Zymbal Gland (Auditory Sebaceous Gland) Carcinoma Presenting as Otitis Externa in a Pet Rat (Rattus norvegicus)

A 1-year-old sexually intact male fancy rat (Rattus norvegicus) was presented for unilateral purulent otitis externa and weight loss. A small amount of bloody discharge was noted at the ostium of the left ear canal. The canal and the surrounding tissue were firm to the touch. Mild left-sided enophthalmos and facial nerve paralysis were noted. Otoscopic examination demonstrated caseous debris, and a swab of the external ear canal demonstrated a mixed population of rods and cocci on cytological evaluation. Bacterial culture of the external ear canal grew Escherichia coli. The rat was initially placed on oral and topical antimicrobial therapy. Despite treatment, the condition of the affected ear deteriorated and the rat continued to lose weight. Other clinical signs observed at this time included a worsening of the facial nerve paralysis, head tilt, and circling. A biopsy sample of the swollen tissue surrounding the left ear canal was collected; however, the rat died the day following the biopsy procedure. Histopathology of the biopsied tissue revealed an invasive neoplasm consistent with a Zymbal gland (auditory sebaceous gland) carcinoma. Zymbal glands are modified sebaceous glands surrounding the ear canal of rodents. Although chemically induced neoplasia of these glands is frequently observed, spontaneous tumor formation is much less common. Tumors of the Zymbal gland should be considered as underlying causes for unilateral otitis externa in rats.