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Small mammal neoplasia (Proceedings)

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Oct 01, 2008

Neoplastic disease is a common clinical entity in exotic small mammals (rabbits, ferrets, rodents). Affected patients can exhibit a wide variety of clinical conditions. The purpose of this presentation is to present an overview of neoplasia in exotics mammals.

Rabbits

Reproductive neoplasia is common in intact, female rabbits over four years of age. Uterine adenocarcinoma is the most frequently diagnosed neoplasia in intact, female rabbits. Although all intact female rabbits are susceptible to developing uterine adrenocarcinoma, certain breeds such as the Dutch, Havana, and French Silver have an incidence of 50-80%.Adenocarcinoma of the uterus is a slowly developing tumor that can have local invasion of the myometrium and peritoneal cavity. Metastatic uterine adenocarcinoma has been documented in the lungs, liver and skeleton.1-3 Clinical signs of uterine adenocarcinoma include reproductive problems such as decreased litter size, fetus retention or resorption, stillbirths or decreased fertility. Hematuria or vaginal discharge can also occur. Cystic mammary glands have occurred in association with this neoplasm. The rabbit patient may exhibit other clinical signs of anorexia, depression or dyspnea if pulmonary metastasis has occurred. Diagnosis is based on the palpation or imaging (via radiography or ultrasonography) of an enlarged uterus or uterine masses in the caudal abdomen. Thoracic radiographs are indicated to evaluate the patient for evidence of pulmonary metastasis. The treatment of choice for this neoplasm is surgical ovariohysterectomy. If the neoplasia is limited to the reproductive tract then the prognosis is good. Prevention of this disease in the female rabbit is ovariohysterectomy prior to two years of age. There is currently no successful chemotherapy protocol for this neoplasm in rabbits.

Lymphosarcoma has been documented in the rabbit patient. Affected rabbits usually present with weight loss, lethargy, anorexia or anemia. The visceral form is the most common in rabbits and young adults and juveniles are the most frequently affected. Diagnosis is similar to that in other mammals.

Neoplasia of the respiratory system in rabbits has been reported but it is uncommon in the author's practice. Thymomas have been documented in both young and adult rabbits. Clinical signs usually include tachypnea or dyspnea due to a mass effect within the pulmonary tissues. Thymomas can be either epithelial or lymphoid in origin. There is currently no treatment available for this neoplasm and the prognosis is grave for these patients.

Cutaneous neoplasia does occur in rabbits and can present in various forms. Documented types of cutaneous neoplasia in rabbits include lipomas, subcutaneous lymphoma, fibroma and basal cell neoplasia. A fine needle aspirate of the mass often helps to identify the type of neoplasm. The treatment of choice for most skin neoplasms is surgical excision. This author has diagnosed malignant melanoma of the plantar aspect of the rear foot in a rabbit patient. The prognosis varies depending on the type of neoplasia and location. Tumors affecting the musculoskeletal system in rabbits include leiomyosarcoma, leiomyoma, osteochondroma and osteosarcoma. Gastrointestinal and nervous system neoplasia is uncommon in rabbits.

Ferrets

Neoplasia is a very common clinical condition in ferrets. Neoplastic disease can occur at any age but is diagnosed most often in ferrets three years of age and older.8 Potential predisposing factors for the development of neoplasia in ferrets in the United States include early neutering which may interfere with the endocrine system development, genetic predisposition, diet, lack of natural photoperiod or exposure to natural sunlight and infectious agents. Surveys of neoplastic disease give indication of the more common types of neoplasia include lymphoma, adrenocortical neoplasia, mast cell neoplasia and pancreatic neoplasia (insulinoma).

Lymphoma occurs in both young and older ferrets. This disease usually involves the peripheral and visceral lymph nodes, liver, spleen, intestine, mediastinum, bone marrow, lung and kidney. Cutaneous involvement has also been described. The hemogram can vary with lymphoma and can show either a lymphocytosis or lyphopenia, anemia or neutropenia. Younger affected ferrets often show a lymphocytosis, but older ferrets with chronic disease usually become lymphopenic. A lymphoblastic leukemia is documented in fewer than 25% of ferrets with lymphoma. Diagnosis is usually based on a combination of testing methods including hematology, imaging and analysis of fine needle aspirate samples, bone marrow analysis or excisional lymph node or tissue biopsies. Classification information for lymphoma in ferrets is available in the literature and is based on the National Cancer Institute's Working Formulation.8

Chemotherapy protocols have been developed for ferrets based on other mammalian protocols. Remission may be possible in some ferrets depending on the organ systems affected and severity of the disease. A complete cure for lymphoma is not possible and ferrets diagnosed with lymphoma require regular recheck hematology and follow-up diagnostics to monitor progress. Palliative therapy for lymphoma in ferrets that are unsuitable candidates for chemotherapy include glucocorticoid administration. Glucocorticoids such as prednisone can actually cause destruction of some tumor cells. Nutritional support is beneficial, such as syringe feeding and vitamin C supplementation.

Adrenocortical disease is frequently diagnosed in ferrets and adrenocortical neoplasia is a common cause of adrenal gland enlargement and disease. Adrenocortical adenomas and adenocarcinomas are neoplastic changes noted in ferrets. Benign adrenocortical hyperplasia can occur as well. Definitive diagnosis of adrenocortical neoplasia is based on histopathological evaluation of the affected adrenal gland from biopsy.9 Surgical therapy is recommended to remove the affected adrenal gland, or in the case of bilateral adrenal gland involvement, a subtotal adrenalectomy on one side and total adrenalectomy on the other is recommended. Nonsurgical forms of therapy are available such as hormone therapy. Neoplastic changes of the adrenal glands are not likely to respond to medical therapy alone.

Pancreatic beta cell tumors (insulinomas) are frequently diagnosed in ferrets in the United States. Ferrets with this disease will usually present with clinical signs of hypoglycemia which can include rear limb weakness, gradual onset of weakness, increased salivation, episodes of collapse, unresponsiveness or even seizurues.9 Surgical and medical management is aimed at regulating the blood glucose level in the patient.

Many other neoplastic diseases occur in ferrets. A copious amount of information is available in the literature and the clinician is directed to these references for a complete summary of neoplastic disease in ferrets.