Unusual splenic B-cell lymphoma in two related Sumatran tigers (Panthera tigris sumatrae)

Lana Krol 1San Francisco Zoo, San Francisco, CA 94132, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616.

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William Vernau 2Departments of Pathology, Microbiology, and Immunology, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616.

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Adrian G. Mutlow 1San Francisco Zoo, San Francisco, CA 94132, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616.

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Sean M. Brady 5Wildlife Health Center, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616.

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Raymund F. Wack 5Wildlife Health Center, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616.

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Sarah Kubly 6West Radiology Services, Oakland, CA 94611.

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Allison L. Zwingenberger 3Surgical and Radiological Sciences, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616.

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William T. N. Culp 3Surgical and Radiological Sciences, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616.

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Carrie Palm 4Medicine and Epidemiology, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616.

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Robert B. Rebhun 3Surgical and Radiological Sciences, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616.

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Abstract

CASE DESCRIPTION

A 14-year-old 120-kg (264-lb) sexually intact male Sumatran tiger (Panthera tigris sumatrae) and its 10-year-old 130-kg (286-lb) sexually intact male offspring were housed separately and evaluated independently after experiencing weeks of ongoing malaise, weight loss, and anorexia.

CLINICAL FINDINGS

Both animals were immobilized and anesthetized for physical examinations and diagnostic testing. Complete blood counts revealed leukopenia and anemia in both tigers. Splenomegaly was identified on abdominal ultrasonography. Cytologic examination and immunohistochemical staining of splenic samples confirmed intermediate to large B-cell lymphoma; no evidence of lymphoma in surrounding organs was noted.

TREATMENT AND OUTCOME

The sire was treated with lomustine and prednisolone. This tiger was euthanized 21 months after initiation of treatment because of chronic progressive renal disease. The male offspring was treated with l-asparaginase but did not respond to the treatment. A splenectomy was performed, and malaise and anorexia resolved. No further chemotherapy was administered, and the male offspring was instead maintained on a low dose of prednisolone. Thirty-two months after diagnosis, the male offspring was still considered to be in remission.

CLINICAL RELEVANCE

To our knowledge, this was the first known report of the diagnosis and management of a splenic B-cell lymphoma in a tiger. Both tigers achieved positive clinical responses and long-term survival by means of different treatment modalities. The finding of such an unusual neoplasm in a male tiger and its male offspring was noteworthy, raising the possibility of a genetic predisposition for this lymphoma type.

Abstract

CASE DESCRIPTION

A 14-year-old 120-kg (264-lb) sexually intact male Sumatran tiger (Panthera tigris sumatrae) and its 10-year-old 130-kg (286-lb) sexually intact male offspring were housed separately and evaluated independently after experiencing weeks of ongoing malaise, weight loss, and anorexia.

CLINICAL FINDINGS

Both animals were immobilized and anesthetized for physical examinations and diagnostic testing. Complete blood counts revealed leukopenia and anemia in both tigers. Splenomegaly was identified on abdominal ultrasonography. Cytologic examination and immunohistochemical staining of splenic samples confirmed intermediate to large B-cell lymphoma; no evidence of lymphoma in surrounding organs was noted.

TREATMENT AND OUTCOME

The sire was treated with lomustine and prednisolone. This tiger was euthanized 21 months after initiation of treatment because of chronic progressive renal disease. The male offspring was treated with l-asparaginase but did not respond to the treatment. A splenectomy was performed, and malaise and anorexia resolved. No further chemotherapy was administered, and the male offspring was instead maintained on a low dose of prednisolone. Thirty-two months after diagnosis, the male offspring was still considered to be in remission.

CLINICAL RELEVANCE

To our knowledge, this was the first known report of the diagnosis and management of a splenic B-cell lymphoma in a tiger. Both tigers achieved positive clinical responses and long-term survival by means of different treatment modalities. The finding of such an unusual neoplasm in a male tiger and its male offspring was noteworthy, raising the possibility of a genetic predisposition for this lymphoma type.

