The number of parasites that may cause illness in a human body approximates one hundred and fifty species, some of the conditions requiring surgery as the definitive treatment
ECHINOCOCCOSIS (HYDATID DISEASE)
Ehinococcosis is a tissue tapeworm infection of humans caused by the larval stage (a hydatid cyst) of Echinococcus granulosus.
The adult worm measuring 5 mm in length and consisting of a scolex and three proglottids is found in the small intestines of dogs, wolves and other canines, sheep, and cattle that are infected from contaminated pastures or water. The terminal or gravid proglottid splits, releasing eggs into the stool. These eggs pass to the external environment and are ingested by an intermediate host (e.g. sheep or human). Humans may ingest the eggs by handling a dog or drinking contaminated water. The embryo is liberated from the ovum in the small intestine (60-70% of these remain at this site) and gains access to the blood stream and thus to the liver, or beyond (in about 10-15% of cases) to the lung, brain, kidney, bones, and other tissues. The resultant cyst grows very slowly (1 mm a month), sometimes intermittently (and may outlive the patient) to produce pressure symptoms and aseptic productive inflammation around the cyst. It is fluid-filled and contains scolices, brood capsules, and 2nd generation (daughter) cysts containing infectious scolices. When the intermediate host is eaten by a carnivore, the scolices are released into the GI tract, where they develop into adult worms.
The dog is the principal definitive host and the sheep the most common intermediate. Human infection, often acquired in childhood during play with infected dogs, is most common in the sheep-raising areas of the world, including Caucasus, West Siberia, Central Asia.