A 48-year-old Somali woman with hip pain.

Figure 1. Noncontrast computed tomography scan of the pelvis. The arrow points to a lytic lesion in the posterior mid-right ilium characteristic of a space-occupying hydatid cyst. Diagnosis: hydatid cyst bone disease. Cysts removed from the iliac bone cavity (figures 1 and 2) were consistent with hydatid cysts due to Echinococcus species. Microscopic examination confirmed the diagnostic features of cystic hydatid disease, including cysts surrounded by prominent laminated acellular layers. Shown is a characteristic cyst with an inner germinal layer and protoscolex with hooklets (figure 3). The fluid and protoscoleces contained within the cysts are known collectively as hydatid sand. Both macroscopic and microscopic features were most consistent with cystic hydatid disease. The cavity was completely curetted and irrigated with several liters of saline before the wound was closed. To support the histological diagnosis, serological testing was performed, and results were positive for immunoglobulin G–class antibodies to Echinococcus species by enzyme-linked immunoassay. Computed tomography (CT) of the chest, abdomen, and pelvis did not show evidence of multifocal disease. The patient initiated therapy with albendazole (200 mg twice per day) with the plan to continue therapy for 8 months. Follow-up CT scans of her pelvis at 1 and 8 months after hospital discharge showed the lytic lesion to be stable. The patient continued to improve and ultimately discontinued albendazole after 8 months of therapy. Hydatid disease is caused by the larval forms of Echinococcus cestodes. The most common species are Echinococcus granulosus and Echinococcus multilocularis, which cause cystic and alveolar hydatid disease, respectively. The geographic distribution of these 2 species differ markedly, with E. granulosus having a worldwide distribution and E. multilocularis being restricted to temperate regions of North America, Europe, and Asia [1]. In sub-Saharan Africa, especially in East and West Africa, cystic hydatid disease is highly endemic [2]. The presence of the laminated acellular layer surrounding the cyst in figure 2, as well as the recent emigration of our patient from an area of endemicity [3], supports a diagnosis of E. granulosus cystic hydatid disease. A laminated layer is not seen surrounding the Figure 2. Cystic structures removed from the pelvic lesion. Note the characteristic variation in size and shape of the cysts. Many of the cystic structures have been cut open to reveal the mostly smooth inner lining. Figure 3. Tissue section of pelvic material from the open biopsy sample (hematoxylin and eosin stain; original magnification, ϫ100). The …