Pathological and histological examination and outcome
The results of a necropsy revealed, a large volume of free fluid in the
thoracic cavity, with blood adhered to the diaphragm, and atelectasis of
the edges of the lung lobes (Fig. 13). A large mass was found in the
cranial mediastinum, approximately 15x15 cm in size, which surrounded
the trachea and esophagus (Fig. 14), and continued up to the aorta.
Enlarged lymph nodes were found in the stomach area. The stomach showed
evidence of mucosal hemorrhages in the pyloric area. The intestines were
without visual pathological changes except for the ileum, where small
hemorrhages on the mucosa and enlarged mesenteric nodes were observed.
The liver was intact, and the spleen was normal on first sight, but on
palpation and later on disection there were obvious changes in the area
of the gastrosplenic vein.The vein was firm, with a thickened wall and
the corresponding lymph nodes were markedly enlarged. While the kidney
parenchyma remained intact, there were also changes in the lymphatic
nodes around the kidneys.
Samples of the fluid on smear testing showed moderate cellularity. Cell
preservation was fair. Cytologically, a small population of lymphocytes
with round nuclei and reduced cytoplasm were observed (cross sections of
the nuclei <1.5x the size of an erythrocyte). These were the
predominant cells on the smears from the fluid and were the second most
common nucleated cells in the pre-made smears. The cells showed mild
anisocytosis and anisokaryosis. Mitotic figures were not found. Low
numbers of neutrophils were found in the smears from the fluid samples
and moderate numbers were found in the pre-made smears. Very low numbers
of eosinophils and macrophages were found.
Microscopic examination of the spleen showed no lesions. The lymph node
adjacent to the spleen had an intact capsule with some retention of
normal cellularity and architecture. However, most of the medullary
sinuses, some vascular spaces, and occasional foci of the lymph node
parenchyma had a population of neoplastic cells. The normal cell
population were instead replaced by monomorphic populations of large
round cells. These cells had slightly ill-defined borders, moderate
amounts of eosinophilic cytoplasm, and varying numbers of oval to
convoluted nuclei with fine chromatin and some with particularly large
nucleoli. Mitotic figures were moderately frequent and occasionally
atypical. There was an increase in cortical small lymphocytes (perhaps
reactive). Additionally the perinodal adipose tissue was infiltrated
with tumourous round cells. The stomach and kidney lymph nodes showed
the same findings as described in the lymph node above.
Findings in the mediastimum lymph node, thymus, and the surrounding
adipose tissue were similar to those described in and around the splenic
lymph node, above. In addition, rare large cells in the thymic sinuses
showed erythrophagocytosis. These cells also resembled the
aforementioned tumourous cells.
The ileum showed a moderate infiltration of plasma cells with
eosinophils present in the mucosa. There were also a moderate number of
eosinophils present in the submucosa, and surrounding glands. No
neoplastic cells were found. Thses findings were indicative of
eosinophilic to plasmacytic duodenal enteritis, ranging from a mild to
moderate level of disease progression
A histopathological examination was repeated to dermine the type of the
tumour, but the exact identity of the tumour cells remained unclear. The
tissue showed large to huge blasts (Fig. 15) with a bizzare appearance,
as they were surrounded by a rich mixture of small (probably T-cell)
lymphocytes,which would support a diagnosis of an agressive form of
equine lymphoma.
Immunostaining was carried out for antibodies against CD3, CD79a. The
residual lymphocytes within the follicles and paracortex show
immunopositive staining for CD79a and CD3 respectively. The atypical
neoplastic cells percolating throughout the sinuses failed to express
CD79a and CD3.
Histopathological examination and immunostaining were not sufficient to
determine the exact type of the tumour, although lymphoma or histiocytic
sarcoma was suspected. Upon repeating the histological examination, the
samples were found to have a greater population of poorly differenciated
cells, and thus the results of which were more compelling towards
supporting the diagnosis of an aggressive lymphoma.
Serological oncomarker testing for thymidine kinase was performed with
the results obtained two weeks after eutanasia however with negative
results 0.696 U/l (< 3).