Lymphocytic Lymphoma in a Ferret with a History of Pancreatic Islet Cell Tumor and Adrenal Tumor: A Case Study

Irina Kim, DVM; Heather L. Tarpley, DVM; Renato S. Sousa, DVM, MS; and Kenneth S. Latimer, DVM, PhD

Class of 2005 (Kim) and Department of Pathology (Tarpley, Sousa, Latimer) College of Veterinary Medicine, University of Georgia, Athens, GA 30602-7388

Signalment – Ferret, Fitch, spayed female, 4-year-old.

Presenting problem – The ferret was presented for scheduled revaluation of the blood glucose concentration four months after a partial pancreatectomy and left adrenalectomy. The owner reported that the ferret appeared to be doing well.

Medical history - Approximately five months prior to presentation, the patient was diagnosed with a probable insulinoma based on intermittent "spacey" episodes and low blood glucose concentrations. The presence of symmetrical ventral alopecia, elevated estradiol levels, and a palpable mass in the area of the left adrenal gland was also suggestive of adrenal disease. The ferret was treated with 0.5 mg/kg oral prednisone BID to stabilize the blood glucose levels until surgery could be performed. An exploratory laparotomy was done to remove the enlarged left adrenal gland and a 5 mm nodule on the right limb of the pancreas. Histopathology of the pancreatic mass was consistent with a benign islet cell tumor. The adrenal mass was suggestive of an adrenocortical spindle cell tumor. Prednisone therapy was tapered over a two-week period and then discontinued. Subsequent monitoring of blood glucose concentrations indicated that glucose values were stable and within the reference range.

Physical examination – On presentation, the patient was bright, alert, and responsive. The alopecia was resolving and the ferret was gaining weight. The heart rate was 300 beats per minute and the respiratory rate was 100 breaths per minute. Body condition score (BCS) was assessed at 2/5 and the patient weighed 720 g. An enlarged, firm, right mandibular lymph node measuring approximately 1.6 x 1.0 cm and an enlarged left prescapular measuring approximately 1.7 x 2.2 cm were palpated. Popliteal lymph nodes were prominent, but surrounded by a large amount of perinodal fat. The remainder of the physical examination was unremarkable.

Diagnostic findings – Blood was collected under isoflurane anesthesia from the jugular vein for a complete blood count (CBC). One week later, the patient returned for a wedge biopsy of the right mandibular lymph node. Thoracic and abdominal radiographs, as well as abdominal ultrasound, were performed.

Complete Blood Count -

Histopathology – Histologic examination of the biopsy sample from the right mandibular lymph node revealed replacement of normal architecture by a diffuse sheet of homogeneous, small, round cells with scant cytoplasm, small nuclei with 1 to 2 evident nucleoli, and mild anisokaryosis. There was 1 mitosis per 10 high power (45x) fields of view (Fig. 1).

Figure 1. A homogeneous population of small lymphocytes is present (Ferret, right mandibular lymph node, hematoxylin and eosin stain).

Immunohistochemical staining indicated that the cells were strongly CD3 positive and CD79a negative. This procedure indicated that the lymphoma was of T cell origin (Fig. 2 and 3)

Figure 2. Neoplastic lymphocytes exhibit strong immunohistochemical staining for CD3 antigen, indicating T lymphocyte origin (Ferret, right mandibular lymph node, CD3 immunohistochemical staining with hematoxylin counterstain).	Figure 3. Neoplastic lymphocytes are unreactive following CD79a staining, demonstrating that the neoplastic lymphocytes are not of B cell origin (Ferret, right mandibular lymph node, CD79a immunohistochemical staining with hematoxylin counterstain).

Radiography - Thoracic abnormalities were not identified on survey radiographs. Metallic hemoclips, consistent with a medical history of previous abdominal surgeries, were identified in the dorsocranial abdomen. The left kidney appeared slightly smaller than the right kidney.

