Wednesday 7 December 2005
Echinococcus granulosus, also called the Hydatid worm, is a cyclophyllid cestode that parasitizes the small intestine of canids as an adult, but which has important intermediate hosts such as livestock and humans, where it causes hydatid disease.
The adult tapeworm is about 5 mm long and has three proglottids ("segments") when intact. Like all cyclophyllideans, E. granulosus has four suckers on its scolex ("head"), and E. granulosus also has a rostellum with hooks.
In canids, E. granulosus causes a typical tapeworm infection, and produces eggs that are passed with the dog’s feces. Intermediate hosts include herbivores such as sheep, deer, moose, kangaroos, and wallabies, and any other organism (including humans) that ingests dog feces.
In the intermediate host, eggs hatch into oncosphere larvae that travel through the blood and form hydatid cysts in the host’s tissues. These cysts can grow to be the size of a softball or basketball, and may contain several smaller "balloons" inside the main cyst.
If the outer cyst ruptures, new cysts can form at a different location in the body. Each smaller section contains several juvenile worms, and dogs may eat millions of them, resulting in very heavy infections. Hydatid cysts occur in organs like the liver, brain and lungs, not in subcutaneous tissue.
Symptoms can include liver enlargement, hooklets in sputum and possible anaphylactic shock when the immune system reacts to ruptured cysts.
pulmonary echinococcosis (pulmonary hydatid cyst)
cardiac echinococcosis (cardiac hydatid cyst)
hepatic echinococcosis (hepatic hydatid cyst)