Equine lymphoma

Clinical Approach to Immune-Mediated Hemolytic Anemia and Thrombocytopenia

Primary immune mediated hemolytic anemia (IMHA) and thrombocytopenia (IMTP) are rare in horses with the conditions more commonly occurring secondary to underlying disease. Several case reports have suggested a link between neoplasia and immune-mediated destruction of platelets and red blood cells. Diagnostic investigations should therefore focus on identifying possible underlying causes such as infections and neoplasia. Immunosuppressive therapy with corticosteroids and azathioprine is the mainstay of treatment but should be used cautiously in cases where underlying infection has not been excluded. Given the frequent association of secondary IMHA and IMTP cases with neoplasia, primary cases generally have a better prognosis.

An Unusual Cause of Femoral Nerve Paresis in a Horse: Disseminated B Cell Lymphoma With Plasmacytoid Differentiation and Direct Neuronal Invasion

A 21-year-old Quarter Horse mare presented with a chronic, progressively worsening left pelvic limb lameness of 3 weeks duration. The initial examination identified a consistent lameness at a walk. Neurological examination showed sensory and gait abnormalities consistent with left femoral nerve dysfunction. The horse minimally advanced the leg cranially and had a shortened stride length at the walk. During the stance phase, the heels of the left hind foot did not contact the ground and the horse quickly took weight off of the limb. Diagnostic imaging (ultrasound and nuclear scintigraphy) examinations did not reveal a cause. Severe lymphocytosis was identified on complete blood cell count (69,600 cells /uL; reference range: 1,500-4,000 cells/uL), suggestive of lymphoma. Postmortem examination revealed focal swelling of the left femoral nerve. Multiple masses were found in the stomach, large colon, adrenal gland, mesentery, heart, and meninges. The entire left pelvic limb was dissected and did not reveal other causes of the gait deficit. Histologic evaluation of the left femoral nerve revealed disseminated intermediate cell size B cell lymphoma, with an immunophenotype suggestive of plasmacytoid differentiation. These lymphocytes infiltrated the femoral nerve at the location of the focal nerve swelling, in addition to other peripheral nerves. This case highlights a horse with an atypical diagnosis of femoral nerve paresis caused by direct neoplastic lymphocyte infiltration, deriving from disseminated B cell lymphoma with plasmacytoid differentiation (neurolymphomatosis). Though rare, disseminated lymphoma with direct nerve infiltration should be considered in horses with peripheral neuropathies.

Contribution of Hemogram Plan in the Horse's Clinical Evaluation

The interpretation of the blood count is essential to help the equine clinician in the diagnosis, prognosis, patient management, and control of equine diseases. Hematologic alterations often reflect the condition of the individual or an overall response to a pathological situation. A thorough clinical examination of the patient is essential to correctly interpret the hematological results. The most common abnormalities in the erythrogram are mainly anemia and polycythemia. The frequent causes of anemia in horses are acute and chronic blood loss, hemolytic anemia, and anemia caused by chronic disease. Evaluation of leukogram, including a total white cell count, a differential cell count, absolute numbers of specific leukocytes can help identify abnormalities that may suggest specific diseases such as a viral or bacterial infection, inflammatory disorders or even a neoplastic process. The platelet count is most often used to monitor or diagnose conditions that cause too much bleeding related with thrombocytopenia; it can be due to multiple mechanisms such as reduction of thrombopoiesis (myeloptisis, myelofibrosis, myeloproliferative disease, and idiopathic medullary aplasias or due to the effect of mielosuppresive drugs), increased peripheral destruction of platelets (immune mediated thrombocytopenia), consumption (dissemined intravascular coagulation) sequestration of the spleen and loss of platelets by idiopathic origin.

Colonic T-cell-rich, large B-cell lymphoma associated with equid herpesvirus 5 infection and secondary trans-colonic fistula in a horse

A 17-y-old Rocky Mountain gelding was presented to the Virginia-Maryland Veterinary Teaching Hospital because of a 4-wk history of anorexia, weight loss, lethargy, and fever of unknown origin. Abdominal ultrasound revealed lymphadenomegaly of the abdominal and colonic lymph nodes, thickening of the wall of the large colon, and a mass associated with the large colon. The horse was euthanized given a poor prognosis. On autopsy, an ~20-cm diameter mass was found within the mesocolon between the right ventral and right dorsal colon. The mass had invaded through the colonic walls and formed a fistula between the 2 involved lumina. On histologic evaluation, the mass consisted of small numbers of large neoplastic lymphocytes, numerous small lymphocytes, and many foamy macrophages. A diagnosis of T-cell-rich, large B-cell lymphoma was made based on immunohistochemical staining for CD79a, CD3, and Iba1; concurrent infection with equid herpesvirus 5 was confirmed with in-situ hybridization (ISH). To our knowledge, neither a trans-colonic fistula resulting from alimentary lymphoma in a horse nor detection of intralesional equid herpesvirus 5 in equine alimentary lymphoma by ISH has been reported previously.