A 14-year-old 120-kg (264-lb) sexually intact male Sumatran tiger (Panthera tigris sumatrae) underwent a physical examination after a 3-week period of malaise, polydipsia, and intermittent anorexia. The tiger was immobilized with butorphanol (0.1 mg/kg [0.045 mg/lb], IM), medetomidine (0.02 mg/kg [0.009 mg/lb], IM), and midazolam (0.2 mg/kg [0.09 mg/lb], IM). Anesthesia was maintained with isoflurane delivered through an endotracheal tube. No major external abnormalities were noted on physical examination. Results of a CBC and serum biochemical analysisa revealed anemia (PCV, 17%; reference interval, 29% to 49.5%), leukopenia (1,650 WBCs/μL; reference interval, 5,700 to 14,000 WBCs/μL), reticulocytosis (2%; reference interval, 0% to 0.3%), and an increased serum alanine transaminase activity (691 U/L; reference interval, 23 to 160 U/L). It was concluded that the tiger had a nonregenerative anemia and leukopenia, with hepatopathy of unknown origin. An abdominal ultrasonographic assessment was attempted, but because of patient size, not all organs were visible at the time. The partial scan did reveal a markedly enlarged spleen with a heterogeneous appearance. The tiger was treated for anemia during the procedure with ferric hydroxide (5.5 mg/kg [2.5 mg/lb], IV). Erythropoietin injections (83.3 U/kg [37.86 U/lb], IM, q 72 h) were delivered by a remote dart injection until the next physical examination 2 weeks later.

For the second physical examination, the tiger was again immobilized for a thorough abdominal ultrasonographic evaluation, collection of splenic and mesenteric lymph node fine-needle aspirate samples, and collection of blood samples for CBC and serum biochemical analysis. Abdominal ultrasonographic findings confirmed the presence of an enlarged spleen and liver with homogeneous echotextures but no enlargement of the mesenteric lymph nodes. The recheck CBC and serum biochemistry valuesa indicated anemia (PCV, 17%), mild azotemia (serum creatinine concentration, 4.1 mg/dL [reference interval, 0.4 to 4.0 mg/dL; median, 1.9 mg/dL; mean, 2.0 mg/dL]), and an increased serum alanine transaminase activity (746 U/L). The WBC count was improved at 5,940 WBCs/μL.

Cytologic examination of the mesenteric lymph node and splenic samples revealed probable lymphoma of the spleen, with marginal zone lymphoma considered the most likely type.b A splenectomy was considered but not performed because of the tiger's marked malaise.1 Owing to the difficulties of administering IV or SC therapeutics, a single-agent chemotherapy approach with oral administration of lomustine was chosen.2,3 Lomustinec (50 mg/m2) was administered orally every 6 weeks, and the tiger received prednisolone (1 mg/kg [0.45 mg/lb], PO, q 24 h). Lomustine has been documented to cause hepatopathy in dogs, and although this has not been documented for cats, given the tiger's hepatopathy, S-adenosylmethionine (17.7 mg/kg [8.05 mg/lb], PO, q 24 h) was administered.4

Four months later, the tiger was again immobilized for a recheck examination. Abdominal ultrasonography revealed a reduction in splenic size, bilateral dilation of the renal pelvises with renal stones, and uroliths in the urinary bladder. A CBC and serum biochemical analysis revealed a WBC count of 4,500 WBCs/μL and an improved PCV of 28%. The serum creatinine concentration was stable at 4.0 mg/dL. Serum alanine transaminase activity was within reference range at 99 U/L. Recheck cytologic examination of fine-needle aspirate samples of the spleen was performed. Findings confirmed lymphoma with intermediate-sized lymphocytes and prominent single central nucleoli, most consistent with marginal zone lymphoma. High-grade lymphoma, however, could not be equivocally excluded without architectural assessment and histologic examination of a splenic tissue specimen.

During the next year, the tiger developed hydronephrosis and ureteral dilation secondary to bilateral ureterolithiasis. The tiger weighed 102 kg (224.4 lb) and underwent a successful ureteral stent placement, with resolution of the hydronephrosis and urolithiasis.5 During management for this condition, the tiger continued to receive the original dosage of lomustine, and the prednisolone dosage was decreased to a maintenance dosage (0.5 mg/kg [0.23 mg/lb], PO, q 24 h). The tiger also received a long-term course of amoxicillin and clavulanate potassiumd (14.7 mg/kg [6.68 mg/lb]) for management of a possible secondary urinary tract infection.

