New research boosts understanding of pain

Ground-breaking research into assessing pain in horses could provide vets with the first definitive protocol for assessing its severity. Work by Rachel Eager and her colleagues at the Royal (Dick) School of Veterinary Studies, Edinburgh University, should also make it easier to evaluate current analgesics (painkillers) — and in the long term, help in the development of new ones.

Rachel, a PhD student whose latest work has been funded by the ILPH, has spent four years researching equine pain assessment systems. Her project offers far more than better ways of keeping horses comfortable, though that in itself is a real benefit.

Rachel, who has also been funded by the British Equine Veterinary Association, the British Veterinary Association Animal Welfare Foundation and the Bransby Home of Rest for Horses, says that the effects of pain can have a profound effect on welfare. They may range from loss of appetite and suppression of the immune system to slower healing, prolonged recovery from surgery and even a greater risk of death.

At one time, some vets and researchers believed horses didn’t feel pain to the same extent as some other animals because they did not show the same reactions. However, that is now believed to be a survival strategy.

“The horse is a prey animal and masks or hides pain to avoid being picked off as weak,” says Rachel. “This means it is difficult to recognise pain and the changes we see may be very subtle. Owners often say that a horse is ‘not quite right’. They can perceive some change in behaviour, but can’t find an objective reason for it.”

Rachel’s preliminary work was prompted by a debate among vets suggesting that it was unnecessary to give analgesics after castration. A survey by the Edinburgh equine pain research group found that 45% did not provide analgesics in this situation and 37% provided them only occasionally.

Vets questioned for Rachel’s project cited the horse’s demeanour or changes in heart rate as indicators of pain. But as she points out, one vet’s definition may not be the same as another’s — and there was a lack of consensus about the level of pain experienced in various conditions and how to manage it.

“Effective pain management relies on good recognition of pain, but there’s no universally accepted and tested model of assessing it and little scientific evidence,” she says.

She set out to provide both, using clinical research on horses undergoing surgery, and post-operative and laminitis cases. All the horses’ owners gave permission for the non-invasive work, no pain was induced by the research and analgesics were given in line with standard practice. Because hospitalisation could be a factor itself, she also looked at its effect on normal, pain-free horses.

Rachel used hi-tech equipment and a range of techniques, including continuous behavioural assessment, human interactive testing, heart rate monitoring and heart rate variability analysis.

Continuous assessment via CCTV cameras allowed Rachel to investigate horses’ “activity budgets”. This included changes in their position, time spent feeding and in exploratory behaviour, restlessness and head position, as well as stamping, weight-shifting and lifting limbs.

An observer outside the stable recorded subtle changes such as ear position and lip tension. Information also came from human interactive testing: the horse’s reactions were noted when an assistant approached it, placed a hand on its neck and applied consistent light pressure down towards the girth.

Heart rate variability analysis carried out with the Animal Health Trust recorded changes in the regularity of beats. Although we talk about horses having an average number of beats per minute, it is now recognised that in a healthy animal there are constant subtle changes.

“It is thought that high variability is a sign of health and adaptability, whereas reduced variability indicates physiological and psychological stress,” says Rachel.

Early results from the project show that horses in pain behave differently from normal horses. Horses who had been castrated standing up, under sedation, stamped “significantly more” in the first 6hr after surgery than those used as a control group. They also shifted their weight more.

Ponies with laminitis spent less time resting a hind limb than normal horses. This is interesting because it is a reduction in the normal behaviour that a clinically sound horse would perform frequently; the laminitic ponies also lifted their forelimbs more frequently than the control animals.

So what practical implications does this have — for vets and horse owners? Rachel, who hopes to continue the research, says that one of the main aims was to develop a “multi-dimensional pain assessment system”. She believes that if this were used as standard, it would be an important tool for assessing the effectiveness of analgesics and would also help vets find the best way of managing pain medically.

“In a hospital situation especially, vets may only see horses for a very short time,” she points out. “If you had a more objective system it could be used not just by vets but also by trained veterinary nurses.”

In the future, it could also help in the evaluation of new analgesics.

“No one has really known how to assess pain, so when you come to assess a new analgesic regime or drug, how do you assess if one is better than another?” she asks.

Vet Richard Payne from Rossdale and Partners says that Rachel’s research will provide a valuable baseline for objective pain assessment. However, he adds that it is important to remember that horses, like people, are individuals.

“If you perform the same operation on 10 people, three might be crying with pain the day after, two might say they experience pain but to a lesser extent, four might talk of discomfort and one might be out jogging!” he says.

“Similarly, you can get Thoroughbreds who think they’re dying when they get a twinge of colic, but stoic Shires or cob types show little sign of pain even when they are in a disastrous state internally.

“Pain assessment has always been a subjective thing, but if this leads to interpretation that is objective rather than subjective, it will be very helpful.”

This veterinary feature was first published in Horse & Hound (31 March, ’05)