Periparturient hypocalcemia in cows: prevention using intramuscular parathyroid hormone.

Intravenously administered parathyroid hormone has recently been reported to prevent parturient paretic hypocalcemia in dairy cows. In the present study, Parathyroid hormone was administered intramuscularly prior to parturition to study its effects on Ca homeostasis and the incidence of periparturient hypocalcemia. Ten Holstein cows were fed a high Ca diet prepartum to predispose them to parturient paretic hypocalcemia. Five cows received intramuscular injections of synthetic bovine parathyroid hormone three times per day beginning about 6 d before parturition and continuing at a reduced dose for about 6 d after parturition. One of five control cows developed parturient paresis, and all five control cows exhibited hypocalcemia within 24 h of calving. None of the treated cows became paretic, and plasma calcium was normal or slightly elevated in these cows during the first 24 h after calving. Parathyroid hormone administration increased plasma concentrations of 1,25-dihydroxyvitamin D and hydroxyproline prior to parturition, suggesting that both intestinal Ca absorption and bone calcium resorption were increased by administration of the hormone. One hormone-treated cow became recumbent and required euthanasia. Although metastatic calcification of soft tissues was not observed, the death of this cow raises the possibility that there are unknown side effects of parathyroid hormone when administered intramuscularly.