Primary muscular hydatid cyst with treatment-induced transcutaneous fistulization

Primary muscular hydatid cyst presenting as a thigh mass with spontaneous fistulization following medical treatment, illustrating the diagnostic challenges and the unusual course of extra-hepatic hydatid disease. A 27-year-old man recently emigrated from rural Morocco was evaluated for a painful, progressively enlarging mass of the left thigh present since December 2023. He reported neither trauma nor chronic disease. Clinical examination showed a large, firm, non-inflammatory medial swelling with no evidence of neurovascular compromise. He was afebrile with no systemic symptoms. C-reactive protein was 40 mg/L and the eosinophil count was normal. The patient was referred to our clinic with a magnetic resonance imaging (MRI) performed before ultrasound because a malignant process had initially been suspected. A full-body computed tomography (CT) scan was subsequently obtained. CT (Figure 1a) and MRI (Figure 1b and c) demonstrated a 12 cm multiloculated cyst within the left adductor magnus muscle, without extramuscular extension. Serological tests were positive: indirect haemagglutination assay (Echinococcosis, Fumouze Diagnostics, Levallois Perret, France: 1:5120, cut-off > 1:160), and IgG western blot (Echinococcosis, LDBIO Diagnostics, Lyon, France) revealed P7, P16, P26–28 specific bands. A diagnosis of primary muscular cystic echinococcosis was established. After multidisciplinary discussion and with the patient’s consent, we planned for a pre-treatment with praziquantel 40 mg/kg/day and albendazole 10 mg/kg/day, followed by a total surgical cystectomy. However, three weeks into medical therapy, the lesion became inflamed and ruptured spontaneously, with cutaneous fistulization and extrusion of hydatid membranes (Figure 2a–d). The patient removed and discarded the membranes himself while on holiday, which precluded microscopy or PCR. By 2 months, the fistula had progressively closed with granulation tissue. MRI showed partial collapse of the cyst with a few residual microcysts and limited peripheral infiltration (Figure 1d and e). Given the favourable course, surgery was reconsidered and medical therapy was continued alone; praziquantel was stopped and albendazole increased to 15 mg/kg/day, achieving a plasma sulfoxide level of 1.46 mg/L (therapeutic range 0.65–5.0). At seven months, near-complete clinical (Figure 2e) and radiological (Figure 1f and g) resolution was observed. Follow-up was still ongoing as of November 2025, with clinical and radiological reassessment planned every 6 months.