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Anaesthesia in the Bornean orangutan

Anaesthesia can be required in orangutans living in conservation centres for a variety of reasons and there are some important considerations to take into account when approaching such cases

The Bornean orangutan, Pongo pygmaeus, is one of three subspecies of orangutan found in Asia. Currently classified as critically endangered, the wild populations have declined by over 50 percent in the last 60 years, leaving an estimated 100,000 remaining in the wild (Ancrenez et al., 2016). The causes of this decline are sadly the usual suspects: habitat loss, illegal traditional medicines and the pet trade, to name a few.

Due to individuals being injured, seized or similarly negatively affected by human actions, hundreds of animals are being housed at multiple conservation programmes within Borneo in order to assist with the rehabilitation and conservation of orangutans.

Orangutans and humans share 97 percent of common DNA (Wong, 2014), so the risk of disease transmission from one to the other is incredibly high. Therefore, it is integral for appropriate PPE to be worn at all times when working in close proximity to these animals. In turn, the orangutan’s physicality and their potential to deliver serious injury should never be underestimated, and suitable safety protocols should be adhered to at all times to minimise risk of injury to both parties.

FIGURE (1) Anaesthesia can be required in orangutans living in conservation centres for a variety of reasons, such as health screening. Pictured here is the author performing auscultation as part of a health check for an infant orangutan

Anaesthesia can be required in orangutans living in conservation centres for a variety of reasons, such as health screening (Figure 1), pre-release, procedure works, etc. Any orangutan undergoing anaesthesia should be fasted for a period of 12 to 24 hours before induction to minimise risk of aspiration under general anaesthesia (Cerveny and Sleeman, 2014). Whilst this is normally suitable for zoo settings, in conservation centres individuals are likely housed in multiple groups where limiting food intake is not possible, or are even on rehabilitation islands where they can freely forage for food, and as such fasting is likely impossible to ascertain.

Route of administration

There are two main options for anaesthetic drug administration: oral or intramuscular. Oral administration brings with it the risk of aspiration, and as such, the intramuscular route is the most commonly preferred. However, studies have shown that the provision of metoclopramide to prevent emesis can negate the risk of aspiration (Miller et al., 2000). If the patient is operant conditioned, IM drug administration can be provided by hand injection; however, it is more likely for the drug administration to be performed by darting from a distance to minimise stress to the individual and risk to human health. Knowing the exact weight of an individual is often not possible, so weight must often be estimated. A study performed by Adams et al. (2003) reported that weights of chimpanzees could be over-estimated by as much as 28 percent, which can lead to incorrect drug dosage provision.

Choosing the right anaesthetic drugs

Anaesthetic drugs in-country are often limited by accessibility, so the ability to tailor drug choice to individuals and circumstance can be restricted.

Ketamine is commonly used at 2 to 4mg/kg (BOSF, 2020) due to its sedative and amnesia abilities, which minimises the impact on relationships between animals and carers post-procedure. In combination with this, xylazine is commonly used at 0.5 to 2mg/kg to provide a greater depth of anaesthesia and muscle relaxation which, given the orangutan’s strong physicality, assists with maintaining safety for both patient and veterinary team. This combination provides greater cardiopulmonary stability than ketamine would on its own, and whilst recoveries are extended, they are more uneventful (Cerveny and Sleeman, 2014).

Another alpha-2 agonist used in conjunction with ketamine is medetomidine at 0.02 to 0.05mg/kg (BOSF, 2020), of which the main benefit is its reversibility with the use of atipamezole at 0.15 to 0.3mg/kg (BOSF, 2020). The ability to use atipamezole or tolazine allows any negative effects of bradycardia to be dealt with accordingly. When required, top-ups of tiletamin and zolazepam at 2 to 4mg/kg, or ketamine and diazepam at 2mg/kg and 0.1mg/kg respectively (BOSF, 2020), can be administered intravenously to maintain anaesthesia in the field when gaseous agents are not available.

The use of ketamine and xylazine in combination has been found to be the most popular by the site where the author worked due to its ability to provide the required anaesthesia whilst being easily reversible. A suitable recovery time of around 40 minutes would be expected post-reversal provision, whilst initial induction time from darting was found to be around five minutes. That being said, when a second dose was required induction time was found to have increased and other studies have shown it to increase to over 15 minutes on occasion (Andeau et al., 1994). Most importantly the speed of onset was not so great that an individual would not have time to find stability within its tree or even descend to the ground prior to onset of anaesthesia, which could lead to serious injury or even death if such an event was not to occur (Cerveny and Sleeman, 2014).

