Spontaneous Lymphoma in a Prairie Dog (Cynomys ludovicianus)

Introduction

Lymphoma is a common lymphoid neoplasm in domestic animals, and several cases in rabbits and rodents have been reported.^1–11 In the dog and cat, lymphomas are classified according to the anatomical location (e.g., multicentric, thymic, alimentary, cutaneous, and solitary), while in most previous reports of lymphoma in rabbits and rodents, neoplastic cells were detected in many organs.^1–8,11 Lymphoma has often developed in rabbits <1 year of age, but it has been reported in three guinea pigs >2 years of age.^1,3,5–8,11 To the authors’ knowledge, lymphoma has not been reported in the prairie dog. The purpose of this case report is to describe the clinical and histopathological findings of lymphoma in a prairie dog.

Case Report

A 4-year-old, female, black-tailed prairie dog (Cynomys ludovicianus) weighing 1.36 kg was presented with a history of a small, reddish skin mass in the left axillary area that had gradually enlarged over 3 months. Fine-needle aspiration of the mass by the referring veterinarian revealed inflammatory cells; therefore, the animal was treated with antibiotics and antifungal agents. Following treatment, the skin mass became smaller over a month, but another skin mass developed in the right flank and grew rapidly. The prairie dog was then referred to the Veterinary Medical Center at the University of Tokyo for further evaluation.

Upon initial admission, the prairie dog had severe dyspnea and anorexia for the previous few days in addition to the two skin masses. Hair had been lost over the masses, which were 1 cm and 4 cm in diameter [Figure 1]. Thoracic radiography revealed bilateral pleural effusion and a dorsally displaced trachea. Ultrasonography confirmed the presence of pleural effusion and a mass within the anterior mediastinum. Thoracocentesis was performed, and 20 mL of serosan-guineous fluid was removed. Cytology of the fluid revealed many neoplastic lymphocytes, leading to a diagnosis of lymphoma.

The prairie dog was kept in an oxygen-rich cage and initially was given furosemide^a (5 mg/kg subcutaneously [SC]). After diagnosis, prednisolone sodium succinate^b (2 mg/kg SC q 24 hours) was administered. By the third day of hospitalization, the dyspnea had resolved and the prairie dog was clinically normal. On the fifth day, the prairie dog was discharged on prednisolone^c (1 mg/kg orally [PO] q 24 hours), enrofloxacin^d (5 mg/kg PO q 24 hours), and cime-tidine^e (10 mg/kg PO q 24 hours) for 7 days.

One week later, the two skin masses were dramatically reduced in size. Thoracic radiographs revealed no pleural effusion, but the mediastinal mass remained. Both skin masses were aspirated, and orthovoltage radiation^f was instituted for the anterior mediastinal mass under general anesthesia delivered via mask inhalation of isoflurane^g and oxygen. Radiation therapy was stopped prematurely at 3 Gy because of anesthetic difficulties. Cytology of the skin aspirates revealed multi-centric lymphoma, and additional irradiation was canceled. Instead, chemotherapy with cyclophosphamide^h (50 mg/m² PO) was instituted in addition to the prednisolone.

On the day following irradiation and administration of cyclophosphamide, the prairie dog became depressed and dyspneic. Cyclophosphamide was subsequently discontinued. Although thoracic radiographs showed no evidence of pleural effusion, depression and dyspnea persisted despite administration of oxygen and supportive care. The animal died 6 days later.

A complete postmortem examination was performed 20 hours after the death. In the thoracic cavity, a small amount of pleural effusion was present, and a 3 × 3 × 4-cm, white mass was found in the anterior mediastinum [Figures 2A, 2B]. A large amount of discolored, hard fat occupied the abdominal cavity. Mesenteric lymph nodes were enlarged, and both kidneys were irregular in shape and adherent to the surrounding fatty tissues. Transverse sections of the kidneys revealed multifocal, whitish, hard lesions with obscure margins in the cortices.

Histologically, pleomorphic neoplastic lymphocytes with a high mitotic index had infiltrated the nodular lesions of the skin, the mediastinal mass [Figure 3], lungs, abdominal fat, mesenteric lymph nodes, spleen, liver, stomach, duodenum, pancreas, intestines, kidneys, adrenal glands, and uterus. The neoplastic lymphocytes were also observed in blood vessels of many organs. In the bone marrow, a large number of neoplastic lymphocytes and a small number of normal marrow cells were observed. Based on these findings, a definitive diagnosis of multicentric lymphoma was made. Immunohistochemistry was conducted using CD3ⁱ (poly-clonal rabbit antihuman T-cell) and CD3^j (purified mouse antirat CD3 monoclonal antibody) for T lymphocytes; CD45RA^k (purified mouse antirat CD45RA monoclonal antibody), BLA36kD^l (monoclonal mouse antihuman B lymphocyte antigen 36kD), and CD79αcy^m (monoclonal mouse antihuman CD79αcy B cell) were used for B lymphocytes. Despite the use of these markers, the authors could not identify the specific signals needed to characterize the lymphoma cell type.

Discussion

Lymphoma and lymphocytic leukemia have been studied and reported in the ferret, rabbit, and other exotic rodent species.^1–11 The present case is the first report of lymphoma in the prairie dog. Histopathological features of the lesions in the case reported here resembled those of lymphoma in the rabbit.^1–3,7,8 Neoplastic lymphocytes were detected in most organs, including the bone marrow, which was compatible with multicentric lymphoma. In previously reported rabbit lymphomas, a leukemic hematological profile and a large number of circulating neoplastic lymphocytes were noted.^1,2 In this prairie dog, a complete blood count was not performed, because the animal was too dyspneic to withstand venipuncture. Based on the postmortem histopathological findings, however, leukemia was suspected because of the presence of neoplastic lymphocytes in many blood vessels.

While prednisolone was initially effective against the lymphoma, clinical signs deteriorated after a small dose of radiation therapy and administration of cyclophosphamide. Further studies are needed to clarify their unfavorable effects on prairie dogs.

Conclusion

Multicentric lymphoma was diagnosed in a 4-year-old prairie dog. Based on this case, the prairie dog may develop lymphoma and lymphocytic leukemia similar to the rabbit and other rodents. Lymphoma should be included in the differential diagnoses of skin masses, pleural effusion, and anterior mediastinal masses in this species. Prednisolone provided a transient, beneficial effect in this case, but further studies are required to determine if radiation therapy and chemotherapy are suitable for treating lymphoma in the prairie dog.