ABSTRACT: Accurate pain assessment is essential to optimal pain management and the maintenance of animal welfare, a fundamental goal of veterinary professionals. Pain assessment in animals is, however, a considerable challenge with no 'gold standard' indicator to guide our decision-making.

What is pain?

Pain provides a protective system, warning of actual or potential damage to the body; discriminating between harmful and harmless situations; allowing escape and withdrawal as well as learning to avoid noxious stimuli or situations. Pain has two components: physiological, which affects the animal functioning and psychological, responsible for negative experiences.

Pain can induce haemodynamic, respiratory and endocrine changes leading to delayed wound healing, prolonging hospitalisation and increasing morbidity and mortality. Chronic pain can be debilitating and fatiguing, with possible impairment of immune function. The negative experience of pain and associated suffering may lead to depression, withdrawal and social isolation.

How do we assess pain in animals?

In human medicine, the predominant method of pain assessment is patient ‘self¬report. In veterinary medicine, patients are not able to communicate pain directly – severity must be attributed through value- judgement of behavioural and physiological changes thought to indicate pain.

The assessment of physiological measures is often considered ‘more objective’ than behavioural indices. Whilst being easily measured, these indices are not pain specific and may be significantly altered by factors such as exercise, feeding, sexual excitement and environmental stress. Modification of ‘normal’ values may also occur in association with disease and with a range of drugs.

Changes in behaviour are most commonly used for the assessment of pain severity and can be classified into four different categories: 

responses that allow behavioural modification through learning to reduce the possibility of further injury protective responses responses that minimise pain and encourage healing response that may elicit help from other individuals.

Clinical experience and an understanding of equine behaviour enable veterinarians, nurses and horse owners to make judgements on pain levels experienced by horses in their care. These assessments are often based on the observer’s subjective opinion of the behaviours shown, their previous experience and their understanding of pain behaviour see Figure 1.

Figure 1: A lowered head position has been associated with both chronic and acute pain states

Currently, there are few validated pain assessment scales or protocols for use in the horse that can guide and improve assessments.

Physiological indicators of equine pain

Heart and respiration rates are the most commonly noted indicators of pain in the horse.4 Veterinary research does not, however, always confirm this viewpoint, with a number of studies finding no change in heart rate in association with painful conditions. No difference in respiration rate has also been found following arthroscopy, castration or exploratory celiotomy.

Physiological measures, such as β-endorphin and cortisol levels, have been identified as potential indicators of equine pain.8, 9 & 10 However, difficulties in measurement and the significant effects of external variables suggest these indicators are impractical in a clinical setting.

What is equine pain behaviour?

Pain behaviour is heavily influenced by species. As prey animals, horses may fail to show obvious responses to pain, instead masking pain to reduce predation. This has led to an assumption that equine pain behaviour is defined by those behaviours seen in severe, abdominal pain. It is well known that pain behaviour is affected by the type, location and duration of pain. Behaviours identified as indicators of equine pain through research or clinical observations are shown in the table.

Most potential behavioural indicators of pain are actually normal behaviours (rolling, for example.These behaviour act as pain indicators when they occur at an abnormal frequency increased weight- shifting in laminitis, for instance) or out of context (such as lateral recumbency during the day).

Subjective assessments

The visual analogue, numerical and simple descriptive scales have been used previously in medicine, enabling patients to record and monitor pain levels (Figure 2). These scales are now used in veterinary medicine and enable comparison between observers and easy record keeping.

Figure 2: Scales used in the assessment of pain

Whilst these scales, are quick, easy to use and allow observers to consider the animal as a whole, there are some limitations. Reliability between different observers is variable. The SDS/NRS limit the sensitivity of recordings. Additionally, observers’ scores have been found to vary with personality, beliefs, mood and past experience.

Furthermore, the failure of an animal to react in a predicted manner may result in the conclusion that significant pain is not being experienced.

Objective assessments

Researchers began defining and measuring behavioural changes seen in horses in potentially painful situations, attempting to reduce variability between observers and over time. This approach used monitoring of the amount of time spent carrying out behaviours such as feeding and lying, and frequency of behaviours, such as kicking, to identify potential behavioural indicators of pain.

To carry out an objective assessment, observers monitor a number of predefined behaviours over a specific time. Behaviours such as posture or locomotion – that take place over a prolonged period of time – are known as ‘state’ behaviours. These can be assessed by timing the exact amount of time the horse spends walking, for example. However, observing a number of behaviours simultaneously can be challenging.

