Eosinophilic leukemia in three African pygmy hedgehogs ( Atelerix albiventris ) and validation of Luna stain

Abstract

Neoplasia is usually encountered in the African pygmy hedgehog at a mean age of 3.5 y, and malignancy is common. Myelogenous leukemias are rarely reported in hedgehogs. We describe 3 cases of eosinophilic leukemia in adult, middle-aged (mean age: 2.3 y) hedgehogs, for which prognosis appears grave. In 1 case, attempted treatment was unsuccessful, and in all 3 cases, the disease course was rapid and all died soon after diagnosis. Blood smear evaluation, along with complete blood count, was critical in making the diagnosis in all cases. Luna stain was validated and used to better visualize eosinophils in cytologic and histologic sections. Electron microscopy confirmed the presence of specific granules in hedgehog eosinophils.

Myeloproliferative diseases (MPDs) are neoplastic diseases affecting the bone marrow with unregulated proliferation of clonal hematopoietic stem cells. This uncontrolled cell proliferation leads to reduction of normal hematopoiesis and invasion of other tissues. ²¹

Eosinophilic leukemia is an infrequent type of MPD and a variant of granulocytic leukemia that has been described in animals and humans.^2,4,14,15,20 Eosinophilic leukemia must be differentiated from other conditions such as hypereosinophilic syndrome, a chronic idiopathic condition in which high numbers of mature eosinophils are seen in the peripheral blood and infiltrating various organs. ⁴ In hypereosinophilic syndrome, the vast majority of the eosinophils are mature and no atypical cells are seen, whereas in eosinophilic leukemia, immature cells can be present in increased numbers in circulation and in tissues, and they predominate in the bone marrow. ²⁰

Usually, eosinophils are easily recognized in cytologic and hematologic specimens given the presence of their characteristic granules. Luna stain was originally developed to detect the cytoplasmic granules within eosinophils. ¹³ Although it is the most common special stain used to improve visibility of eosinophils in histologic sections in domestic animals,^5,7 this stain has not been validated in the African pygmy hedgehog, to our knowledge.

Neoplasia is commonly reported in the African pygmy hedgehog (Atelerix albiventris). ⁶ Hedgehogs have an average life span in captivity of 4–6 y, although some may live up to 8 y. Neoplasia has been reported to occur in animals ranging from 1 mo to >5 y of age, with a mean age of 3.5 y (Done L, et al. Necropsy lesions by body system in African hedgehogs (Atelerix albiventris): clues to clinical diagnosis. Proc Joint Conf Am Assoc Zoo Vet and Am Assoc Wildl Vet; Nov 1992; Oakland, CA). ¹⁶ The most commonly reported neoplasms in African pygmy hedgehogs are mammary gland adenocarcinoma, intestinal lymphoma, and oral squamous cell carcinoma. ¹⁶ Myelogenous leukemia has been reported rarely in hedgehogs.^8,10,17 In the current report, we describe the clinical presentation and clinicopathologic features of 3 cases of eosinophilic leukemia in African pygmy hedgehogs in the United States and the validation of Luna staining to recognize eosinophils in this species.

Case 1

A 3-y-old male hedgehog was presented to the referring hospital because of a 2-wk history of ataxia and weakness. The animal weighed 432 g, and had lost >25% of its body weight from previous visits. While under general anesthesia, physical examination revealed a mass in the mid abdomen and 3 additional cervical subcutaneous masses. Subsequently referred to the College of Veterinary Medicine, University of Georgia (Athens, Georgia), the hedgehog was found to have lost additional weight (404 g), and had left forelimb lameness. Mites were also present over the axillary regions. Under isoflurane general anesthesia, ^a blood was collected from the cranial vena cava and submitted for a complete blood cell count (CBC). The CBC was performed with an automated analyzer ^b and revealed marked leukocytosis (Table 1). Most of the cells were atypical and were classified as “other.” The atypical cells were large (15–20 µm diameter), round cells with moderate nuclear-to-cytoplasmic (N:C) ratio and abundant, lightly to moderately basophilic cytoplasm containing variable quantities of small, round, eosinophilic granules (Fig. 1A). The nuclei were round to reniform and had an open, ropy, chromatin pattern. These cells were interpreted to be immature eosinophils. Rare non-granulated cells containing a single nucleolus (presumed myeloblasts) were also present. Fine-needle aspirates of the abdominal and skin masses were also taken and submitted for cytologic examination. The skin and abdominal masses were cytologically similar, and were mainly composed of large, individualized, round cells (88% of nucleated cells) compatible with the immature cells seen in circulation. Occasional mitotic figures and binucleated cells were noted. Low numbers of mature eosinophils and neutrophils (6% of nucleated cells) were scattered throughout. Occasional lymphocytes and plasma cells (6% of nucleated cells) were also seen. The presence of high numbers of eosinophilic precursors (39.2 × 10⁹/L and 88% of nucleated cells in the blood and the mass aspirates, respectively) and the lack of orderly maturation or well-organized left shift in the granular cells suggested that these were neoplastic eosinophils and not part of an inflammatory response. Eosinophilic leukemia was considered to be the most likely diagnosis.

