Tapeworm cyst. X-ray showing a hydatid cyst (round, upper centre) in the liver of a 72 year old female patient. This cyst is caused by a parasitic tapeworm larva (Echinococcus sp.). Humans are not a natural host of the parasite, but may become infected from ingesting eggs in food that has been contaminated by the faeces of an infected animal, usually a sheep or a dog. Cysts may be formed in the lungs, brain, liver or kidneys.