During anesthesia for stent placement, fine-needle aspirate samples of the spleen were collected. Splenic samples were evaluated by immunocytochemical staining and molecular clonality assessment with feline PCR primers.e Immunocytochemical immunophenotyping results were equivocal. Molecular clonality testing resulted in a reproducible clonal spike with multiple B-cell primer sets and a broad polyclonal hump with T-cell primers indicative of B-cell lymphoma.6

Sixteen months after the initial diagnosis of lymphoma, the tiger was euthanized as a result of the progression of renal disease and subsequent severe malaise, lethargy, weight loss, and polydipsia. The tiger's final body weight was 99 kg (217.8 lb). Final hematologic tests revealed elevated BUN (> 180 mg/dL) and serum creatinine (> 20 mg/dL) concentrations with a PCV of 35% and severe leukopenia (830 WBCs/μL). On necropsy, the patient's end-stage kidney disease was attributed to previous urolith renal obstruction and consequent decreased renal perfusion as evidenced by generalized bilateral pyelectasia, chronic papillary necrosis, and severe interstitial fibrosis.f No evidence of lymphoma was found on histologic examination of sections of the liver or spleen. However, histologic examination did reveal lymphoma focally affecting both kidneys and also effacing a mesenteric lymph node (Figure 1). Ultimately, it could not be determined whether the renal failure was the result of medullary crest necrosis, the presence of renal lymphoma, or both. The neoplastic lymphocytes had intermediate-sized nuclei, approximately 9 to 12 μm in diameter, often with large, prominent single central nucleoli. Mitotic figures were rare. Immunohistochemical staining of sections of the kidneys and affected mesenteric lymph node showed CD20 and CD79a expression by neoplastic lymphocytes but no immunoreactivity with CD3 antibodies, indicative of B-cell lineage.6,e The final diagnosis was marginal zone lymphoma, a type of B-cell lymphoma. By the time of death, this tiger had received a total of 12 doses of lomustine.

Figure 1—
Figure 1—

Photomicrograph of a section of mesenteric lymph node tissue of a 15.5-year-old 99-kg (217.8-lb) sexually intact male Sumatran tiger (Panthera tigris sumatrae) that had a diagnosis of primary splenic B-cell lymphoma 16 months earlier. The tiger had received a total of 12 doses of lomustine. Ultimately, the tiger was euthanized as a result of progressive renal disease and subsequent severe malaise, lethargy, weight loss, and polydipsia. Notice that the neoplastic lymphocytes have intermediate-sized nuclei that are approximately 9 to 12 μm in diameter, often with large, prominent single central nucleoli (arrowheads). Mitoses are rare. H&E stain; bar = 10 μm.

Citation: Journal of the American Veterinary Medical Association 257, 12; 10.2460/javma.257.12.1288

The 10-year-old 130-kg (286-lb) sexually intact male offspring of the already described Sumatran tiger was immobilized for a physical examination approximately 4 years after its sire's initial presentation, to investigate several months of weight loss, intermittent vomiting, lethargy, and general malaise. A blood sample had been obtained from the lateral tail vein several days prior to the immobilization and revealed leukopenia (2,700 WBCs/μL) and anemia (PCV, 23%), with no abnormalities detected on serum biochemical analysis.a The tiger was immobilized with medetomidine (0.03 mg/kg [0.014 mg/lb], IM) and ketamine (3 mg/kg [1.4 mg/lb], IM); anesthesia was maintained with isoflurane via an endotracheal tube. Abdominal ultrasonography revealed marked splenic enlargement with mild hypoechoic parenchyma and several poorly defined hypoechoic nodules. The remaining ultrasonographic examination findings were unremarkable. Fine-needle aspirate samples of the spleen were collected. Cytologic examination of the splenic samples revealed numerous intact round cells exhibiting some atypia, interpreted as high-grade lymphoma.g

In consideration of treatment options, there were concerns about the invasive nature of splenectomy as well as difficulties of administering IV therapeutics to this animal.1–3 The tiger was trained for voluntary SC injections, and a single-agent injectable chemotherapy protocol with l-asparaginase was chosen for the initial therapy.2 Oral administration of lomustine was considered but ultimately not chosen because of potential excretion of cytotoxic metabolites in the urine and thus exposure of the tiger's mate that was in close quarters.2 The tiger's body mass was estimated at 2.71 m2, and l-asparaginaseh (11,100 U/m2, SC) treatment was prescribed weekly for 4 weeks. The tiger was also receiving prednisolone (2 mg/kg [0.9 mg/lb], PO, q 24 h) and vitamin B12 injections (60 μg/kg [27.3 μg/lb], SC, q 7 d). Approximately 2 months after diagnosis of lymphoma, the tiger continued to have signs of lethargy, decreased appetite, ongoing weight loss (115 kg [253 lb]), and anemia (PCV, 28%). The l-asparaginase treatment was considered inadequate, and a splenectomy was performed. The removed spleen weighed 1.75 kg (3.85 lb), and portions were submitted for histologic examination, immunohistochemical staining, and evaluation of molecular clonality by use of PCR assay.e On histologic examination, the spleen contained numerous nodular expansions of neoplastic lymphocytes that also infiltrated into the adjacent red pulp (Figure 2). In the nodular expansions, neoplastic lymphocytes had intermediate-sized nuclei that were approximately 12 μm in diameter, often with a large, prominent, single, central nucleolus (Figure 3). The cells were larger (nuclear diameter, 12 to 20 μm) in the adjacent red pulp, sometimes forming sheets, and mitotic figures were frequent in these areas. Immunohistochemical staining revealed that the neoplastic cells had strong cytoplasmic and membranous expression of CD20 (Figure 4), a high Ki67 labeling index (50% to 60% of neoplastic lymphocytes in the red pulp–infiltrated areas), and no expression of CD3. Molecular clonality assessment with feline PCR primers produced an unequivocal, reproducible clonal spike with B-cell primers.6,e The final diagnosis was diffuse large B-cell lymphoma that most likely developed as a progression from initial marginal zone lymphoma.

