Safely immobilizing wildlife: Etorphine under scrutiny

For veterinary examinations, treatments, translocations, or management interventions, large wild animals must be immobilized safely and reliably. Because of its potent, rapid, and reversible action, etorphine is an important immobilizing drug in wildlife medicine. At the same time, the drug can cause significant cardiopulmonary side effects - including respiratory depression, pulmonary hypertension, and impaired oxygenation.

“Etorphine plays a crucial role in the immobilization of certain wildlife species. That makes it all the more important to understand exactly how it affects their physiology,” says Gabrielle Stalder, head of the Wildlife Medicine Working Group at FIWI, Vetmeduni. “In our controlled model, we can capture processes in considerable temporal and physiological detail that would be very difficult to measure in an immobilized wild animal under field conditions.”

Early changes

A key focus of the research project was how rapidly the physiological changes develop. Using electrical impedance tomography (EIT) - which continuously tracks changes in lung ventilation via an electrode belt around the chest - the researchers were able to monitor pulmonary ventilation continuously and noninvasively, while simultaneously recording, among other variables, arterial and pulmonary arterial pressures.

“Just three minutes after etorphine administration, mean pulmonary arterial pressure increased markedly - an indication of developing pulmonary hypertension,” explains Martina Mosing, head of the Clinical Unit of Anesthesiology and Intensive Care Medicine at Vetmeduni. “As time progressed, respiratory rate and the EIT-based tidal impedance variation (TIV) - a proxy for the tidal volume per breath - declined, accompanied by a deterioration in oxygenation.” The EIT data showed a pronounced reduction in overall ventilation without substantial changes in the regional distribution of ventilation.

Also notable was the recovery following naltrexone, a medication that reverses the effects of etorphine: ventilation recovered rapidly, whereas pulmonary arterial pressure and oxygenation did not return to baseline to the same extent during the observation period.

Pharmacological strategies in direct comparison

In a randomized, controlled crossover study, the team investigated the cardiovascular effects of etorphine and the potential of two agents - BIMU-8 and vatinoxan - to mitigate its side effects.

Etorphine led to a reduced respiratory rate, pulmonary hypertension, dysrhythmias, and tachycardia. BIMU-8 lowered both pulmonary and systemic blood pressure but was also associated with severe tachydysrhythmias. Vatinoxan increased cardiac output but did not reduce etorphine-induced pulmonary hypertension. It was also associated with a lower respiratory rate.

“The results highlight how complex it is to pharmacologically modulate these side effects. An improvement in a single parameter does not automatically translate into an overall improvement in the animal’s physiological state,” Stalder explains.

Focus on the oxygen cascade

Another component of the project examined the entire so-called oxygen cascade - from lung ventilation and gas exchange in the lungs, through oxygen transport in the blood, to oxygen utilization in tissues.

Etorphine impaired several steps of this cascade: the animals developed a reduced arterial partial pressure of oxygen but an increased partial pressure of carbon dioxide in the arterial blood (hypoxemia and hypercapnia), oxygen delivery to the tissues (DO₂) decreased, and the oxygen extraction ratio rose as a compensatory response. Neither BIMU-8 nor vatinoxan could effectively prevent these changes compared with the control treatment. In this study, the measured parameters of the oxygen cascade normalized following naltrexone.

Together, the three publications paint a consistent picture: the effects of etorphine extend beyond reduced respiration. Changes in ventilation, the pulmonary circulation, gas exchange, and oxygen transport interact and can significantly compromise the organism’s oxygen supply.

Implications for wildlife management

In free-ranging wildlife, the immediate post-immobilization phase is particularly difficult to study scientifically. As the drug takes effect, respiration and circulation change within minutes; at the same time, continuous and comprehensive monitoring in the field is often only possible to a very limited extent.

By contrast, the controlled sheep model allows multiple physiological levels to be recorded simultaneously and with high temporal resolution. This makes it possible to trace when individual changes occur and how they are interconnected. The findings can help to better interpret observations from wildlife practice and to develop more targeted monitoring and supportive measures.

These three publications thus provide an important experimental foundation for better understanding the physiological risks of chemical immobilization in large wild animals and, in the long term, for developing safer strategies for their veterinary care and management.

“Our goal is not to question the use of etorphine,” Stalder emphasizes. “We want to understand the mechanisms behind its side effects and thereby build a scientific basis for making immobilization safer for wild animals.”

International collaboration and a strong contribution from early-career researchers

The research project was conducted by FIWI together with the Clinical Unit of Anesthesiology and Intensive Care Medicine at Vetmeduni and implemented in collaboration with international partner institutions. Contributors include researchers from the University of Pretoria and the University of the Witwatersrand in South Africa, Massey University in New Zealand, the University of Helsinki, and BOKU in Vienna.

Early-career scientists Hathaipat Rattanathanya, first author of two of the three publications, and Anna Binetti, first author of the third publication, both made major contributions to conducting the studies and analyzing the extensive physiological datasets. “We are very proud of our early-career researchers, who contributed to the success of this project with great dedication and scientific independence,” emphasize senior authors Stalder and Mosing.

 

Scientific articles:

Frontiers | Early cardiovascular and respiratory changes after etorphine immobilization and naltrexone reversal in sheep

Effects of a selective 5-HT4 agonist and an alpha-2 adrenoceptor antagonist on etorphine-induced impairment across the oxygen cascade in immobilised sheep (Ovis aries): a randomised, prospective, and controlled trial | BMC Veterinary Research | Springer Nature Link

Cardiovascular effects of a selective 5-HT4 agonist and an alpha-2 adrenoceptor antagonist in etorphine immobilised sheep (Ovis aries) - a randomised, prospective, and controlled trial | BMC Veterinary Research | Springer Nature Link

 

Scientific contact:
Dr. Gabrielle Stalder
Forschungsinstitut für Wildtierkunde und Ökologie (FIWI)
Veterinärmedizinische Universität Wien (Vetmeduni)
Gabrielle.Stalder@vetmeduni.ac.at