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Case report 

A Hidden Threat: A Case Report on Pheochromocytoma in a 
Horse (Equus ferus caballus) 
Romelia Pop1, Ecaterina Semzenisi1, Claudiu-Nicușor Ionică2, Dragoș Hodor1, Cristian M. Crecan3, Iancu A. 
Morar5), Alexandru Florin Lupsan3, Zsofia Daradics4, Mirela Alexandra Tripon5, Denisa Bungărdean6, Otilia 
Bulmez5, Maria Popescu6, Valeria Ciulu-Angelescu3, and Alexandru-Flaviu Tăbăran1 

 
1 Affiliation 1; Department of Pathology, Faculty of Veterinary Medicine, University of Agricultural Sciences 

and Veterinary Medicine of Cluj-Napoca, Calea Manaștur, 400372 Cluj-Napoca, Romania 
2 Affiliation 2; Department of Animal Nutrition, Faculty of Veterinary Medicine, University of Agricultural 

Sciences and Veterinary Medicine of Cluj-Napoca, Calea Manaștur, 400372 Cluj-Napoca, Romania 
3 Affiliation 3; Department of Anaesthesiology and Surgery, University of Agricultural Sciences and Veteri-

nary Medicine of Cluj-Napoca, 400372 
4 Affiliation 4; Department of clinical sciences, University of Agricultural Sciences and Veterinary Medicine of 

Cluj-Napoca, 400372 
5 Affiliation 5; Department of Obstetrics and Reproduction, University of Agricultural Sciences and Veteri-

nary Medicine Cluj-Napoca, 40037 
6 Affiliation 6; Equine Clinic, University of Agricultural Sciences and Veterinary Medicine Cluj-Napoca, 

40037 
* Correspondence: romelia.pop@usamvcluj.ro; Tel: +40744263975 

 
Abstract: Pheochromocytomas are rare neuroendocrine tumors in horses, originating from chromaffin cells within the adrenal medulla. This case 
report describes a 15-year-old Friesian horse that presented with progressive ataxia, muscular weakness, and lateral recumbency, leading to eutha-
nasia. During necropsy, a 2x3 cm dark-red mass with focal necrosis was diagnosed in the left adrenal gland, along with perirenal hematoma and 
hemoperitoneu. Histopathological analysis confirmed an adrenal pheochromocytoma, characterized by nests of polygonal to spindle-shaped cells 
with small amount of cytoplasm and low mitotic activity.. The clinical signs were likely due to catecholamine hypersecretion and the acute hemoperi-
toneum was caused by the tumor rupture and hemorrhage. Pheochromocytomas, though often diagnosed post-mortem, can cause life-threatening 
cardiovascular and systemic effects, underscoring the importance of histopathological evaluation and the need for improved antemortem diagnostic 
techniques in equine practice. 

Keywords: Pheochromocytoma; Equine neuroendocrine tumor Adrenal gland neoplasm; Histopathology 
 

 

 1. Introduction 
 Pheochromocytoma, a rare neoplasm in horses [1,2], arises from chromaffin [3] cells lo-

cated within the adrenal medulla [4]. Histopathologically, these tumors are characterized by the 
proliferation of neuroendocrine cells, which are responsible for the production and secretion of 
catecholamines [3]. While pheochromocytomas are uncommon in equine species, their discovery 
typically occurs either incidentally during necropsy or upon investigation of unexplained clinical 
symptoms [4,5]. From a pathologic perspective, pheochromocytomas in horses often present as 
well-encapsulated, lobulated masses, ranging from a few to several centimeters in diameter [6]. 
These tumors are typically located within or adjacent to the adrenal gland [4]. Grossly, pheo-
chromocytomas may appear firm and reddish-brown, with areas of hemorrhage or necrosis, re-
flecting their variable growth rates and vascularization [6]. Histologically, pheochromocytomas 
are composed of polygonal to spindle-shaped cells arranged in nests or cords. These cells exhibit 
a finely granular cytoplasm due to the presence of catecholamine-containing secretory vesicles 
[7]. The nuclei are generally round to oval, with prominent nucleoli, and mitotic figures are rare, 

Received: 14.11.2024 

Accepted: 11.12.2024 

Published: 19.03.2025 

DOI: 10.52331/cvj.v30i1.46 

 

 

Copyright: © 2025 by the 

authors. Submitted for pos-

sible open access publica-

tion under the terms and 

conditions of the Creative 

Commons Attribution (CC 

BY) license (http://crea-

tivecommons.org/li-

censes/by/4.0/). 



