Tremors in white rhinoceros (Ceratotherium simum) during chemical immobilisation

Abstract

White rhinoceros (Ceratotherium simum) are susceptible to developing muscle tremors during chemical immobilisation induced by potent opioid receptor agonists. Whether these tremors result directly from the actions of the opioids or from other physiological changes associated with immobilisation is unknown. A pilot study on 8 boma-managed chemically immobilised rhinoceros was conducted using different supportive interventions for the animal?s cardiorespiratory systems to test whether these interventions had an effect on tremors during chemical immobilisation. The pilot study revealed that butorphanol, a partial opioid agonist/antagonist, combined with nasotracheal oxygen insufflation, compared to the control, was the only intervention that decreased the observed tremor intensity and adequately stabilized the rhinoceros cardiorespiratory system in the immobilised rhinoceros. With this knowledge and using the same drug protocol (etorphine and azaperone and hyaluronidase) and supportive interventions (butorphanol and nasotracheal oxygen insufflation), a field study was conducted to quantify tremors, both objectively and subjectively, and record various physiological responses of 14 rhinoceros during a 25 minute chemical immobilisation period. Butorphanol was injected intravenously 6 minutes after the rhinoceros became laterally recumbent. Tracheal oxygen insufflation was also administered from this time. Occurrence (intensity) of tremors was assessed every minute throughout the 25 minute immobilisation period, both subjectively by human observation, and objectively by accelerometer data loggers placed on the front leg. Arterial blood pH, oxygen and carbon dioxide levels, electrolytes and plasma catecholamine concentrations were measured at 5 minute time points. The tremor intensity was highest (5 minutes 28 counts/min) just after the animals became recumbent, but decreased (3 counts/min) after butorphanol and nasotracheal oxygen insufflation was administered. Tremor intensity was correlated with the mean pH, arterial partial pressure of oxygen, serum potassium and median plasma adrenaline concentration. High tremor intensity occurred when plasma adrenaline concentrations were elevated and when hypoxaemia and acidaemia were at their worst. Hypoxaemia and acidaemia, both physiological stressors, were correlated with the increased plasma adrenaline concentrations. These correlations indicate that changes in blood oxygenation and pH could be the driving force behind the changes in the tremor intensity. Butorphanol and nasotracheal oxygen insufflation corrected the hypoxia and acidaemia and reduced tremor intensity. Therefore, tremor intensity could possibly indicate the severity of the pathophysiological effects of the capture drugs on a rhinoceros cardiorespiratory system.