Hydatidosis is one of the most serious helminthic diseases of human being with worldwide distribution, caused by larval form of the cestode worm of Echinococcus. According to various authors, primary subcutaneous and musculoskeletal echinococcosis is extremely rare [1]. Fine needle aspiration cytology (FNAC) of hydatid cysts has been described [2]. Hydatid disease should be considered in the differential diagnosis of every subcutaneous cystic mass, especially in areas where the disease in endemic [3].

A 68-year female complained of painless swelling over posterior aspect of left thigh which was gradually increasing in size since last five months and dragging type of pain in left leg, since last one month. There was no history of trauma, fever or weight loss. On physical examination, a soft, irreducible, non-tender swelling of size 15 X 10 X 3 cm was present over posterior aspect of left thigh. It was non-inflammatory, cystic mass with the normal looking overlying skin.

General physical examination, liver and kidney profile and other routine haematological and biochemical investigations revealed no abnormality. Chest X-ray and Ultrasonography of the abdomen were normal. The patient was referred to Cytopathology Department for FNAC, with clinical as well as radiological diagnosis of soft tissue neoplasm, without prior clinical suspicion of hydatid disease. 

Fine Needle Aspiration (FNA) of the swelling was done using 25 gauze needles and 01 ml of brownish watery fluid with flakes was aspirated. The post FNAC period was uneventful. There were no post-procedure complications like urticaria or anaphylactic reaction. The fluid was centrifuged and cyto-spin smears were prepared from the sediment. Also, smears were prepared from the aspirate material in the needle hub. The smears were stained by Haematoxylin and Eosin (H&E), May- Grunwald Giemsa (MGG) and Papanicolaou (PAP) stains. Few smears were left unstained for any special stain if required subsequently.

Microscopic examination revealed numerous hooklets and laminated membrane. No inflammatory cells were seen. The laminated membrane appeared as fragments of acellular material with delicate parallel striations staining deep blue. The hooklets were about 20-40um in size, semitranslucent, refractile, triangular or sickle-shaped with an inner semi-translucent core of the same shape. With these cytological findings, a diagnosis of hydatid cyst was made.                

Cyto-spin smears from the aspirated fluid showed presence of mixed inflammatory infiltrate predominantly consisting of eosinophils along with amorphous necrotic debris in the proteinaceous background.

Reviewed ultrasonography of the swelling done subsequently showed a well-defined, ovoid, thick walled cystic lesion measuring 15 X 10 cm with posterior acoustic enhancement, in the subcutaneous plane of left thigh. The lesion was around 03 mm deep from the skin surface and superficial to the muscular plane. It showed floating collapsed membranes within. No calcification was evident.

Clinical and radiological examination done subsequently to look for evidence of hydatid disease in other organs did not show involvement of any other organ or site. The patient also did not give any past history of hydatid cyst in any other organ. So final diagnosis of primary subcutaneous Hydatid cyst of left thigh (posterior aspect) was made. As typical features of Hydatid disease were present on cytology and later corroborated by radiology, so serological tests which are expensive tests with low diagnostic sensitivity and specificity, were not done.

Hydatid disease caused by the larvae of the cestode Echinococcus occurs worldwide. The common species are E. granulosus and E. multilocularis. The former is more prevalent and causes unilocular cysts, while E. multilocularis is rare and causes multilocular cysts. The definitive host is dog, sheep, goat and cattle. Humans are the coincidental intermediate hosts. When infected ova are consumed by the human host, embryos develop in the proximal small intestine and reach the liver through the blood resulting in one or more echinococcal cysts. Some of the embryos may pass through the hepatic capillaries (first filter) and enter the pulmonary circulation (second filter) [4]. 

A few may pass the pulmonary capillaries to enter the general stream and lodge in various organs, causing hydatid disease in other organs and sites. Dissemination through lymphatic channels has also been suggested as a possible mechanism and accounting for cases with solitary cysts in uncommon sites [3,4]. Another mechanism may be direct spread from adjacent sites [5].

The liver is the most frequently involved organ (75%) followed by lungs (15%) although hydatid cyst may develop in almost any part of the body, including soft tissues [6]. Musculoskeletal or soft tissue hydatidosis account for about 0.5% to 5.0% of all echinococcal infections in endemic areas. The frequency of subcutaneous tissue involvement associated with involvement of other solid organs, has been reported to be approximately 2%. A solitary primary subcutaneous localization is an extremely rare entity, even in countries where the echinococcus infestation is endemic and real incidence is not known. The mechanism of primary subcutaneous localization of hydatid cyst is still not clear [3,7,8].