Cluj Vet J 2025, vol 30, issue 1 57 of 60 

indicating the typically slow-growing nature of these tumors. However, more aggressive variants can demon-
strate higher mitotic activity [1] and local invasion into surrounding tissues, including blood vessels. Hemor-
rhage, necrosis, and vascular thrombosis are commonly observed.  

Special histochemical stains, such as chromogranin A, synaptophysin, and neuron-specific enolase, are 
useful in confirming the neuroendocrine origin of the tumor cells. Immunohistochemical analysis further aids 
in differentiating pheochromocytomas from other adrenal or metastatic neoplasms, with pheochromocytomas 
showing positive staining for chromaffin markers [8]. Additionally, electron microscopy may reveal dense 
core granules within the cytoplasm, corresponding to the secretory vesicles laden with catecholamines[7]. 

 Given the neuroendocrine origin and variable behavior of pheochromocytomas, their histopathologi-
cal features are critical for diagnosing and determining potential malignancy. Although these tumors are gen-
erally low aggressive in horses, malignant pheochromocytomas can invade adjacent tissues or metastasize [9].  

 This case report explore the pathological and histopathological findings of a pheochromocytoma in a 
horse, highlighting its cellular morphology and histological characteristics. While histopathological evalua-
tion contributes to a greater understanding of tumor biology, this report also highlights its potential to guide 
future advancements in ante-mortem diagnosis, prognosis, and treatment approaches. 

 
2. Case description 

A fifteen-year-old intact male Friesian horse (Equus ferus caballus) was referred to the Equine Clinic at 
the Cluj-Napoca Faculty of Veterinary Medicine for evaluation due to an array of progressive clinical signs, 
including abnormal gait, ataxia, muscular weakness, and, eventually, lateral recumbency. The horse’s condi-
tion deteriorated rapidly despite supportive care, culminating in a state of recumbency. 

 The initial clinical examination was conducted with the patient in right lateral recumbency. Vital pa-
rameters showed a heart rate of 96 beats per/minute, which was irregular, and a respiratory rate of 16 breaths 
per minute, with eupnea. a body temperature of 38.6°C, a dehydration score of 10%, icteric mucous mem-
branes in the mouth, icteric and congested subconjunctival mucosa, nystagmus, and a body condition score 
of 2-3/5. Peristalsis in the left abdomen was present. The patient had not defecated or urinated during the 
clinical examination, so urinary catheterization was performed. The urine was brownish-red in color and low 
in quantity, approximately 3.5 L. 

For a more complex neurological assessment, the patient was suspended in a sling and attempts were 
made to assist them in rising with the help of an electric elevator. Using the sling, the patient was able to 
maintain a quadrupedal position for approximately 12 hours, showing normal appetite for both food and 
water throughout this period. At the end of this time, the patient presented with muscle fasciculations and 
was left in recumbency to rest. Subsequently, the patient's condition deteriorated and he was no longer able 
to maintain a quadrupedal position regardless of the support provided. For the next three days, the patient 
was supported through parenteral therapy (fluid therapy, parenteral nutrition, analgesia, anti-inflammatory,  
gastric and hepatic protectants). 

 
Clinical Pathology 

Table 1. Clinical parameters assessments 
Biochemical examination: Glucose 176 mg/dL [71-141 mg/dL]), 

ASAT 892 U/L [100-525 U/L] 
ALP 361 U/L [10-335 U/L] 
Total bilirubin 4.69 mg/dL [0-2.40 mg/dL 

Hematological examination: Neutrophils 11.88 109/L [2.3-9.5] 
Lymphocytes 11.1 109/L [17-68] 
Eosinophils 0.3 109/L [1-8] 

Cerebrospinal Fluid (CSF) ex-
amination: 

Cytology No significant findings 
Microbiological exames negative 
Proteins 0.16 g/dL [<0.005 g/dL]. 

Peripheral blood Babesia spp.(piroplasmosis) PCR positive 
 

Biochemical examination: high levels were found for glucose (176 mg/dL [71-141 mg/dL]), ASAT (892 
U/L [100-525 U/L]), ALP (361 U/L [10-335 U/L]), total bilirubin (4.69 mg/dL [0-2.40 mg/dL]), CK (3530 U/L [21-
400 U/L]), GLDH (90 U/L [0-15 U/L]). The renal profile was within normal limits. 



