Tremors in white rhinoceroses (Ceratotherium simum) during etorphine–azaperone immobilisation

Little is known about the mechanisms causing tremors during immobilisation of rhinoceros and whether cardiorespiratory supportive interventions alter their intensity. Therefore, we set out to determine the possible mechanisms that lead to muscle tremors and ascertain whether cardiorespiratory supportive interventions affect tremor intensity. We studied tremors and physiological responses during etorphine–azaperone immobilisation in eight boma-held and 14 free-living white rhinoceroses. Repeated measures analysis of variance and a Friedman test were used to determine differences in variables over time and between interventions. Spearman and Pearson correlations were used to test for associations between variables. Tremor intensity measured objectively by activity loggers correlated well (p < 0.0001; r(2) = 0.9) with visual observations. Tremor intensity was greatest when animals were severely hypoxaemic and acidaemic. Tremor intensity correlated strongly and negatively with partial pressure of oxygen (PaO(2)) (p = 0.0003; r(2) = 0.9995) and potential of hydrogen (pH) (p = 0.02, r(2) = 0.97). It correlated strongly and positively with adrenaline concentrations (p = 0.003; r(2) = 0.96), and adrenaline correlated strongly and negatively with PaO(2) (p = 0.03; r(2) = 0.95) and pH (p = 0.03; r(2) = 0.94). Therefore, hypoxaemia and acidaemia were likely associated with the intensity of tremors through their activation of the release of tremorgenic levels of adrenaline. Tremors can be reduced if circulating adrenaline is reduced, and this can be achieved by the administration of butorphanol plus oxygen insufflation. Furthermore, to assist with reducing the risks associated with rhinoceros immobilisation, tremor intensity could be used as a clinical indicator of respiratory and metabolic compromise.