Review of genetic aspects of radiological alterations in the navicular bone of the horse.

Navicular disease or podotrochlosis has long been known to cause forelimb lameness in horses. It had been proposed that the development of podotrochlosis has similarities to the human osteoarthritis (OA) complex. Alterations of the navicular bone can be made visible early in life only on the basis of radiographs. Reports on the prevalences of navicular disease indicate that radiological alterations in the navicular bone are present in different warmblood populations at frequencies of between 14.9% and 87.6%. Genetic factors play an important role in the development of the radiological signs. Estimates of heritability using animal threshold models range from h2 = 0.09 to h2 = 0.40. Estimated additive genetic correlations between radiological changes in the navicular bone and other orthopaedic health traits indicated that they mostly develop genetically independently of each other. There was a negative genetic correlation between radiological changes in the navicular bone and the number of tournament entries and placings. It has also been shown that reduction of radiological changes of navicular bones and improvement of breeding values for performance of riding horses can be achieved if selection is based on breeding values for these traits simultaneously. An optimised markerset was developed to detect quantitative trait loci (QTL) for pathologic changes in the navicular bone of Hanoverian warmblood horses. The horse genome was scanned using 214 highly polymorphic microsatellites chromosome-wide significant QTL were located on equine chromosomes (ECA) 2, 3, 4, 10, and 26. Genome-wide significant QTL were on ECA2 and on ECA10. Unravelling QTL associated with navicular disease will enhance selection progress for a healthy limb constitution in horses.