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et al. 2012; Barstow and Dyson 2015; Dyson 2016a). Lameness-associated performance problems are most frequently caused by bilateral foot, fetlock, carpal, or distal hock joint pain or bilateral proximal suspensory desmitis (Dyson 2002, 2009, 2010a). Common causes of back pain that instigate performance problems include impinging dorsal spinous processes, osteoarthritis of the thoracic and/or lumbar articular process articulations, and sacroiliac joint region pain (Jeffcott 1980; Walmsley et al. 2002; Dyson and Murray 2003; Girodroux et al. 2009; Barstow and Dyson 2015). Another very important cause that is likely to affect equine performance is muscle pain that can be either focal (i.e. muscle cramps, strains or trauma) or generalised (i.e. sporadic or chronic exertional rhabdomyolysis or chronic exertional myopathies) (Valberg 2018). A potentially under-recognised cause of exercise-associated hindlimb muscle pain and lameness is aorta-iliac thrombosis (TAI), which is a progressive vascular disease characterised by thrombus formation of the caudal aorta or iliac arteries and subsequent impairment of blood supply to the large muscles of the hindlimbs (Brama et al. 1996; Rijkenhuizen et al. 2009; Rijkenhuizen and Pokar 2019). Another musculoskeletal disorder to consider in the differential diagnosis is temporomandibular joint disease, which reportedly may present with nonspecific symptomatology such as obscure performance and riding problems (Jørgensen et al. 2015). Oral pain is also a well-accepted potential cause for
performance and rideability issues. Horses are often able to mask signs of oral pain by adapting their eating patterns and behaviour until the underlying disorder becomes too advanced to cope with (Moine et al. 2017). However, signs of oral pain may become evident when the horse is ridden, which may be related to the impact of bits and bridles (Thomas 2020). The use of bits might either be the primary cause of oral pain (Cook 2003; Cook and Kibler 2019) or aggravate existing dental problems (Pehkonen et al. 2019; Thomas 2020). There is evidence that horses ridden with a bit and bridle have a higher prevalence of oral ulcerations compared with unridden horses and that bit type influences the location and severity of lesions (Tell et al. 2008; Bj€
ornsd ottir
et al. 2014). In another study, however, the prevalence of oral lesions did not differ between different types of bit or bitless bridles but was associated with overtightened nosebands (Uldahl and Clayton 2019). There is widespread anecdotal evidence that common equine dental problems (such as soft tissue injuries caused by sharp enamel points, presence of the first premolar teeth, abscessed teeth, dental fractures, periodontal disease, abnormalities of eruption and malocclusions) might cause significant pain and in turn could be linked to impaired performance and occurrence of undesirable behaviours during ridden work (Griffin 2009; Anthony et al. 2010; Bryant 2014; Thomas 2020). In accordance herewith, a recent study established a link between bit-related behavioural problems and dental pain caused by periapical infection in cheek teeth of horses (Pehkonen et al. 2019). Similar to these findings in horses, studies in human athletes have provided evidence that dental disease had substantial negative impact on their sports performance (Needleman et al. 2013, 2016). Visceral pain, originating from abdominal or pelvic
viscera, is the third main type of cause to consider in the differential diagnosis of performance and rideability issues in horses. The gastrointestinal tract is presumably the most
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frequent source of visceral pain in horses, wherefore this type of abdominal visceral pain has received the most research and clinical attention (Robertson and Sanchez 2010). There are various gastrointestinal problems that may result in chronic intermittent colic and adversely affect equine performance and behaviour. A well-known example is equine gastric ulcer syndrome (EGUS), for which there is evidence of a significant association with poor performance (Vatistas et al. 1999; Jonsson and Egenvall 2006; Franklin et al. 2008; Sykes et al. 2015). The underlying mechanism has not yet been established, but it has been proposed that decreased performance may arise as a direct consequence of the gastric pain per se (Vatistas et al. 1999; Andrews 2008; Sykes et al. 2015), or potentially because of referred pain that might also cause other types of aversion, such as reaction to the girth (Millares-Ramirez and Le Jeune 2019). There has also been reported an association between EGUS and behavioural changes such as nervousness, aggression and self-mutilation (McDonnell 2008; Sykes et al. 2015), why it is reasonable to expect that EGUS may also affect the horse’s rideability by causing behavioural problems during ridden work. Other possible causes for chronic abdominal pain related directly to the gastrointestinal tract include inflammatory disorders, parasite infestation, ulceration, intestinal spasms, mesenteric tension, neoplasia and partial obstruction of the intestinal lumen due to, for example, sand impaction, adhesions, stricture, intussusception, enterolithiasis or intra-abdominal abscesses (Archer 2008; Cot
e 2019). As a
direct consequence of the inflicting pain, all of these disorders have the potential to adversely affect performance and rideability. Although less common, chronic visceral pain may also originate from other abdominal or pelvic organs such as the peritoneal cavity, liver, biliary tracts, pancreas, organs of the urinary system and organs of the reproductive system (Archer 2008; Cot
e 2019). Research is generally
lacking in regard to confirmation of a potential association between these causes for abdominal and pelvic pain and the occurrence of performance and rideability issues, but since the pain has an independent impact on performance and behaviour regardless of the underlying cause, such an association is considered very plausible. Of these less common causes, it is especially referred urogenital pain causing hypersensitivity/hyperalgesia in the lower back, flanks and/or hindquarters that have been established as a potential cause of performance and behavioural changes in horses (Christoffersen et al. 2007; Lindegaard et al. 2009). The phenomenon of referred pain is well-established in human medicine where several specific somatic areas of referred pain have been linked to specific visceral organs, for
example epigastric region for the stomach, lumbar region/ flank for the urinary tract and lowest abdominal quadrants for the reproductive organs (Giamberardino et al. 2010; Kaufman and Jones 2018). This can be explained on the basis of convergence of somatic and visceral afferents on the same secondary neuron within the dorsal horn of the spinal cord, which can cause the brain to misinterpret the visceral nociceptive input as somatic in origin since the nociceptive system is primarily accustomed to somatic inputs (Giamberardino 1999; Dunckley et al. 2005; Christoffersen et al. 2007; Lindegaard et al. 2017). Performance and behaviour problems in mares have also been linked to ovarian pain that occasionally occurs in association with impending ovulation (Cox and DeBowes 1987; Chenier 2008;
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