The mechanism of the primary subcutaneous localization is unclear [2, 4]. The ingested parasite's ova penetrate the intestinal wall, join the portal system, and reach the liver, where most of them are caught in the hepatic sinusoids . A few ova may pass through the liver (first filter) and reach the lung (second filter) and the systemic circulation, causing hydatid disease in other organs [1, 2]. A possible dissemination through lymphatic channels has also been reported. This accounts for cases with solitary cysts in uncommon sites [3–5]. The direct spread from adjacent sites may be another mechanism of infection .
In our case, the hydatid cyst was located subcutaneously. The patient had not undergone previous surgery for any hydatid cysts, which were never found in other organs. Therefore, our patient was diagnosed as having a primary subcutaneous hydatid cyst.
In a large series of patients from Greece, the frequency of extra-hepatic and extra-pulmonary hydatidosis was 9% . However, in different series, the frequency of subcutaneous tissue involvement, which is usually associated with involvement of other solid organs, has been reported to be approximately 2% [1, 7, 8]. Primary isolated hydatid cysts located in the abdominal wall remain extremely rare, however, even in geographic areas in which echinococcal infestation is frequent [3, 4].
The clinical course is non-specific and depends on the site of involvement, the size of the cyst, and the pressure caused by the enlarged cyst . Usually, it presents as an inert, painless, non-inflammatory mass without any deterioration of the patient's general condition [4, 9]. However, if super-infected or cracked, the cyst can simulate an abscess or a cancer [8, 9].
Radiological imaging (ultrasonography, computed tomography, and MRI) is useful in rendering the diagnosis, showing the size, localization, relationship to adjacent organs, and type of the cyst. It can also be used to search for another hydatid location [1, 4]. The radiological findings of a thick cyst wall, calcifications, daughter cysts, and a germinative membrane separated from the cyst wall are all specific to hydatid cysts [1–4]. Enhancement of the peri-cystic soft tissues can be considered an MRI feature suggestive of soft-tissue hydatid disease . Serology is a useful tool that confirms the diagnosis, although it is rarely positive for cysts in extra-hepatic and extra-pulmonary locations (25%) [1, 4, 8]. It is furthermore associated with false-negative and false-positive results .
The best treatment option is complete surgical excision of the intact cyst, which avoids leakage of cyst content that can cause anaphylaxis and local recurrence [1, 2, 8]. If the ideal surgery is impossible, the cyst content (fluid, membrane, and daughter cysts) has to be removed intra-operatively and the cyst pouch has to be irrigated with scolicidal solutions [1, 2]. Other options include percutaneous treatment under ultrasound guidance with needle aspiration irrigation of scolicidal solutions, as well as medical treatment with the use of albendazole [2, 8].