Sebaceous adenitis associated to thymoma in rabbits

Esther van Praag, Ph.D.

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The etiology of sebaceous adenitis – also called inflammation of the sebaceous glands - is not well understood; it is considered idiopathic, inherited or endocrine. In rabbits, the disorder appears to have an inherited autoimmune origin, accompanied by a defect of the fatty acids metabolism. An autoimmune origin also established in dogs, after immunohistological analysis of skin samples, but also from the successful treatment with cyclosporine, an immunosuppressant drug.

Different types of thymoma have been observed in rabbits:

· Thymoma, with neoplastic development of the gland;

· Lymphocytic thymoma, with increased presence of lymphocytes;

· Cystic thymoma, with accumulation of fluid in the gland.

Clinical characteristics

The first clinical manifestations of sebaceous adenitis resemble those of skin allergy: inflamed sebaceous gland with progressive destruction of the glands and the adjacent hair follicles, accompanied by inflammation of the hair follicles (mural lymphocytic folliculitis). The condition worsens over time. Hair growth stops, the fur is thinning, and alopecic patches appear. The skin becomes erythematous, with abnormal thickening (hyperkeratosis). Scales adhere tightly to the skin. Infiltration of lymphocytes into the basal layer of the epidermis (interface dermatitis) is furthermore observed in rabbits. This causes changes in the basal cells of that layer, necrosis of keratinocytes and occasionally inflammation of the follicular-dermal interface (interface folliculitis).

Sebaceous adenitis in rabbits can occur in patches or be progressive, with non-pruritic scaling on the head. It later spreads to the neck, the pelvic region, and the rest of the body. Lesions are often symmetrical over the head and abdomen.

Diagnosis

Radiography of the chest helps rule out thymoma.

Angie Lai

Chest x-ray from a rabbit suffering respiratory distress. There is an increased soft tissue opacity in the cranio-ventral lung fields. The mass, tumor, abscess or granuloma leads to a dorsal displacement of the heart and trachea. In rabbits, this situation relates often to the presence of an abscess, to thymoma or to a primary lung tumor. Further tests include an ultrasound guided or blind fine needle aspirate.

Inflammation of the sebaceous glands is often mistakenly diagnosed as skin allergy. As a result, it is treated inappropriately. Skin problems such as fungal dermatitis, parasitic infestation or defluxion will often be considered, when the proposed skin allergy treatment fails to bring an improvement. This skin condition should furthermore be differentiated from thymoma-associated exfoliative dermatitis, cutaneous lymphoma, or dermatitis caused by autoimmune hepatitis.

Treatment

Depending on the health condition of the rabbit, the type of thymoma, and prognosis, different treatment options are available.

· Radiation therapy has brought remission and decrease of the size of the tumor in rabbits suffering from thymoma, with neoplastic development of the gland;

· Corticosteroids, e.g., prednisolone, can bring relief in case of lymphocytic thymoma;

· Drainage of fluids has successfully brought relief in case of cystic thymoma, with accumulation of fluid in the gland.

Since growth of thymoma is slow, it is sometimes chosen not to treat. The tumor will increase in time and compress heart and lungs, leading to compression of the heart and increase respiratory distress.

If thymoma is accompanied by sebaceous adenitis, a regular brushing also helps to remove the dandruff, which is a source of secondary bacterial infection.

Treatments with an antifungal, corticosteroid or an immunosuppressant drug did not bring improvement. The administration of fatty acids, vitamin A or retinoids (e.g. isotretinoin, etretinate) can be attempted, but the toxicity of these compounds must be taken into account.

The combined administration of cyclosporine (5 mg/kg, PO, sid), medium chain triglycerides and essential fatty acids, accompanied by topical application of propylene glycol sprays was successful in the treatment of sebaceous adenitis in a rabbit. Two months later, remission was observed, with new hair growth. Since this treatment is costly, euthanasia should be considered as a humane alternative, so as not to unnecessarily prolong the rabbit's pain and suffering.

Bella, before undergoing radiotherapy
Confirmed sebaceous adenitis associated to thymoma
Side view of the head Nancy Martin	Top of head Nancy Martin
Nancy Martin
A few months after radiotherapy, Bella developed a strange gait... Nancy Martin

References

Florizoone K. Thymoma-associated exfoliative dermatitis in a rabbit. Vet Dermatol 2005;16:281-284.

Jassies - Van der Lee A, van Zeeland Y, Kik M, Schoemaker N. Successful treatment of sebaceous adenitis in a rabbit with ciclosporin and triglycerides. Vet Dermatol 2009;20:67-71.

Quesenberry KE, Carpenter JW. Ferrets, rabbits and rodents. Clinical Medicine and Surgery. St Louis, USA: Saunders; 2004.

White SD, Linder KE, Schultheiss P, Scott KV, Page G, Taylor M, Best SJ, Walder EJ, Rosenkrantz W, Yager JA. Sebaceous adenitis in four domestic rabbits (Oryctalagus cuniculus). Vet Dermatol 2000;11:53-60.

A special thanks to Angie Lai (Singapore), to Nancy Martin (USA) for sharing radiographs and pictures of the rabbit Bella, respectively, and to Debbie Hanson (USA) for making the video.

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