scolicide chemicals, such as cetrimide, may also be used during surgery to sterilize excision sites. In contrast, infections by adult worms in dogs can be successfully treated with praziquantel, and it is advisable to confine dogs and/or use purgatives to facilitate the collection and disposal of infected faeces. Preventing dogs from becoming infected involves eliminating offal and other potentially infected material from their diets, curbing their hunting behaviour, properly disposing of carcases in the field, and culling wild and feral dogs. Several countries have developed highly successful hydatid eradication campaigns based around dog management and treatment. Recently, a recombinant vaccine has been developed to prevent hydatid formation in domestic herbivores, and is undergoing further evaluation. While control may be possible in situations involving pastoral cycles, there will be many problems accessing wildlife involved in sylvatic cycles.

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