The CT scan of the abdomen reveals a fluid-filled cystic mass with an irregular margin in the fourth segment of the liver. The mass communicates with the gallbladder and is associated with a small amount of free fluid around the liver and in the peritoneal cavity. These imaging findings in the setting of eosinophilia, an associated allergic reaction, and a history of acutely worsening abdominal pain with sudden pressure applied to the abdomen are consistent with a ruptured hydatid cyst.
Echinococcosis, otherwise known as hydatid or alveolar cyst disease, is an infection caused by the larval stage of small taeniid-type tapeworms of the Echinococcus species. Echinococcus is a zoonotic parasite primarily in the lifecycle between domestic dogs (definitive host) and domestic ungulates (intermediate host). Echinococcosis is found in worldwide studies and has a prevalence rate between 1% and 10% in endemic areas, such as China, the Middle East, and South America.
Three forms of human hydatid disease are recognized: Echinococcus granulosus and Echinococcus vogeli produce unilocular cystic lesions, whereas Echinococcus multilocularis produces multilocular alveolar lesions that are locally invasive. E vogeli is uncommon and is occasionally found in the South American highlands. E multilocularis is more common than E vogeli, but it is probably not the etiologic organism in this case. It is different from E granulosus in that it remains in a proliferative phase, is always multilocular, and survives in wild canines as the definitive hosts and small rodents as the intermediate hosts.
The adult form of E granulosus (3-5 mm long) inhabits the intestines of definitive hosts (which are most commonly dogs, but it can also be found in coyotes or wolves). It has three proglottides: immature, mature, and gravid. The gravid proglottid splits into eggs that can be found in the feces of the definitive host. Intermediate hosts, such as humans, sheep, cattle, and goats, get infected by consuming plants that are contaminated by the feces of affected animals or by direct contact with an affected animal. After humans ingest the eggs, they hatch into embryos in the small intestine. The embryos penetrate the intestinal mucosa, enter the portal circulation, and are carried to the liver.[2,3] Some are destroyed in the liver while others form into hydatid cysts. A small percentage of the eggs may pass through the liver and form cysts in other parts of the body, including the lungs, central nervous system (CNS), spleen, and pancreas. After the developing embryos localize in a specific organ, they transform and develop into larval echinococcal cysts. This process is referred to as primary echinococcosis.
The cyst is composed of two layers: the endocyst, which is filled with clear fluid, and the pericyst, which is a fibrous capsule that develops as a host response to the growth of the echinococcal cyst.[2,3] Nutritive substances that contribute to the cyst's growth pass through the pericyst. The pericyst encompasses the endocyst, which is of larval origin. It is composed of an outer laminated layer, or hyaline membrane, and an inner multipotential germinal layer. Daughter cysts develop from the inner aspect of the germinal layer, as do germinating cystic structures called brood capsules. New larvae, called protoscoleces, develop in large numbers within the brood capsule. The cysts typically expand slowly over a period of years, at a rate of approximately 1-3 cm per year.
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