The effects of natural exposure to high levels of zinc and cadmium in the immature pony as a function of age.

To study the effects of environmental exposure to zinc and cadmium in immature foals, five pregnant ponies were raised within 2.9 km of the New Jersey Zinc Smelter in Palmerton, Pennsylvania. The mares and their foals were kept outdoors on timothy hay and orchard grass. The foals were examined daily for signs of illness and blood samples were taken monthly for estimation of serum zinc, copper, and ceruloplasmin levels. The foals were sacrificed at 2.5, 4.5, 8.5, 13.5, and 18.5 months of age. Necropsy revealed generalized osteochondrosis in joints of the limbs and cervical vertebrae, lymphoid hyperplasia, and eosinophilia. Two of the foals had developed mild lameness. The concentrations of zinc, cadmium, copper, lead, magnesium, and calcium were determined in liver, kidney cortex, and pancreas. The concentration of cadmium and zinc were the only elements that were greatly elevated in all three tissues as compared to control animals. The concentration of cadmium was directly correlated with age in the three tissues (e.g., 23.9 to 212.7 micrograms/g wet wt in kidney cortex), whereas zinc was significantly increased (range 132 to 954 micrograms/g wet wt in liver) but there was no correlation with age. It was concluded that the development of osteochondrosis is associated with increased exposure to zinc and possibly cadmium. The classical signs of cadmium toxicosis, such as renal damage and osteomalacia, were not observed.