Filarial dance--sonographic sign of filarial infection.

A 17-year-old boy presented with a 3-month history of swelling and vague pain in the scrotum. US revealed multiple anechoic cyst-like lesions in the body of left epididymis. These cysts showed tubular echogenic internal structures with peculiar twirling motion. This was recognised as the sonographic filarial dance-sign of live adult filarial worms. The boy subsequently underwent needle aspiration of the lesion, which microscopically demonstrated microfilaria of Wuchereria bancrofti. Our report includes an online video clip that will help familiarise readers with the filarial dance.