Instead, a sample is taken by recording the behaviour being carried out every 30 seconds for example, giving an indication of the amount of time spent carrying out different behaviours. Recordings of state behaviour can be kept in a table as in Figure 3.

Figure 3: Recording of state behaviour using point sampling

Short, discrete behaviours, such as stamping, are known as ‘event’ behaviours. Event behaviour can be monitored by counting the number of times the behaviour occurs in a specific time period. R
ecordings of event behaviour are often a tally (Figure 4).

Figure 4: Recording of event behaviour Behaviour Tally (5 minute observation)

Problems in pain assessment

Assessing pain severity experienced by an individual animal is not a precise science. Individual horses may respond to pain in different ways. Pain tolerance and expression may vary with motivation, stress, previous experience, breed, location and severity.14, 15 & 16 Traditional notions of breed characteristics may affect pain assessment, a problem which is especially prominent in equines where native breeds -Shetland ponies, for example – are considered more hardy than warmblood breeds.

Behavioural and physiological indicators of pain are affected by stress, such as new environments and anaesthetic drugs, factors which should be taken into account when assessing pain severity. These problems highlight the importance of thorough case histories, including behavioural profiles giving information such as the horse’s normal reaction to people and novel environments and any behavioural habits.

Practical clinical pain assessment

Tools for the assessment of pain in a clinical setting must be practical, reliable and sensitive. Training and experience should improve observer assessment.

Veterinary nurses often spend more time with patients on a daily basis and, therefore, are frequently best placed to identify and judge changes in pain behaviour. Assessments should be made carefully considering the influence of potential external factors, such as environmental stress and drug effects.

Objective assessments, if given sufficient time, give a more accurate indication of behavioural change and are less open to subjective interpretation. Subjective scales can be useful, particularly if direction is given on variables to be observed. Additionally, as an overview, recognising limitations of reliability, subjective scales can provide a method for recording and monitoring pain severity.

In conclusion, a good understanding of behavioural and physiological responses to pain and consideration of individual characteristics and external influences will give the most accurate indication of pain severity in the equine patient (Figure 5).

Figure 5: Pain can lead to depression and withdrawal

Careful observation of pain responses and potential external influences will enable prompt action to be taken to improve the animal’s welfare and should, therefore, form an integral part of the nursing process. 

Author

Rachel Eager BSC MSC PhD

Rachel carried out her PhD research at The Royal (Dick) School of Veterinary Studies investigating pain assessment in horses. She now works as research advisor for The Brooke, an international equine welfare charity working in countries such as India, Pakistan, Ethiopia and Egypt.

References

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9.   MCCARTHY, R. N., JEFFCOTT, L. B. and CLARKE, I. J. (1993) Preliminary studies of the use of plasma beta-endorphin in horses as an indicator of stress and pain. Journal of Equine Veterinary Science, 13: 216-219.

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13.   SANFORD, J. (1992) Guidelines for detection and animals: Initiatives and experience in the United Kingdom. In: Animal Pain (Ed. by C.E.Short & A. Van Poznak), pp 51 5-524. London: Churchill Livingstone

14.   Kitchell, R. L. (1987) Problems in defining pain and peripheral mechanisms of pain. Journal of the American Veterinary Medicine Association 191: 1 195-1 199.

15.   MATTHEWS, N. S. (1992) Pain Control in the Horse: A Review of Equine Pain Models. In: Animal Pain (Ed. by C.E.Short & A. Van Poznak), pp 403-407. London, U.K.: Churchill Livingstone

16.   ARCHER, N., JOHNSTON, A. M. and KHALID, M. (2004) Differences in the acute pain responses of two breeds of lamb following castration and tail docking with the
rubber ring method. Animal Welfare 13: 135-141.

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18. TAYLOR, P. M., PASCOE, P. J. and MAMA, K. R. (2002) Diagnosing and treating pain in the horse – Where are we today? Veterinary Clinics of North America – Equine Practice 18: 12-22.

Further reading

BATESON, M. and MARTIN, P. (2007) Measuring behaviour An introductory guide. Cambridge University Press, U.K.

McGREEVY, P. (2004) Equine behaviour: a guide for veterinarians and equine scientists. Saunders assessment of pain and distress in experimental U Pain behaviour is heavily influenced by species. As prey animals, horses may fail to show obvious responses to pain, instead masking pain to reduce predation

Veterinary Nursing Journal • VOL 25 • No7 • July 2010 •