Abdominal Ultrasonography - Abdominal ultrasonography identified multiple hypoechoic lymph nodes throughout the abdominal cavity. The medial iliac lymph nodes were mildly enlarged and a small amount of peritoneal effusion was present. Abnormalities were not identified in the liver, spleen, kidneys, bladder, right adrenal gland, or the right limb of the pancreas.

Problems –

Problem 1 – Peripheral and abdominal lymphadenopathy

The differential diagnosis for lymphadenopathy includes inflammation, lymphoid hyperplasia, and neoplasia. Lymph node cytologic aspirates or biopsy may help to identify the underlying pathology. A wedge biopsy of the right mandibular lymph node showed a diffuse population of small lymphocytes with scant cytoplasm as described above. These findings were compatible with lymphocytic lymphoma. Abdominal lymphadenopathy was noted on ultrasonographic examination of the abdominal cavity. Without aspirates or biopsies, the intra-abdominal presence of lymphoma could not be confirmed. Underlying gastro-intestinal disease such as coronavirus or Helicobacter sp. infections also could lead to enlarged, reactive, abdominal lymph nodes.

Problem 2 - Moderate anemia

The complete blood count (CBC), taken at the time of peripheral lymphadenopathy, revealed a moderate anemia with a decreased red blood cell (RBC) count and hemoglobin (Hb) concentration. The reticulocyte count and plasma protein concentration were not assessed. Anemia may reflect red blood cell loss, destruction, decreased or ineffective erythropoiesis, or hemodilution from intravenous fluid administration. The blood sample was collected under isoflurane anesthesia. In ferrets, isoflurane anesthesia has been associated with a transient but marked decrease in hematocrit (Hct), RBC and Hb concentration values. Greater than 30% difference was noted in these indices when comparing pre-anesthetic and anesthetic blood samples from individual healthy ferrets. The mechanism of this phenomenon is unknown.

When the CBC was repeated 22 days later (on the conscious patient), the Hct and Hb values were just slightly below reference intervals. Given the variations in the published reference ranges for ferrets, these values could fall within the low end of the reference range. Without evidence of hemorrhage, hemolysis, or regeneration, the initial change in Hct was attributed to the effects of isoflurane.

Problem 3 – Mature neutrophilia with lymphopenia

On day 22, the total white blood cell count was within the reference range but the leukocyte differential count indicated a mature neutrophilia with a slight lymphopenia. This leukogram pattern was suggestive of stress associated with endogenous cortisol release.

Diagnosis – T-cell lymphocytic lymphoma

Discussion

In a retrospective study of ferret neoplastic diseases, lymphoma was identified as the third most common form of neoplasia following pancreatic islet cell tumors and adrenocortical cell tumors. The ferret in this case study was afflicted with all three neoplasms. The slow onset of lymphoma and the apparent lack of clinical signs other than lymphadenopathy are more characteristic of the adult-onset form of lymphocytic lymphoma described in ferrets.

The etiology of lymphoma is multifactorial and likely reflects genetic, environmental, and, possibly, infectious causes. Incidents of cluster outbreaks of lymphoma in related or cohabitating ferret populations have led to the speculation of a possible infectious etiology such as Aleutian parvovirus and feline leukemia virus, but definitive links have not been demonstrated.

Lymphoma in ferrets can affect peripheral and visceral lymph nodes, spleen, liver, thymus, bone marrow, kidneys, lungs, and gastrointestinal tract. In addition, lymphoma can occasionally involve the nervous system and skin. Anatomic staging of lymphoma in ferrets has followed the established guidelines for canine lymphoma: Stage I is localized to a single tumor site, Stage II involves multiple sites on same side of the diaphragm, Stage III includes the involvement of the spleen and lymph nodes on both sides of the diaphragm, and Stage IV describes multiple sites on both sides of the diaphragm.

Diagnosis of lymphoma is based on the presence of neoplastic lymphoid cells. Tissue biopsy is recommended to evaluate both cellular morphology and tissue architecture. The complete blood count, serum biochemical profile, thoracic and abdominal radiographs, abdominal ultrasonography, and tissue and bone marrow biopsy or aspirates are useful for staging the disease.

