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multifactorial, containing both genetic and environmental elements. Heritability estimates for OC vary considerably, but are rarely higher than 0.35, which implies that most of the variation is explained by environmental factors. There are many environmental factors that have been shown to be related to OC, but they can be divided into two major categories: those somehow influencing the biomechanical loading of the juvenile joints; and those related to the intake of several nutrients.
Genetics There are strong indications that genetics play an important part in OC. Osteochondrosis does occur in feral horses, but the prevalence is much lower (about 2.5% in the tarsocrural joint and even 0% in the femoropatellar joint) (Valentino et al. 1999). Further, the disease is more than rare and in fact virtually absent in ponies (Bj€ 2007; Vo^
ornsd ottir et al. 2000; Strand et al. ute et al. 2011). Hence, either genetic factors or
management factors are at the base of the high incidence in many other horse breeds; it is most probably a combination of both.
a)
Heritability Many studies have aimed at determining heritability estimates (h2) for OC. However, values vary widely (for a review, Distl 2013), which is caused by the fact that this area is complicated and that there are many confounding factors. Any genetic study is heavily dependent on the definition of the phenotype and in the case of OC the situation is severely complicated by the fact that the phenotype is not stable over time, as lesions tend to heal until a certain age. In OC, the phenotype is determined radiographically and an animal may be positive on radiography at age 6 months and free of the disease at age 12 months, thus showing either a positive or a negative phenotype depending on the moment of examination while still being genetically exactly the same animal. Further, animals presented for selection events (from which many studies have used radiographic data) are usually heavily preselected, as owners do not want to present OC- positive animals. Therefore, prevalence figures for offspring of given sires based on these studies are virtually always underestimations. Also, the number of positives may be influenced by the number of radiographic views taken, as
b)
Fig 4: a) Arrows: perichondrial vessels becoming surrounded by/incorporated into bone. Arrowheads: vessels in distal portions of cartilage canals exiting normally from subchondral bone after incorporation (from a 3-week-old foal). a) Arrows point to where vessels fail to exit and histological lesion was found. Both images are perfused and cleared slabs (from a 1-week-old foal). (From Olstad et al. 2008a.)
a) b) c)
Fig 5: Lesions of osteochondrosis associated with ongoing ossification. Images of three-dimensional volume-rendered models of blocks containing permanent barium angiograms are shown. A, Axial; D, dorsal; L, lateral; Pl, plantar. a) Two-week-old foal, distal intermediate ridge of tibia, oblique distal view: a perfused vessel descends on the cranial aspect of the process to terminate within the growth cartilage immediately superficial to a triangular indented defect in the subchondral bone plate. The vessel terminus was surrounded by a spherical bone opacity (arrows), compatible with a separate centre of ossification, seen as an early manifestation of the repair process. b) Three-week-old foal, intermediate coronoid process, distal view: there is a circular indented defect in the subchondral bone plate (arrows). c) Seven-week-old foal, lateral trochlear ridge, distal view. There is a circular indented defect in the subchondral bone plate (arrows), partially filled by a hemispherical bone opacity, representing the repair process. (From Olstad et al. 2008c.)
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