Multicentric mast cell tumors in a horse.

The research article details a case of multicentric mast cell tumors in a 6-year-old female Rocky Mountain horse. The horse originally presented symptoms of draining tracts and distal limb subcutaneous edema on both left front and hind limbs. The diagnosis was made through fine-needle aspiration and cytological examination, revealing predominantly eosinophils and atypical mast cells. The case is unique for its primary symptoms as well as multicentric involvement, and underscores the importance of various staining techniques in differentiating among eosinophilic lesions in horses.

Background and Objective

• The study was initiated to document and evaluate a unique case of multicentric mast cell tumors (MCTs) in a horse, where the primary symptoms were draining tracts and distal limb subcutaneous edema.
• The researchers aimed to confirm the cytologic diagnosis and provide insights into equine MCTs, which have rare occurrence in horses.

Case Presentation

• The subject was a six-year-old female Rocky Mountain horse who had been showing symptoms of draining tracts and subcutaneous limb edema for two weeks before presentation.
• During the medical examination, fluid was collected from both affected limbs using fine-needle aspiration.

Cytologic and Histologic Evaluation

• Microscopic analysis of direct smears of the aspirated fluid showed high cellularity and a predominance of eosinophils along with moderately atypical granulated mast cells.
• Based on these findings, the initial cytologic diagnosis was mast cell tumor (MCT) with noteworthy eosinophilic infiltration, with an alternate diagnosis being eosinophilic granuloma.
• To confirm the diagnosis, surgical biopsy samples of the lesion from both limbs were obtained and evaluated through histopathology.
• Sections from the biopsies were stained with hematoxylin and eosin (H&E), toluidine blue, and Luna stains, which respectively highlighted general cellular structures, mast cells, and eosinophils.
• The histologic examination substantiated the cytologic diagnosis of MCT with traces of mast cells and eosinophils.

Immunohistochemical Findings and Conclusion

• The examination also revealed that the mast cells in the biopsied tissues strongly expressed CD117, a marker particularly found in mast cells and related tumors, thus confirming the presence of the MCT.
• This study thereby registered as the first documented case of cutaneous mast cell neoplasia in a horse, where the primary symptoms were draining tracts and distal limb subcutaneous edema across multiple limbs.
• The findings reinforce the use of complementary diagnostics like cytology and histopathology, and the importance of using different staining techniques, in effectively differentiating MCTs from other eosinophilic lesions in horses.