Like dogs and cats, horses can develop hypercalcemia due to several disorders, including chronic renal failure, vitamin D toxicosis, and primary hyperparathyroidism. The most common cause of hypercalcemia in horses is chronic renal failure. The equine kidney is important in the excretion of calcium; therefore, impaired renal calcium excretion associated with normal intestinal calcium absorption may explain the hypercalcemia found in these horses.
Humoral hypercalcemia of malignancy has been reported to be associated with gastric squamous cell carcinoma, adrenocortical carcinoma, squamous cell carcinoma of the vulva, lymphosarcoma, and ameloblastoma. These horses have hypercalcemia, hypophosphatemia, increased serum concentrations of PTHrP, and decreased serum concentrations of PTH.
Intoxication with ergocalciferol or cholecalciferol has been reported in horses. Ingestion of plants containing 1,25-dihydroxyvitamin D–like compounds (Solanum malacoxylon, S sodomaeum, Cestrum diurnum, Trisetum flavescens) causes typical clinical signs of vitamin D intoxication, including hypercalcemia.
Primary hyperparathyroidism is a rare disorder in ponies and horses. As in dogs and cats, hypercalcemia, hypophosphatemia, and high serum PTH concentrations are reported in horses with the disorder. Additional tests to exclude other conditions associated with hypercalcemia may include measurement of PTHrP and vitamin D metabolite concentrations.
As in other species, the definitive treatment of equine hypercalcemia is treating or removing the underlying cause. Unfortunately, the cause may not be readily apparent, and supportive measures (eg, fluid therapy, diuretics, and/or glucocorticoids) must sometimes be used to enhance urinary excretion of calcium and to decrease the serum calcium concentration.