Use of a Fentanyl Transdermal Patch for Post-Surgical Analgesia in a Mexican Gray Wolf (Canis lupus baileyi)
Marjorie L. Nard, DVM
Southwest Wildlife Rehabilitation and Educational Foundation, Scottsdale, AZ, USA
A Mexican gray wolf (Canis lupus baileyi) underwent four orthopedic surgeries over a period of 3 years. Knowledge of this wolf’s normal behavior suggested inadequate analgesia and significant stress in the first two surgeries. A fentanyl transdermal patch (Duragesic®, Janssen Pharmaceuticals) was used for analgesia after the next two surgeries. Postsurgical behavior suggested the wolf received acceptable analgesia from the fentanyl patch for 72 hours. The patch provided analgesia and reduced stress to the wolf by minimizing animal handling.
The mere presence of humans, let alone handling by humans, evokes a profound stress response in wild animals. Compounded by pain from surgery and/or trauma, the end physiologic and psychologic response can be detrimental to healing and even survival.2,3 Minimizing both stress and pain will improve recovery. Suffering animals seldom eat well and generally wild animals are very suspicious of odd smelling food making oral pain medications unpredictable. Repeated analgesic injections relieve pain, but also increases the stresses due to handling which ultimately degrades the relationship of the animal with the caregiver.
The efficacy of pain medications is subjective and is usually based on behavioral observations. Variations in body posture, wakefulness and response to interaction with caregiver have been described as the most reliable evaluations of pain. Wild animals innately hide disabilities and those highly debilitated by trauma and/or surgery cannot express painful behaviors. Considering that conditions that cause pain in people will cause pain in animals, behavioral evidence of pain should not be a requirement for intervention.3 This case report describes postsurgical pain control in a Mexican gray wolf (Canis lupus baileyi) with minimal animal handling using a fentanyl transdermal patch.
Fentanyl is an opioid analgesic drug with agonist activity. Opioids in general are short acting and provide analgesia with few adverse effects.6 The most frequent side effects are constipation and sedation, but respiratory depression and bradycardia can occur.2,6 Fentanyl is a class II narcotic and requires accurate record keeping in accordance the Controlled Substance Act.1 As a pure agonist, fentanyl may be inactivated when used in combination with agonist-antagonists, such as butorphanol. The fentanyl transdermal patch has been used in people for chronic cancer pain and has seen increasing use in companion animals.4,6
The fentanyl transdermal system (Duragesic, Janssen Pharmaceuticals, Titusville, NJ) is an adhesive patch that allows constant-rate, dermal absorption of the drug from a reservoir across a release membrane.6 The patch is applied to clipped, clean, dry skin in an area where the animal will not chew or scratch it. Water will not affect absorption, but direct heat, solvents or alcohol may increase absorption of the drug. Patches are available in four sizes: 25, 50, 75, 100 µg. A Duragesic-50 is designed to provide 50 µg/h for 72 hours, but individual variations occur.4,5 Recommended use is 25 µg patch in dogs and cats less than 10 kg, 50 µg patch in 10–20 kg dogs, 75 µg patch in 20–30 kg dogs and 100 µg patch in dogs over 30 kg.2,4
In dogs, blood levels of fentanyl have been reported at a mean of 1.6 ng/ml, which corresponds with therapeutic levels reported in humans.5 Lag time to the steady state was reported at up to 24 hours after application of the patch.5 Mean elimination half-life was reported at 1.39 hours after patch removal.5 In cats absorption of fentanyl from a patch applied to the lateral thorax or abdomen was more rapid and blood levels lasted longer, but were lower than those reported in dogs.5,7
A female Mexican gray wolf underwent four orthopedic surgeries over a period of 3 years. The surgeries were humeral osteotomy, two carpal arthrodeses and amputation of a forelimb. Pain control in the humeral osteotomy and first carpal arthrodesis was intravenous butorphanol intraoperatively at a dose of 0.1 mg/kg, followed by oral butorphanol at a dose of 0.4 mg/kg. Pain control was successful in the humeral osteotomy but not in the first arthrodesis. Failure of pain control following the first arthrodesis was based on observations. The wolf did not eat well, completely rejected food that contained a pill (antibiotic and butorphanol) and was described by the keeper as irritable and unsociable. Posture was difficult to evaluate due to physical limitations.
The first arthrodesis failed and a second, more aggressive arthrodesis was performed. Immediately after induction of general anesthesia the fentanyl patch was applied. Hair was clipped on the back of the neck. A Duragesic 50 was applied to the skin. The patch was manually held in place for 1 hour. The patch was covered with a soft bandage to help keep it in place. Pre-emptive pain control for this surgery was intraoperative morphine at 0.5 mg/kg IV and the same dose immediately postoperatively. The wolf was anesthetized every 72 hours to change the fentanyl patch and the leg bandage. A total of three patches were used. One year later a fentanyl patch was again used for pain control after amputation of a forelimb; the same application procedure was used. Morphine was not used intra- or postoperatively for the amputation.
Behavioral observations, made by the same keeper in all instances, were subjective but remarkable. The wolf was alert within 2 hours of the second arthrodesis and 1 hour of the limb amputation. The animal consumed a full meal within 12 hours of each surgery and even consumed food containing an antibiotic tablet. The animal’s caregiver described it as minimally irritable and nearly as sociable as prior to the surgeries. Specifically, the wolf snorts, snaps, growls, urinates and defecates when it is uncomfortable. When comfortable, the wolf responds submissively to the keeper and does not snort, snap and growl and does not defecate but occasionally urinates.
Other factors could contribute to the improvement in postsurgical behavior when the fentanyl transdermal patch was used in this Mexican gray wolf. Our overall subjective impression was improvement in the level of pain. The keeper’s knowledge of the wolf’s normal behavior was essential to reaching this conclusion. Ease of fentanyl administration, minimal animal handling and timely return to food consumption were significant advantages hastening the healing process.
Ideally the fentanyl patch should be applied 24 hours prior to surgery allowing drug levels to reach steady state. Since the manufacturer suggests increased rate of drug absorption may occur if direct heat is applied to the patch, we maintained direct contact with the patch on the wolf to enhance absorption. This avoids extra pre- and postsurgical handling. Intraoperative morphine was considered necessary to provide pre-emptive analgesia for the second arthrodesis. The wolf’s behavior suggested this approach was acceptable. Agonist-antagonist analgesics such as butorphanol should be avoided as they may inactivate the fentanyl.
The fentanyl patch appeared to provide adequate analgesia for the suggested 72 hours. The soft bandage covering the patch was well tolerated. The patch may contain residual fentanyl after 72 hours and should be disposed of properly to avoid exposure to other animals or children. The manufacturer recommends flushing the patch down the toilet.
Using a class II narcotic requires meticulous record keeping which is not unreasonable. We keep only a few patches on hand and obtain them from a local pharmacist. The cost is $21 for a Duragesic 50 µg/h patch in our area. At an average cost of $7 per day and with minimal animal handling, the fentanyl transdermal patch provided acceptable postsurgical analgesia in a Mexican gray wolf.
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