Comparison of the acute stress response in southern white rhinoceros (Ceratotherium simum simum) immobilised with four different drug protocols

Abstract

Efforts made to ensure conservation of the southern white rhinoceros (Ceratotherium simum simum) include dehorning and translocation, which necessitate chemical immobilisation. Rhinoceros experience high levels of stress during capture, and numerous immobilisation drug combinations have been proposed, with each claimed to be less stressful and safer than others. The purpose of this study was to evaluate the differences in the effects of four commonly-used drug protocols on the acute stress response in white rhinoceros. Eight sub-adult males were captured in the southern Kruger National Park area and housed in individual bomas. The rhinoceros were chemically immobilised with four etorphine-based drug protocols every two weeks for a 10-week period, using a cross-over design. The protocols used were etorphine + sterile water (control), etorphine + azaperone, etorphine + medetomidine, and etorphine + midazolam. All animals received intravenous butorphanol fifteen minutes after going down into lateral recumbency. Venous blood samples were collected at three time points during immobilisation (0, 20 and 40 min). The following were measured: noradrenaline, adrenaline and their metabolites, dopamine, serotonin, cortisol, glucose, neutrophil: lymphocyte ratio, haematocrit and leukocyte coping capacity. Differences in biomarkers between drug protocols were analysed using a linearmixed model. All rhinoceros mounted an acute stress response, as shown by high catecholamines, blood pressure and heart rate. Noradrenaline was lower (p = 0.008) with the midazolam protocol compared to the etorphine-only control and medetomidine protocols and was also lower with the azaperone protocol compared to the medetomidine protocol (p = 0.008), but not compared to the etorphine-only control protocol. There were no other differences in biomarkers between the azaperone and midazolam protocols, and the control protocol. With the medetomidine protocol, glucose was higher compared to etorphine alone (p = 0.003), cortisol was lower compared to the azaperone protocol (p=0.027), neutrophil: lymphocyte ratio was higher than with azaperone and midazolam protocols (p < 0.001) and haematocrit was lower compared to all protocols (p < 0.001). We found that the addition of azaperone and midazolam yielded a minimal dampening in the stress response when compared to the etorphine control, although noradrenaline was lower with the addition of midazolam. The medetomidine protocol showed the most differences in the stress biomarkers, with the highest N: L ratio and glucose concentration, and significantly lowered haematocrit, which may negatively affect tissue oxygenation. These differences seen with medetomidine may be related to the direct effect of the drug on some of the biomarkers. The study found that the sedative and tranquilizing drugs currently in use have a limited ability to alter the stress response in rhinoceros chemically immobilised with etorphine, highlighting the need for research into the addition of other anxiolytic drugs to reduce etorphine-induced effects during rhino immobilisation procedures.