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

The research article presents a case study of an unusual parasitic infection in a horse’s retained testis, identified as Strongylus vulgaris, and its visualization through ultrasound.

Case Background

• The subject of the study is a 3-year-old Quarter horse stallion presenting with unilateral left cryptorchidism, a condition where one or both of the testes fail to descend.
• The horse was noted for the absence of the left testis in the scrotal bursa, and an enlarged right testis.
• Through an ultrasonographic examination, the left testis was found at the abdominal level and two small hyperechogenic lines, similar to a cylindrical cavitary foreign body, were observed within a non-homogeneous area with blurred margins.

Parasitic Invasion

• The testis was removed via laparoscopy, a minimally invasive surgical procedure. The cut section of the testis revealed two live adult nematodes (parasitic worms) emerging from 2mm blood-filled tracks within the testicular parenchyma (functional tissue).
• These parasites were removed, cleaned in a sterile saline solution, and morphologically identified as Strongylus vulgaris, a common horse parasite.
• Upon histological examination, the tracks left by the parasites were noted to be characterized by lymphoplasmacytic infiltrates, with heavy haemorrhage invading surrounding structures.

Scientific Significance

• Instances of erratic migration of S. vulgaris have been reported in retained testes of stallions before, but it is considered uncommon. This case adds to the very few cases reported in literature, highlighting the rarity of such an aberrant localization of S. vulgaris in the cryptorchid testis.
• In addition, the ultrasound findings related to the presence of the parasite and its tracks or tunnels (the so-called ‘train track’) are also reported, which may be helpful in diagnosing similar cases in the future.