Cluj Vet J 2025, vol 30, issue 1 58 of 60 

Hematological examination: the results showed , neutrophilia, lymphopenia and eosinopenia. Red blood 
cell and platelet counts were within physiological limits. 

Cerebrospinal Fluid (CSF) examination: a CSF sample was collected, which appeared normal with char-
acteristic color and no changes in cytological, microbiological (negative), or hematological findings. However, 
the biochemical examination revealed high protein levels of 0.16 g/dL [<0.005 g/dL]. 

Peripheral blood sample: a blood sample was taken to diagnose a potential infestation with Babesia 
spp.(piroplasmosis). The blood smear confirmed a positive diagnosis due to the presence of intracellular par-
asitic forms and the diagnosis was further confirmed by a positive PCR result. 

Despite the treatments administered over the three days of recumbency, the patient's condition deterio-
rated significantly, with a comatose neurological status, accentuated nystagmus, and continued high liver and 
muscle values. 

Postmortem examination  
The case was deemed suitable for euthanasia, and a full necropsy was subsequently performed to ascer-

tain the underlying cause of the animal’s clinical decline. 
On gross examination, the body was markedly anemic, with retroperitoneal massive hemorrhage 

spreading from the left perirenal area, acute hemoperitoneum (measuring 2 liters), prominent hepatic steatosis 
and mild lung congestion. Within the left adrenal gland, a dense, dark-hemorrhagic, focally necrotic mass 
measuring approximately 2x3 cm was observed within the left adrenal gland. The mass expanded from the 
adrenal`s medulla, was moderately compressive on the surrounding adrenal cortex and was moderately ele-
vating the capsule (Figure 1).  

 
Figure 1. Left Adrenal Gland. Horse. An expansile mass with a focal area of necrosis compressinbg the 

surrounding adrenal tissue (arrow) is observed within the adrenal gland. CT-cortex, MD-medulla 

 

 
Histological examination  

The adrenal gland was fixed in 10% formalin for 48 hours, then dehydrated in ethanol, cleared in xylene, 
and infiltrated with paraffin at 58°C for 5 hours. Using a rotary microtome, 2 µm sections were cut, 
deparaffinized in xylene, and rehydrated through graded ethanol solutions. After rinsing in water, the 
sections were stained with hematoxylin for 5 minutes, rinsed, then stained with eosin for 2 minutes. The slides 
were dehydrated, cleared in xylene, mounted with Permount, coverslipped, and prepared for microscopic 
analysis.. Microscopic evaluation of the adrenal gland confirmed the presence of a neoplastic mass. The tumor 
was composed of nests and clusters of polygonal to spindle-shaped neoplastic cells. These cells were arranged 
in a well-defined trabecular pattern, supported by a fine fibrovascular stroma. The cytoplasm of the neoplastic 
cells was abundant and finely granular. Nuclei were round to oval, exhibiting a stippled chromatin pattern 
with occasional prominent nucleoli. The mitotic figures were rarely observed.. Additionally, the tumor 
displayed areas of extensive hemorrhage and necrosis, which correlated with the gross findings of 
hemorrhagic changes (Figure 2). The further histopathological analysis supported the diagnosis of 
pheochromocytoma, with the tumor arising from the chromaffin cells of the adrenal medulla. This histological 



Cluj Vet J 2025, vol 30, issue 1 59 of 60 

presentation, coupled with the observed hemorrhagic manifestations, shows that the pheochromocytoma 
rupture induced the acute hemoperitoneum and systemic symptoms observed before the animal’s death. 

 

 

Figure 2. Histological features of a pheochromocytoma. Horse. The mass consists of polygonal to spindle-shaped cells 
arranged in a trabecular pattern, interspersed by a fine fibrovascular stroma (black arrow). The neoplastic cells display 

a small amount of eosinophilic cytoplasm (red arrow), while their nuclei range from round to oval, exhibiting a 
stippled chromatin pattern with occasional prominent nucleoli.. H&E stain. Ob. x4 (Image A), Ob. x10 (Image B), Ob. 
x20 (Image C), Ob. x100 (Image D). Barr 500 µm (Image A), 200 µm (Image B), 100 µm (Image C), 20 µm (Image D). 

