The normal life cycle of Echinococcus requires infested meat to be eaten by a dog or other carnivore. The larva enters the gut and grows into a tapeworm, which then passes eggs out in the faeces to contaminate grass and soil. The normal hosts are cattle, sheep and other grazing animals that eat the contaminated grass and are eventually killed by the Echinococcus infestation in their body. This allows the carcass to be eaten by meat-eating animals, and the life cycle of the parasite starts again. If a human eats food that has been contaminated by the faeces of an infected animal (usually dogs or other meat-eating animals), the larva migrates to the liver, lung, spleen or brain, where it forms a cyst that remains lifelong. The disease is rare in developed countries, but widespread in South America, around the Mediterranean, in east Africa and central Asia.
After the cyst forms in the body, it usually remains dormant for many years, often causing no symptoms. Over a decade or more the cyst slowly enlarges, until the pressure it exerts on its surroundings causes problems. With liver cysts, there may be pain in the upper part of the abdomen, nausea, vomiting and jaundice. In the lung, the cysts may cause shortness of breath, pain and even part of the lung to collapse. In the brain symptoms occur earlier, and even a small cyst may cause convulsions or severe headaches. If a cyst ruptures, the reaction in the body to the sudden release of a large number of larvae may cause sudden death or severe illness and the formation of multiple cysts in other parts of the body. If multiple cysts are present, the long-term outlook is grave.
The condition is diagnosed by seeing the cyst on a CT or ultrasound scan. Specific antibody blood tests can be performed to determine whether or not a person has a cyst somewhere in their body, but discovering the actual site of the cyst may then prove very difficult. The blood test remains positive long term after an infection.
If possible, a cyst should be removed surgically. It is vital for the surgeon not to rupture the cyst during its removal, because the spilled larvae can then spread through the body. In other cases, or as an additional form of treatment, potent medications may be prescribed to kill the larvae, but the cyst will remain. Provided the disease is not widespread, the results of treatment are good. Dogs in affected areas can be treated regularly to prevent them carrying the disease.