Histologically, lymphomas have been described by the appearance of the neoplastic cell population. Small noncleaved cell (SNC) lymphomas, immunoblastic large cell (IB) lymphomas and immunoblastic polymorphic (IBP) lymphomas are considered high grade neoplasms and statistically are associated with a shorter mean survival time than low grade diffuse small lymphocytic (DSL) lymphoma.

Immunohistochemical staining with T-cell and B-cell markers, such as anti-CD3 and anti-CD79a antibodies, has been employed in the study of canine lymphoma as prognostic indicators. As with people, these studies demonstrated that B-cell lymphomas have a better clinical prognosis than T-cell lymphomas. In dogs, high grade lymphomas are associated with better initial response to chemotherapy but a decrease in overall survival time. Due to the limited number of studies of lymphoma in ferrets, little information exists on immunophenotype and grade classification of ferret lymphoma and subsequent response to treatment.

Clinically, lymphoma in ferrets can be described in two general forms that appear to be associated with age. Ferrets younger than 2 years of age generally present with an acute, aggressive, multicentric form of lymphoma. The neoplastic cells are typically high grade SNC or IB lymphoma cells. Frequently, a mediastinal mass is present but peripheral lymphadenopathy may not be apparent. Clinical signs of juvenile lymphoma relate to the organs affected and may include vomiting, diarrhea, and organomegaly. Coughing or dyspnea may be observed if a compressive, intrathoracic mass is present. Non-specific clinical signs such as anorexia, lethargy, and weight loss are frequently seen. Lymphocytosis has been commonly reported in juvenile lymphoma. Lymphoblasts or abnormal lymphoid cells are not typically observed in stained blood smears, but bone marrow involvement is more likely with the juvenile form of lymphoma. Juvenile lymphoma in ferrets carries a guarded prognosis.

Mature ferrets with lymphoma most often present with chronic, nonspecific signs such as weight loss and lethargy. Peripheral lymph nodes are frequently enlarged and firm. Care must be taken when interpreting the size of popliteal and axillary lymph nodes; large fatty deposits surround these lymph nodes in mature ferrets. Splenomegaly due to extramedullary hematopoeisis or neoplastic infiltration may be palpable. As the disease progresses, other body systems may be affected resulting in variable clinical signs. On the CBC, lymphopenia is more commonly reported than lymphocytosis. The histological classification of the lymphoma cells is variable and may include SNC, IB, IBP and DSL. The diffuse small cell lymphoma (DSL) is classified as low grade malignancy and is associated with a longer survival time. Treatment is aimed at diminishing clinical signs. If remission is achieved, it may last for months to years. In the retrospective study by Erdmans et al of 60 ferrets with lymphoma, several mature ferrets lived for years (mean survival of 3.1 years) without any treatment following the clinical diagnosis of neoplasia.

Treatment options for lymphoma in ferrets include surgical resection of a localized tumor, chemotherapy, or a combination of the two. Radiation therapy and alternative medicines also have been attempted. Various chemotherapeutic protocols have been described for ferrets using oral and intravenous drugs. Given the difficulty of repeated venous catheterizations, implanted vascular access ports are useful to maintain long-term venous access. In general, lymphomas involving the liver, intestines, stomach, and bone marrow reportedly respond poorly to chemotherapy.

Therapy and Patient Outcome

A chemotherapy protocol consisting of L-asparaginase, oral prednisone and cyclophosphamide was started three weeks after initial presentation and diagnosis of lymphoma. Two months into treatment, the patient continued to do well clinically and tolerated the drugs used in the treatment protocol. The affected lymph nodes had decreased in size and firmness, but were still enlarged. Prior treatment of this ferret with prednisone (near the time the pancreatic and adrenal neoplasms were discovered) may have potentially reduced the tumor burden of this patient resulting in initial subclinical disease.

References

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Acknowledgement

The image "Sleeping Ferret Study" by Selina Siu is from her artwork website and is used with permission.

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