In addition to amorphous debris, mixed inflammatory cells and
plate-like structures consistent with cholesterol crystals, careful
examination at high power showed refractile sickle-shaped
structures diagnostic of Echinococcal hooklets.
Rare degenerated forms suggestive of scolices were
also identified.
Diagnosis: Hydatid cyst.
Discussion: Echinococcus granulosus is a tissue cestode for which man sometimes becomes an intermediate host (dogs or foxes commonly serve as definitive hosts). The mechanism of infection involves ingestion of plant material or soil contaminated with proglottids and/or eggs. The most common site for echinococcal cyst formation in man is the liver (in up to 70% of cases), though other organs including lung and brain may be involved. Unilocular cysts develop with an inner germinal membrane which gives rise to daughter cysts. Daughter cysts give rise to individual scolices; such forms floating free within the cyst cavity are known as hydatid sand. Cysts grow and develop over a period of years. When they rupture (either spontaneously or secondary to trauma), anaphylactic shock and/or spread to other visceral organs may occur.
Given an appropriate clinical history (i.e. residence in an endemic zone) and appropriate serologic findings (see below), cysts may be deliberately aspirated as part of definitive surgical excision - a procedure which still carries a risk of dissemination or anaphylactic shock. In this case, the diagnosis was suspected prior to surgery. Examination of the surgical specimen revealed only a fibrotic cyst wall and amorphous debris, with no diagnostic parasitic forms seen.
The most common structures seen in cytologic preparations are the individual hooklets. Well-preserved scolices are seldom identified. Cytology cannot differentiate between disease caused by E. granulosus and E. multilocularis (the agent of alveolar hydatid disease); clinical and radiologic correlation is required.
Serologic studies including enzyme immunoassays currently offer a high degree of sensitivity and specificity in the diagnosis of hydatid disease.
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