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EQUINE VETERINARY EDUCATION / AE / MARCH 2016
a)
b)
Fig 7: a) Post mortem specimen of a 7.5 month-old-foal with a very severe OC lesion of the lateral trochlear ridge of the femur. The foals were known to have accidentally slipped at age 1 week. Upon radiographic examination at that age, only distension of the femoropatellar joint was visible. b) Radiographic re-examination at age 4.5 months showed a large radiolucent area with surrounding bone sclerosis (arrowheads) in the lateral femoral trochlea. (From Denoix et al. 2013.)
increasingly recognised as an important developmental orthopaedic disorder (McIlwraith 1986). Present-day prevalence in nonpreselected populations is reported as more than 60% (Wittwer et al. 2006; Van Grevenhof et al. 2009a). This huge increase in importance of OC over recent decades is concomitant with profound changes of the management of horses, especially sport horses.
Recent changes in the equine industry and horse husbandry Horses have grown considerably taller within a relatively short period. In the wild, there is no equid, from onager to zebra, which has an average height at the withers of over 16 hands (~1.6 m). The Dutch Warmblood horse has increased in average withers height with 1 mm/year over the past 25 years. In other breeds, such as the Andalusian (or Pura Raza Epa~
nola) this increase is even more. An increase of
1 mm may seem minimal, but it should be realised that the development from two-hands-tall (~20 cm) Hyracotherium or Eohippus to the Prezewalski horse that has an average height at the withers of 12–14 hands (~1.2–1.4 m) took 65 million years. If Eohippus had increased in height at the same rate as the Dutch Warmblood, the present-day horse would stand 634,600 hands (or slightly over 40 miles, 63 km) tall! Horses are seasonal breeders with foals typically being
born in spring, which in nature gives them the best chance to survive the critical early juvenile phase of life. By setting the official ‘birthday’ of horses at 1st January (1st August for the southern hemisphere), we have prompted a demand for early foals, as these will be older and thus on average
stronger in their age class, which is an advantage during the first years of competition. Such a policy has, however, consequences for the management of these foals. In many parts of the world, weather conditions in mid-winter do not permit keeping mares and foals outside for long periods of time. Forcibly, the animals will be confined to stables for most of the time, which severely hampers their possibility to exercise. Many studies have been conducted on the effect of exercise on the development of the equine musculoskeletal system (for a review see Birch et al. 2013) and as yet free pasture exercise for 24 h a day in moderately sized plots comes out as the optimal exercise condition for raising foals (Van Weeren and Denoix 2013).
© 2015 EVJ Ltd Obesity and related hormonal imbalances and metabolic
disorders such as diabetes are currently rampant and a serious human health concern in many parts of the world. The phenomenon is, however, on the increase in horses too. In a recent study a prevalence as high as 31% was reported (Robin et al. 2015). The causes are similar: excessive high- energy nutritional intake and insufficient exercise. Insulin resistance has been reported as a consequence (Carter et al. 2009), although the relationship with obesity does not seem to be straightforward in the horse (Bamford et al. 2015). Alterations in insulin metabolism may affect the process of endochondral ossification directly along the pathways discussed earlier. In some horse breeds selection pressure is high, as is the
case in the racing breeds and in Warmbloods; in other breeds, such as in most pony breeds and Coldbloods, selection pressure is much less. It may not be coincidental that the prevalence of OC is generally much higher in the breeds subjected to a high selection pressure, and those having a very narrow genetic base such as the Friesian horses, than in the breeds that are subject to much less intense, if any, selection pressure. Some of the traits we tend to select for, such as early maturity in the racing breeds and ever-taller horses in many Warmblood breeds, may well be related to the vulnerability for the development of certain manifestations of OC.
How to manage equine osteochondrosis in a practical sense? In practice, OC is commonly dealt with surgically through the arthroscopic removal of loose fragments and/or debridement of affected areas. The prognosis of these interventions is good to excellent for most affected joints (for a review, McIlwraith 2013). Further, the effect of minor lesions on ultimate performance has been shown to be minimal, if any (Robert et al. 2006; Verwilghen et al. 2013), which means that in some cases no intervention may carry a good prognosis as well. Overall, the disorder is manageable and direct clinical impact is limited. Nevertheless, often horses are operated for cosmetic and/or commercial reasons and the indirect economic and welfare impact remains, as well as the substantial loss of potential breeding stock.
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