What Is Your Diagnosis? Multiple Subcutaneous Lumps in a Horse

A 21-year-old Irish Cob gelding was hospitalized, presenting with multiple subcutaneous nodular masses disseminated across the trunk, neck, head, and ventral abdomen. The two larger masses, which presumably appeared about 3 months prior to clinical admission and had been rapidly growing, were located on the lateral aspects of the neck and measured 12 and 10 cm in diameter, respectively (Figure 1). Throughout the rest of the body (head, trunk, withers, both shoulders, and ventral abdomen), the masses exhibited considerable variability in size, ranging from 0.5 to 6 cm in diameter. On dermatologic external examination, all nodules appeared firm, non-painful, and with- out pruritus, alopecia, ulceration, or fistulation. Fine-needle aspiration (FNA) cytology was performed on the mass located on the left side of the neck (Figure 2), and concurrently, a surgical excisional biopsy of the same subcutaneous nodule was performed under local anesthesia for histopathologic examination, fixed in 10% neutral buffered formalin, and routinely processed.