Anesthesia, Chemical Restraint and Pain Management in Venomous Snakes
Dissociative, anesthetic and analgesic drugs are valuable tools for the humane management of illness and injury in venomous snakes. Their value lies primarily in making veterinary procedures safer, more comfortable and less stressful for the animal. A secondary benefit is that the handler's risks are lowered.
Minimally invasive procedures can be performed with just a local anesthetic such as 2% Lidocaine. In our experience the use of a local anesthetic can also extend the depth and duration of general anesthesia since it blocks painful stimulus that could otherwise bring the patient out of anesthesia.
Painful procedures should be performed with the aid of analgesic (pain relieving) drugs as well as anesthesia. Pain relief for both the short term procedure and the longer term recovery period is very appropriate for invasive surgical procedures. Prolonged pain and stress can be immunosuppressive and all invasive or painful procedures should be done with good analgesics already on board to prevent post-operative discomfort. Buprenorphine 0.02 mg/kg IM is a good drug for pain management in reptiles. Surgery patients should be premedicated as it can take many hours for this drug to reach peak effectiveness. A combination of Buprenorphine and Butorphanol (a shorter acting analgesic) may also be appropriate.
Surgical procedures and some types of lengthy examinations will also generally require general anesthesia. Our "gold standard" for inducing general anesthesia in all venomous snakes is isoflurane. Snakes can be induced by placing them in a sealed box of known volume with a measured quantity of the drug injected into a cotton ball inside. A 5% concentration is sufficient for induction. Click here to learn more about this procedure. Other induction methods include piping isoflurane through a surgical mask placed on the end of a snake tube or directly on a snake's head, or intubation of a premedicated (sedated) snake in a tube. A photograph of the latter procedure is shown here. Once a snake is induced and has lost its righting reflex, it may be intubated with a non cuffed endotracheal tube and maintained on isoflurane and oxygen.
We have heard multiple anecdotal reports from veterinarians who have used sevoflurane in reptiles, and they report that it is hard to distinguish from isoflurane. There are no obvious benefits to justify sevoflurane's higher cost, and at least one well known clinician states that sevoflurane produces less stable anesthesia in reptile patients than isoflurane.
Chemical restraint can be induced in reptiles by means of dissociatives (Ketamine, Tiletamine), tranquilizers or muscle relaxants (Midolazam, Diazepam, Zolazepam) or a combination of the two (Telazol). Other sedative agents such as Propofol (an intravenous hypnotic agent) may also be used, either as a pre-anesthetic drug prior to inducing general anesthesia or by themselves for some types of surgical procedures. Painful/invasive procedures should never be performed with a dissociative alone, as these drugs induce chemical immobility without lessening sensitivity to pain.
Patients at high risk of self-injury from struggling or severe stress on conscious restraint are good candidates for premedication with a sedative before their examination. This Egyptian banded cobra (Naja annulifera) has managed to injure his own mouth while inside a properly sized tube for a veterinary examination, a rather difficult trick. This particular patient has a history of self-injury on restraint, including self envenomation and mouth trauma from biting handling tools. A more extensive writeup on this animal's case can be found here. Sedation is the best protocol for an animal that injures itself on conscious restraint.
Sedation is a risk in debilitated patients, as some drugs may depress respiratory, cardiovascular and immune system functions. Care must be taken when deciding whether these drugs are more of a risk to the patient than the stress of conscious handling. We have had excellent results with Diazepam (0.1 to 0.8 mg/kg IM) as an effective sedative that does not appear to cause noticeable respiratory or cardiovascular depression in elapids and vipers. The effects are quite obvious at 0.2 mg/kg, without inhibiting thermoregulation, while the upper end of the range produces profound lethargy.
We briefly tried the veterinary standard Acepromazine, but quickly gave up because it seemed to be completely ineffectual. Apparently we are not alone in this opinion; other veterinarians at NAVC also reported that this drug has highly variable results in snakes.
