1-year-old, female, quarter horse

Draining tract at the base of the left ear


A 1-year-old quarter horse filly presented for a left peri-aural draining tract, which had been present since she was 2 months old. Prior to presentation, a small lump and wound formed near the draining tract, between the left eye and the left ear. This area was clipped and cleaned by the referring veterinarian, after which this lump resolved and wound scabbed over. At the time of examination, a small scab persistent near the left ear base. The patient was refractory to palpation of that area.


CT Images

The above video displays transverse reconstructions of the head in series in the following order: precontrast bone, precontrast soft tissue window, postcontrast soft tissue window. The right side of the patient is on the left side of the image.

• Dorsal of the right horizontal ear canal is a medium, lamellar, tooth-like mineral structure. This was continuous with the temporal bone zygomatic process by a thick, smooth osseous extension. The regional soft tissues are normal in size, enhancement, and definition. No local fluid accumulation or tracts towards the right pinna were detected.

• No left temporal region anomalous dental structures are detected. The zygomatic process of the left temporal bone has a small, round, geographical cortical defect.

• Bilaterally, the axial aspects of the mandibular condyloid processes have broad regions of flattened, hypoattenuating subchondral bone (R>L) and focal to coalescing medullary hypodensities.


• Right heterotopic polydontia (aka dentigerous cyst, temporal teratoma, ear tooth) is present.

• Bilateral mandibular coronoid process osteochondral defects are likely age-related findings, and may resolve with maturation.

• The patient was diagnosed with a right-sided dentigerous cyst, without associated draining tract. No left-sided dentigerous cyst or peri-aural cystic structure was seen. No further diagnostics or treatments were performed at that time.

• In equids with heterotopic polydontia, aberrant tooth germ cells are displaced with the first branchial cleft towards the temporal region. This can be unilateral or bilateral, and is commonly associated with a sinus tract at the base of the pinna.


• Fessler JF. Heterotopic polyodontia in horses: nine cases (1969-1986). JAVMA 1988.

• Stieger-Vanegas SM and Hanna A. The role of computed tomography in imaging non-neurologic disorders of the head in equine patients. Frontiers 2022.

• Carmat JL, Kneissl S, Rawlinson JE et al. CT appearance of the temporomandibular joint in 1018 asymtomatic horses. Vet Rad Ultrasound 2016.