Abstract
A 4-year-old male European domestic cat was presented with dysorexia, weakness and depression. Normocytic normochromic non-regenerative anaemia, leucopaenia and thrombocytopenia were detected. Rubriblasts were detected both in the blood and in the bone marrow. Tests of blood chemistry revealed no alterations of renal and hepatic function and a positive reaction to FeLV antigen was detected in the cat's serum. Neoplastic cells did not show positive to cytochemical reactions against granulocytes, monocytes and lymphocytes. According to haematological and bone marrow cytological findings, a diagnosis of erythremic myelosis (AML6er) was made. Histopathology showed extramedullary haematopoiesis in the liver, spleen, kidney and lymph nodes and chronic nephropathy and degenerative signs in the liver.
A 4-year-old male European domestic cat was presented because it had been suffering dysorexia, weakness and depression for 2 days. Clinical examination revealed it had pale mucous membranes, dyspnoea and cyanosis. The cat's temperature was 38°C. The lymph nodes and spleen were mildly enlarged; however, no masses were detectable on radiological examination. The laboratory findings are shown in Table 1. Normocytic normochromic non-regenerative anaemia (PCV 13.1) and thrombocytopenia (32×103/μl) were detectable. A leucogram showed a low number of leucocytes, mainly mature neutrophils, and more than 13×103/μl round cells, resembling rubriblasts, were detected together with many erythroblasts. These were mainly medium-sized (10.8–13.8 μm diameter), with a high nuclear/cytoplasmic ratio (mean value 3.07) and strongly basophilic cytoplasm. The nucleus was round and central with scanty chromatin and one or two well-defined large nucleoli (mean 2.97 μm) were detectable. Some erythroblasts showed an altered synchronism in maturation of the cytoplasm and nucleus.
Laboratory findings and reference values (Jain 1986a) from the cat affected by erythremic myelosis
Feline leukaemia virus (FeLV) antigen (p27) was detected in the serum (SNAP, IDEXX lab. Inc.); however, a test for feline immunodeficiency virus (FIV) was negative. Blood chemistry showed only a mild increase of gamma-GT (12.8 U/l) and urea (20.3 mmol/l) while no alterations were detected in other hepatic or renal markers. Increases of total protein (81 g/l) and albumin (52 g/l), probably due to dehydration, were detected.
According to these features, a myeloproliferative disease was suspected and because of rapid deterioration euthanasia was performed the day after. Bone marrow smears were immediately made and the cat was submitted for necropsy. To identify potentially neoplastic cells, blood and bone marrow smears were stained with May Grünwald-Giemsa and tested for cytochemical reactions: α-naphtylacetate esterase (NAE) for monocytes; fluoride-inhibited NAE for lymphoid cells; chloracetate esterase (CAE) and myeloperoxidase (MPX) for granulocytes and periodic acid-Schiff (PAS) for erythroid precursors (Miale 1982). In bone marrow aspirates, more than 87% of nucleated cells were of the erythroid type (myeloid/erythroid ratio 0.12) and most of these were similar to those detected in the blood. Based on morphology they were classified as rubriblasts (Jain et al 1986a) and some mitotic and atypical cells were also present. Ineffective erythropoiesis was supported by a low percentage of erythroblasts (less than 2%). Occasionally some azurophilic granules were detectable in the cytoplasm of neoplastic cells. Both granulocytic and megakaryocytic lines were strongly depressed. Cytochemical staining for granulocytic, monocytic or lymphocytic lineage were all negative, except for a weak positivity for NAE. At necropsy a marked splenomegaly and hepatomegaly were observed while only a mild generalised enlargement of the lymph nodes was detectable. Perirenal fat necrosis was also observed. The most consistent histopathological findings were a mild (kidney) to moderate (lymph nodes) to marked (liver, spleen) extramedullary haematopoiesis. The lymph nodes showed severe, diffuse, follicular hyalinosis and atrophy. The liver showed diffuse mild to moderate degenerative changes with cholestasis. Focal membranous glomerulonephritis was the main feature of the kidney.
A final diagnosis of erythremic myelosis was made and, according to the French-American-British (FAB) classification adapted to cats (Jain et al 1991), it was classified as AML6er.
