Knowledge
Vet Insight
By Camilla Nock MA(cantab) VetMB MRCVS. Certificate in Equine Sports Medicine
Osteochondrosis & OCD Genetics
A disease of genetics or management?
OCD can occur in any joint, but is most commonly diagnosed in fetlocks, stifles and hocks. The facet joints of the vertebrae of the neck are also commonly affected sites, and more rarely knees, elbows and shoulders. Lameness may be seen in foals with large lesions, usually in association with visible distension of the joint, but more commonly lameness first occurs when the horse enters training. OCD is however often found on routine screening radiographs of older horses, for instance during a pre-purchase examination, with no evidence of lameness. Whilst lameness usually occurs on just one leg the opposite joint is also affected in more than 50 percent of horses with stifle or hock OCD.
Photography courtesy of Camilla Nock
Osteochondrosis (OC) is a failure in the process of bone formation from a cartilage framework. OCD stands for Osteochondritis Dissecans and specifically refers to those cases that develop fragments. Most commonly diagnosed by X-rays that show areas of flattening or fragmentation of the joint surface, lesions are thought to result from damage to the small blood vessels supplying this developing bone in the foetus or foal. Defects may be present in new born foals, or may develop in the first few months of life. It is also possible for lesions to repair and resolve without intervention, however by the time the horse is 12 months old further progression or repair is unlikely to occur.
Prevalence of osteochondrosis varies, particularly between different breeds. The most commonly affected breed appears to be warmbloods with as many as 67.5 percent of Dutch Warmbloods yearlings affected in one survey. This is significantly higher than the propotion of Thoroughbreds affected, whilst diagnosis of OC in ponies is rare. This supports the role of genetics in the development of lesions, and has driven selection programs like those implemented by the Royal Dutch Warmblood Studbook (KWPN). Candidate sires with evidence of osteochondrosis in the hocks (since 1984) and stifles (since 1992) were rejected for breeding, however this strict program did not lead to significant reductions in the incidence of osteochondrosis in the population so a grading scale has since been adopted, including screening of the first crop of offspring, alongside DNA testing. So why was this selection process unsuccessful? Firstly the sires were screeened as mature horses, and so those individuals that had lesions as foals that subsequently resolved will not have been detected, but may have carried genes associated with development of osteochondrosis. Secondly, recent genetic studies into osteochondrosis have shown a complex
A severe case of stifle OCD with mutliple fragments
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Polo Times, September 2020
A hock OCD with a large fragment
www.polotimes.co.uk
