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.
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.
Radiography of the chest helps rule out thymoma.
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.
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.
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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.