Assessing pain in horses

Pain has a function. It serves to warn and protect the body against (more) potential tissue injury so that the body is allowed to heal. Pain is therefore very important for survival. As riders and caretakers, it is important that we are able to recognise and acknowledge when a horse is in pain.

 

 

When an animal is in pain, this often leads to changes in behaviour. An animal may become depressed (reduced interaction or social behaviour, less responsive), assume a certain posture (lowered head carriage, fixed stare, rigid stance, reluctance to move) or become less active to reduce their pain level (often with orthopaedic pain), or they may become hyperactive (for example during colic). When pain becomes chronic, these symptoms may continue (lameness, depression, aggression), but they may also develop other signs of stress or discomfort, such as tremors, shivering, grimacing, stereotypic behaviour (head shaking, pawing, licvking, tongue-playing, kicking), etc. On the long term other issues such as weight loss, loss of interest in grooming or other social behaviour, distrubed sleeping patterns, immune suppression and stress related disorders may develop.

The expression of pain through behaviour is influenced by many things, such as the severity of pain, the localisation, the environment and the individual’s character, learned experience and breed.

 

Types of pain 

There are different kinds of pain. Nociceptive pain is caused by tissue damage. Pain receptors called “nociceptors” are present throughout the body. They percept pain directly, or via chemicals that are released by inflammation. This kind of pain can be treated well with non-steroidal anti-inflammatory drugs (NSAID) and/or opioids.  

  • Somatic pain is a nociceptive pain that arises from skin, connective tissue, muscle or bones, charachterized by localized, sharp, stabbing or throbbing pain.  
  • Visceral pain is also a nociceptive pain, but originates from organs in the chest or abdomen. This pain can often not be clearly localized and gives a piercing, cramping or pressing sensation.  

Besides nociceptive pain, there is also “neuropathic pain”, which originates from damage or disease of the nervous system. This type of pain can be challenging to diagnose the cause of, and may lead to unexplained chronic pain in humans. 

 

Assessing pain 

Assessing pain in humans is quite different from assessing pain in animals. Humans can usually communicate about their level of pain verbally, or point at pictures on a scale to indicate level of pain. Unfortunately, we can’t use such systems as the Visual Analogue Score or the Numerical Rating Scale with horses. Besides not being able to use conversation, there are also some issues that may prevent us from being able to interpret pain through behaviour reliably. For instance, prey species such as the horse, have evolved the ability to mask obvious signs of pain under specific circumstances (i.e. stress, the presence of a predator or threat: unfamiliar environments, people). Still, some behaviour based pain scoring systems have been developed for horses that have been proven to be reliable. Some of these scoring systems have been specifically developed for use in horses under very specific conditions (head pain, pain after colic surgery, etc), but there is also a more general pain scoring system available. In all of these scoring systems, it is very important that they are proven to measure pain correctly and that this does not depend on the user/person. 

 

Composite Pain Scale 

The Composite Pain Scale is a numerical scale that has been validated in horses for orthopaedic pain (Bussiers et al. 2008, Lingegaard et al. 2010) and somatic/visceral pain after colic surgery (Pritchett et al. 2003, Graubner et al. 2011, van Loon et al. 2014). However, this does not mean that the scale can’t be used in other situations, it just means that this hasn’t been validated yet.  

The Composite Pain Scale integrades physiological parameters (respiratory rate, heartrate, etc.) and behavioural parameters, such as responses to stimuli (sound, touch) and spontaneous (interactive) behaviour. The user therefore has to be capable of taking these measurements. For veterinarians, the Composite Pain Scale is an objective “day-to-day” pain evaluation tool in horses with acute pain. It can also be used to evaluate the effect of pain medication.  

 

Facial Expression Scales 

Facial expression has been used to assess pain levels in various animal species, including humans. In horses, this has lead to the development of the “Equine Pain Face”, also called the “Horse Grimace Scale”.  

A study by Dalla Costa et al. 2014, tested the Horse Grimace Scale on horses undergoing routine castration. Horses were evaluated before and 8 hours after surgery. The study design made sure that all horses were accustomed to their environment, so that expression of behaviours was not inhibited. They found that changes in facial expressions in the horses were detectable without the need of approaching the horse, and by observers with differing expertise with only the horse grimace scale manual for guidance.  

They concluded that the Horse Grimace Scale may provide insights into the experience of pain in horses in their own environment, and therefore be a useful tool in the assessment of horse welfare on-farm.

Another study by Gleerup et al. 2014, looked at the Equine Pain Face in horses that were given one of two painful stimuli that have also been used in studies on pain in humans. One stimulus was a tourniquet at 240 mmHg (which would otherwise usually be applied under sedation), the other was the application of capsaicin cream (the pungent constituent of chilli peppers) to the skin. Both pain inductions produced an acute, moderate and reversible pain reaction, visible as changes in the Equine Pain Face (Table 1). It is important to note that facial expressions were not static but changed whenever the horses reacted to something in the surroundings. This is normal and should therefore be kept in mind when observing a horse. Also, not all identified features of the pain face were present simultaneously at all times. Therefore, all features should be assessed before a conclusion can be drawn.

Table 1: Description of the features of the equine pain face

Pain face feature 

Detailed description 

Asymmetrical/low ear position 

The distance between the bases of the ears increased during pain inductions because the ears tended to drop slightly down to each side of the head with an outward rotation of the ears. The movement pattern of the ears changed. During pain sessions, the horses spent significantly more time with ears moving ‘asymmetrically’ and/or ‘low’ and less time with attentive ears and ears forward. 

