A link between chronic back pain and aggression in horses has been discovered by French researchers.
A study has probed aggression in horses.
The study team said it was known that sudden or severe (acute) pain was known to increase the risk of aggressive or violent behaviour in humans and animals.
However, to their knowledge, their findings provide the first experimental evidence of such a link between persistent and long-lasting (chronic) pain and aggression.
The findings suggest chronic pain may act in ways similar to those of acute experiences.
"Chronic discomfort or pain may often be overlooked when facing 'bad tempered' individuals, whether humans or animals," researchers Carole Fureix, Hervé Menguy and Martine Hausberger concluded.
"This experimental study confirms the importance of including chronic discomfort or pain as a major factor in interpersonal relations and models of aggression."
The findings of Fureix and Hausberger, from Animal and Human Ethology laboratory at the University of Rennes 1, and Menguy, from the chiropractic practice of St Jacques de la Lande, were published in the open-access journal, PLoS ONE, in 2010.
The researchers noted that studies to date had focused mainly on the impact of acute rather than chronic painful experiences.
They set about testing the hypothesis that, in horses, aggression towards humans - a common source of accidents for professionals - could be linked to regularly reported vertebral problems in riding horses.
In the study, chiropractic back examinations and standardised behavioural tests were made independently on the same group of 59 horses, all kept under similar conditions.
In the study, the researchers investigated whether there was an association between vertebral problems, assessed by chiropractic examination, and behavioural problems - that is aggressiveness towards humans - assessed by using standardised behavioural tests in 59 riding school horses of varied ages and sexes, living under similar conditions.
Evaluations were made separately, each involving a different experimenter blind to the results of the other evaluation.
The 59 tested horses comprised 44 geldings and 15 mares.
Examinations were based on bony and soft tissue manual palpation for localised regions of vertebral stiffness based on spinal mobilisation and palpable areas of muscle.
Horses were either classified by the practitioner as totally unaffected, slightly affected (one slightly affected vertebra) or severely affected (at least two severely affected vertebrae).
Before the spine examination, horses were submitted to five standardised behavioural tests, routinely used in different studies on human-horse relationship.
The tests involved observing the horses' reactions to varying degrees of approach, ranging from standing motionless in their box for five minutes, to approaches that involved touching their necks, a sudden approach holding a saddle, and halter fitting.
Both threats and positive reactions were noted. Threats always consisted of ears back and could vary from simple threats (that is, looking with ears laid back), threats to bite, to threatening approaches, stretching the neck or approaching towards the experimenter with ears laid back.
No kicking or threats to kick were observed.
Positive behaviours included upright ears, sniffing, licking, nibbling and chewing.
Horses were classified as "aggressive" on the basis of at least one aggressive reaction in one test.
Horses' responses in the behavioural tests clearly reflected the evaluation of their spine, the researchers found.
While about half the slightly affected and totally unaffected horses (8 from 16 horses) showed aggressiveness, more than 75 per cent of the severely affected horses did.
Remarkably, the researchers noted, only two of the 43 severely affected horses showed consistent positive behaviour towards the experimenter, whereas about half of the other horses did.
Severely affected horses showed fewer positive reactions towards humans than totally unaffected or slightly affected horses, their findings revealed.
"The more the vertebrae were affected, the less positive reactions the horses made."
Their findings show that most horses severely affected by vertebral problems were prone to react aggressively towards humans (33 of 43 horses), compared with unaffected or slightly affected horses (9 of 16 horses).
"The more affected they were, the fewer positive reactions they exhibited," they wrote.
"Horses share with humans a high prevalence of back problems, considered as one of the most common and least understood clinical problems in sporting horses ..."
Thirty-five per cent of 805 racehorses were found to have back problems in one study, while 100 per cent of the western horses examined in another suffered from thoracolumbar (thoracic and lumbar vertebrae) problems.
"Vertebral problems may elicit at least discomfort, and are assumed to indicate the presence of back pain," they said.
"Most of the horses affected by vertebral problems keep on working, as discomfort/potential pain is generally underestimated through lack of direct measurements and owners' personal interpretations of behaviour they assume to reflect discomfort or pain.
"Horses," they noted, "are behaviourally 'mute', even when in great pain, maybe because non-disclosure of pain is a valuable survival strategy to avoid predation.
"Severe back problems could be discovered after owners had observed increased aggressiveness when grooming or saddling and when the horses were not just categorised as 'horses with bad temper'.
Behavioural problems with horses and especially aggression towards humans are regularly reported and constitute a common source of accidents involving veterinarians or caretakers, they noted.
"Recent studies show that horses tend to adopt generalised attitudes towards known and unknown humans in different situations, revealing for some of them a form of 'hostility state'."
Discussing their findings, the authors said it was possible that horses build a "memory" of negative associations between work with humans and discomfort or potential pain, as they do for positive actions.
"More recent data even strongly suggest an ability of horses to anticipate the positive or negative outcome of an interaction with humans.
"Such memories are long-lasting and generalised to unfamiliar humans.
"Being ridden regularly while in pain may certainly lead to such associations ..."
Pain and aggression are definitely related, they said, whether on a chronic or acute basis, but the relation may well be underestimated in cases of chronic pain.
The five vertebral areas (cervical, thoracic lumbar, sacral and coccygeal) of the horse's skeleton.