Serum concentrations of etorphine in juvenile African elephants

Eleven juvenile African elephants were given etorphine hydrochloride (2.19 +/- 0.11 micrograms/kg of body weight; mean +/- SD) as a single IM injection; 3 elephants were given additional etorphine (0.42 +/- 0.09 micrograms/kg) IV. After immobilization, each elephant was maintained in lateral recumbency by administration of a 0.5% halothane/oxygen mixture or by administration of multiple IV injections of etorphine. At postinjection hours 0.25 and 0.5 and at 30-minute intervals thereafter, blood samples were collected via an auricular artery, and serum concentrations of etorphine were determined by use of radioimmunoassay. The highest mean serum concentration of etorphine in 6 elephants given a single IM injection and subsequently maintained on halothane and oxygen was 1.62 +/- 0.97 ng/ml at postinjection hours 0.5; thereafter, the mean serum concentration decreased steadily. In 4 elephants maintained in lateral recumbency with multiple IV administrations of etorphine, a correlation was not found between the time to develop initial signs of arousal and serum concentrations of etorphine before arousal. After administration of the initial immobilizing dose of etorphine, the interval between successive IV administrations of etorphine decreased.