Table 1.

Eosinophilic leukemia in 3 African pygmy hedgehogs (Atelerix albiventris): comparison of complete blood counts performed.*

CBC	Case 1		Case 2	Case 3	Reference interval 1
	Pre-tx	Post-tx
Hematocrit	0.37	0.32	0.26	0.18	0.36 ± 0.07 (0.22–0.64) L/L
Hemoglobin	137	111	83	82	120 ± 28 (70–211) g/L
RBC	4.3	3.67	4.07	1.8	6 ± 2 (3–16) × 1012/L
MCV	85.3	87	62.6	101	67 ± 9 (41–94) fL
MCH	31.9	30.3	20.3	46	11–31 pg
MCHC	373	349	324	450	340 ± 50 (170–480) g/L
Platelets	137	142	Decreased	178	226 ± 108 (60–347) × 109/L
MPV	11.6	10	ND	15.6	NA
WBC	85.4	2.0	51.9	365	11 ± 6 (3–43) × 109/L
Segmented	25.6 (30%)	0.220 (11%)	13.5 (26%)	11 (3%)	5.1 ± 5.2 (0.6–37.4) × 109/L
Bands	7.7 (9%)	0 (0%)	2.1 (4%)	0	NA
Lymphocytes	2.6 (3%)	0.6 (32%)	5.2 (10%)	3.6 (1%)	4 ± 2.2 (0.9–13.1) × 109/L
Monocytes	0 (0%)	0.1 (6%)	4.2 (8%)	0	0.3 ± 0.3 (0–1.6) × 109/L
Eosinophils	7.7 (9%)	0.3 (17%)	0	350.4 (96%)	1.2 ± 0.9 (0–5.1) × 109/L
Basophils	2.562 (3%)	0.04 (2%)	0	0	0.4 ± 0.3 (0–1.5) × 109/L
Other	39.28 (46%)	0.6 (32%)	26.99 (52%)	0	0 × 109/L
Nucleated RBC	2	3	1	50.6	0/100 WBC

*

CBC = complete blood count; MCH = mean corpuscular hemoglobin; MCHC = mean corpuscular hemoglobin concentration; MCV = mean corpuscular volume; MPV = mean platelet volume; NA = not available; ND = not done; Pre-tx = before chemotherapy treatment; Post-tx = after chemotherapy treatment; RBC = red blood cell count; WBC = white blood cell count.

Figure 1.

Eosinophilic leukemia in African pygmy hedgehogs (Atelerix albiventris). A. Peripheral blood smear from case 1. Note the large, immature cells, with high nuclear-to-cytoplasmic (N:C) ratio and numerous, small, pink, cytoplasmic granules (arrow). A segmented eosinophil is also present (arrowhead). Inset: In some areas, these immature cells predominated (arrows). Modified Romanowsky stain. Bar = 20 µm. B. Bone marrow smear from case 1. The hypercellular marrow consists predominantly of immature myeloid cells with pink granules (arrows). Modified Romanowsky stain. Bar = 20 µm. C. Peripheral blood smear from case 2. Image of circulating neoplastic eosinophils represented by large cells with high N:C ratio and numerous, small, red cytoplasmic granules (arrow). Notice the similarity to the cells in panel A. Modified Romanowsky stain. Bar = 20 µm. D. Bone marrow smear from case 2. Immature myeloid cells with pink granules represent the predominant cell type. Modified Romanowsky stain. Bar = 20 µm. E. Transmission electron micrograph of liver from case 2. Note representative ultrastructural features of neoplastic eosinophils within a hepatic sinusoid. The eosinophils have many electron-dense granules. Bar = 2 μm. Inset. Crystalloid structures (arrow) typical of eosinophil-specific granules. F. Peripheral blood smear from case 3. The cytoplasmic granules stain strongly positive (bright red) with Luna stain. Note that many of these cells have large, round nuclei, typical of more immature cells. Luna stain. Bar = 20 µm.