Erythroleukaemia is a myeloproliferative disease characterised by an abnormal proliferation of erythroid precursors in the bone marrow. The designation of erythremic myelosis is usually related only to erythroid series even if some authors define both pure erythroid and myeloerythroid proliferations as erythroleukaemias (Schalm 1975a, Harvey et al 1978). In veterinary medicine erythremic myelosis is uncommon, and most cases have been encountered in cats (Saar 1968, Watson 1974, Maede et al 1979, Falconer et al 1980). The FAB classification, adapted to veterinary medicine, allows an identification of erythroleukaemia from myelodisplastic syndrome in bone marrow aspirates where more than 50% of all nucleated cells (ANC) are erythroid cells (Jain et al 1991). In erythremic myelosis a blast cell count that includes rubriblasts, with a predominance of rubriblasts, is higher than 30% of ANC while in erythroleukaemia (AML6) they are mainly non-erythroid blast cells (myeloblast and monoblasts) (Jain et al 1991, Fournel-Fleury et al 1994).
Haemocytometry and bone marrow cytology showed a strong proliferation of erythroid cells resembling rubriblasts (Fig 1). Extramedullary haematopoiesis in lymph nodes, spleen, liver and kidney is probably related to serious anaemia in the absence of effective bone marrow regeneration. Myeloid and megakaryocytic series were consequently depressed as shown also by leucopenia and thrombocytopaenia in the blood.
Bone marrow smear from the cat affected by erythremic myelosis. Some blast cells classifiable as rubri-blasts are detectable. May Grünwald-Giemsa stain, 100 ×.
The cat was positive to FeLV antigen in serum. This is a very sensitive test but somewhat less specific (Lappin & Turnwald 1994). Unfortunately no additional testing (IFA, isolation) was undertaken to prove the FeLV status or to characterise the sub-type of the virus. Some authors (Harvey et al 1978, Shimada et al 1995) reported that erythroleukaemia is sometimes associated with FeLV infections. C type virus, similar to feline leukaemia virus, has been demonstrated by electron microscopy in the bone marrow of some cats with myeloproliferative disease (Schalm 1975a). The progression from erythremic myelosis to erythroleukaemia and finally to granulocytic leukaemia has also been reported in some cases (Schalm 1975b, Engelman et al 1986). The cat examined was euthanised 1 day after initial presentation and this evolution was not detectable but the presence of few azurophilic granules in some neoplastic cells suggests a possible progression to a mixed form.
Positive cytochemical reactions to granulocytic, monocytic or lymphocytic markers were not found to any significant degree. Positive esterase and peroxidase reactions are usually good indicators of granulocytic (CAE+, MPX+, NAE–), monocytic (CAE–, MPX–, NAE+, NaF/NAE–) or lymphoid (CAE–, MPX–, NAE+, NaF/NAE+) cell lineages both in humans and animals (Miale 1982, Jain 1986b). In humans, a strong, diffuse/granular PAS positivity is reported together with a slight NAE positivity in erythroleukaemia (Miale 1982). In veterinary medicine, PAS positivity was not detected in feline erythroleukaemia by some authors (Maede & Murata 1980, Jain 1986b) and our results seem to confirm these reports.
Haematology and blood chemistry did not suggest pathological alterations except for a strong non-regenerative anaemia partially masked by dehydration, with thrombocytopaenia and panleucopaenia due to neoplastic substitution of staminal cells. The haematological, chemical and histopathological features are in agreement with those reported by other authors (Maede & Murata 1980). Degenerative hepatic signs and chronic nephropathy were not followed by any alteration in hepatic or renal function, except for a higher gamma-GT activity. This parameter is usually a good indicator of cholestasis in cats and it is more sensitive compared with ALP in this species (Hall 1998) and gamma-GT increases are expected in hepatobiliary diseases. A marked extramedullary haematopoiesis also in the periportal area with a mild cholestasis probably induced the release of the gamma-GT from the membranes of the biliary tree.
Footnotes
Acknowledgements
This work was supported by MURST (60%). The authors wish to thank Dr Valentina Spagnolo for technical assistance.