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III. Alleviation and Prevention of Pain in Animals

In many countries, regulations dictate that pain as an outcome of procedures that people carry out on animals can and must be minimized. Guidelines for research state clearly that investigators bear the responsibility to predict and prevent pain in their animal subjects. If it is not known whether pain may occur, the mandated oversight committee should ask for a pilot study to gauge the potential for pain during the proposed procedure, and ways of alleviating any resulting pain must be planned1a,1b.

Preventing pain in animals is not an insurmountable task, since nearly every available drug for people's pain has been tested extensively for safety and effectiveness in animals. Appropriate doses of effective pain relievers are therefore available for many species2.

Researchers may argue that introducing a pain-alleviating drug will interfere with the variable they wish to study. Drugs that interrupt pain can change respiration or cardiovascular function and certainly affect nerve transmission and endocrine responses. On the other hand, this is exactly why the potential interactions between pain and the variable that a researcher wishes to measure make it necessary for good experimental design to eliminate pain. The animal's pain-and its reaction to that pain-may confound the outcome without a researcher realizing the interaction is taking place.

This concern must be addressed logically, and the potential effects of analgesics placed in the context of all of the sources of variation in a study. To control chronic pain in animals in a research setting, use of non-pharmacological techniques may be possible and indeed more appropriate3, 4.

Despite considerations of how pain or its relief may affect measured responses to an experimental manipulation, it is the ethical responsibility of a research worker to provide an adequate justification for withholding analgesic drugs5.

Types of pain relief

Pain is the signal of tissue damage perceived by the brain (see pain and distress biology). Anesthesia produces a loss of consciousness: i.e., the brain is not perceiving the pain signals that are coming in6.

Anesthesia is appropriate for long duration or invasive manipulations such as surgeries.

Analgesics interfere with pain transmission to the brain while retaining consciousness7.

Analgesics to relieve pain after surgical procedures include opiate drugs related to morphine. The opioid drugs act in the spinal cord and brain to limit the input of pain information. Analgesic drugs that include aspirin, and the so called non-steroidal anti-inflammatory agents (NSAIDs), act to limit the signals that get through to the brain and also act in the tissues to decrease inflammation during and after surgery, thus limiting the byproducts of inflammation that cause pain8.

Preemptive Analgesia

To avoid pain, analgesics can be given during or even before surgery. This is called preemptive analgesia9.

In fact, giving analgesics before any noxious stimulus (eg, surgery) can actually reduce the degree of pain afterwards. On a note of caution, there is still little information concerning interactions between analgesics and anesthetics, and almost no information concerning appropriate analgesics and dose rates for use in non-mammalian species10.

Examples of Analgesics for Animals

An opiate drug called buprenorphine is often recommended as an analgesic for animals, since there is good evidence that it lasts for six to twelve hours in humans and in a number of animal species. This drug can be given in a flavored gelatin as "Buprenorphine Jello" to provide post-operative pain relief. The treat without the drug should be introduced before the planned painful procedure. Rats are initially cautious of jelly pellets, but once one pellet has been eaten, subsequent ones are eaten as soon as they are offered11.

Other choices for opiates commonly used in animals are patches for use on the skin, such as the fentanyl patch that is a frequent choice for dogs and pigs.

The analgesic NSAIDs include carprofen for use in rodents. It produces less loss of appetite in mice than buprenorphine.

Caregivers can choose a drug for pain relief based on the duration of procedure. For those working with small animals, be aware that they can rapidly lose body heat, so a heating pad during surgery is recommended12.

The use of analgesics of different classes in combination often can provide more effective pain relief than any single agent alone. This has similar advantages to the widely used, balanced anesthesia. Lower doses of individual compounds are given, achieving the desired relief with minimal undesirable side-effects13.

For specific drug information see "A Reference Source for Analgesia and Analgesics in Animals".

Individual and Species Differ

Animal caregivers should watch out for individual, strain, and species differences in the response to analgesics. The role of genetic differences among species, strains, and individuals in their response to drugs is well known. Individual differences in responsiveness are apparent in both people and animals. Of note, with the production of transgenic animals that carry a gene inserted by genetic engineering, it is possible that the transgene could alter the response to drugs, including analgesics14.

NEXT: Humane endpoints


Readings and Resources on Pain Alleviation and Prevention

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