Abstract
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; r2 = 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 (PaO2) (p = 0.0003; r2 = 0.9995) and potential of hydrogen (pH) (p = 0.02, r2 = 0.97). It correlated strongly and positively with adrenaline concentrations (p = 0.003; r2 = 0.96), and adrenaline correlated strongly and negatively with PaO2 (p = 0.03; r2 = 0.95) and pH (p = 0.03; r2 = 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.
Highlights
Management and conservation of the white rhinoceros necessitates the use of potent opioids such as etorphine, thiafentanil, fentanyl and carfentanil for immobilisation (Kock et al 1995; Radcliffe & Morkel 2014)
These findings suggest that hypoxaemia and acidaemia may have been associated with tremor intensity by stimulating the release of catecholamines
In the bomamanaged white rhinoceroses, we found a decrease in the tremor intensity after butorphanol was administered, but this decrease was no different compared to when the animals received sterile water in the control trial
Summary
Management and conservation of the white rhinoceros necessitates the use of potent opioids such as etorphine, thiafentanil, fentanyl and carfentanil for immobilisation (Kock et al 1995; Radcliffe & Morkel 2014). During immobilisation opioids produce well-known side-effects, including muscle tremors, tachycardia, hypertension and respiratory depression (with subsequent hypoxaemia, hypercapnia and acidaemia), which may result in lifethreatening complications (Atkinson et al 2002; Buss et al 2015, 2016; Fahlman 2008; Moreira 2010; Portas 2004; Radcliff, Ferrell & Childs 2000). Of these side-effects, the least studied is muscle tremors. The potent opioids used for chemical immobilisation have been implicated as a direct cause of muscle tremors in several different species (Burroughs et al 2012b; Moreira 2010) and, in white rhinoceroses, tremors have been found to increase with additional doses of etorphine (Heard, Olsen & Stover 1992; Kock et al 1995)
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