Filarial dance sign

The filarial dance sign refers to a twirling motion of microfilariae (e.g. W. bancrofti) in dilated lymphatic channels. It is identified as a characteristic sign of scrotal filariasis.

The dilated channels are identified with the absence of color flow on color Doppler study and the microfilariae as curvilinear echogenic undulating structures within it. Generally, 5-6 microfilariae are seen per channel.

Practical point

It is important to note that this sign is highly characteristic of filarial infection in the right clinical context but is by no means pathognomonic. It has also been reported on scrotal ultrasound in individuals with no history of exposure to filariae, but who clinically have epididymal obstruction. Indeed in a large published series from the UK, a country in which filariasis is not endemic, most of the affected patients had undergone a vasectomy ⁴. 

Histopathologically, the randomly moving echogenicities in one of these reported cases comprised multiple macrophages admixed with partially-broken down clumped together spermatozoa ²; although other explanations have also been posited as the origin of these appearances ⁴.

History and etymology

The filarial dance sign was first described by Fernando Amaral (fl. 2019), a Brazilian radiologist, and colleagues in 1994, in the scrota of 14 patients infected by Wuchereria bancrofti ³.

Differential diagnosis

• dancing megasperm