Pseudohyperparathyroidism in a mare associated with squamous cell carcinoma of the vulva.

An 18-year-old Appaloosa mare was examined because of squamous cell carcinoma of the vulva, anorexia with pronounced weight loss, and hypercalcemia. The tumor had developed rapidly over a period of 3 months and externally extended ventrally involving the perineum and the dorsal aspect of the udder. Necropsy examination demonstrated a large primary squamous cell carcinoma of the vulva, perineum, and mammary gland with metastases to the supramammary, sublumbar, deep inguinal, and mediastinal lymph nodes. No gross renal lesions were observed and, histologically, there was only mild vacuolation of renal tubular epithelium. Based on the normal concentration of serum parathyroid hormone, the absence of evidence of hypervitaminosis D, and normal renal function, a diagnosis was made of hypercalcemia of malignancy or pseudohyperparathyroidism. The mechanism responsible for hypercalcemia was not determined, but the histologic type of the neoplasm and the clinical course suggested possible production of a humoral hypercalcemic factor by the neoplasm, similar to that demonstrated in certain types of human squamous cell carcinoma.