Acute vitamin D2 (ergocalciferol) toxicosis in horses: case report and experimental studies.

Acute accidental vitamin D2 (ergocalciferol) toxicosis was diagnosed in a 6-month-old foal with extensive lesions of soft tissue mineralization. In an experimental study, three 18-month-old horses were given ergocalciferol per os at a rate of 9,300, 22,200, or 47,200 IU/kg of body weight/day for 21 days. Clinical signs or lesions were not seen in horses given the low and intermediate doses, whereas the horse receiving the highest dose developed clinical signs and lesions similar to those noted in the foal. Signs included depression, loss of appetite, weakness, limb stiffness with impaired mobility, and cessation of growth or weight loss. Gross and histologic lesions of mineralization of various soft tissues, especially of the endocardium and wall of large blood vessels, were seen in the foal and the horse given the high dose. Marked, persistent, hyperphosphatemia (7.0 to 13.0 mg of P/dl of serum) developed in each horse. The horse given the intermediate dose remained normocalcemic. Horses given the low and high doses became hypercalcemic (13.6 to 14.5 mg of Ca/dl of serum), but serum calcium concentrations varied from day to day and both horses were normocalcemic at necropsy (12.4 to 12.7 mg of Ca/dl of serum). Distal metacarpal bone ash concentrations of calcium, phosphorus, and magnesium of the foal were mg/g of bone ash) 400.5, 180.5, and 5.30, respectively. In the horses, treatment with ergocalciferol also had no significant effect on serum magnesium (1.88 to 2.18 mg/dl of serum) or distal metacarpal bone ash concentrations of calcium (352.5 to 362.5 mg/g of bone ash), phosphorus (182.5 to 184.0 mg/g of bone ash), or magnesium (5.48 to 6.02 mg/g of bone ash).