Thymoma in Rabbits
Rabbits normally have a large thymus that is placed cranial to the heart and extends into the thoracic inlet. Thymoma, thymic lymphoma and thymic carcinoma have all been reported in rabbits. Thymic lymphoma and carcinoma are rare in rabbits.The overall incidence of thymoma in rabbits is low (around 7% of reported neoplasms in 55 colony rabbits) with no apparent sex predilection. The mean age at presentation is 6.7 years based on one survey of 19 cases. Rabbit thymomas are generally slow growing and are potentially locally invasive tumors that rarely metastasize.
Rabbits may present with difficulty breathing if they have a cranial mediastinal mass. Since rabbits are obligate nasal breathers,open-mouth breathing is generally not seen. Signs of a rabbit in respiratory distress can include nostril flaring, tachypnea and increased abdominal excursions. Bilateral exophthalmos with third eyelid prolapse may be seen if there is pooling of blood in the retrobulbar venous plexus, secondary to cranial vena cava compression (Fig. 1). Vascular compression can also lead to edema of the head, neck and forelimbs. A heart murmur may also be present due to the compression of the heart by the mass.
Suspected paraneoplastic syndromes in rabbits include hemolytic anemia and exfoliative dermatitis. Paraneoplastic hyercalcemia is questionable in rabbits with thymoma; however, there is one report which described hypercalcemia (14.7 mg/dL) in a rabbit with thymoma which likely represented non-neoplastic high normal calcium as the reference range for canine patients. Rabbits with thymoma may present with generalized scaling and sebaceous adenitis.
Rabbits with thymic masses may have a normal complete blood count, or an elevated white blood cell count, or anemia. Lymphocytosis is often noted and heterophilia has also been reported. Thoracic radiographs are important to diagnose a mediastinal mass. If a mass is seen, we recommend an ultrasound-guided fine-needle aspiration for cytologic evaluation be perormed. Cytologic evaluation of cases with thymoma generally yields predominantly mature lymphocytes as opposed to lymphoblasts, which would support a diagnosis of lymphoma. Computed tomography with contrast is recommended to determine the location and extent of the thoracic mass, especially if surgery or radiation therapy are treatment options.
Treatment options for thymoma in rabbits can include chemotherapy, surgery, or radiation therapy. Consultation with an oncologist is recommended when clients are interested in pursuing chemotherapy or radiation therapy. At this time, clients should be informed of risks, benefits and potential side effects of treatment options. During treatment and evaluation, it is important to consider rabbits are prey species with specialized gastrointestinal physiology and do not tolerate the stress of treatment in the same manner as a dog or cat.
Anecdotal information is available on the use of various chemotherapeutic agents in rabbits. If rabbits have underlying disease conditions such as pasteurellosis or encephalitozoonosis, chemotherapy may be contraindicated. Additionally, side effects from chemotherapy can include severe anemia, enteritis, typhlitis, and nephrotoxicity. Rabbits with thymomas have been treated with immunosuppressive therapy, including prednisone. Steroids should be used with caution in rabbits and risks of immunosuppression should be discussed with clients prior to use.
Surgical excision of thymoma has been reported in rabbits, but the risk of surgical or anesthetic-related complications is high. In three rabbits treated with surgery alone, postoperative survival ranged from 8 months to 3 years. In a series of rabbits treated by median sternotomy for mediastinal masses, 7 of 14 rabbits survived 6 months or longer. One rabbit died during surgery, 6 died within 10 days after surgery, 1 survived 6 to 12 months after surgery, 4 survived 12 to 14 months after surgery and 2 survived more than 24 months after surgery.
Radiation therapy (RT) appears to be a good treatment option for rabbits with thymomas. In a recent study of 19 rabbits treated with radiation, median overall survival was 313 days; when 3 rabbits that died acutely during the first 14 days of treatment were excluded, median survival was 727 days. Complications associated with radiation were uncommon and included radiation-induced myocardial failure, radiation pneumonitis and alopecia. Tumors generally respond rapidly and decrease in size after a few treatments. Risk factors to consider with radiation therapy include the number of anesthetic episodes and radiation-associated side effects, including late-term side effects such as pulmonary or myocardial fibrosis. The cost of radiation therapy should also be discussed with clients as cost maybe a factor given the number of treatments necessary.