WildLife Group of the SAVA
and its high lipophilicity enables rapid penetration of the blood-brain barrier . 12 Thiafentanil is a synthetic analogue of fentanyl and it has an exclusive affinity for the µ -opioid receptors found throughout the central nervous system . 14 , 15 It is rapidly absorbed , which is thought to result in quicker inductions and has a shorter duration of action than etorphine . Thiafentanil is rapidly metabolised , resulting in a lower risk of
3 , 16 – 18 renarcotization due to prolonged agonistic activity . Both etorphine and thiafentanil are fully reversed by naltrexone , an opioid antagonist that displaces etorphine and thiafentanil from the opioid receptors . 2
Owing to the wide variety of wild ungulate species , it is often difficult to make well-informed recommendations on the use of opioids in different species . Domestic animal models are commonly used for opioid immobilisation studies although it may not always be accurate to extrapolate data from these studies to all wild ungulate species . 19 – 21 Impala appear to be a popular representative species used for opioid studies but their susceptibility to stress , sensitivity to opioid-induced respiratory depression and variable response to opioids often makes this a difficult species to work with .
To thoroughly compare the physiological responses of ungulates to etorphine and thiafentanil , Wildlife Pharmaceuticals conducted a cross-over study in both impala and blesbok . The study was a collaboration with Murdoch University and formed part of the PhD of Dr Silke Pfitzer , who was investigating opioid-induced respiratory depression in African ungulates and ways to mitigate it .
Eight impala and eight blesbok were used in the study and each animal was immobilised twice : once with each of the following treatments , given at random :
1 . 0.09 mg / kg etorphine ( Captivon 98 , Wildlife Pharmaceuticals )
2 . 0.09 mg / kg thiafentanil ( Thianil , Wildlife Pharmaceuticals )
Etorphine and thiafentanil are commonly used in combination with other sedatives or tranquilisers to improve induction and the quality of immobilisation ; and lower doses of 0.04 – 0.05 mg / kg are often used due to the opioid-sparing effects of these sedative and tranquilisers . Since the opioids were used on their own , the higher dose of 0.09 mg / kg was selected for this study to ensure the safe handling of the animals and to prolong the monitoring period to 40 minutes after recumbency . This worked out to a total dose of 5 - 5.5 mg opioid per blesbok ( mean weight of 58 kg ) and 2.8 – 3.8 mg opioid per impala ( mean weight of 37 kg ). All of the animals were darted with 1 ml P-type Pneu-Darts . Only female animals were used to limit variability within the results and reduce fighting during group-housing . The treatments were allocated at random with a one week wash-out period between treatments . Inductions and recoveries were timed , the animals were monitored for vital signs and arterial blood gasses were measured .
In the blesbok , induction times were similar for both etorphine ( 2.5 minutes ) and thiafentanil ( 2.2 minutes ). In contrast , in the impala , induction times were significantly faster when thiafentanil was used ( 2 minutes ) compared to etorphine ( 3.9 minutes ). This is clinically relevant when animals are darted in the field since faster induction times mean reduced exposure to capture stressors and a decreased risk of losing animals as they run away after darting .
As expected , the impala appeared to be much more susceptible to opioid-induced respiratory depression regardless of the opioid used . They developed severe respiratory compromise and hypoxaemia , particularly at the start of monitoring . Temporary cessation of respiration at the start of monitoring was observed in some of the impala but not in any of the blesbok . With both etorphine and thiafentanil , the impala had low respiratory rates and were hypercapnic and severely hypoxaemic . In contrast , the blesbok had good respiratory rates with both etorphine and thiafentanil and PaCO 2 values indicated that they were not hypercapnic and were adequately ventilating during immobilisation . Interestingly , the PaO 2 values of the blesbok revealed that while they were only moderately hypoxaemic when darted with thiafentanil ( mean PaO2 of 68 mmHg ), they were severely hypoxaemic when darted with etorphine ( mean PaO 2 of 54 mmHg ). This highlights not only the differences in the
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