Following diagnosis, chemotherapy was begun (cytarabine ^c 100 mg/m² subcutaneously [SC q12h] for 2 d, and prednisone ^d 2 mg/kg orally [PO] q12h). Weight loss continued, and the patient was then given 1 oral dose of ivermectin ^e at 0.5 mg/kg, and started on tramadol ^f 15 mg/kg PO q6h and mirtazapine ^g 1.35 mg/kg PO q12h. Because of severe bone marrow suppression by day 4 postinitiation of the chemotherapy treatment, the patient was started on orbifloxacin ^h 20 mg/kg PO q12h, metoclopramide ⁱ 0.2 mg/kg PO q8h, and diazepam ^j 1 mg/kg PO q12h.

The clinical status of the animal continued to deteriorate with worsening of lethargy, anorexia, diarrhea, and vomiting. Famotidine ^k 1 mg/kg SC q12h, maropitant citrate ^l 2 mg/kg SC q12h, and sucralfate ^m 10 mg/kg PO q8h were started. Despite supportive care, the hedgehog died 2 d after the last CBC (8 d after cytarabine treatment; Table 1) and was subjected to cosmetic autopsy. At the time of autopsy, the animal weighed 342 g. Petechiae and ecchymoses were present in the skin and subcutaneous tissues. Dark brown, hemorrhagic feces stained the perineum, and moderate amounts of hemorrhagic feces were in the colon. Multiple, 3–6 mm, pale tan, friable nodular masses were in the right and left cervical subcutaneous tissues, the thoracic cavity, ribs, mesentery, and in the spleen. The liver was swollen and friable with a prominent reticular pattern on the capsular and cut surfaces.

A sample of the bone marrow was taken soon after euthanasia and routinely stained for cytologic examination (Fig. 1B). The bone marrow was hypercellular, with no visible iron deposits and normal megakaryocytes; the myeloid-to-erythroid (M:E) ratio was markedly increased, indicating a large expansion of the myeloid line, and relative erythroid hypoplasia. In a 500 cell count, the majority of the cells (68%) were immature myeloid cells ranging from promyelocytes to metamyelocytes, many of which contained variable numbers of round eosinophilic granules. Rare dysplastic features such as N:C maturation asynchrony, or megalocytic cells, were noted in this population. Myeloblasts comprised 7% of the nucleated cells, and band and segmented granulocytes comprised 15% of the nucleated cells. Erythroid cells in different stages of maturation represented the remaining 10% of the nucleated cells.

Samples of bone marrow, liver, spleen, heart, lung, stomach, and intestine, as well as the mediastinal, cervical, and abdominal masses, were submitted and processed for routine histologic evaluation and Luna stain. Luna stain is performed on standard paraffin sections and employs a combined hematoxylin–Biebrich scarlet solution to demonstrate eosinophil granules, which stain red against a blue background. This combined solution was obtained by using Weigert iron hematoxylin and 1% Biebrich scarlet solution as described elsewhere. ¹³ Histologically, all organs examined were diffusely infiltrated by cells similar to those seen in the marrow and peripheral blood. Although the granules were not readily visible histologically, ~60% of the neoplastic cells were positive for Luna stain, helping to identify them as immature eosinophils. A final diagnosis of myeloid leukemia with eosinophilic differentiation and metastasis to multiple organs was confirmed. The immediate cause of death was presumed to be cardiac dysfunction resulting from myocardial necrosis associated with hypoxia.

