Hemangiosarcoma (HSA) is a malignant tumor originating from vascular endothelial cells, characterized by rapid growth, aggressive biological behavior, and a high metastatic potential (1). This neoplasm represents approximately 2% of all canine tumors and it is overrepresented in specific breeds such as German Shepherds, Golden Retrievers, and Labrador Retrievers (1, 2). While most frequently observed in the spleen, right atrium of the heart, liver, and skin, HSA can also develop in less common sites, including the kidneys, retroperitoneum, central nervous system, and urinary tract (3, 4). In dogs, the clinical presentation is often no specific, with signs such as lethargy, anorexia, and acute collapse depending on the tumor location and stage (5).

Despite its frequent occurrence in domestic animals, cases involving wildlife are rarely documented, highlighting a critical gap in understanding this neoplasm in free-ranging species (6, 7). Advanced imaging modalities such as ultrasonography, computed tomography (CT), and magnetic resonance imaging (MRI) are essential for diagnosing and staging HSA in animals. These techniques allow clinicians to evaluate tumor size, vascular involvement, and potential metastases (6, 8). However, imaging findings alone are often insufficient to differentiate malignant HSA from benign masses such as hemangiomas or hematomas (3, 5). Definitive diagnosis relies on histopathological examination, supported by immunohistochemical markers such as Vimentine and specifically vascular markers such as factor VIII, CD31 (PECAM), and CD34 (4, 9–11). The biological behavior and prognosis of HSA vary significantly based on the tumor's primary location. For instance, heart and splenic HSA is associated with a high metastatic rate (16%−89%) and a median survival time (MST) of less than 3 months following surgical intervention (3, 5). Conversely, renal and retroperitoneal HSA exhibit lower metastatic rates and comparatively better survival times when managed surgically, with median survival exceeding 200 days in some cases (3, 6). Tumor size, invasion into adjacent tissues, and the presence of metastases significantly influence prognosis (2, 9). This variability underscores the importance of detailed diagnostic and staging protocols to guide clinical decision-making. In wildlife, HSA is not reported in red fox, and the available literature on neoplasms is limited to isolated case studies in this species (12–14). Neoplastic diseases in wild species may serve as valuable indicators of environmental health, reflecting exposure to contaminants, habitat degradation, or other stressors (15). As apex predators or sentinel species, wild carnivores such as the red fox (Vulpes vulpes) are particularly important for monitoring ecosystem dynamics and health (7, 12–15). This report documents the first known case of primary renal hemangiosarcoma in a red fox, combining advanced diagnostic imaging, cytological evaluation, and histopathological analysis. The fox originated from a sparsely urbanized area in southern Italy and presented with clinical signs of renal impairment. This case not only broadens the understanding of hemangiosarcoma in wild carnivores but also emphasizes the critical role of wildlife health surveillance in assessing ecological stability and addressing conservation challenges.

An adult male red fox (Vulpes vulpes, Linnaeus, 1758), was admitted on October 28, 2024, to the Wildlife Rescue Center (CRAS) “Federico II” in Naples, at the Veterinary Hospital of the Local Health Authority (ASL) Napoli 1 Centro, after being found in a sparsely urbanized area. The fox showed depression and posterior paraparesis. Clinical examination revealed normal temperature and pulse, pink mucous membranes, and mild dehydration. There was a mild nociceptive deficit in the hind limbs and pain upon abdominal palpation, suggesting possible intra-abdominal pathology. The fox was sedated with an intramuscular combination of medetomidine (30 μg/kg) and ketamine (5 mg/kg), followed by the placement of a 22-gauge venous catheter and administration of isotonic fluids (Ringer's lactate at 50 ml/kg). Following clinical deterioration, the animal died during hospitalization and was subsequently submitted for post-mortem examination.

