Field Anesthesia for Bison
Bison can be extremely difficult to anesthetize. Major complications of anesthesia can include: bloat, regurgitation, and hypoxemia. Bison are also predisposed to the development of capture myopathy, which can occur during or after anesthesia.
Bison may require anesthesia for a variety of reasons: obstetrical procedures, capture of escaped animals and traumatic injury are all common. Pre-anesthesia fasting is desirable but these are often emergency procedures and pre-operative fasting is not possible. Bison are similar to domestic cattle in many respects and anesthesia of an animal with a full rumen pack can result in bloat or regurgitation. Xylazine or general anesthesia with volatile agents will lead to decreased rumen motility, bloat and regurgitation. These can be fatal complications and tend to be more severe in large, mature animals.
Ruminants are prone to hypoxemia during general anesthesia. Positioning in dorsal or lateral recumbency will exacerbate hypoxemia. Alpha-2 agonist drugs such as xylazine or medetomidine will also cause hypoxemia.
Bison become stressed very quickly. Stressed animals tend to over-ride the sedative effects of alpha-2 agonist drugs, necessitating the use of relatively high dosages of these agents to produce recumbency. Stressed animals are prone to capture myopathy (CM). The acute form of capture myopathy is a shock-like syndrome that can result in death within hours. The sub-acute form of CM often results in muscle damage, myoglobinuria and potentially, kidney failure. The chronic form is characterized by severe muscle damage and is often manifested in bison as ruptured gastrocnemius muscles. It is extremely important to keep chase times to a minimum, and try to avoid excitement prior to induction.
Hyperthermia can be a further complication. Hyperthermia increases metabolic oxygen demand and increases the risk of capture myopathy. If immobilization is anticipated, it should be planned for the cool hours of the day.
Trauma during induction of anesthesia is not uncommon. It is particularly important to control the rest of the herd during induction as other members of the herd may traumatize the drugged animal.
There is no information concerning the abortifacient effects of xylazine in bison cows. The risk of abortion in the last third of pregnancy may be increased with this drug and the owner should be informed of this possible risk.
It is important to note that every bison immobilization is a high risk procedure and owners should be warned of this risk prior to immobilization.
Sedation and anesthesia of captive bison
Bison restrained in a head gate and squeeze can often be sedated with intravenous xylazine. The tail vein can be used for the injection of small volumes. Xylazine can be use as the sole agent given IV for standing sedation. Sedation may be enhanced by the addition of acepromazine or butorphanol, given IV. All three drugs may also be combined to produce deep sedation, and possibly recumbency. If short term general anesthesia is desired, higher doses of xylazine can be administered IV, via the tail vein, to produce recumbency. This is followed by ketamine via the jugular vein. If access to the jugular vein is needed, the head must be adequately restrained to avoid injury to handlers.
Anesthesia may be prolonged with 5% guaifenasin and additional boluses of ketamine given to effect. Use guaifenasin cautiously as I have seen toxicity in two bison at what appeared to be a relatively low dose. It would be premature to say these animals are overly sensitive to the drug but it should be used cautiously until more information is available.
Xylazine should be antagonized at the end of the procedure. If IV ketamine has been used, do not administer the antagonist for 10-15 minutes to avoid undesirable effects of the ketamine.
Bison calves can be induced to anesthesia with IV ketamine plus diazepam. This combination will give 5-10 minutes of light anesthesia. In depressed calves diazepam and butorphanol can be used IV for sedation. If the calf is difficult to work with, xylazine and butorphanol can be administered IM prior to induction with ketamine-valium.
Immobilization of free ranging bison
Bison may require immobilization for a variety of reasons. Remote delivery of immobilizing agents necessitates the use of potent agents that can be delivered in relatively small volumes. Various drugs have been used in the past, but morbidity and mortality has been high.