Intubation and maintenance of anaesthesia

FIGURE (2) In the surgery setting, endotracheal intubation is performed as standard to allow maintenance of anaesthesia. Note the intubated orangutan’s canines, able to inflict serious injury

Whilst intubation is generally not performed within the field for sedation, in the surgery setting endotracheal intubation is performed as standard to allow maintenance of anaesthesia via isoflurane and provision of oxygen (Figure 2). The utilisation of propofol was used if the plane of anaesthesia achieved was not suitable for intubation. Intubation was generally performed in dorsal recumbency with the head extended off the table to extend the neck. In turn, pulling the tongue forward and the use of a laryngoscope allowed increased visualisation.

Orangutans commonly hypersalivate which is further heightened by the provision of ketamine; the use of lidocaine on the glottis assists in minimising this (Cerveny and Sleeman, 2014).

All great apes possess relatively shortened tracheas which can result in accidental intubation of one primary bronchus, in turn leading to reduced oxygenation capacity (Fowler, 1995). As such, auscultation and IPPV should be performed at intubation to ensure evenness of chest inflation.

Maintenance of anaesthesia when intubated can be achieved with the use of isoflurane; however, it is important to note its potent vasodilatory properties which can result in severe hypotension. Indeed, studies on other great apes have shown that maintenance on a non-rebreathing circuit can be achieved with 0.5 to 1.0 percent isoflurane to 100 percent oxygen (Horne et al., 1997).

Monitoring anaesthesia

FIGURE (3) Monitoring under anaesthesia is essential both in the field and in a hospital setting; pictured here is the author monitoring the anaesthesia of a male adult orangutan

Monitoring under anaesthesia is essential both in the field and in a hospital setting, each presenting its own individual challenges. The field has restricted access to equipment, so you’re unlikely to be prepared for every scenario; however, to compensate for this it is unlikely that a full general anaesthetic would ever be performed. On the other hand, you have access to all equipment on-site in the surgery setting (Figure 3), but general anaesthesia brings with it an increased risk of health compromise, and the likelihood of more invasive procedures being performed in this scenario is greater.

A heart rate of 64 to 114 can be expected (Fahlman et al., 2006), whilst a respiratory rate of 24 to 120 is considered as within an expected range (Horne, 2001). Body temperature will always be affected under anaesthesia; within the tropics the risk of hypothermia is greatly reduced but the risk of hyperthermia is much higher so procedures should not be performed in direct sunlight or hot rooms as far as possible, in order to maintain a rectal body temperature between 34.7 and 38.6°C (Naples et al., 2010).

Throughout each procedure the author always had use of a pulse oximeter to monitor the patient’s oxygen saturation as, due to the weight of the animal, a severe restriction on lung ability can occur if the patient is positioned incorrectly, or otherwise has respiratory compromise (eg via tuberculosis or air sacculitis). The probe should be positioned upon either the tongue or prepuce with the author finding digits not an ideal choice given their thickened skin texture.

Anaesthesia in great apes is especially risky in individuals over the age of 30 years, with a 30-fold increase of anaesthetic death (Masters et al., 2007), whilst many of the individuals the author worked with were at a 26-fold risk increase due to their varying sickness or illnesses. The risk of aspiration pneumonia when working with such orangutans is high due to the prevalent condition of laryngeal air sacculitis and so a cuffed ET tube should always be utilised (Fowler, 1995).


Recovery from anaesthesia after the administration of a reversal agent is often fast, and as such the reversal agent should always be administered to the patient once in a suitably safe environment for their recovery.

Concluding remarks

All three orangutan species are facing serious risk from threat of extinction, and reliance on captive settings for preservation of the species is not suitable both due to the logistics, with widespread hybridisation occurring in captive settings, but also morally and ethically as they are a wild animal. Nothing will ever replace the amazement of seeing a wild orangutan and they belong where they should be, in the wild.

There are many ways we can help fight against the extinction of these creatures, the first step being to learn more about what we can do to help, with many legitimate sites offering suitable ways to assist the fight against extinction.

In 2019 the author was lucky enough to be invited to assist the veterinary team at Samboja Lestari, one of the Borneo Orangutan Survival Foundation (BOSF) in-country project sites. The author would like to provide special thanks to all of the staff members at the site who made the time both enjoyable and informative. Particular gratitude must be expressed to the veterinary team (Figure 4) for their support, knowledge and willingness to involve the author in their conservation work.

FIGURE (4) The author was able to work alongside the veterinary team at Samboja Lestari, one of the Borneo Orangutan Survival Foundation in-country project sites in Borneo (Pictured from left to right: Dr Agnes, Dr Machmudi, the author, Dr Tadin, Dr Felita, Dr Made)

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