This case report describes the first documented occurrence of primary renal hemangiosarcoma in a free-ranging red fox and contributes to the limited body of literature on neoplastic diseases in wild carnivores. In dogs, hemangiosarcoma is well recognized as an aggressive vascular neoplasm with a predilection for the spleen, right atrium, liver and, less frequently, the kidneys. Renal hemangiosarcoma is considered an uncommon localization even in this species, and data on its biological behavior are relatively scarce compared with splenic or cardiac forms. The present case shows that, in the red fox, renal hemangiosarcoma may follow a highly aggressive clinical course, characterized by extensive intra-abdominal dissemination. The clinical presentation was nonspecific, with generic signs that preceded acute decompensation due to hemorrhage or rupture of the mass. Advanced diagnostic imaging techniques played a central role in the characterization of the renal lesion. Ultrasonography revealed a large heterogeneous mass replacing the caudal pole of the right kidney, with mixed echogenicity and associated fluid accumulation. Computed tomography further defined the lesion as a voluminous, heterogeneously hypoattenuating mass with peripheral or ring-like contrast enhancement. On contrast-enhanced CT, the spleen showed a small hypodense area with moderate enhancement, no appreciable abnormalities were detected in the heart, and an enlarged lymph node was observed with regular and homogeneous contrast enhancement. In dogs, such imaging features are often associated with malignant vascular tumors, large hematomas or necrotic neoplasms, and do not by themselves allow hemangiosarcoma to be distinguished from other malignant renal tumors, such as sarcomas, carcinomas or lymphomas. Nonetheless, cross-sectional imaging allowed accurate localization of the lesion, assessment of the degree of renal involvement and identification of abdominal effusion, providing key elements for clinical decision-making and for planning the pathological investigation. Cytological examination of the mass yielded findings consistent with a malignant mesenchymal neoplasm. The predominance of spindle to oval cells with marked anisocytosis and anisokaryosis, prominent nucleoli and occasional mitotic figures on a strongly hemorrhagic background is in line with the cytological appearance of vascular sarcomas in domestic species. However, as frequently reported for hemangiosarcoma, marked hemodilution and lesion friability limited the diagnostic yield, precluding definitive classification. This underscores a well-known limitation of fine-needle aspiration in vascular tumors, in which histopathology and immunohistochemistry remain indispensable for an accurate diagnosis. Histologically, the tumor was a non-encapsulated, infiltrative neoplasm extending from the renal hilus to the cortex, with invasion of the ureteral and bladder wall. The neoplastic cells formed irregular blood-filled vascular channels and compact whorls, and showed marked pleomorphism and a high mitotic index, all features typical of malignant endothelial neoplasms. Immunohistochemical expression of Vimentin and von Willebrand factor (factor VIII– related antigen) in neoplastic cells confirmed their mesenchymal and endothelial nature, supporting the diagnosis of hemangiosarcoma. The presence of similar neoplastic proliferations in the peritoneum, spleen, intestinal wall, associated lymph node and at the level of the right atrial epicardium indicates widespread metastasis, mirroring the metastatic pattern described in canine hemangiosarcoma and highlighting the highly malignant behavior of this tumor in the red fox. The metastatic lesion observed on the epicardium of the right atrium is particularly noteworthy. In dogs, primary cardiac hemangiosarcoma classically arises in the right atrium or auricle and may coexist with, or metastasize to, abdominal organs. In the present case, the small epicardial focus was histologically and immunohistochemically compatible with a metastasis from the primary renal tumor rather than a distinct primary cardiac lesion. This finding illustrates the ability of renal hemangiosarcoma to disseminate to typical target sites of vascular neoplasms and reinforces the need for accurate necropsy with systematic sampling of multiple organs in cases involving wildlife. From an ecological and epidemiological perspective, neoplastic diseases in free-ranging wildlife remain underestimated and underreported, mainly due to limited diagnostic resources, the absence of systematic necropsy programs and the perception that tumors are less relevant than infectious or toxicological hazards. This case helps to partially fill this gap by documenting a malignant renal neoplasm in a synanthropic carnivore and demonstrating that advanced diagnostic approaches commonly used in companion animals can also be successfully applied to wildlife. The involvement of the urinary tract and the extensive peritoneal dissemination further raise questions about the possible role of environmental or toxic cofactors, although no specific etiological agents were identified in this animal, the case underscores the importance of integrating tumor surveillance into broader environmental health assessment systems. This report has some limitations inherent to descriptive studies on single cases. The absence of long-term clinical follow-up, the inability to perform comprehensive toxicological analyses and the lack of comparable cases in the same area limit the possibility of formulating causal hypotheses or drawing conclusions at the population level. Nevertheless, the detailed clinical, imaging and pathological characterization, combined with integration into an established surveillance network, demonstrates the feasibility and added value of intensive diagnostic investigation even in wild animals.