Relief of acute and chronic pain in animals is an important part of the practice of veterinary medicine. Human pain medications
frequently are used in small-animal and equine practice. However, food-animal practitioners must follow the Animal Medicinal
Drug Use Clarification Act (AMDUCA), which limits extra-label drug use to treatment when an animal's health is threatened,
it is suffering or death might result from failure to treat.
Pain research in animals can be difficult to perform. Pain responses in animals vary greatly, and the severity of clinical
signs does not always match the severity of a lesion. Techniques used in humans, such as pain-score verbalization, cannot
be performed in animals. Concealing pain is a survival instinct for cattle, since they are a prey species, which further hampers
research. For all of these reasons, development of a consistent pain model for cattle is difficult. And without a reliable
pain model, it's hard to critically evaluate the efficacy of pain medications on these patients.
With this in mind, the scarcity of clinically relevant studies on pain responses and pain relief in cattle is not surprising.
The good news is research is being conducted. Dr. Hans Coetzee and his colleagues at Kansas State University are one group
trying to find answers to some of these questions.
Adaptive responses to pain make chronic pain more difficult to treat than acute pain. The longer the duration and/or the more
intense the pain, the more difficult it is to treat, requiring higher and/or more frequent doses of analgesics.
Preventive pain management is preferred and can be performed for most elective and emergency surgical procedures. Local and
general anesthetics, some tranquilizers and sedatives can alleviate much of the immediate pain associated with surgical procedures,
but the effects are short-lived and do not address the longer-term post-operative pain associated with inflammation following
surgery. Proper use of non-steroidal anti-inflammatory drugs (NSAIDs) can greatly reduce longer-term post-operative pain.
Although preventive pain management in cattle is preferred, more often practitioners are faced with making decisions about
managing patient pain following an injury or presentation of disease. The drug classes used systemically for treating pain
are opioids, alpha-2 adrenergic agonists and NSAIDs. Since the mechanism of analgesia is different for drug classes, combinations
should be considered for systemic drugs or systemic and local analgesia.
Opioids used commonly in veterinary practice are buprenorphine, butorphanol, fentanyl, meperidine, methadone, morphine and
oxymorphone. These drugs have a short duration of action and disappointing analgesic properties in ruminants compared to other
species, so they are not widely used systemically in food animals. They frequently are used for local anesthesia. Opioids
are controlled substances and are not approved for use in food-producing animals, so the tenets of AMDUCA have to be met before
they are selected. Opioids can inhibit rumenoreticular contractions, and some cause abnormal behaviors such as propulsive
walking and hypersensitivity/hyperexcitability, which can be dangerous for animals and personnel.
Butorphanol is the most widely used opioid in food animals. The recommended dose is 0.05 mg/kg subcutaneously every four hours
to six hours. Butorphanol might be indicated for short-term analgesia for acute, severe post-operative pain. Suggested meat
and milk withdrawals are four days and 72 hours, respectively. There are anecdotal reports of increased appetite following
Transdermal fentanyl is used in other animal species, but absorption and analgesic efficacy in cattle is unknown. Rumenosalivary
recycling of this drug might prolong the effects but can pose problems with establishment of withdrawal times. Constant-rate
infusion techniques of analgesic cocktails have been used in food animals and might benefit animals with severe, acute pain.
Alpha-2 adrenergic agonists
For food animals, the drug most commonly used in this class is xylazine. It has analgesic and sedative effects. The analgesic
effects are short lived (less than one hour), but the sedative effects can last greater than 24 hours, which makes them poor
choices for long-term pain management. Xylazine might be indicated alone or in combination with butorphanol for post-operative
pain, especially if sedation is a desired outcome.