Hydatid disease is caused by the parasite Echinococcus granulosus. The disease is mainly present in rural areas where dogs are used for herding live stock, especially sheep, and occurs frequently in Mediterranean countries, in Australia,and in South America.
Dogs are the normal host for the adult parasite, and hundreds of worms may exist in their intestinal tract. Sheep, cattle, herbivores and humans are intermediate hosts for the parasite and are infected after contact with dog feces. In heavily endemic areas, about 50% of dogs and up to 90% of sheep and cattle are infected with E. granulosus. Eggs are passed by the dogs and can be ingested by intermediate hosts.
Once inside the intermediate host, the parasitic eggs hatch and embryos penetrate the intestinal mucosa to enter lymphatic and venous channels.
Most embryos are filtered by the liver and lungs with the remaining parasites reaching other organs, including the brain, spleen, kidneys, and the musculoskeletal system.Viable embryos transform into cysts which grow at a rate of approximately 1cm per year. The wall of the hydatid cyst is composed of two layers: the endocyst, a germinal layer, and the ectocyst, a proteinaceous membrane. A dense fibrous capsule containing co lagen, the pericyst, is formed by the host.
Echinococcal cysts usually develop in the liver (75% of cases) but may occur in any part of the body. The lesions are often asymptomatic for many years and are discovered incidentally on US or CT scans. Hydatidosis can also be detected by serologic tests. Classic symptoms of hepatic hydatid cyst include upper abdominal pain and hepatomegaly.
Treatment consists of surgical removal of the cyst or antiparasitic drug therapy. If left untreated, a hepatic hydatid cyst may rupture into surrounding structures such as the liver parenchyma, biliary system, peritoneum, GI tract, or pleura. Hydatic cyst rupture is the major complication of echinococcal disease.
On abdominal plain film, curvilinear or ringlike calcifications can be seen in the right upper abdominal quadrant in about 20–30% of cases. However, calcifications do not necessarily indicate death of the parasite.
The appearance of the hydatic cyst on US is variable and depends on the stage of evolution and maturity. The lesion may appear as a well-defined anechoic cyst, as an anechoic cyst except for hydatid sand, as a multiseptate cyst with daughter cysts, as a cyst with a floating membrane, or finally, as a densely calcified mass.
ultrasound images Echinococcal cyst on US. Ultrasound reveals either a well-defined anechoic cystic-like lesion
ultrasound images a cystic lesion with a floating membrane (arrow)
ultrasound images or a dense and heterogeneous nodule (arrowhead