hydatid cyst

Definition: Hydatid disease (cystic echinococcosis) is caused by the larval forms of Echinococcus tapeworms. E. granulosus is the most frequent parasite, whereas E. multilocularis causes a more aggressive clinical disease.

Images

 Disseminated hydatidosis with parasitic pulmonary artery obstruction. Autopsy

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Hydatid disease is caused by ingestion of eggs of echinoccal species.

For Echinococcus granulosus, the definitive hosts are dogs, and sheep are the usual intermediate hosts. For Echinoccus multilocularis, foxes are the most important definitive host, and rodents are intermediate hosts. Humans are accidental intermediate hosts, infected by ingestion of food contaminated with eggs shed by dogs or foxes. Eggs hatch in the duodenum and invade the liver, lungs, or bones. Unilocular cysts caused by E. granulosus are most common. Multilocular cysts are caused by E. multilocularis.

Morphology

Cysticerci may be found in any organ, but the more common locations include the brain, muscles, skin, and heart. Cerebral symptoms depend on the precise location of the cysts, which includes the meninges, gray and white matter, sylvian aqueduct, and ventricular foramina. The cysts are ovoid and white to opalescent, rarely exceeding 1.5 cm, and contain an invaginated scolex with hooklets that are bathed in clear cyst fluid.

The cyst wall is more than 100 μm thick, is rich in glycoproteins, and evokes little host reaction when it is intact. When cysts degenerate, however, there is inflammation, followed by focal scarring, and calcifications, which may be visible by radiography.

About two-thirds of human E. granulosus cysts are found in the liver, 5% to 15% in the lung, and the rest in bones and brain or other organs. In the various organs, the larvae lodge within the capillaries and first incite an inflammatory reaction composed principally of mononuclear leukocytes and eosinophils.

Many such larvae are destroyed, but others encyst. The cysts begin at microscopic levels and progressively increase in size, so that in 5 years or more, they may have achieved dimensions of more than 10 cm in diameter.

Enclosing an opalescent fluid is an inner, nucleated, germinative layer and an outer, opaque, nonnucleated layer. The outer nonnucleated layer is distinctive and has innumerable delicate laminations as though made up of many layers of gelatin.

Outside this opaque layer, there is a host inflammatory reaction that produces a zone of fibroblasts, giant cells, and mononuclear and eosinophilic cells. In time, a dense fibrous capsule forms. When these cysts have been present for about 6 months, daughter cysts develop within them. These appear first as minute projections of the germinative layer that develop central vesicles and thus form tiny brood capsules.

Scolices of the worm develop on the inner aspects of these brood capsules and separate from the germinative layer to produce a fine, sandlike sediment within the hydatid fluid.

Hydatid disease is caused by hydatid cysts:

• Echinococcus granulosus :
  • classic form of hydatid cyst
  • affects liver, lung, kidneys, spleen, brain, musculoskeletal system, heart

• Echinococcus multilocularis
  • alveolar hydatidosis (multilocular hydatidosis)

Structural components of the hydatid cyst :

 outer acellular laminated membrane
 germinal membrane
 protoscolices, attached and budding from the membrane

Localization

 pulmonary echinococcosis (pulmonary hydatid cyst)
 hepatic echinococcosis (hepatic hydatid cyst)
 cardiac echinococcosis (cardiac hydatid cyst)

See also

 infectious diseases

• helminthiases