Hydatid cysts are fluid filled sacs formed by infection with the parasite (tapeworm) Echinococcus granulosus. The parasite eggs are ingested, hatch and pass through the gut wall to enter the blood stream and travel to certain organs. The liver is the most common site of infection although other organs such as the spleen or lungs can sometimes be infected also.

Infection is acquired through contact with animals, particularly dogs that have been fed offal or raised on farms with sheep or other livestock. Certain regions in the world such as South America, the Middle East, Western China and rural Australia have a higher incidence of hydatid cyst. Infection in people who have lived only in Australian cities is rare.

In many patients, hydatid cysts do not cause symptoms. When diagnosed, they have usually been present for many years (even from childhood) and are often only discovered on an ultrasound or CT performed for other reasons. However in some patients hydatid cysts can cause complications such as leakage into the abdominal cavity, infection to form liver abscesses, obstruction of veins that carry blood to or from the liver, or may block bile ducts leading to pain, jaundice, cholangitis (infection of the bile) or pancreatitis.

Tests required to diagnose hydatid cysts may include

• Ultrasound
• CT
• MRI
• Blood tests (serology or antigen tests)

Many hydatid cysts are already inactive (no longer alive) when diagnosed and do not need any treatment. Active cysts require treatment which may include

• Anti parasitic medication
• Needle aspiration and injection to destroy the parasite within the cyst
• Surgery to remove the cyst

If the parasite is infecting bile ducts, causing blockage or other complications, an ERCP may be required to remove the parasite from the ducts.