T-cell-rich, large B-cell lymphoma in the brain of a horse

T-cell-rich, large B-cell lymphoma (TCRLBCL) is the most commonly diagnosed type of lymphoma in horses. Here we describe the clinical signs, neuropathology, immunohistochemistry (IHC), and PCR for antigen receptor rearrangement (PARR) analysis results of a TCRLBCL in the brain of an 8-y-old male Quarter Horse that was euthanized after acute anorexia, tremors, head pressing, falling, blindness, incoordination, and seizures. Autopsy revealed a firm, smooth, pale-yellow mass that expanded both lateral ventricles and the adjacent subcortical white matter. Histologically, the mass consisted of a densely cellular neoplasm composed of large, CD79+ neoplastic B-lymphocytes admixed with sheets of small, CD3+ reactive T-lymphocytes, Iba1+ histiocytes, MUM1+ plasma cells, and rare eosinophils supported by a fine fibrovascular stroma. Formalin-fixed, paraffin-embedded tissue scrolls were retrieved and subjected to PARR analysis, which revealed a clonal reaction in the immunoglobulin gene and a polyclonal reaction for the T-lymphocyte receptor gene, consistent with a neoplastic B-lymphocyte and reactive T-lymphocyte proliferation. The diagnosis of TCRLBCL was suspected histologically and confirmed based on IHC and PARR analysis.

Association of globulin concentrations with prognosis in horses with lymphoma

Introduction

Lymphoma is the most common hemopoietic neoplasia in horses. Common clinicopathologic abnormalities in equine lymphoma include hyperglobulinemia, hypoalbuminemia, hyperfibrinogenemia, anemia, thrombocytopenia and lymphocytosis. Hypoglobulinemia has been reported in other species with lymphoma, however it has not been well-described in horses. The objectives of this study were to examine the prevalence of hypoglobulinemia in equine lymphoma, and to identify prognosis and clinicopathological abnormalities associated with serum globulin concentrations.

Methods

Ninety-six horses with lymphoma were investigated in this retrospective study. Patients were allocated into groups based on serum globulin concentration. Survival analysis was performed to determine risk factors associated with globulin concentration and outcome.

Results

Nineteen horses were hypoglobulinemic (≤2.1 g/dL), 63/98 were normoglobulinemic (2.2-4.3 g/dL), and 16/98 were hyperglobulinemic (≥4.4 g/dL). Hyperglobulinemia was associated with a higher anion gap (P = 0.0005), lower bicarbonate (P = 0.006), sodium (P = 0.03) and chloride concentrations (P = 0.002), and higher total protein than hypoglobulinemic horses (P < 0.0001). For location, 37% of horses with mucocutaneous lymphoma were hypoglobulinemic, compared to none in the hyperglobulinemic group (P = 0.02). Survival times were significantly different between low, normal and high globulin groups (P = 0.0002, median [range] survival times: 333 [1-3792], 43 [1-4,001] and 4 [1-129] days, respectively). Significant risk factors for shortened time to death were hyperglobulinemia (HR 2.4, P = 0.02), T cell lymphoma (HR 3.5, P < 0.0001), and multicentric (HR 3.1, P = 0.0008) and mediastinal (HR 6.4, P = 0.006) forms of lymphoma. Lack of chemotherapy was associated with shortened survival time (HR 4.5, P < 0.0001). B cell lymphomas (P < 0.0001) and mucocutaneous lymphoma location (P < 0.0001) were associated with longer survival times.

Discussion

Serum globulin concentrations are associated with location of lymphoma, clinicopathologic abnormalities, and survival times in equine lymphoma.