Case 2

A 2-y-old male hedgehog was presented to Texas A&M University College of Veterinary Medicine Zoological Service (College Station, Texas) for evaluation of an ulcerated, 7 × 5 × 5 mm mass on the dorsal surface of the right hind foot. The animal was overweight (410 g). Under isoflurane general anesthesia, ^a the mass was surgically removed and diagnosed as a histiocytic sarcoma. The animal was subsequently discharged. Seven weeks later, the animal returned with weight loss (355 g), ataxia, hind limb paresis, and progressive hyporexia; the previously removed mass had not recurred. Blood samples were obtained under general anesthesia ^a from the cranial vena cava and submitted for CBC. The CBC was analyzed ⁿ and revealed marked leukocytosis characterized by neutrophilia with a left shift, including bands and rarely metamyelocytes, monocytosis, and intermediate-to-large (12–30 µm) cells, classified as “other” (Table 1), representing ~52% of the reported cells. These large cells had a high N:C ratio and numerous, small, red cytoplasmic granules that often obscured the nucleus (Fig. 1C). Rare large, immature cells with a round nucleus, but lacking cytoplasmic granules, and rare mature, segmented eosinophils were also present. Neutrophils often displayed cytoplasmic basophilia and anisocytosis, suggesting abnormal maturation. Mild anemia was also present, along with polychromasia, which could indicate regeneration. Platelets were decreased. Based on the high number and morphologic features of the large unclassified cells, eosinophilic leukemia was considered the most likely diagnosis. The animal died in its cage shortly after anesthetic recovery.

An autopsy was performed and revealed marked hepatomegaly and splenomegaly. The bone marrow from the right femur was mottled red to tan and turgid. Bone marrow impression smears were obtained, and tissues collected at autopsy were fixed and processed routinely for light and electron microscopy. In a 500 cell count, 88% of the cells present in the bone marrow impression smears were immature eosinophils, similar to those observed in the blood smear, 8% of the cells were neutrophils and neutrophil precursors, and the remaining 4% were erythroid cells, lymphocytes, and plasma cells (Fig. 1D). On histologic examination, blood vessels and organs were diffusely infiltrated by a population of 15–35 μm neoplastic round cells. The cells had distinct cell borders and a mild-to-moderate amount of a granular bright eosinophilic cytoplasm. Nuclei were variably round to indented to irregular, with coarsely stippled chromatin and inconspicuous nucleoli. Anisocytosis and anisokaryosis were mild with a moderate mitotic index, averaging 6 per ten 40× fields. Single cell necrosis and degranulation were common features, the latter of which was often associated with intravascular aggregates of karyorrhectic cellular debris, vascular endothelial necrosis, thrombosis, and hemorrhages, resembling typical features of acute tumor lysis syndrome. ⁹ The marrow cytoarchitecture was severely effaced by neoplastic eosinophils (myelophthisis). Neoplastic cells identical to those seen in circulation composed up to 80% of the myeloid population. This was supportive of a myeloproliferative neoplasm. Luna staining was performed on liver sections, revealing strong positive cytoplasmic staining. Control tissue from a different healthy hedgehog confirmed positive Luna staining of hedgehog eosinophils compared with nonstaining neutrophils. Additionally, histologic examination of the central nervous system revealed marked, multifocal white matter spongiosis with mild, multifocal axonal degeneration involving the cerebrum, cerebellum, brainstem, and spinal cord. This finding was strongly supportive of wobbly hedgehog syndrome. ³

For electron microscopy, formalin-fixed tissues were immersed in 2% glutaraldehyde, stained with 1% uranyl acetate in water, dehydrated, infiltrated, and polymerized in epoxy. ^o Neoplastic cells observed within hepatic sinusoids ranged from 10 to 20 μm, and had a high N:C ratio (Fig. 1E). The euchromatic nuclei were pleomorphic, large, and central to paracentral, with peripheral mildly aggregated chromatin and occasionally a single prominent nucleolus. Approximately two-thirds of the neoplastic cells had irregularly indented nuclei, and one-third of the cells had regularly round nuclei. The cytoplasm of these cells contained numerous electron-dense granules (~0.5 µm diameter). Approximately 40–90% of the granules were round and moderately and homogeneously electron dense (primary granules). Approximately 10–60% of the granules were smaller, round to oval, and more electron-dense, often demonstrating a crystalloid structure (specific granules; inset of Fig. 1E). These microgranules were larger than those of other hedgehog species such as the European hedgehog (Erinaceus europaeus) and the long-eared hedgehog (Hemiechinus auritus). ¹⁸

