Small mammal neoplasia (Proceedings)

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.

Rats and Mice

Mammary neoplasia is the most common type of neoplasm diagnosed in both rats and mice. Mammary neoplasia is most common in female rodents but males can occasionally develop mammary tumors.In rats, mammary neoplasms are usually benign mammary fibroadenomas. Although benign, mammary fibroadenomas in rats can become enormous, growing to up to 50% of the animal's body weight.In mice, the majority of mammary tumors are adrenocarcinomas and carcinomas.There is a genetic predisposition in some rodents and the incidence of mammary neoplasia in some rodent strains is as high as 70%.Several different mouse mammary tumor viruses (MMTV) have been identified including retroviruses. The treatment of choice for both rats and mice is complete surgical excision of the affected tissue. Recurrence is quite common in the remaining mammary tissue and clients should be informed of this prior to resection.

Guinea Pigs

Skin neoplasms are well documented in guinea pigsThe most common type of dermal neoplasia in the guinea pig is trichofolliculoma, which is a benign neoplasm of the basal cells. This neoplasm usually presents in the lumbosacral area as a firm, round nodule. Fibrosarcomas, lipomas and sebaceous adenomas have also been documented in guinea pigs. Surgical excision is the treatment of choice for skin neoplasms. This author diagnosed a very large, locally invasive dermal neoplasm (pilomatrixoma) in a guinea pig. An external skin stretching system was used successfully in this guinea pig to allow for primary skin closure after surgical resection.

Mammary neoplasia occurs in both female and male guinea pigsThese tumors are characterized by a soft tissue mass effect in one or both mammary glands. The majority of mammary neoplasms in guinea pigs are benign fibroadenomas. Approximately 30% are adenocarcinomas with metastatic potential. Wide excision of the mammary tissue and local lymph nodes is recommended.

Lymphosarcoma occurs in guinea pigs. In guinea pigs the lymphosarcoma is caused by a type C retrovirus. Clinical signs can include lymphadenopathy and a rough haircoat. Splenomegaly and/or hepatomegaly can also occur. Metastasis can occur rapidly in tissues including the visceral organs. Clinical pathology changes can include a marked leukocytosis with a white blood cell count in the range of 25,000- 500,000 cell/µL. Fine needle aspirates of the peripheral lymph nodes or abdominal ultrasound guided aspirates may aid in the diagnosis. Response to chemotherapy has been noted in some cases.

Pulmonary neoplasia has been documented in guinea pigs. The most common nonhematopoietic neoplasm of the guinea pig is pulmonary adenoma of bronchogenic origin.12 There is currently debate as to whether this disease process is truly neoplastic as the pathologic changes seen with this disease are similar to those seen with adenomatous hyperplasia.

Chinchillas

Neoplasia is rare in chinchillas despite their relatively long life spans in captivity. There is a scarcity of documentation in the medical literature. There was one report of malignant lymphoma in a chinchilla from 1953.12 This author has diagnosed one case of thoracic neoplasia (thymoma) in an adult, male chinchilla.

References

Harkness JE, Wagner JE. Neoplasia. The biology and medicine and rabbits and rodents. Fourth edition. 1995. Williams & Wilkins, Media, PA, pp 251-259.

Paul-Murphy J. Reproductive and urogenital disorders [rabbits]. In Hillyer EV, Quesenberry KE (eds): Ferrets, Rabbits and Rodents: Clinical Medicine and Surgery. Philadelphia, PA, WB Saunders Co, 1997, pp 202-211.

Stein S, Warshaw S. Rabbits. In: Laber-Laird K, Swindle MM, Fleknell P (eds). Handbook of rodent and rabbit medicine. Pergamon Veterinary Handbook Series, Elsevier Science, Inc. Tarrytown, NY, 1996, pp 183-218.

Deeb BJ. Respiratory disease and the Pasteurella complex. In: Hillyer EV, Quesenberry KE (eds): Ferrets, Rabbits and Rodents: Clinical Medicine and Surgery, Philadelphia, PA, WB Saunders Co, 1997, pp 189-201.

Vernau KM, Grahn BH, Clarke-Scott HA, et al. Thymoma in a geriatric rabbit with hypercalcemia and perioditic exophthalmos. J Am Vet Med Assoc 1995; 206: pp 820-822.

Hillyer EV. Dermatologic diseases [rabbits]. In Hillyer EV, Quesenberry KE (eds): Ferrets, Rabbits and Rodents: Clinical Medicine and Surgery. Philadelphia, PA, WB Saunders Co, 1997, pp 212-219.

Gentz EJ, Carpenter JW. Neurologic and musculoskeletal disease. In Hillyer EV, Quesenberry KE (eds): Ferrets, Rabbits and Rodents: Clinical Medicine and Surgery. Philadelphia, PA, WB Saunders Co, 1997, pp 220-226.

Brown SA. Neoplasia [ferrets]. In Hillyer EV, Quesenberry KE (eds): Ferrets, Rabbits and Rodents: Clinical Medicine and Surgery. Philadelphia, PA, WB Saunders Co, 1997, pp 99-114.

Rosenthal KL, Quesenberry KE. Endocrine diseases [ferrets]. In Hillyer EV, Quesenberry KE (eds): Ferrets, Rabbits and Rodents: Clinical Medicine and Surgery. Philadelphia, PA, WB Saunders Co, 1997, pp 85-98.

Donnelly TM. Disease problems of small rodents. In Hillyer EV, Quesenberry KE (eds): Ferrets, Rabbits and Rodents: Clinical Medicine and Surgery. Philadelphia, PA, WB Saunders Co, 1997, pp 307-327.

Fallon MT. Rats and mice. In: Laber-Laird K, Swindle MM, Fleknell P (eds). Handbook of rodent and rabbit medicine. Pergamon Veterinary Handbook Series, Elsevier Science, Inc. Tarrytown, NY, 1996, pp 1-38.

Schaeffer DO, Donnelly TM. Disease problems of guinea pigs and chinchillas. In Hillyer EV, Quesenberry KE (eds): Ferrets, Rabbits and Rodents: Clinical Medicine and Surgery. Philadelphia, PA, WB Saunders Co, 1997, pp 260-327.

Ritzman TK. Use of an external skin stretching device in a guinea pig. ExoticDVM, Vol 2.6, Jan 2001, pp 31-35.