Efficacy and safety of naltrexone hydrochloride for antagonizing carfentanil citrate immobilization in captive Rocky Mountain elk (Cervus elaphus nelsoni).

We evaluated efficacy and safety of naltrexone for antagonizing carfentanil immobilization in 12 captive Rocky Mountain elk (Cervus elaphus nelsoni) using a randomized incomplete block experiment. In three replicate trials, elk were hand-injected with 10 micrograms carfentanil citrate/kg body weight intramuscularly. Fifteen min after each elk became recumbent, we administered naltrexone HCl (25% of dose intravenously, 75% subcutaneously) dosed at 0 (control), 25, 50, or 100 mg/mg carfentanil; after an additional 15 min of immobilization, controls received 500 mg naltrexone HCl/mg carfentanil. Elk were immobilized in 34 of 36 attempts; the mean (+/-SE) induction time was 3.1 +/- 0.2 min. Regardless of dose, all elk stood < 9 min after receiving naltrexone; controls remained immobilized until they received antagonist. Mean recovery times did not differ with increasing naltrexone dose (P = 0.31) or among individuals (P = 0.16). None of the elk receiving 100 or 500 mg naltrexone/mg carfentanil renarcotized, but three of eight and seven of nine elk receiving 50 and 25 mg naltrexone/mg carfentanil, respectively, showed signs of mild renarcotization 8 to 24 hr later (P = 0.0002). We observed no adverse clinical effects in elk receiving < or = 500 mg naltrexone/mg carfentanil. Based on these data, we recommend 100 mg/mg carfentanil as a minimum effective dose for rapidly antagonizing immobilization and preventing renarcotization.