Hydatid disease in man is caused principally by infection with the larval stage of the dog tapeworm Echinococcus granulosus. It is an important zoonotic (acquired from animals) parasitic infection of humans, which follows accidental ingestion of tapeworm eggs excreted in the faeces of infected dogs.
Three species of tapeworm are of importance in human hydatid disease worldwide; E. granulosus, E. multilocularis and E. vogeli. However, E. multilocularis and E. vogeli have not been found in the UK. E. granulosus, which causes cystic echinococcosis (CE), is the only species of the tapeworm found in the UK. The natural definitive hosts of CE are the domestic dog (Canis l. familiaris) and foxes, chiefly the Arctic fox (Alopex lagopus) found in the Arctic Region of the Northern Hemisphere, and the red fox (Vulpes vulpes) found in urban and rural habitats of the UK, Europe and North America.
E. granulosus is distributed throughout most of the world, especially in areas where sheep are raised, and is endemic in Asia, North Africa, South and Central America, the USA, Canada, and the Mediterranean region. It is common throughout Europe, and in the UK there are well-documented 'hotspots' of infection in Wales and the Western Isles of Scotland. Approximately 10-20 human cases are reported in the UK each year and most arise following exposure abroad. In many countries, hydatid disease is more prevalent in rural areas where there is close contact between man and dogs and the various domestic animals which act as intermediate vectors.
In the UK, the important intermediate hosts for E. granulosus are sheep; infection rates may be high in these animals, over 90% of cysts may be fertile. Current evidence suggests that the main areas for hydatid disease in Wales are Powys, Monmouthshire and farms on the southern slopes of the Brecon Beacons and the Black Mountains. A pocket of disease is also present in the part of South Herefordshire adjacent to South Powys.
The cysts of E. granulosus may take many years to produce clinical symptoms. Cysts may be asymptomatic throughout the infected individual's life and be found only at autopsy, during surgery or when X-rays or imaging studies are taken for other reasons. The occurrence of symptoms depends on the location of the cyst in the body and its size; the liver is the organ most commonly affected. There is an inflammatory reaction in the surrounding tissue, with formation of a fibrous, encapsulating membrane. If the outer laminated wall of the cyst calcifies, the cyst then remains asymptomatic. In locations such as the abdomen, where growth of the cyst is not restricted by anatomical structures, it can grow very large and contain several litres of fluid. Rupture of the cyst presents the greatest danger for the patient and may be fatal.
Sheep and other intermediate hosts such as cattle acquire hydatid disease by grazing on pastures contaminated with dog faeces containing eggs. Each egg hatches in the small intestine of the sheep where it penetrates the gut wall. This larval stage of the parasite (called an oncosphere) is carried via the bloodstream to target organs in other parts of the body (liver, lungs, brain, muscles, etc.) where they develop by expansion into a hydatid cyst (called a metacestode). This hydatid cyst is usually fluid filled, containing brood capsules in which are protoscolices produced by asexual reproduction.
Dogs are, in turn, infected by ingesting meat or viscera containing such cysts, for example by eating an infected rodent, or being fed infected sheep meat and viscera. The fertile cysts they ingest develop into adult tapeworms in the small intestine where they reproduce sexually. New reproductive segments (proglottids) of the worm are produced. After about a month, the animal host begins to excrete proglottids containing infective eggs in its faecal matter. E. granulosus lives in the dogs' intestine for about a year but ceases to produce eggs after 6 - 10 months. Eliminated segments of the worm in dog faeces have been reported to migrate some distance from faecal matter over grass or garden vegetables before expelling eggs that subsequently adhere to the vegetation. Humans are intermediate hosts; they do not play a role in the biological cycle but may act as agents perpetuating the disease by feeding dogs infected meat and viscera.
There are no specific signs of hydatid disease in farm animals. Infection in food animals is in nearly all cases confined to the lungs and the liver; infected organs must be condemned and destroyed.
Dog owners should practice good hygiene when handling their animals. In endemic areas, it particularly important to wash the hands after handling dogs, avoid contact with dog faeces, and to prevent dogs from soiling the immediate environment. Dogs should not be allowed to roam or to have access to raw sheep meat or viscera. All sheep carcasses should be disposed of properly and immediately. All dogs, especially those in rural endemic areas should be treated at 6 weekly intervals with a wormer containing Praziquantel. Vegetables, salads and fruit should be thoroughly washed before consumption.
Diagnosis is made by a combination of clinical, imaging, serological and molecular techniques. Human disease is treated by surgical removal of the cyst with supplementary chemotherapy (Mebendazole or Albendazole). Surgical removal may not prevent other cysts growing and causing further problems.
There are currently no effective drugs or vaccines to protect humans against the disease.
Last reviewed: 20 May 2011