Figure 2—
Figure 2—

Photomicrograph of a section of splenic tissue of a 10-year-old 115-kg (253-lb) sexually intact male Sumatran tiger that was the offspring of the tiger described in Figure 1. Notice the numerous white pulp nodular expansions (arrows) of neoplastic lymphocytes that also infiltrate into adjacent red pulp. H&E stain; bar = 250 μm.

Citation: Journal of the American Veterinary Medical Association 257, 12; 10.2460/javma.257.12.1288

Figure 3—
Figure 3—

Photomicrograph of a section of a splenic white pulp nodule from the same male offspring tiger as in Figure 2. Notice that the neoplastic lymphocytes have intermediate-sized nuclei that are approximately 9 to 12 μm in diameter, often with a large, prominent, single, central nucleolus (arrowheads). Mitoses are rare. H&E stain; bar = 10 μm.

Citation: Journal of the American Veterinary Medical Association 257, 12; 10.2460/javma.257.12.1288

Figure 4—
Figure 4—

Photomicrographs of sections of splenic tissue of the same male offspring tiger as in Figures 2 and 3. A—Notice that the neoplastic lymphocytes also infiltrate the red pulp and, in this location, are larger (nuclear diameter, 12 to 20 μm) and mitoses (arrows) are more frequent than in the nodular white pulp expansions depicted in Figure 3. H&E stain; bar = 10 μm. B—Immunohistochemical staining reveals that lymphocytes in the nodular white pulp expansions have strong membranous CD20 expression. Immunoperoxidase stain with red-brown reaction product and hematoxylin counterstain; bar = 10 μm.

Citation: Journal of the American Veterinary Medical Association 257, 12; 10.2460/javma.257.12.1288

The tiger showed marked improvements in appetite, attitude, energy, and weight gain within a week after the splenectomy. The tiger was receiving omeprazole (1 mg/kg, PO, q 24 h) and a lowered dosage of prednisolone (0.8 mg/kg [0.36 mg/lb], PO, q 48 h) for maintenance. Because of the lack of response, l-asparaginase treatment was discontinued. No alternative chemotherapy was deemed necessary. At a 1-month recheck evaluation of hematologic values, the tiger's PCV had increased to 34%. However, a leukocytosis of 15,700 WBCs/μL was noted, with a neutrophilia of 14,287 cells/μL (reference interval,a 3,200 to 12,700 cells/μL). The tiger received amoxicillin and clavulanate potassiumd (12.6 mg/kg [5.73 mg/lb], PO, q 12 h) for 10 days. On recheck evaluation 1 month later, leukocytosis was again noted (17,600 WBCs/μL) with ongoing neutrophilia (15,100 cells/μL). Enrofloxacini (5 mg/kg [2.3 mg/lb], PO, q 24 h) was prescribed for 10 days. Fourteen days later, recheck CBC again demonstrated leukocytosis (15,100 WBCs/μL) with neutrophilia (13,700 cells/μL) but a stable PCV of 39% that was within the reference interval. Despite the inflammatory changes, the tiger was clinically normal, so it was assumed that the leukocyte changes were corticosteroid related and no further treatments were pursued. Six months after splenectomy, the tiger was eating well, active on exhibit, and breeding with its mate. Monthly hematologic rechecks revealed chronic but stable leukocytosis and neutrophilia, with no anemia. Eleven months after diagnosis and 9 months after splenectomy, the tiger reached a maximum body weight of 148 kg (326 lb). At 32 months after diagnosis, the male offspring was still considered to be in remission.

