Mitigating the Adverse Effects of Etorphine-based Capture in White Rhinoceros

Mitigating the Adverse Effects of Etorphine-based Capture in White Rhinoceros

Robin D. Gleed, BVSc, MA, MRCVS, DVA, DACVAA, DECVAA, MRCA, Professor of Clinical Sciences, Department of Clinical Sciences

Leith C. R. Meyer, BSc Hon, BVSc, PhD, Associate Professor Veterinary Pharmacology, Department of Paraclinical Sciences, University of Pretoria, South Africa

White rhinoceros (Ceratotherium simum) are classified as “near-threatened” due to poaching and must be managed actively to ensure their conservation. Management of wild rhinoceros requires capture by chemical immobilization; unfortunately, this induces profound physiological derangements, including severe hypoxemia. The hypoxemia is likely due to the adverse cardiopulmonary effects of the potent opioid used for immobilization, etorphine, combined with extreme exertion during capture in the field. Hypoxemia may result in morbidity and mortality. In other species given etorphine, hypoxemia has been attributed to pulmonary hypertension.

Observing the effects of etorphine on pulmonary blood pressure and flow in white rhinoceros habituated to captivity in bomas without the confounding effects of exertion or other drugs would help determine some of the mechanisms responsible for hypoxemia in these animals. Drugs that might be co-administered with etorphine could then be tested for their ability to mitigate the hypoxemia first in captivity and then in the field. The findings would underpin evidence-based recommendations that could result in immediate improvement in the safety of capture for white rhinoceros and contribute to their conservation.

In year one, the first aim is to measure partial pressure of arterial oxygen (PaO2), pulmonary vascular pressures (pulmonary artery pressure [PAP], pulmonary artery occlusion pressure [PAOP]), pulmonary flow (cardiac output [Q]), and other cardiopulmonary variables in boma-habituated white rhinoceros immobilized with etorphine only. The second aim is to compare these values to those measured in white rhinoceros immobilized with etorphine plus azaperone or etorphine plus midazolam to determine whether co-administration of these synergistically-acting immobilizing drugs mitigates etorphine-induced hypoxemia. Eight subadult, male, white rhinoceros will be captured, habituated to captivity, and then assigned to receive each of the three protocols in random order, separated by a 2 week washout period. This will take 6-8 weeks in total.

In year two, the objective is to measure the same variables in free-ranging white rhinoceros immobilized via aerial darting with etorphine plus azaperone or etorphine plus midazolam to determine the effects of helicopter pursuit on these variables and whether either azaperone or midazolam mitigates hypoxemia. Thirty rhinoceros will be assigned via block randomization to receive either etorphine plus azaperone (n = 15) or etorphine plus midazolam (n = 15). A month or less will be required to immobilize all 30 rhinoceros.

For all observations, immediately after immobilization, rhinoceros will be pushed into lateral recumbency and instrumented. Arterial blood pressure and respiratory gas tensions will be measured using an auricular arterial catheter. Pulmonary vascular pressures and thermodilution cardiac output will be measured using a custom, 180 cm catheter equipped with a pressure port, a balloon, and a thermistor. The catheter will be inserted into a branch of the jugular vein under ultrasonographic guidance, a technique already developed by our group. The tip of the catheter will then pass through the right heart into the pulmonary artery. Measurements will be made 20, 30, 40, 50, and 60 minutes after positioning in lateral recumbency. We anticipate that etorphine-induced pulmonary hypertension and associated hypoxemia will occur during boma and field immobilizations and that at least one of the two drugs co-administered with etorphine will mitigate or eliminate these effects.