Acute vitamin D3 toxicosis in horses: case reports and experimental studies of the comparative toxicity of vitamins D2 and D3.

Abstract

Acute vitamin D toxicosis was diagnosed in 2 horses fed a grain ration containing 1,102,311 IU of cholecalciferol (vitamin D3)/kg (500,000 IU/lb) for about 30 days. Horse 1 died acutely with extensive mineralization of cardiovascular and other soft tissues. Horse 2, which had severe clinical signs and clinicopathologic changes of toxicosis, was treated with nonsteroidal antiinflammatory drugs and recovered in about 6 months. In an experimental study, the toxicity of ergocalciferol (vitamin D2) and cholecalciferol was compared in 2 horses (No. 3 and 4) given the respective vitamins at a daily dosage of 33,000 IU/kg of initial (day 0) body weight for 30 days. Except for slight loss in body weight (8%) during the 1st few days of treatment, horse 3 remained clinically normal. Horse 4 developed limb stiffness and tachycardia, became anorectic, weak, and recumbent, lost 29% of body weight, and had polydipsia and polyuria. Horses 2, 3, and 4 developed persistent hyperphosphatemia. Horse 2 remained normocalcemic whereas horses 3 and 4 became hypercalcemic by day 28. In horse 3, serum vitamin D2 metabolite concentrations on days 0, 1, 14, and 26 were: vitamin D2 (ng/ml) = less than 5.0, 5.7, 71.4, and 188.0; 25-hydroxy-vitamin D2 (ng/ml) = less than 5.0, less than 5.0, 43.1, and 117.5; and 1,25-dihydroxy-vitamin D (pg/ml) = 19.7, 23.2, 25.0, and 45.7, respectively. In horse 4, serum vitamin D3 metabolite concentrations on the same days were: vitamin D3 (ng/ml) = less than 5.0, 110.0, 1,049.0, and 887.0; 25-hydroxy-vitamin D3 (ng/ml) = less than 5.0, 18.9, 201.0 and 182.0; and 1,25-dihydroxy-vitamin D (pg/ml) = 21.5, 18.9, 25.2, and 21.6, respectively. Urine of horses 2 and 4 became acidic (pH 6). Horses 2, 3, and 4 became hyposthenuric, but the decrease in urine specific gravity (sp gr) in horse 3 occurred only after 3 weeks of treatment and was only moderate (sp gr, 1.018 to 1.021) and nonprogressive. Hyposthenuria was evident in horse 4 on day 4 (sp gr, 1.028), and was progressive and marked (sp gr, days 28 to 32: 1.006 to 1.009). Urine sp gr of horse 2 ranged from 1.002 to 1.007. Fractures were demonstrated radiographically and histologically in the costochondral junctions of horses 3 and 4. Mineralization of cardiovascular and other soft tissues developed in horses 3 and 4, with lesions being more severe and having a wider tissue distribution in horse 4.