King cobras may show significant clinical signs of stress during extended conscious restraint. They may begin to produce a thick, copious salivary mucous if they are restrained for more than a few minutes at a time. Their heartbeat and respiration may become abnormally fast or slow. Other stress-induced histological changes are likely to be occurring. Sedation is the most humane protocol for these patients when lengthy procedures are called for. They are not particularly difficult patients to physically restrain, but they evidence some very alarming stress-induced symptoms which are probably best managed with medication.
Neotropical rattlesnakes (Crotalus durissus), fer-de-lance (Bothrops atrox and asper), bushmasters (Lachesis species) and Western diamondbacks (Crotalus atrox) are also patients at high risk of injury during extended conscious restraint. These animals have a poorly attached occipital condyle, minimal supporting musculature around the spine and a tendency to struggle violently during examination. In the Crotalus species, physical restraint around the head and neck may result in visible swelling and thickening of the tissues in that region. Stressed animals may exhibit unusual behavior and refuse to feed for a prolonged period. While it is not too difficult to restrain these snakes so that they cannot injure their handlers, it is much more difficult to keep them from causing injury to themselves. Options for these patients include tube restraint and pinning for short duration procedures such as the quick injection of a sedative drug. A snake may also be briefly restrained with Gentle Giant tongs (the other tong brands are not recommended for this use) for a rapid injection delivered with a veterinary pole syringe. Sedation is probably the most humane veterinary protocol for these patients as it seems to result in the least physical and behavioral disruption.
Monitoring a patient under any form of general anesthesia or sedation is a crucial part of the procedure. A wholly or partially anesthetized snake may not be breathing on its own, and you will have to closely watch respiration and other vital signs. You should be prepared to ventilate (breathe for the snake). It is appropriate to ventilate with oxygen during surgical anesthesia and with room air during the recovery period. A pulse oximeter is a valuable tool, but continual manual palpation of the heartbeat and visual observation of the respiratory process is generally sufficient. Your patient also will not be able to thermoregulate, so you will need to maintain the animal at a consistent temperature. The monitoring process begins when the snake is first induced and ends only when the animal regains conscious mobility. Depending on the procedures and the chemical agents used, induction to full recovery can be as rapid as 15 minutes or may be as long as several hours. Careful and constant attention to the patient is needed during this time.
If isoflurane flow is titrated fairly low (starting at 5% and moving down to 2.5% for maintenance and down to 1.5% towards the end of the operation) and a local nerve block is used, the patient's recovery will be much more rapid than if the snake is maintained on 5% iso and oxygen for the entire operation. As a general rule you should administer the minimum amount of drug that will keep your patient humanely sedated, free of pain and immobile during the surgical procedure as this will give you the least systemic depression and the best recovery.
Recovery from anesthesia in snakes is caudal to cranial; induction is cranial to caudal. That means that they lose muscle control and reflexes starting with the head, and regain responsiveness starting with the tail. This is a very convenient physiological mechanism as it allows a good safety margin while the veterinarian is performing surgical procedures. A handler should be constantly monitoring muscle tone and withdrawal reflex in the tail. When responsiveness is detected, the patient is no longer at a surgical plane of anesthesia, but there is still plenty of time before the head will regain consciousness. Rattlesnakes in particular come with their own pre-installed warning bells when a patient is beginning to recover. They will not rattle before they are conscious, but have strong muscle tone holding the rattle up off the ground that is easily seen in a recovering patient. A venomous snake demonstrating tail reflex should be head-tubed for the safety of the handler who is monitoring the recovery process.
"Physical restraint alone, although sometimes more economical
and quick, is often problematic. Many reptiles are capable of maiming
and/or killing handlers. Persistent struggling will result in muscle
contraction and consequent damage, possible hyperkalemia and lactic
acidemia. Although bound large reptiles appear immobile they may still
be isometrically contracting skeletal muscle."