Primary hydatid cyst of psoas muscle in an adolescent.

Hydatid disease, a zoonotic infection caused by Echinococcus granulosus, primarily affects the liver and lungs but can involve other areas, including muscle tissue, though muscle involvement is rare. This case describes an adolescent girl with a psoas muscle hydatid cyst presenting with abdominal pain, a palpable mass and polymenorrhoea. Imaging and serology confirmed the diagnosis, and treatment included albendazole therapy and surgical excision via transperitoneal laparotomy, followed by a successful recovery with no recurrence at an 18-month follow-up.Hydatid disease is transmitted from definitive hosts, like dogs, to humans. Diagnosis can be challenging due to non-specific symptoms, and it typically relies on ultrasound, CT and serology. Surgical removal combined with antiparasitic drugs remains the primary treatment, though newer methods like ultrasound-guided aspiration are emerging. This case underscores the need to consider hydatid disease in differential diagnoses of cystic masses in endemic areas, particularly in paediatric cases.