Lymphomatosis as a Cause of Abdominal Pain and Distension in Two Adult Horses

Two equine patients presented separately with severe abdominal distention, colic, lethargy, and decreased appetite. An ante-mortem diagnosis of lymphoma was reached in each case based on peritoneal fluid cytology. Due to a poor prognosis, the horses were humanely euthanized. Post-mortem examination with histology and immunohistochemistry confirmed both cases as lymphoma: alimentary B-cell lymphoma of the distal jejunum and cecum in one case, and T-cell lymphoma of the cecum in the second case. Both cases exhibited extensive metastasis with peritoneal and pleural serosae covered in small nodules and plaque like masses consistent with lymphomatosis. These cases document a unique presentation of lymphoma in equine patients presenting as peritoneal lymphomatosis with ascites.

Less Typical Courses of Rhodococcus equi Infections in Foals

This article aims to present several interesting and less typical courses of Rhodococcus equi infections in foals, collected during the 2019-2021 foaling seasons in some Polish studs. The study was conducted by the Division of Veterinary Epidemiology and Economics, Warsaw University of Life Sciences-SGGW, and concentrated on ultrasonographic contribution to diagnostics and treatment of the disease. Among many standard cases of rhodococcal pneumonia, some rare ones occurred. The aforementioned issues include the potential contribution of rhodococcal infection to a grave outcome in a prematurely born filly, lost as a yearling, so-called "extrapulmonary disorders" (EPD), a hypothesis of inherited immunodeficiency with grave outcome in a breeding dam line from one stud, and macrolide-induced anhidrosis. The main benefit of this report would be to supplement the general picture of clinical rhodococcosis.

Molecular characterization of equine thymidine kinase 1 and preliminary evaluation of its suitability as a serum biomarker for equine lymphoma

Background

Thymidine kinase 1 (TK1) plays a key role in the synthesis of deoxythymidine triphosphate (dTTP) and is thus important for DNA replication and cell proliferation. The expression of TK1 is highest during S-phase, and it is rapidly degraded after mitosis. In cancer cells, TK1 is upregulated, resulting in leakage of excess TK1 into the blood. Consequently, serum TK1 has been used as a diagnostic and prognostic cancer biomarker, mainly in human medicine. The aims of this work were to characterize equine TK1 and to evaluate its suitability as a serum biomarker for equine lymphoma.

Results

Equine TK1 was cloned, expressed in E. coli and affinity purified. The purified recombinant horse TK1 showed broad substrate specificity, phosphorylating pyrimidine deoxyribo- and ribonucleosides and, to some extent, purine deoxynucleosides, including anticancer and antiviral nucleoside analogues. ATP was the preferred phosphate donor. Serum TK1 activity was measured in samples collected from horses with confirmed or suspected lymphoma and control horses with and without concurrent diseases. Serum TK1 activity levels were significantly higher in horses with lymphoma (p <  0.0005) and suspected lymphoma (p <  0.02) and in tumour-free groups with diverse diseases (p <  0.03) than in controls without concurrent diseases. There was a significant difference between the lymphoma group and the tumour-free group with diverse diseases (p <  0.0006). Furthermore, receiver operating characteristic analysis revealed a sensitivity of 0.86, a specificity of 0.95 and an AUC (area under the curve) of 0.92 compared to the controls without concurrent diseases, with a sensitivity of 0.97, a specificity of 0.71 and an AUC of 0.88 when compared with the tumour-free group with diverse diseases.

Conclusion

Equine TK1 showed high specific activity and broader substrate specificity than human TK1. Anticancer and antiviral thymidine analogues were efficiently phosphorylated by horse TK1, suggesting that these analogues might be good candidates for chemotherapy in horses. Serum TK1 activity was significantly higher in horses with lymphoma than in controls. ROC analysis indicated that serum TK1 could serve as a promising cancer biomarker in horses.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12860-021-00399-x.

Nonclassified Lymphoma as a Cause of Radial Nerve Paralysis in a Horse

A 4-year-old American Quarter Horse gelding was evaluated for acute non-weight-bearing lameness of the right thoracic limb with swelling in the right shoulder region. Physical examination revealed radial nerve paralysis of unknown etiology. The primary differential diagnosis was musculoskeletal trauma. Ultrasonography of the right shoulder region identified a heterogeneous mass that extended from the point of the shoulder to the thoracic inlet. Cytologic analysis of fluid collected by fine needle aspirate of the mass was consistent with large cell lymphoma. Based on the cytological findings, locally invasive neoplasia was diagnosed and considered the likely cause of the radial nerve paralysis. Because of the poor prognosis, the horse was euthanized, and postmortem examination confirmed the diagnosis of a nonclassified large cell lymphoma that extended from the deep tissues of the right pectoral muscle group into the thoracic inlet and pleural cavity, as well as the right brachial plexus. The mass in the region of the brachial plexus encompassed and mechanically compressed all of the nerves within the area, resulting in the clinical sign of radial nerve paralysis. Although neoplasia as a cause of radial nerve paralysis is rare, it should be considered as a differential diagnosis, regardless of age.