This is a depolarizing muscle relaxant that has been used for rapid knock-down of bison. In theory, the dose is titrated to produce a locomotor paralysis, yet maintain activity in the respiratory muscles. In practice, respiratory depression is common, and muscle relaxation/paralysis without anesthesia is extremely stressful. Mortality is high with this drug and may be the result of respiratory depression or capture myopathy. Its use is considered inhumane.
Xylazine has been used as the sole agent for immobilization of bison. High dosages have been attempted but are often ineffective. Animals may appear to be sedated but will flee when approached. Bison immobilized with xylazine as the sole agent often succumb to bloat or acute capture myopathy.
Xylazine combined with ketamine can be an option for immobilization of bison. The major drawback of this combination is that a large volume is needed if commercial preparations are used. A large bull may require 5-10 mls of xylazine plus 40 mls of ketamine. Another drawback of this combination is that premature antagonism of the xylazine can result in rigidity and convulsions from the ketamine fraction.
Here are some viable options for bison immobilization:
This combination has been used for capture of free-ranging wood and plains bison. Carfentanil is combined with xylazine at various combined doses. Bison on high carfentanil dose ratios will demonstrate increased motor activity and excitement prior to induction. If the animal demonstrates head and/or limb movement on approach, additional xylazine may be administered IV to improve muscle relaxation. Following the procedure, carfentanil should be antagonized with naltrexone (as opposed to naloxone for humans).
Complications of opioid immobilization can include hypoxemia, hypoventilation, regurgitation and hyperthermia. Renarcotization is not usually a problem if naltrexone is used to antagonize carfentanil. Unfortunately, the extreme human safety hazards that accompany exposure to carfentanil prevents the routine use of this combination by most individuals.
The combination of xylazine and Telazol will produce an immobilizing agent with a relatively small, workable volume for capture. The low end of dose ranges is often effective in calm animals but the high end of the dose range may be required in fractious or wild animals. The drug is mixed by adding 100 mg/ml xylazine to a 500 mg vial of powdered Telazol. The resulting mixture has a volume of approximately 2.8 ml and contains approximately 90 mg/ml of xylazine and 180 mg/ml of Telazol. The mixture will produce approximately 1 hour of anesthesia, and will provide adequate analgesia for most minor procedures. The major complications that can be encountered with this combination are:
Hypoxemia: Bison should be maintained in sternal recumbency when possible and should receive 10 – 15 liters/min of oxygen by nasal insufflation.
Bloat and/or regurgitation: If possible animals should he fasted for 24-48 hours prior to immobilization, and immobilization times should be kept to a minimum. This is particularly important in large, unfasted animals.
Hyperthermia: This can result from stress prior to induction and may contribute to capture myopathy.
Xylazine should be antagonized following the procedure. Since the Telazol dose is relatively low, recoveries are generally smooth. Rougher recoveries may be noted if a high dose of Telazol was used for induction. Telazoline or atipamizole (as opposed to yohimbine) should be used to antagonize xylazine in bovids. Tolazoline doses can be split between IV and IM routes. Yohimbine will not effectively antagonize xylazine-induced sedation in bovids and should not be used as the reversal agent.
Preliminary work has suggested that xylazine-Telazol is a useful alternative for bison immobilization as long as steps are taken to avoid adverse side effects.
Volatile anesthesia may be used for prolonged procedures. Isoflurane is preferable to halothane as fewer arrhythmias will be encountered in stressed animals. Induction can be achieved with IV xylazine-ketamine in restrained animals, or IM xylazine-Telazol in unrestrained animals. Intubation is similar to cattle and should be performed manually. It is very important to fast animals for 24-48 hours prior to general anesthesia whenever possible as ruminants are prone to bloat and regurgitation during volatile anesthesia. Animals under 150 kg can be anesthetized with a small animal machine. Animals over 150 kg require a large animal circuit.
Bison are very prone to complications during immobilization and anesthesia. There is a risk of morbidity or mortality with any immobilization and clients should be made aware of these risks prior to anesthesia.