  

4. Discussion 
Pheochromocytomas, though rare in equines, represent a critical pathology due to their potential to 

cause significant systemic effects, largely through catecholamine hypersecretion. This case highlights the im-
portance of recognizing the neoplasm's pathologic and histopathologic presentations to guide accurate diag-
nosis and appropriate management. The presence of pheochromocytoma in this 15-year-old Friesian horse 
adds to the limited but growing body of literature documenting such tumors in horses, where they are often 
identified post-mortem or following a sudden onset of severe clinical signs [1,3]. The gross and histologic 
features observed in this case are consistent with previously reported descriptions of equine pheochromocy-
tomas. The presence of a well-encapsulated, firm mass in the adrenal gland, with associated necrosis and 
hemorrhage, mirrors findings from earlier studies [6]. The dark-red color of the mass and its dense nature are 
suggestive of its high vascularization, which, in combination with areas of hemorrhage, reflects the potential 
for these tumors to cause acute, life-threatening hemorrhagic events, as seen in the severe hemoperitoneum 
and perirenal hematoma in this case. Histopathologically, the tumor displayed nests and trabecular arrange-
ments of polygonal to spindle-shaped cells, with small amount of cytoplasm [7]. This cellular architecture is 
typical of pheochromocytomas, and the low mitotic index further supports its generally slow-growing nature 
[1]. However, the areas of necrosis and hemorrhage within the tumor may suggest a more aggressive variant, 
or the result of vascular compromise, which may be found in pheochromocytomas [9]. The clinical manifesta-
tions in this case, including progressive weakness, ataxia, and lateral recumbency, are not specific to pheo-
chromocytoma but may reflect the cardiovascular and neuromuscular effects of excess catecholamine release. 
Elevated catecholamine levels can lead to neuromuscular dysfunction by increasing sympathetic stimulation, 
which can disrupt normal muscle tone and coordination. This may result in muscle weakness, impaired motor 
control, and uncoordinated movements, which could explain the observed symptoms of ataxia and recum-



Cluj Vet J 2025, vol 30, issue 1 60 of 60 

bency in this case. However, these clinical signs were compounded by the presence of the acute hemoperito-
neum, likely precipitated by hemorrhage from the adrenal mass. This scenario underscores the importance of 
considering pheochromocytoma in differential diagnoses when horses present with nonspecific but rapidly 
deteriorating clinical signs, especially in cases where significant cardiovascular disturbances are observed. 
From a diagnostic standpoint, pheochromocytomas in horses are often diagnosed incidentally, as was the case 
here, following euthanasia and necropsy. This delayed diagnosis highlights the challenges in identifying such 
tumors antemortem. Veterinary imaging, such as ultrasonography or advanced imaging techniques like CT 
and MRI, could offer valuable diagnostic insight when pheochromocytoma is suspected based on clinical or 
laboratory findings, such as unexplained hypertension [4]. Nonetheless, these diagnostic tools are rarely em-
ployed in equine practice due to financial and logistical constraints. 

The histological findings in this case further confirm the neuroendocrine nature of the tumor, with the 
characteristic chromaffin cell morphology and staining properties. Special histochemical and immunohisto-
chemical techniques, such as chromogranin A and synaptophysin staining, are valuable in distinguishing phe-
ochromocytomas from other adrenal neoplasms or metastatic tumors [8]. 

This case report contributes to the limited pool of documented equine pheochromocytomas, empha-
sizing the necessity of a thorough histopathological evaluation to confirm the diagnosis. The tumor's potential 
to cause severe hemorrhage and the subsequent development of hemoperitoneum in this horse exemplify the 
life-threatening nature of pheochromocytomas when not promptly identified. Future studies should aim to 
improve the antemortem diagnostic techniques for pheochromocytoma in equines, which could enhance early 
detection and potentially improve clinical outcomes. 

 
Author Contributions: R.P.: Conceptualization, Data curation, Writing – original draft. C.N.I.: Writing – review & 

editing. R.P. & C.N.I.&D.H.: Investigation, Writing – review & editing A.F.T,. C.M.C., I.A.M, A.F.L, Z.D., M.A.T, D.B., O.B., 
M.P., V.C.A, Validation, Writing – original draft, R.P.& C.N.I  Writing – review & editing A.F.T. All authors have read and 
agreed to the published version of the manuscript”. 

Funding: Not applicable  

Institutional Review Board Statement: Not applicable 

Data Availability Statement: The data that support the findings of this study are available from the Department of 
Veterinary Pathology, University of Agriculture Science and Veterinary Medicine, the case has a registration number. Data 
are, however available from the authors upon reasonable request and with the permission of the Department of Veterinary 
Pathology, University of Agriculture Science and Veterinary Medicine. 

Acknowledgments: Not applicable 

Conflicts of Interest: Not applicable  

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