In majority of cases with atypical locations, the disease is asymptomatic, while some patients may clinically present with non-specific complaints like pain or by its pressure effects on surrounding tissues. Usually it presents as painless, non-inflammatory, slow growing soft tissue masses which may mimic other pathological conditions such as soft tissue tumours in some cases. In these cases, FNAC may be used for initial evaluation of such lesions, as was done in the present case. FNA is conventionally contraindicated in a suspected case of hydatid cyst because of risk of anaphylaxis and dissemination. However, this risk has been overemphasized in the past as there are many reports on cytological diagnosis of hydatid disease without complications. FNAC appears to be a safe and useful method in the diagnosis of hydatid cyst, although arguments regarding this issue persist. Cytologic diagnosis of hydatid cysts has been reported in the lung, omentum, muscle, soft tissue, brain, orbit and joints [2,3,4,9].

Hydatid disease can be diagnosed by serology and imaging studies, but these techniques are not definitive. However, all serological tests have low diagnostic sensitivity and specificity. These serological tests are complementary to pathological and radiological tests and if on cytology and radiology characteristic features of hydatid cyst are present, then serological tests may not be done, especially in developing countries where they are expensive tests and not readily available in all diagnostic center [3].

A confirmatory diagnosis can only be made by biopsy or fine needle aspiration cytology. On FNA, the diagnosis is established by demonstrating scolices, hooklets or laminated membrane. However, all the three components may not be visualized in any one given case. Special histochemical stains can be used to highlight these different components. The cyst membrane takes up the periodic Schiff stain while hooklets are better stained by Masson’s trichrome and Ziehl Neelsen stains [4]. 

The best treatment option is complete surgical excision of the intact cyst which avoids leakage of cyst contents. If it is not possible, the cyst contents can be removed intraoperatively and the cyst pouch irrigated with scolicidal solutions. The combination of adjunctive chemotherapy with anti-helminthics pre and post operatively is also recommended to cover the risk of dissemination [6,7].

In a patient with a presumptive diagnosis of cystic hydatid disease, FNA is the primary method of confirming hydatid disease.

Mostly hydatid cyst can be diagnosed on the basis of clinical history and imaging findings. However, when it is present in atypical locations, the diagnosis can be challenging. FNAC in collaboration with radiological investigations is very useful in the preoperative diagnosis of such clinically unsuspected hydatid cysts in atypical locations.

1. Bothale, K.A. et al. “Diagnosis of Primary Hydatid Cyst of Thigh by Fine Needle Aspiration Cytology.” Indian Journal of Medical Microbiology, vol. 33, no. 1, 2015, p. 151.

2. Kim, A.R. et al. “Fine Needle Aspiration Cytology of Hepatic Hydatid Cyst: A Case Study.” Korean Journal of Pathology, vol. 47, no. 4, 2013, pp. 395–398.

3. Bagga, P.K. et al. “Primary Subcutaneous Inguinal Hydatid Cyst: Diagnosis by Fine Needle Aspiration Cytology.” Journal of Clinical and Diagnostic Research, vol. 8, no. 8, 2014, pp. FD11–FD13.

4. Handa, U. et al. “Cytomorphology of Hydatid Disease.” The Internet Journal of Tropical Medicine, vol. 2, no. 1, 2004.

5. Safioleas, M. et al. “Echinococcal Cyst of the Subcutaneous Tissue: A Rare Case Report.” Parasitology International, vol. 57, no. 2, 2008, pp. 236–238.

6. Dirican, A. et al. “Subcutaneous Hydatid Cysts Occurring in the Palm and the Thigh: Two Case Reports.” Journal of Medical Case Reports, vol. 2, no. 1, 2008, p. 273.

7. Ousadden, A. et al. “A Solitary Primary Subcutaneous Hydatid Cyst in the Abdominal Wall of a 70-Year-Old Woman: A Case Report.” Journal of Medical Case Reports, vol. 5, no. 1, 2011, pp. 1–3.

8. Burgazli, K.M. et al. “Unusual Localization of a Primary Hydatid Cyst: A Subcutaneous Mass in the Paraumbilical Region.” European Review for Medical and Pharmacological Sciences, vol. 17, no. 13, 2013, pp. 1766–1768.

9. Basarir, K. et al. “Primary Muscular Hydatidosis Mimicking Soft Tissue Tumour: A Report of Five Cases.” Journal of Orthopaedic Surgery, vol. 16, no. 3, 2008, pp. 368–372.