Introduction

• The thymus of rabbits is large, persists into adulthood, and is located in the cranial mediastinum. 
• Benign (thymoma) and malignant (thymic lymphoma, thymic carcinoma) neoplasms have been reported. Thymomas are more common.
• Cause: unknown. Genetics or previous infections have been suggested as predisposing factors in some cases.
• Signs: clinical signs are generally associated with space-occupying mass in the mediastinum.
• Diagnosis: based on clinical signs, imaging and cytology/histology.
• Treatment: radiation therapy is now the treatment of choice, although surgery also available.
• Prognosis: relatively good with radiation therapy.

Presenting signs

• Dyspnea - generally progressive, but it can go unnoticed by owners until the rabbit presents with an acute onset of dyspnea, tachypnea and open mouth breathing.
• Exercise intolerance.
• Anorexia and lethargy (with consequent gastrointestinal stasis Gastric dilation and stasis in some cases).
• Bilateral exophthalmos Eye: exophthalmos and prolapse of the third eyelid .
• Swelling/edema of head, neck and forelimbs has also been seen due to impaired venous return to the heart.

Acute presentation

• The condition can be presented acutely, probably due to inability of owners to detect more chronic signs such as exercise intolerance and tachypnea.

Age predisposition

• May be more common in older rabbits (older than 3 years old).
• Median age of presentation is 6 years old.

Public health considerations

• Thymomas can be concurrent with other zoonotic infections such as mycobacteriosis.

Cost considerations

• Advance imaging techniques, surgery and radiation therapy may be very expensive.

Special risks, eg anesthetic

• Surgical treatment is technically challenging and carries high risk of complications.
• Radiation therapy involves repeated frequent anesthetic episodes.

Pathogenesis

Diagnosis

Treatment

Sequelae

Further Reading

Publications

Refereed papers

• Recent references from PubMed and VetMedResource.
• Dolera M, Malfassi L, Mazza G et al (2016) Feasibility for using hypo fractionated stereotactic volumetric modulated arc radiotherapy (VMAT) with adaptive planning for treatment of thymoma in rabbits: 15 cases. Vet Radiol Ultrasound 57 (3), 313-320 PubMed.
• Andres K M, Kent M, Siedlecki C T et al (2012) The use of megavoltage radiation therapy in the treatment of thymomas in rabbits: 19 cases. Vet Comp Oncol 10 (2), 82-94 PubMed.
• Sanchez-Migallon D G, Mayer J, Gould J et al (2006) Radiation therapy for the treatment of thymoma in rabbits (Oryctolagus cuniculus). J Exotic Pet Med 15 (2), 138-144 VetMedResource.
• Florizoone K (2005) Thymoma-associated exfoliative dermatitis in a rabbit. Vet Dermatol 16 (4), 281-284 PubMed.
• Morrisey J K, McEntee M (2005) Therapeutic options for thymoma in a rabbit. Semin Avian Exotic Pet Med 14 (3), 175-181 VetMedResource.
• Wagner F, Beinecke A, Fehr M et al (2005) Recurrent bilateral exophthalmos associated with metastatic thymic carcinoma in a pet rabbit. JSAP 46 (8), 393-7 PubMed.
• Clippinger T L, Bennett R A, Alleman A R et al (1998) Removal of a thymoma via median sternotomy in a rabbit with recurrent appendicular neurofibrosarcoma. JAVMA 213 (8), 1140-1143 PubMed.
• Vernau K M, Grahn B H, Clarke-Scott H A et al (1995) Thymoma in a geriatric rabbit with hypercalcemia and periodic exophthalmos. JAVMA 206 (6), 820-2 PubMed.
• Kostolich M & Panciera R J (1992) Thymoma in a domestic rabbit. Cornell Vet 82 (2), 125-129 VetMedResource.
• Fox R R, Meier H, Crary D D et al (1971) Hemolytic anemia associated with thymoma in the rabbit. Genetic studies and pathological findings. Oncology 25 (4), 372-382 PubMed.

Other sources of information

• Lewis W (2013) Mediastinal Masses and Other Thoracic Surgery. In: BSAVA Manual of Rabbit Surgery, Dentistry and Imaging. Gloucester, UK. pp 257-268.
• Antinoff N (2009) Mediastinal Masses in Rabbits: Another Therapeutic Option. In: Proceedings of the Association of Exotic Mammal Veterinarians. Milwaukee, Wisconsin, USA. pp 67.
• Bennett R A & Rosenthal K L (2007) Thymoma in Rabbits: Diagnosis and Treatment Options. In: Proc of NAVC. pp 1609-1611.
• Quesenberry K (1997) Lymphoproliferative Disorders. In: Ferrets, Rabbits and Rodents Clinical Medicine and Surgery. 2nd edn. Saunders, Philadelphia. pp 216-219.