Immune-mediated haemolytic anaemia and thrombocytopenia in 25 adult equids: 1997-2016

BACKGROUND

Information concerning clinical presentation, conditions associated with immune-mediated haemolytic anaemia (IMHA) and thrombocytopenia (IMTP) and outcome in equids is lacking. Previous case reports suggest that immune-mediated disease and neoplasia are associated.

OBJECTIVES

Characterise the clinical presentation, clinicopathologic data, underlying conditions, treatment and outcome of IMHA and IMTP cases in equids. We hypothesise that IMHA with concurrent thrombocytopenia occurs more often than IMHA or IMTP alone, and that neoplasia is commonly associated with these immune diseases and cases frequently have a poor prognosis.

STUDY DESIGN

Retrospective case-control study.

METHODS

Medical records were reviewed from 1997 to 2016. Twenty-five equids were diagnosed with IMHA, IMTP or IMHA with thrombocytopenia by Coombs test or flow cytometry. Controls were equids presented for nonimmune-mediated disease immediately prior to and after study animals. Fisher's exact test was used to compare between groups for categorical variables (P < .05). Results reported as odds ratios (OR) and 95% confidence intervals (CI). Unpaired t test and Mann-Whitney test were used to compare between groups for continuous variables (P < .05).

RESULTS

Neoplasia incidence was significantly higher in the study population (28%) versus controls (8%) (P = .04). Equids with primary disease were more likely to survive to discharge than equids with secondary disease (8/9 vs 7/16; P = .03; OR = 13.3; 95% CI: 1.3-134.7). Survivors had a significantly lower blood urea nitrogen (BUN) than those that died or were subjected to euthanasia (survivors, 6.1 ± 2.5 mmol/L vs nonsurvivors, 9.9 ± 3.1 mmol/L P = .003). The odds of short-term mortality were higher in horses presenting with increased BUN (OR = 19.5; 95% CI, 1.8-214.1; P = .009).

MAIN LIMITATIONS

Retrospective nature of the study, small case numbers and limited long-term follow-up.

CONCLUSIONS

Primary IMHA/IMTP cases have a reasonable prognosis and warrant treatment. Secondary cases have a poor prognosis, and are frequently associated with cancer. BUN may have utility as a prognostic indicator for IMHA/IMTP cases.

Pericardial effusion and congestive heart failure in a horse with multicentric lymphoma

A 20-year-old Warmblood mare was evaluated for acute-onset tachycardia and a one-week history of lethargy and ventral oedema. Haematology revealed leucocytosis with neutrophilia. Ultrasonographic examination revealed pericardial effusion and cardiac tamponade consistent with right-sided congestive heart failure. Abdominal ultrasonography demonstrated loss of definition of the right kidney and a structure adjacent to the kidney suggestive of a renal neoplasm. A tentative antemortem diagnosis of multicentric neoplasia, most likely lymphoma, was made. The horse was subjected to euthanasia due to a grave prognosis. Postmortem examination and histological evaluation were consistent with lymphoma, including infiltration of the cardiac muscle with neoplastic cells. Although lymphoma has been reported to affect the heart, congestive heart failure due to cardiac and pericardial neoplastic infiltration has not been previously reported as the presenting complaint. This case report describes the diagnostic approach to pericardial effusion and congestive heart failure in a mare with lymphoma.

Species-specific regulation of angiogenesis by glucocorticoids reveals contrasting effects on inflammatory and angiogenic pathways