Angled eye 

There is tension of the levator anguli oculi medialis muscle, giving the upper eyelid a very characteristic angled appearance. 

Withdrawn and tense stare 

The quality of the glance changes to become withdrawn and tense. 

Nostrils square-like 

The nostrils are dilated sideways; especially the centre/inside wing of the nostril may be tense. This is most obvious during inspiration. This changed the shape of the nostrils from the normal elongated shape to a striking edged square shape. 

Tension of the muzzle 

There is increased tonus of the lips and tension of the chin resulting in an edged shape of the muzzle. 

Tension of the mimic muscles 

There is tension of the muscles visible on either side of the head, especially the zygomaticus muscle and the caninus muscle. This accentuated the appearance of these muscles on both sides of the head. The masseter muscle is tightened in some of the horses as a result of clenched jaws.  

 

Some differences have been found between these two studies: 

  • Dalla Costa et al. 2014 found ears back as a pain indicator in horses, while this study found a different ear position to be relevant (ears more to the side and down). 
  • The characteristic angled appearance of the eye has not previously been described in detail, but study done by Dalla Costa et al 2014 did describe a tension above the eye. They also found orbital tightening in painful horses, while Gleerup et al. 2014 found a widening of the eyes and increased exposure of the sclera.  
  • The orbital tightening and backward ears of castrated horses did resemble the appearance of the horses in the second study that were dozing off, suggesting that the Horse Grimace Scale describes a combination of pain and fatigue as a consequence of the surgical stress response. Fatigue may be an important factor when evaluating horses post-surgery. It may also explain the differences in facial expressions between the horses in both studies.  
  • The horses in the second study displayed increased interaction with the observer during pain stimulation. This is in contrast to previous research, reporting a reluctance to interact with humans when in pain (Price et al. 2003; Pritchett et al. 2003; Bussieres et al. 2008; Lindegaard et al. 2010; Graubner et al. 2011; Sutton et al. 2013). This discrepancy in behavioural response may be explained by the intensity of the pain and whether horses were used to their environment. The observer in the second study was a familiar person associated with positive reinforcement training, which has been shown to increase contact seeking behaviour (Sankey et al. 2010). Though the reluctance to be handled when in pain is obviously a very important observation, the contact-seeking behaviour may be interesting in relation to the early detection of pain by horse owners. Horses with acute or low-grade pain may increase their contact-seeking behaviour when they percieve themselves to be in a safe environment.
Fig 1 - Can you recognise pain in your horse
Credit: Karina Beck and Cristina Wilkins
 

It may be clear that more research is needed to answer many of the questions about pain expression in horses. However, in the meantime the results from these studies may help us identify when our horses may be in pain. Not every horse is the same. Some horses make it very obvious when they’re not happy, while others may be more stoic. Some horses internalize stress or pain because they’ve been taught (perhaps earlier on in life) that humans do not aprove of the behaviour a horse may show when it’s trying to express itself. Perhaps the equine pain face score will help us read some of the more stoic or introvert horses. 

It is always useful to know your horse well, so that subtle differences in behaviour or expression can be noticed. By studying your horse it’ll become easier to notice differences. Have a look at your horse and try to identify the areas of the face that have been described in these studies. Look at them under differenct circumstances so that you become familiar with your horse's facial expression. There is also a free Horse Grimace Scale app available. If you have any questions about this article or your horse, please give us a call. 

 

Interesting videos and literature

A few interesting videos on Youtube about recognising issues under saddle, explained by one of Britain’s most eminent equine orthopaedic specialists, Dr Sue Dyson. These videos are a valuable learning tool to help riders, owners, trainers and vets to recognise musculoskeletal pain and subtle lameness sooner, to improve the welfare and performance of the ridden horse. 

 

E Dalla Costa M Minero D Lebelt D Stucke E Canali MC Leach (2014) Development of the Horse Grimace Scale (HGS) as a Pain Assessment Tool in Horses Undergoing Routine Castration. PLoS One. 2014; 9(3): e92281. Link: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0092281 

KB Gleerup B Forkman C Lindegaard PH Andersen (2015) An equine pain face. Vet Anaesth Analg, 2015, 42, 103–114. Link: https://www.sciencedirect.com/science/article/pii/S1467298716301325 

G Bussieres C Jacques et al. (2008) Development of a composite orthopaedic pain scale in horses. Research in Veterinary Science 85 294306 

C Lindegaard MH Thomsen S Larsen et al. (2010) Analgesic efficacy of intra-articular morphine in experimentally induced radiocarpal synovitis in horses. Vet Anaesth Analg 37, 171–185 

LC Pritchett C Ulibarri MC Roberts et al. (2003) Identification of potential physiological and behavioral indicators of postoperative pain in horses after exploratory celiotomy for colic. Appl Anim Behav Sci 80, 31–43 

C Graubner V Gerber M Doherr et al. (2011) Clinical application and reliability of a post abdominal surgery pain assessment scale (PASPAS) in horses. Vet J 188, 178–183. 

JPAM Van Loon VSM Jonckheer-Sheehy W Back et al. (2014) Monitoring equine visceral pain with a composite pain scale score and correlation with survival after emergency gastrointestinal surgery. Veterinary Journal 200, 109-115. 

 

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