African pygmy hedgehogs (Atelerix albiventris), also known as four-toed hedgehogs, are small, spiny insectivores that have become increasingly popular as domestic animals in many countries in the Americas, Asia, and Europe (1). To ensure the wellbeing of these small, spiny insectivores, owners are advised to schedule veterinary check-ups every 6 months. This precaution is particularly crucial as African pygmy hedgehogs often conceal signs of pathology until the conditions reach an advanced stage, resulting in significant organ damage (2). The increasing prevalence of hedgehogs as companion animals has yielded valuable insights into their health and diseases. Notable conditions observed include periodontal disease, traumatic exophthalmia, bronchopneumonia, dilated cardiomyopathy, bacterial gastroenteritis, renal failure, endometrial polyps, limb constriction by hairs or threads, wobbly hedgehog syndrome, Caparinia tripilis acariasis, pyoderma, and neoplasms (3).

Neoplastic processes have been identified as one of the leading causes of mortality in captive hedgehogs, accounting for 25% of clinical problems (4–6). The average age of hedgehogs with neoplasms is approximately between 2.5 and 3 years (6), with a life expectancy of 5 years and exceptionally up to 8 years (7). In many studies, epithelial neoplasms are the most common, followed by round cell tumors and mesenchymal cell tumors (5–7). In addition to the mentioned neoplasms, there are other less frequent neoplasms such as endometrial stromal sarcomas, leiomyoma, adenoleiomyoma, adenocarcinoma, lymphoma, oral squamous cell carcinoma, schwannoma or neurofibrosarcoma, plasma cell tumors, hemangiosarcoma, fibrosarcoma, osteosarcoma, undifferentiated or poorly differentiated sarcomas, mammary gland tumors, mast cell tumors, sebaceous carcinoma, and lipoma (8). Additionally, it is important to note that the rate of malignancy is high (up to 85%), which often results in death (5, 6).

Consequently, a deeper understanding of neoplastic processes, including their incidence, pathophysiological behavior, and diagnostic approaches, is essential for informed treatment planning and accurate prognosis assessment. To contribute to the expansion of knowledge and provide a better understanding of neoplastic processes affecting hedgehogs, this study presents a report on specific type of neoplasm called “fibrous abdominal mesothelioma.” Mesothelioma is a type of cancer formed in the layer of cells that cover internal organs (7). This contribution aims to enhance the collective understanding of such conditions within the scientific community.

Initially, in this clinical case, because the mass was found in the abdomen, gastrointestinal lymphoma was suspected. This type of tumor of the hemolymphatic system is the second most common in land hedgehogs, corresponding to 11% of the neoplasms in this species. Since it is a round cell tumor, it has a strong diagnostic approach in cytology, which is why it was decided to take this type of sample in the first instance, finding no compatibility of findings (7). The cytological evaluation revealed the presence of moderate cellularity, with degenerated neutrophils. In addition, cells exhibited oval morphology of moderate pleomorphism, nuclei were ovoid in shape, and there was an increased nucleus: cytoplasm ratio. In mesotheliomas, it is common to find neoplastic cells that can vary from exclusively epithelial types to mixed forms, where characteristic pointed cells are also found (17).

The histological and morphological characteristics of the tumor in this study are similar to those of canine species mesothelioma. Mesotheliomas are rare neoplasms described in different species such as bovines, horses, canines, felines, and humans (18). The etiology of mesothelioma in hedgehogs is still unknown. Mesotheliomas have been associated with chronic asbestos inhalation in humans and canines (19, 20). There are few cases with the presence of asbestos bodies in dogs (21). Several mechanisms through which asbestos leads to benign and malignant diseases have been reported. However, there are currently no effective means to determine which patients exposed to asbestos will develop malignant neoplasms and which will not (22–25).

The microscopic histopathological results are similar to those previously described for canine mesothelioma, where a proliferation of neoplastic mesothelial cells has been observed. These cells form solid areas or cover cystic spaces and tubular structures, presenting a mucinous matrix pattern (26). The neoplastic cells are classically cuboidal with scant to moderate amounts of eosinophilic cytoplasm (27), supporting the diagnosis of mesothelioma. Generally, these tumors show marked anisocytosis and anisokaryosis, pleomorphism, bi-, and multinucleation, with mitosis occurring more frequently (28).

The result from the IHC staining demonstrated a high specificity for both normal and neoplastic mesothelium cells which are similar to those described using the calretinin marker in humans with this type of tumor (29). Additionally, the sections of the excised tissue were positive for mesothelin, a marker that participates in cell adhesion and cell–cell recognition, which is expressed in normal tissues and is overexpressed in different tumors such as malignant mesothelioma in humans (30). The tumor cells were negative in the for cytokeratins 5/6 (CK5/6) in the immunohistochemical study. These immunohistochemical markers of epithelioid mesothelioma are considered as valuable by authors (31).

