Pathologic features of iodide toxicosis in calves

Forty Holstein-Friesian heifer calves were divided into four groups of ten each and dosed orally with ethylenediamine dihydriodide to provide 0, 50, 250, or 1,250 mg of iodide/animal each day. Five calves in each group were given thyrotropin-releasing hormone at 4-week intervals. Two calves given 1,250 mg of iodide (active ingredient)/day died by 70 days and had severe bronchopneumonia. At 6 months, the remaining 38 calves were killed. Pneumonia was evident in calves given iodide, with the severity apparently dose-related. Squamous metaplasia of tracheal epithelium occurred in all calves given 1,250 mg of iodide. Similar changes were seen in the interlobular duct of the parotid gland of two calves given 1,250 mg of iodide and in one calf given 250 mg. Scanning electron microscopy revealed changes in the tracheal mucosa, the severity of which were related to the dose of iodide fed. Changes in the thyroid glands of calves given 1,250 mg of iodide included large follicles, flat epithelium, and large amounts of colloid. In calves given 1,250 mg of iodide plus thyrotropin-releasing hormone, the histologic features of the thyroid glands were similar to controls. Serum vitamin A concentrations were less in calves given 1,250 mg of iodide relative to that of control calves. In general, calves were able to compensate fpr excessive dietary iodide after an initial period of an apparent increased susceptibility to respiratory tract infection.