Discussion

Lymphoma is the most common hematopoietic neoplasm of domestic cats and dogs.7 In cats, cases of lymphoma are most often small-cell, low-grade gastrointestinal tract lymphomas of T-cell origin.1,7 In contrast to cats, most lymphomas in dogs are high-grade, multicentric lymphomas of B-cell origin.7

Lymphoma has become an increasingly common diagnosis in felids at zoo facilities, having been reported in at least 12 species of exotic felids.8–11 Reports on malignant lymphomas in exotic felids indicate primarily a T-cell origin, although 1 African lion (Panthera leo) was reported to have a diffuse large B-cell lymphoma.8–11 The reported lymphoma cases were multicentric with involvement of local organs and lymph nodes but with no peripheral lymphadenopathy.8–11

Historically, lymphoma in domestic cats has been associated with FeLV and FIV.7,12 It is surmised that the oncogenic retrovirus RNA is copied into host cell DNA, causing host cell transformation and ultimately neoplasia.12,13 However, as a result of widespread testing and vaccination for FeLV, the incidence of retro-virus-associated lymphoma has decreased since the 1980s.13 This appears to be true of exotic felid species as well. There is 1 known report11 of an FIV-positive cheetah (Acinonyx jubatus) with multicentric T-cell lymphoma, but most reports of exotic felid species confirm the negative FIV and FeLV status of the patients; therefore, lymphoma was classified as nonretroviral induced.8–11 The tigers in this report had both been confirmed negative for FeLV antigen and FIV antibody during physical examinations conducted over the course of their lives. Consequently, their lymphomas were not associated with retroviruses.

Immunophenotyping of lymphoma in both of these tigers indicated B-cell lymphoma, confirmed with reproducible clonal results of B-cell loci molecular clonality assessment by use of PCR assay.6 More specifically, both tigers had splenic marginal zone lymphoma, with progression to diffuse large B-cell lymphoma in the male offspring tiger. Splenic marginal zone lymphoma is typically an indolent or low-grade type of lymphoma that has characteristic cytomorphology. It has been well described in dogs but is uncommon in cats.14 Marginal zone lymphoma may progress to diffuse large B-cell lymphoma over time.14 Because of the uncommon nature of the lymphoma type reported in this species, it is interesting to note that it developed in a related pair. Cancers develop as a result of a number of factors, and genetics play a role in the development of neoplasia in both humans and other animal species. In mice, the likelihood of developing lymphoma has been linked to a number of genetic predispositions, and in domestic cats, there is propensity for the Siamese breed to develop lymphoma.7,12,15 The development of lymphoma in tigers is also likely multifactorial, and a rare lymphoma type in a sire–male offspring pair suggests a possible familial predisposition for this unusual neoplasm. Multigeneration tracing of these tigers’ lineages showed no evidence of inbreeding. Lymphoma as a cause of death was not found in available records of ancestral tigers. However, the ancestors also died at a younger age than either of the 2 tigers of the present report. Splenic-origin B-cell lymphoma, specifically marginal zone lymphoma, should be considered a differential diagnosis in tigers with malaise and splenomegaly, particularly geriatric Sumatran tigers with a history of neoplasia in their familial lines.

There are multiple recommended treatment modalities for intermediate- to high-grade lymphoma in canids and felids. Generally, splenic lymphoma of B-cell origin has a better prognosis than intermediate- to large-cell lymphoma of T-cell origin.16 Systemic multiagent treatments are usually recommended, as these tend to have higher response and remission rates than single-agent treatments.7 In cases where lymphoma is limited to a single site, such as the spleen, surgery such as splenectomy can be successful without chemotherapy, although patients that undergo both tend to have a longer survival time.1,2,7,15,17 The 2 tigers of the present report were treated with different modalities. The elder tiger was considered too debilitated to undergo a successful splenectomy, and chemotherapy was chosen instead. A multiagent therapy course was also not the best option for this patient; such a protocol would have entailed repeated IV access, which would have required frequent immobilizations and anesthetic procedures. This tiger's treatment therefore consisted of lomustine administration in combination with a corticosteroid. The tiger's attitude and appetite improved, with improved hematologic results for approximately 625 days from the time of the first lomustine dose to the time of death. It was assumed that the tiger had responded well to chemotherapy and had been in remission for some time, but postmortem histopathologic findings indicated that lymphoma was present in the kidneys and > 1 mesenteric lymph node, with no evidence of lymphoma in the liver or spleen. Similarly, the male offspring was initially treated with a single-agent drug therapy, and splenectomy was also not initially pursued because of concerns about the invasive nature of the procedure and possibly prolonged recovery time. Because of concerns about the health safety of its mate, l-asparaginase was chosen instead of lomustine. When this tiger did not respond to l-asparaginase, splenectomy was then performed. The clinical and hematologic improvement after splenectomy was rapid and dramatic, and further chemotherapy was not pursued. After 180 days of excellent attitude and an increase in body weight and appetite, this tiger was considered to be in remission. Thus, effective management was achieved in both tigers by use of different treatment approaches.