Immunohistochemestry (IHC) markers are not validated for the species of this study; however, according to the prospectus, they present reactivity in other species, with reactivity to 80 a 100% (32), its demonstrated when polyclonal antibodies against human recombinant calretinin are used, calretinin is 80–100% sensitive for malignant mesothelioma (33, 34), with matches (35), when calretinin are positive in 95% of epithelioid mesotheliomas; staining is often strong and diffuse and must be both nuclear and cytoplasmic. While to mesothelin showed sensitivity and specificity of 0.73 (CI, 0.68–0.77) and 0.90 (CI, 0.84–0.93), respectively (36).

In a 2007 study, researchers confirmed the value of cytokeratin 5/6 as a marker of mesothelioma in effusion samples. Effusion, the buildup of excess fluid, is a common symptom of pleural and peritoneal mesothelioma, and this is similar as in this study (37). This marker is not effective for all mesothelioma cell types, it is weak or negative for fibrous mesothelioma, which is the least common and most difficult to treat asbestos-related cancer cell type in domestic animals (38). However, this marker showed reactivity to 51% of peritoneal mesothelioma cells (35). Mesothelioma has three distinct histological patterns, namely epithelioid, fibrous (sarcomatoid), and mixed cell (biphasic). These tumors may present foci of chondroplasia and osteoid formation or mineralized bone, which were not present in this case. The fibrous variant of malignant mesothelioma somewhat resembles fibrosarcoma, while the biphasic variant contains areas of carcinomatous and sarcomatoid features (39). Each type can be diffuse or localized. In the first group, the epithelial and mixed types predominate, while among the localized forms, the fibrous type is more common (40). Additional subtypes reported in canine species include deciduoid peritoneal mesothelioma, cardiac mesothelioma with granular cell morphology, lipid-rich pleural mesothelioma, and cystic peritoneal mesothelioma (41). Macroscopically, mesotheliomas appear primarily as fragile, pedunculated nodules with villous projections on the serosal surface (42). In contrast, sclerosing (mesenchymal) mesotheliomas are large, firm, multinodular, irregularly shaped masses with smooth surfaces (41), which tend to spread and thicken through fibrous adhesions on the serous surfaces of the abdomen. Affected animals show severe decline and are generally diagnosed clinically as having generalized peritonitis (42). In this clinical case, there were several points of interest: on the one hand, there is difficulty in diagnosing this type of neoplasia that is not described previously for this species. There have been no studies describing ultrasound results in hedgehogs, and it was difficult to distinguish the neoplastic nodules due to the hydronephrosis present and the accumulation of intra-abdominal fluid, which required a second examination to confirm the presence of a mass. Furthermore, the cytological characteristics of reactive mesenchymal cells due to inflammatory processes are very similar to those of neoplastic mesenchymal cells (29, 40). On the other hand, the biopsy after the removal of the mass was the method that allowed us to analyze the morphological findings, cyto- and anatomopathological findings and the immunohistochemical reactivity, confirming the suspicion of mesothelioma. Finally, the morphological characteristics described in the results led to the final diagnosis of localized malignant fibrous peritoneal mesothelioma, demonstrating compliance with at least three nuclear criteria for cellular malignancy (43).

Approximately 2 weeks after the tumor removal surgery, the hedgehog was monitored, showing clinical improvements, without signs of pain and with recovery of appetite. However, 1 month after this check-up, the owners reported the death of the hedgehog, following an acute deterioration over 2 days.

In conclusion, the microscopic lesions identified exhibit characteristics indicative of neoplastic transformations arising from mesenchymal cells with a mesothelial origin within the abdominal serosa, specifically of a fibrous nature. This study serves to highlight the viability of fibrous mesothelioma as a plausible diagnosis for abdominal neoplasia in terrestrial hedgehogs.

The original contributions presented in the study are included in the article/Supplementary material, further inquiries can be directed to the corresponding author.

The animal studies were approved by Comité Ético Científico de UDLA. The studies were conducted in accordance with the local legislation and institutional requirements. Written informed consent was obtained from the owners for the participation of their animals in this study.

IT: Formal analysis, Investigation, Methodology, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing, Software. JB: Data curation, Formal analysis, Supervision, Validation, Writing – original draft. CI: Conceptualization, Data curation, Investigation, Methodology, Writing – original draft. JL: Formal analysis, Investigation, Methodology, Software, Writing – original draft. ME: Conceptualization, Data curation, Methodology, Visualization, Writing – original draft. KF-C: Data curation, Investigation, Methodology, Visualization, Writing – original draft.