What is the “train track” in the retained equine testis?

A 3-year-old Quarter horse stallion with unilateral left cryptorchidism was referred for the absence of the left testis in the scrotal bursa and an enlarged right testis. The ultrasonographic examination revealed the presence of the left testis at the abdominal level. Two small hyperechogenic lines were observed running parallel and resembling a cylindrical cavitary foreign body, within a non-homogeneous area with blurred margins. The testis was removed by laparoscopy and macroscopically, the testis's cut section revealed the presence of two live adult nematodes emerging from 2 mm blood-filled tracks within the testicular parenchyma. The parasites were isolated and washed in sterile saline solution and morphologically classified as Strongylus vulgaris. Histologically, the tracks were characterized by a lymphoplasmacytic infiltrates with abundant haemorrhage invading the surrounding structures. S. vulgaris erratic migration has been reported in the retained testis of stallions, but this localization is considered uncommon; the case here reported is one of the few cases reported in the literature highlighting the rarity of S. vulgaris aberrant localization into the equine cryptorchid testis. This case also reports the ultrasonographic findings related to the parasite presence and track.