Hydatid cyst (echinococcosis) of the liver
DEFINITION AND AETIOLOGY
Hepatic hydatid disease is a parasitic infection caused mainly by the larval stage of Echinococcus granulosus. Dogs and other canids are definitive hosts; sheep and other livestock are intermediate hosts. Humans are accidental intermediate hosts after ingesting eggs from food, water or hands contaminated with dog faeces. The liver is the commonest site because portal venous blood carries oncospheres from the intestine to hepatic sinusoids.
LIFE CYCLE AND PATHOGENESIS
After ingestion, eggs release oncospheres that penetrate the intestinal wall and enter portal circulation. In the liver they develop into fluid-filled cysts with an outer host-derived pericyst, laminated acellular membrane and inner germinal layer. The germinal layer produces brood capsules, daughter vesicles and protoscolices (hydatid sand). Cysts enlarge slowly and compress adjacent parenchyma or bile ducts. Rupture may be contained, communicating with bile ducts, or free into the peritoneum. Leakage can cause secondary implantation and potentially severe anaphylaxis.
CLASSIFICATION AND MORPHOLOGY
The WHO-IWGE ultrasound classification describes CE1 (unilocular active cyst), CE2 (multivesicular/daughter cysts), CE3a (detached membranes), CE3b (daughter cysts in a partly solid matrix), CE4 (heterogeneous degenerative contents) and CE5 (thick calcified wall). CE1–CE2 are active; CE3 is transitional; CE4–CE5 are usually inactive. Cyst size, stage, number, location and communication with bile ducts determine treatment.
CLINICAL FEATURES
Many cysts are asymptomatic and discovered incidentally. Symptoms develop from mass effect: dull right-upper-quadrant pain, fullness, hepatomegaly, nausea or early satiety. Biliary communication may cause jaundice, cholangitis, biliary colic or pancreatitis. Rupture can produce acute abdominal pain, urticaria, bronchospasm or anaphylactic shock. Infection causes fever and a picture resembling pyogenic liver abscess. Thoracic extension or transdiaphragmatic rupture may cause cough, chest pain or expectoration of cyst contents.
INVESTIGATIONS
Ultrasound is first-line and stages the cyst; daughter cysts, detached membranes and hydatid sand are characteristic. Contrast CT defines number, calcification, relation to vessels and biliary tree, and complications. MRI/MRCP is useful when biliary communication is suspected. Serology (Echinococcus IgG) supports diagnosis but a negative result does not exclude disease, particularly in intact or extrahepatic cysts. Check liver tests, full blood count and inflammatory markers. Avoid diagnostic aspiration unless performed as part of a planned specialist procedure because of leakage and anaphylaxis risk.