Glucocorticoids are potent inhibitors of angiogenesis in the rodent in vivo and in vitro but the mechanism by which this occurs has not been determined. Administration of glucocorticoids is used to treat a number of conditions in horses but the angiogenic response of equine vessels to glucocorticoids and, therefore, the potential role of glucocorticoids in pathogenesis and treatment of equine disease, is unknown. This study addressed the hypothesis that glucocorticoids would be angiostatic both in equine and murine blood vessels.The mouse aortic ring model of angiogenesis was adapted to assess the effects of cortisol in equine vessels. Vessel rings were cultured under basal conditions or exposed to: foetal bovine serum (FBS; 3%); cortisol (600 nM), cortisol (600nM) plus FBS (3%), cortisol (600nM) plus either the glucocorticoid receptor antagonist RU486 or the mineralocorticoid receptor antagonist spironolactone. In murine aortae cortisol inhibited and FBS stimulated new vessel growth. In contrast, in equine blood vessels FBS alone had no effect but cortisol alone, or in combination with FBS, dramatically increased new vessel growth compared with controls. This effect was blocked by glucocorticoid receptor antagonism but not by mineralocorticoid antagonism. The transcriptomes of murine and equine angiogenesis demonstrated cortisol-induced down-regulation of inflammatory pathways in both species but up-regulation of pro-angiogenic pathways selectively in the horse. Genes up-regulated in the horse and down-regulated in mice were associated with the extracellular matrix. These data call into question our understanding of glucocorticoids as angiostatic in every species and may be of clinical relevance in the horse.

Linfoma mediastinal em equino - relato de caso

RESUMO Uma égua SRD, com 20 anos de idade, foi encaminhada ao hospital veterinário com histórico de emagrecimento, perda de apetite e aumento de volume na região peitoral e no pescoço. Aos exames físico e laboratorial, constatou-se que o animal apresentava taquicardia, taquipneia, hiperproteinemia, hiperfibrinogenemia e hipoalbuminemia. À auscultação cardíaca, os sons das válvulas pulmonar e aórtica não foram audíveis. Realizaram-se as avaliações eletrocardiográfica e ecodopplercardiográfica transtorácica bilateral, nas quais se observaram presença de efusão pleural e deslocamento caudal do coração. Na varredura ultrassonográfica ao redor do coração, constatou-se a presença de uma massa na base do coração. Para tentar amenizar o quadro respiratório, foi realizada drenagem torácica. A efusão pleural apresentava característica serossanguinolenta, com padrão citológico inflamatório. Algumas horas após retornar a baia, o animal veio a óbito. Realizou-se a necropsia, na qual pôde ser observada a presença da massa posicionada ao redor do tronco braquiocefálico. Na análise histopatológica, diagnosticou-se o linfoma difuso de células pequenas e médias.

Ocular involvement of multicentric malignant B-cell lymphoma in a ewe. A case report

An 8.5-year-old, 98 kg female mountain sheep presented with bilateral exophthalmos with reduced retropulsion of the globes, impairing physiologic eyelid closure, sanguineous ocular discharge, as well as swelling of the eyelids and periocular skin. Bilateral vitreal hemorrhage hindering examination of the fundus was further noticed. Systemic signs included reduced general demeanour, presence of a firm mass in the left half of the mammary gland, multiple masses in the area of the vulva and a mass between the shoulder blades. Complete diagnostic work-up, i.  e. complete blood count, blood chemistry and coagulation status, fine needle aspiration of periocular swellings and incisional biopsy of the vulvar masses revealed a diagnosis of malignant B-cell lymphoma. Due to the deterioration in general demeanour and rapid progression of exophthalmos, resulting in bilateral corneal ulceration, despite symptomatic medical treatment, the ewe was humanely euthanized. Gross necropsy and histopathology of select tissue samples confirmed the diagnosis of multicentric malignant B-cell lymphoma. To the authors' knowledge, this is the first report of multicentric malignant B-cell lymphoma involving the ocular adnexa in sheep.

Blindness associated with nasal/paranasal lymphoma in a stallion

A 29-year-old stallion presented with bilateral blindness following the chronic purulent nasal drainage. The mass occupied the right caudal nasal cavity and right paranasal sinuses including maxillary, palatine and sphenoidal sinuses, and the right-side turbinal and paranasal septal bones, and cribriform plate of ethmoid bone were destructively replaced by the mass growth. The right optic nerve was invaded and involved by the mass, and the left optic nerve and optic chiasm were compressed by the mass which was extended and invaded the skull base. Histologically, the optic nerves and optic chiasm were degenerated, and the mass was diagnosed as lymphoma which was morphologically and immunohistochemically classified as a diffuse large B-cell lymphoma. Based on these findings, the cause of the blindness in the stallion was concluded to be due to the degeneration of the optic nerves and chiasm associated with lymphoma occurring in the nasal and paranasal cavities. To the best of our knowledge, this is the first report of the equine blindness with optic nerve degeneration accompanied by lymphoma.

A database survey of equine tumours in the United Kingdom

REASONS FOR PERFORMING STUDY

Survey data on equine tumours are sparse compared with other species and may have changed over time.

