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How is Pain Relieved?

This section does NOT describe how to treat particular painful conditions but does describe the different types of treatment that are generally used to control pain. 

If you have an animal suffering from unacceptable pain it should be treated as soon as possible and you are advised to seek help from a veterinary surgeon.

Three groups of drugs commonly used to control animal pain are:

    Opioids e.g. Morphine, Buprenorphine, Fentanyl

    Anti-inflammatory drugs e.g. Phenylbutazone, Flunixin, Carprofen, Aspirin and Dexamethasone.

    Local anaesthetics e.g. Lignocaine, Bupivacaine, Procaine.

Other drugs are also used to treat pain. All these drugs are administered either systemically, locally, epidurally or intrathecally (Otto & Short 1998).

Opioids:

    Work by their action on several different cell membrane receptor subtypes (Mu, Delta, Kappa)

    Different opioids activate (agonists) or inactivate (antagonists) these receptors or can have both actions.

    Tolerance to their effects can develop

    The onset and duration of their effects are different according to how they are administered.

    Effectiveness varies with the type of pain and species being treated.

    Unwanted effects e.g. respiratory depression, sedation, constipation can limit their usefulness.

    Antagonists e.g. naloxone can rapidly reverse their effects.

Anti-inflammatory Drugs:

There are two types, steroids and non-steroids (NSAIDs)

Steroids are more often used for controlling severe inflammation, rather than pain.

NSAIDs (non-steroidal anti-inflammatory drugs) are commonly used for pain relieve (analgesics)

    They have analgesic and anti-inflammatory effects, both of which are beneficial.

    They work by inhibiting either or both of two enzymes (Cyclo-oxygenase - COX-1 and COX-2).

    Different NSAIDs can have selective actions on the two enzymes.  This can reduce unwanted effects.

    They work on the brain and spinal cord (central nervous analgesic actions) as well as on damaged tissues (peripheral actions).

    The onset and duration of their effects are different according to how they are administered.

    Effectiveness varies with the type of pain and species being treated.

    Unwanted effects e.g. gastric irritation, can limit their usefulness (Wallace et al 1990).

Pain relieve from treatment with a NSAID can be seen after an experimental study of the effects of castration of lambs with a bloodless castrator (Burdizzo) (Molony et al 1997).  The NSAID either alone or in combination with local anaesthetic reduced the plasma cortisol response, the time spent in abnormal postures and the time spent trembling. 

Anti-inflammatory drugs also reduce the plasma cortisol responses after scoop dehorning (Sutherland et al 2002a) and surgical castration (Stafford et al 2002) of cattle.

However, NSAIDs do NOT significantly reduce pain behaviour or plasma cortisol responses associated with rubber ring castration and / or tail docking of lambs or calves (Graham et al 1997; Price & Nolan 2001; Stafford et al 2002). 

Other analgesic drugs

Various drugs are used to treat pain though their main use as sedatives, anaesthetics, anti-depressants or anti-convulsants.  Xylazine, an alpha2 agonist, used frequently in cattle and sheep for sedation / anaesthesia (Grant & Upton 2001) was effective for controlling pain associated with mulesing and knife docking of lambs (Grant 2002) but was ineffective for controlling pain associated with scoop dehorning (Stafford et al 2003) and rubber ring castration and tail docking of lambs (Molony et al 1993b; Scott et al. 1996; Grant 2002).

Local Anaesthetics

    Local anaesthetics act in the periphery blocking the signals from painful stimuli reaching the brain. Some act quickly but for only one or two hours (lignocaine), some take longer to become effective but last for longer (bupivacaine lasts for about four hours).

    They effectively prevent, reduce or delay acute pain resulting from 'surgical' procedures and husbandry practices e.g. dehorning. See movies comparing lamb castration, tail docking with and without local anaesthesia. Local anaesthesia should work provided there is sufficient volume to reach all the affected nerves and sufficient time is given for the anaesthetic to act.

    Local anaesthetics are sometimes used to help identify the source of pain e.g. use of nerve blocks in lame horses.

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