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ear, notably the stylohyoid and petrous temporal bones. While a specific aetiology has not been defined, it is probable there are different aetiologies in different cases, with consequent ankylosis of the temporohyoid joint (Hassel et al. 1995; Walkeret al. 2002; Peaseet al. 2004; Diverset al. 2006; Aleman et al. 2008). This ankylosis places abnormal forces on the petrous temporal bone, especially when there is movement of the tongue and larynx. The hyoid apparatus is linked in series to the larynx and tongue, so normal forces during eating/vocalisation/tongue movement can cause acute excessive pressure on the ankylosed temporohyoid joint and fracture of the petrous temporal or stylohyoid bone in horses with THO. It is sometimes noted that an underlying event, such as passage of a nasogastric tube, dental floating, mastication of firm feeds, or excessive vocalising, predisposes a horse to acute neurological signs (Walker et al. 2002). Crib-biting is a repetitive oral behaviour of horses in which the horse grasps a fixed object with the incisor teeth, contracts the neck muscles such as the sternocephalicus and sternohyoideous (thus pulling on the larynx) and draws air into the cranial oesophagus (McGreevy et al. 1995). The frequency of crib-biting behaviour varies between horses and within individuals, but may occur as often as every 10–20 s (Marsden 2008). The behaviour is often associated with stress or pain, or after eating a high concentrate feed (Waters et al. 2002). Prevalence of crib-biters within the general horse population has been found to range from 2.8% (Redbo et al. 1998) to 10.5% (Waters et al. 2002), with 4.4% reported in the most recent paper by Albright et al. (2009). Crib-biting has been suggested to be more prevalent in populations of horses with limited turnout (Waters et al. 2002; Bachmann et al. 2003) and found to be more common in Thoroughbred horses with a possible genetic predisposition, at 13.3% (Albright et al. 2009), all of which are lower than the 31% prevalence in the population of horses with neurological disease associated with THO. Unfortunately housing and management information was not available for the horses in this report. This is the first paper to evaluate a potential relationship between crib-biting and neurological disease associated with THO. There are 2 possible ways in which THO and crib-biting could be associated. The act of crib-biting could put chronic repeated excessive pressure on the hyoid apparatus and stylohyoid bone, making horses that crib more prone to develop the arthritis seen in THO. The excessive chronic traction on the stylohyoid bone could lead to inflammatory joint disease of the temporohyoid joint. Degenerative joint disease in other articular joints is known to occur with chronic increased pressure (Hough 1997). This may help explain the bilateral osteoarthropathy found in many cases. More likely, horses with subclinical THO of another aetiology could be predisposed to acute clinical fracture and neurological disease due to the excessive forces that crib-biting puts on the ankylosed temporohyoid joint. Thoroughbreds have a higher prevalence of crib-biting than the general population of horses, yet our study is the third to find a higher number of Quarter Horse-types with neurological signs associated with THO. This further suggests that crib-biting does not cause THO, but rather increases the risk of neurological signs of THO in horses that already have the osteoarthropathy. Yet no significant difference was found in the time to develop THO between horses that had the crib-biting behaviour in comparison to those that did not. Without knowledge of when the ankylosis process started relative to progression to the fracture and neurological signs, it is possible that the complete sequence of events occurs faster in crib-biters. This information could be used for early detection of THO in crib-biting horses and consideration of the prophylactic ceratohyoidectomy in those horses since they might benefit the most from the procedure. Crib-biting has been shown to be related to release of endogenous opioids and is possibly related to stress, pain and increasing basal beta-endorphin or cortisol levels (Lebelt et al. 1998; McGreevy and Nicol 1998a). It is therefore possible that a source of pain or stress (such as THO) could lead a horse to crib or crib more frequently, accounting for the association between crib-biting and neurological signs associated with THO. However, there has been no link between crib-biting and other degenerative joint disease or pathology of the head. It also seems unlikely that horses suffering arthritic pain from THO would choose to crib to alleviate their pain. More likely, crib-biting was an acquired behaviour that preceded the THO and combination of both causes an increased risk of neurological disease associated with THO. This retrospective case series shows a high prevalence of crib-biters in a population of horses with neurological disease associated with THO. It does not prove a causal relationship between crib-biting and THO, but is the first paper to bring to light an interesting possible correlation of risk. Owners of horses who crib-bite are already warned that crib-biting leads to excessive incisor wear (Houpt 1986), and has been linked to weight loss, gastric or gastroduodenal ulceration (Nicol et al. 2002), gas colic (White 1997), epiploic foramen entrapment (Archer et al. 2004), large colon impactions (Hillyer et al. 2002), prolonged gastrointestinal transit times (McGreevy et al. 2001) and equine motor neuron disease (de la Rúa-Domènech et al. 1997). A potential link between crib-biting and neurological disease associated with THO might further encourage owners to attempt to prevent crib-biting in their horses. There are numerous methods of preventing crib-biting, which are controversial and have variable efficacy, including the crib-biting collar, crib-biting muzzle, hog rings between incisors, acupuncture and a variety of myectomy and neurectomy surgical techniques (McGreevy and Nicol 1998b). Management changes directed at providing a calm environment and natural setting are considered most effective. These focus on increasing forage feeding, increasing time spent eating, living in a pasture situation with other horses, minimising stress, and covering any possible crib-biting surfaces with © 2010 EVJ Ltd 471 EQUINE VETERINARY EDUCATION / AE / septe�fber 2�b1�b