Case 3

A 2-y-old male hedgehog was presented to the Colorado State University College of Veterinary Medicine (Fort Collins, Colorado) for removal of a 1-cm diameter dermal nasal mass. The mass was consistent with severe lymphocytic granulomatous dermatitis on initial histopathology. The patient initially did well but was reevaluated 3 mo later for weight loss and mass regrowth. The mass was removed again, and histopathology was reported as granulomatous eosinophilic inflammation. The patient became lethargic and anorexic following surgery. Blood was collected under isoflurane general anesthesia ^a for CBC and revealed marked leukocytosis (Table 1). Collected blood was analyzed, ^b as in case 1. The vast majority (97%) of the leukocytes were large round cells that contained granules resembling those of eosinophils. The nuclei of these cells ranged from rounded or reniform to appropriately segmented. Given the high eosinophil count, with a high percentage of immature forms, eosinophilic leukemia was considered as the most likely diagnosis. The immature cells were large, with rounded nuclei and an indistinct-to-prominent nucleolus. Luna staining of the blood smear stained the cytoplasmic granules (Fig. 1F). The hedgehog died 1 h after venipuncture, and an autopsy was declined.

Our report describes 3 cases of eosinophilic leukemia in the African pygmy hedgehog. In each case, the disease progressed rapidly, as all hedgehogs died soon after diagnosis. All animals were adult to middle aged (mean age: 2.3 y), which is consistent with reports of other neoplastic diseases in hedgehogs. ¹⁶ In one of the cases presented (case 2), a histiocytic sarcoma had also been diagnosed. This finding is consistent with previous reports in which 10% of hedgehogs with cancer had more than 1 neoplasm. ⁶ In all cases, clinical signs at presentation were nonspecific and included weight loss. Ataxia was observed in 2 of the cases (cases 1 and 2). In case 2, a final diagnosis of wobbly hedgehog syndrome was reached after histologic examination of the central nervous system; in case 1, a cosmetic autopsy was elected and examination of the nervous system was not performed, preventing a final evaluation.

In all 3 cases, moderate-to-marked leukocytosis characterized primarily by immature eosinophils was present and ranged from 1.2 to 8.5 times the upper reference interval (Table 1). ¹ Eosinophilic leukemia should be differentiated from hypereosinophilic syndrome. In both conditions, myeloid hyperplasia with eosinophilic predominance is seen in the bone marrow and peripheral blood. However, the presence of high numbers of immature eosinophils in our cases, the marked increase in the M:E ratio, the dysplastic changes observed, and the infiltration of immature cells in other organs were characteristic of eosinophilic leukemia. Immature eosinophils (hematopoietic cells with eosinophilic granules but no nuclear segmentation) predominated within the marrow and multiple organs (cases 1 and 2). Crystalloid structures typical of eosinophil-specific granules were also seen ultrastructurally in neoplastic cells (inset of Fig. 1E).

Eosinophils are usually recognized easily in cytologic and hematologic specimens given the presence of characteristic pink granules; however, these may not be as evident in histologic specimens. Luna staining is the most common special stain used to improve visibility of eosinophils in histologic sections.^5,7 Using healthy hedgehog tissue as a control, we demonstrated positive staining in both the control and all eosinophilic leukemic cases. Eosinophils were markedly positive, and Luna staining was widespread for cases 2 and 3. Interestingly, only 60% of cells were positive for Luna in case 1. The lack of widespread, marked positivity could be the result of either the immaturity of many of the cells or altered granule content in the atypical neoplastic population. Antibodies for a specific eosinophil peroxidase and CD453 and CD348 have been used to confirm eosinophil lineage in dogs and cats, respectively^14,19; however, the validity of these antibodies has not been verified in other species and they were not attempted in any of our cases.

Eosinophilic leukemia is commonly classified as a chronic leukemia.^4,20 This classification is likely because of the degree of differentiation, such as the presence of the characteristic eosinophilic granules, which permits the recognition of the cells as eosinophil precursors. In our cases, however, the immaturity of the cells and the rapid disease progression was more consistent with an acute leukemia.

Although there is no literature-based evidence about the efficacy of chemotherapy for the treatment of eosinophilic leukemia in hedgehogs, cytarabine was chosen because of its ease of administration and its reported use in people with hypereosinophilic syndrome. ¹² In our report, the chosen dosage was based on dosages used in dogs and cats. At this dose, it appeared to result in a marked reduction in circulating neoplastic cells (Table 1). The other drugs were used in an attempt to control the clinical course of the disease; dosages were all determined based on scaling. ¹¹ However, despite treatment, the hedgehog died shortly after diagnosis, possibly, in part, because of toxicities of chemotherapy. With that being said, conclusions about the efficacy of cytarabine or any chemotherapy regimen cannot be inferred from a single case. Future cases and treatment attempts may provide more information about how to best manage this type of cancer.