Lymphoma in both domestic and exotic felids has been previously documented. This report highlighted the diagnosis and treatment of 2 unusual cases of splenic B-cell lymphoma in a sire–male offspring pair of Sumatran tigers. These cases were unique because of the rarity of this lymphoma type in exotic felids as well as the successful treatment of both cats with different modalities. Genetic predisposition for this neoplasm is suspected in this line of tigers. Splenic B-cell lymphoma should be considered as a differential diagnosis in tigers, particularly Sumatran tigers, with clinical signs of malaise, weight loss, and splenomegaly.

Acknowledgments

The authors declare that there were no conflicts of interest.

The authors thank Dr. Jason Kidd of SAGE Veterinary Centers, Dr. Laura Garrett of the University of Illinois College of Veterinary Medicine, and Dr. Alice Morassi of NorCal Blue Pearl Veterinary Specialists for their guidance with the San Francisco Zoo case. A special thanks to Dr. Anne Avery of Colorado State University for her consultation on splenic flow cytometry.

Footnotes

a.

Zoological Information Management System (ZIMS) [database online]. Minneapolis: Species360, 2020. Available at: zims.Species360.org. Accessed May 28, 2020.

b.

Clinical Diagnostic Laboratory, School of Veterinary Medicine, University of California-Davis, Davis, Calif.

c.

Wedgewood Pharmacy, Swedesboro, NJ.

d.

Clavamox, Zoetis, Parsipanny, NJ.

e.

Leukocyte Antigen Biology Laboratory, University of California-Davis, Davis, Calif.

f.

Anatomic Pathology Service, School of Veterinary Medicine, University of California-Davis, Davis, Calif.

g.

Antech Diagnostics, Sacramento, Calif.

h.

KRS Global Biotechnology, Boca Raton, Fla.

i.

Baytril, BayerDVM, Shawnee Mission, Kan.

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  • Figure 1—

    Photomicrograph of a section of mesenteric lymph node tissue of a 15.5-year-old 99-kg (217.8-lb) sexually intact male Sumatran tiger (Panthera tigris sumatrae) that had a diagnosis of primary splenic B-cell lymphoma 16 months earlier. The tiger had received a total of 12 doses of lomustine. Ultimately, the tiger was euthanized as a result of progressive renal disease and subsequent severe malaise, lethargy, weight loss, and polydipsia. Notice that the neoplastic lymphocytes have intermediate-sized nuclei that are approximately 9 to 12 μm in diameter, often with large, prominent single central nucleoli (arrowheads). Mitoses are rare. H&E stain; bar = 10 μm.

  • Figure 2—

    Photomicrograph of a section of splenic tissue of a 10-year-old 115-kg (253-lb) sexually intact male Sumatran tiger that was the offspring of the tiger described in Figure 1. Notice the numerous white pulp nodular expansions (arrows) of neoplastic lymphocytes that also infiltrate into adjacent red pulp. H&E stain; bar = 250 μm.

  • Figure 3—

    Photomicrograph of a section of a splenic white pulp nodule from the same male offspring tiger as in Figure 2. Notice that the neoplastic lymphocytes have intermediate-sized nuclei that are approximately 9 to 12 μm in diameter, often with a large, prominent, single, central nucleolus (arrowheads). Mitoses are rare. H&E stain; bar = 10 μm.

  • Figure 4—

    Photomicrographs of sections of splenic tissue of the same male offspring tiger as in Figures 2 and 3. A—Notice that the neoplastic lymphocytes also infiltrate the red pulp and, in this location, are larger (nuclear diameter, 12 to 20 μm) and mitoses (arrows) are more frequent than in the nodular white pulp expansions depicted in Figure 3. H&E stain; bar = 10 μm. B—Immunohistochemical staining reveals that lymphocytes in the nodular white pulp expansions have strong membranous CD20 expression. Immunoperoxidase stain with red-brown reaction product and hematoxylin counterstain; bar = 10 μm.

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