OBJECTIVES

To describe the most frequently diagnosed equine tumours recorded by a diagnostic pathology laboratory over 29 years, to identify background factors associated with tumour type, and to identify any changes in the tumours diagnosed or the background of cases submitted during the study period.

STUDY DESIGN

Observational; cross-sectional analysis of records of a diagnostic pathology laboratory.

METHODS

The records of all neoplastic equine histology submissions to the University of Bristol (January 1982-December 2010) were accessed from a database, and a list of diagnoses compiled. The 6 most commonly diagnosed tumour types were analysed using logistic regression to identify background factors associated with tumour type. The overall population of equine tumour submissions and the relative frequency of diagnosis of the most common tumour types were compared between decades.

RESULTS

There were 964 cases included. The most frequently diagnosed tumours were: sarcoid (24% cases), squamous cell carcinoma (SCC) (19%), lymphoma (14%), melanoma (6%), gonadal stromal tumour (6%) and mast cell tumour (MCT) (4%). With sarcoid, Thoroughbred/Thoroughbred cross and gelding as reference categories: increasing age was significantly associated with the odds of each of the other tumour types, mares were at reduced risk of SCC, Arab/Arab cross had a higher risk of MCT, Cob/Cob cross had an increased risk of SCC and MCT, and ponies had an increased risk of melanoma. The mean age of submissions increased in each successive decade and the breed composition became broader. Sarcoids and lymphoma formed a smaller proportion of diagnoses in later decades.

CONCLUSIONS

The types of tumours submitted to this laboratory have changed over the last 3 decades. Current data inform clinicians and researchers and further studies are warranted to follow trends.

Impact of karyotype organization on interlocus recombination between T cell receptor genes in Equidae

The T cell receptor (TCR) genes (TRA, TRB, TRD and TRG) reside in 3 different chromosomal regions. During the maturation of T lymphocytes, the TCR genes are rearranged by site-specific recombination, a process that also predisposes T cells to aberrant rearrangements. Illegitimate recombination between the TCR genes occurs at a low level in healthy individuals, but this frequency may correlate with the risk of lymphoma. The aim of this work was to investigate interlocus recombination in equids. Illegitimate rearrangements were studied in peripheral blood lymphocytes by FISH with painting and BAC probes and by sequencing of PCR products, and the frequencies of recombination were assessed in horses and 4 other equids. The presence of several trans-rearrangement products between the TRA and TRG genes was verified by PCR in all investigated equids. Frequencies of trans-rearrangements in horses are higher than in humans, and colocalization of the TCR genes on the same chromosome increases the incidence of trans-rearrangements between them. The orientation of the TCR genes does not impact interlocus recombination itself but does affect the viability of cells carrying its products and consequently the number of trans-rearrangements observed in lymphocytes.

Solitary (primary) uveal T-cell lymphoma in a horse

A 22-year-old Australian stockhorse gelding was presented with anterior uveitis in the right eye which was nonresponsive to anti-inflammatory therapy. Clinical examination revealed corneal edema and vascularization, marked hypopyon, and thickening of the dorsal iris, which was confirmed by ultrasonography. Hematologic and biochemical analyses, abdominal and thoracic ultrasonography, and abdominocentesis with cytologic and biochemical analysis revealed no significant abnormalities. Cytological examination of an aqueous humor sample revealed a population of predominantly large lymphoblasts with high nuclear-to-cytoplasmic ratio, round or irregular nuclei, clumped nuclear chromatin, multiple large prominent nucleoli, and a small volume of basophilic cytoplasm. The cytologic diagnosis was intraocular lymphoma. Biopsy of the right submandibular lymph node revealed no evidence of neoplastic invasion. Euthanasia and a complete necropsy were performed and revealed no evidence of neoplasia in any tissue other than the right eye, which had an extensive, well-defined infiltrate of neoplastic lymphocytes expanding the ciliary body and iris, infiltrating the ciliary epithelium, and extending into the pars plana and peripheral choroid. Immunohistochemistry confirmed that neoplastic cells expressed the T-cell marker CD3. To the authors' knowledge, this is the first description of primary, solitary uveal T-cell lymphoma in a horse. Although apparently rare, lymphoma should be considered in horses with uveitis, even when inflammation is unilateral and in the absence of extraocular signs of neoplasia. Aqueocentesis and cytological examination provided an antemortem diagnosis in this case and should be considered as a diagnostic tool for investigation of uveal thickening and hypopyon.