Physiologic Outcomes after Thiafentanil and Xylazine Immobilization in Free-Ranging Moose (Alces alces)

Effects on physiology were evaluated in 49 free-ranging moose (Alces alces), 11 adult males, 36 females, two of unknown sex) in Minnesota, USA, immobilized by helicopter darting with thiafentanil (10 mg) and xylazine (30 mg) in February 2020 and March 2021. Pursuit time (PT), induction time (IT), recumbency time (RT), recovery time (RC), temperature (T), and body position were recorded. For 14 females, respiratory rate (RR), pulse rate (PR), and pulse oximetry (SpO2) were obtained before and at 1, 5, and 10 min after 4 L/min nasal oxygen supplementation was started; pH, lactate (Lac), arterial oxygen (PaO2), and carbon dioxide (PaCO2) tensions were obtained pre-treatment and 5 min post-treatment. Immobilizations were reversed with naltrexone (200 mg) and tolazoline (800 mg) intramuscularly. Median PT and IT were 4 min; a longer dart needle significantly reduced IT (P=0.0274). Most (98%) remained sternal; 77% held their head upright. Median RC was 3 min. Temperature remained ≤41.2 C. Median RR, PR, and SpO2 were RR=20 breaths/min, PR=70 beats/min, SpO2=91% pre-treatment and RR=30 breaths/min, PR=72 beats/min, SpO2=97% 5-min post-treatment. Median blood gas values were pH=7.45, Lac=5.62 mmol/L, PaCO2=44 mmHg, and PaO2=65 mmHg pre-treatment and pH=7.48, Lac=4.99 mmol/L, PaCO2=41.1 mmHg, and PaO2=78 mmHg 5-min post-treatment. Physiologic improvement from oxygen supplementation was clinically relevant but not statistically significant. All moose survived immobilization. Thiafentanil and xylazine combination provided safe and effective immobilization in free-ranging moose.