Multinodular Malignant Cutaneous Mast Cell Tumor in a Horse With Generalized Pruritus and Reactive Fibrosis: A Case Report.

The research article presents a unique case report on a horse diagnosed with a rare form of malignant skin tumor known as Mast cell tumor (MCT), marked by distinct symptoms such as multiple skin lesions, generalized itching, and a fibrous reaction in the affected tissues.

Background of the Case

• The subject of the study was an 18-year-old Friesian stallion that had been experiencing intermittent and progressive skin lesions on its left forelimb along with intense, generalized itching and apathy.
• The symptoms were temporarily responsive to glucocorticoids and antibiotics.
• The horse was alert but had a poor physical condition and demonstrated significant generalized itching.

Physical Observations

• A severe enlargement was noted in the left forelimb of the horse, surrounded by multiple firm masses ranging between 2 and 20 cm. These masses were ulcerated and showed focal necrosis.
• A single, 7 cm round firm mass was also observed on the left dorsal neck of the horse.
• The horse’s skin exhibited generalized alopecia (hair loss) and scaling with ulcers and erosions caused by itching.

Diagnostic Procedures

• A skin smear test was performed on the horse’s affected leg. The test revealed severe eosinophilic (white blood cells) and neutrophilic (type of immune cell) inflammation, along with pleomorphic bacteria.
• The skin and underlying tissue biopsies identified an unusual concentration of atypical mast cells, indicative of a malignant Mast Cell Tumor (MCT).

Necropsy Results

• The horse was eventually euthanized, and a necropsy done after that revealed a significant fibrous reaction in the longitudinal sections of the affected limb.
• The tumor was detected in only a few histological slides.

Conclusion

• The diagnosis process for equine MCT can be challenging due to the intense accompanying fibrous reaction in the affected tissues.
• Mast cell tumor should be suspected in cases observing eosinophilic infiltration in the targeted tissue and unexplained generalized itching.