Osteochondrosis: genetic causes and early diagnosis
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By Prof. Celia M Marr
Osteochondrosis (OC) is a common lesion in young horses affecting the growing cartilage of the articular/epiphyseal complex of predisposed joints at specific predilection sites. In the young thoroughbred, it commonly affects the stifles, hocks, and fetlocks. As this condition has such important impact on soundness across many horse breeds, it is commonly discussed in Equine Veterinary Journal. Four recent articles covered causes of the disease, its genetic aspects, and a new and very practical approach to early diagnosis through ultrasound screening programmes on stud farms.
OC is a disease of joint cartilage. Cartilage covers the ends of bones in joints, and healthy cartilage is central to unrestricted joint movement. With OC, abnormal cartilage can be thickened, collapsed, or progress to cartilage flaps or osteochondral fragments separated from the subchondral bone leading to osteochondrosis dissecans (OCD). OC and OCD can be regarded as a spectrum rather than two discrete conditions.
Certain joints are prone to OC & OCD, and there is some variation between breeds on which joints have the highest prevalence. In Australian thoroughbreds, 10% of yearlings had stifle OC, 8% had fetlock OC, and 6% had hock OC. The prevalence data may seem very high, but thoroughbred breeders may take some comfort in learning that similar, and indeed slightly higher prevalences, are reported in the warmblood breeds, standardbreds, and Scandinavian and French trotters. Heavy horse breeds have the highest prevalences.
In an article discussing progress in OC/OCD research, Professor Rene Van Weeren concludes that the clinical relevance of OC is man made. In feral horses, where there is no human influence on mating pairings, OC does occur but at much lower prevalence than in horse breeds selected for sports or racing. Similarly, in pony breeds where factors other than speed and size are desirable characteristics, OC is also rare. These facts suggest that sports and racehorse breeders have inadvertently introduced a trait for OC along with other, desired traits. There is a strong link between height and OC, suggesting that one of the desired traits with unintended consequences is height. This is of particular relevance in sports horses: the Dutch warmblood has become taller at a rate of approximately 1 mm per year over the past decades, which might not seem much but it is still an inch in 25 years. Van Weeren points out that if the two-hands tall Eohippus or Hyracotherium, the browsing forest-dweller with which equine evolution started some 65 millions of years ago, had evolved at this speed, the average horse would now have stood a staggering 40 miles at the withers.