Developmental Bone Disorders
Developmental orthopaedic disease (or DOD for short) is the name given to a group of bone growth disorders that commonly occur in young horses. The DOD group of disorders all result from defects in normal bone formation as young horses grow and develop. The breeds most commonly affected are those that are large, fast-growing, light horse breeds, particularly the standardbred and the thoroughbred. DOD lesions are very common and cause a significant economic loss to the horse industry. Lesions are reported to be present in 20-25% of Thoroughbreds, with a quarter of these occurring in the vertebrae in the neck and the remainder in the leg bones. However only 5-10% of the lesions are severe enough to cause obvious clinical signs or X-ray changes. Surveys in Standardbreds have shown that the incidence may be even higher with one X-ray study finding that only 25% of the young horses studied did not have DOD lesions of some degree!
NORMAL BONE DEVELOPMENT
Development of the skeleton of a foal starts during the first month of pregnancy. Initially the skeleton is composed of cartilage, however as the unborn foal develops this cartilage is slowly converted into bone. This conversion of cartilage to bone initially occurs in the central shaft of the bone, followed by the ends of the bone, which are also called the "epiphyses". By the time the foal is born almost all the cartilage has been converted to bone. However, in each long bone in the limb there are two areas where growth cartilage remains - in the growth plates at each end of the bone shaft, and a layer under the articular (joint) cartilage at the end of the bones.
After birth the foal's limbs continue to lengthen for at least 18 months and this occurs from the growth plates. The cartilage cells in the growth plates multiply rapidly and are pushed towards the main shaft of the bone where they gradually change in formation, produce bone matrix, calcium and phosphorus is laid down and mature bone is eventually formed. In this way the bone lengthens and develops. Eventually the growth plates close and growth ceases and cartilage only remains at the end of the bones where they form joints. This process is known as "endochondral ossification".
It is when this process of cartilage conversion to bone goes wrong, that DOD problems develop. Abnormal bone stress or loads may disrupt the blood supply to the cartilage and prevent its conversion to bone. Thickened areas of cartilage can alter the rate of bone growth leading to limb deviations and bent legs. Abnormal areas of thickened cartilage can be left within the bone ends or if a joint surface is involved, a piece of abnormal cartilage may separate from the bone and float free in the joint cavity, causing irritation and eventually arthritis.
TYPES OF DOD
There are a number of different types of DOD lesions in young horses. It is thought that specific joints have "windows of vulnerability" when they are particularly sensitive to damage.
Physitis (or Epiphysitis)
Physitis is due to thickened growth cartilage in the growth plates of bones - it is most common in the vertebral bones of the neck and at the top of the knee. It can also occur above the hock and fetlock. Often all the limbs are affected to some degree. In the case of the knee the disorder is usually most obvious at 12-18 months of age and it gives the knee a "dished-in" appearance in front. It is often referred to as "open knees". When the fetlock is affected it is usually most evident at 5-9 months and results in enlarged fetlocks which may develop an "hour glass" appearance.
Affected horses may not be lame, or may only have mild and intermittent lameness or stiffness. Severe cases may have warmth and pain when the joints are palpated and affected foals may not play as much as their paddock mates, spending more time lying down. The condition is usually self-limiting and disappears as growth slows or the growth plates close unless bone cysts develop.
OCD (Osteochondritis Dissecans)
OCD occurs when development problems occur in the cartilage adjacent to a joint. The abnormal cartilage may crack when placed under physical stress and flaps may protrude into the joint or may even break off and float free in the joint. The cartilage fragments rub on the cartilage of the opposing bone and can cause juvenile arthritis or degenerative joint disease with chronic lameness. The sites most commonly affected are the stifle and hock and less commonly the fetlock and shoulder. Typically the joints are distended and there may be mild lameness which becomes more severe once the horse begins work. In the hock OCD can cause bog spavin without lameness.
Bone cysts are similar to OCD lesions however instead of cartilage cracking on the edge of the joint surface the defect may occur deeper within the end of the bone. Joint fluid can leak into this cavity resulting in a fluid filled bone cyst. Bone cysts are most common in the stifle, knee, fetlock, pastern and hock. Bone cysts are usually not discovered until after young horses are put into work when they may or may not cause lameness. They may be an incidental finding when X-rays are taken for other reasons
Angular Leg Deformities
Foals can be born with angular leg deformities ("bent legs") or they can develop over weeks or months. Those that develop after birth are often the result of DOD problems such as physitis. Abnormal cartilage development can slow bone growth and if this occurs unevenly, with one side of the leg growing slower than the other, leg deviation may occur. Angular deformities are most common in the knee followed by the hock. The angular deformity may also be due to collapse of the small bones within the knee or hock due to a defect in bone development.
Swelling and developmental changes in the vertebrae of the neck can lead to narrowing of the vertebral canal and compression of the spinal cord. This in turn can cause the typical symptoms of wobbler syndrome where the horse becomes inco-ordinated, particularly in the hind limb. Wobbler syndrome can be caused by anything that damages the spinal cord in the neck region but developmental bone disorders are the most frequent cause in horses less than 4 years old. Signs are usually first seen when the horse is put into work and are often triggered by a traumatic incident such as pulling back when tied-up or rearing and falling down.
CAUSES OF DOD The cause of DOD has not been fully determined but appears to be multifactorial, with the major factors being rapid growth rates, trauma or stress on the growth plates or joint cartilage, a genetic predisposition and nutritional imbalances. The presence of two or more of these factors significantly increases the risk of DOD.
Rapid growth seems to be one of the major factors in the development of DOD. The larger the body size and the faster the growth rate the greater the risk of DOD developing. The high-risk period for the development of bone and joint related abnormalities appears to be from 3 to 9 months of age. This correlates with the peak of lactation in the mare, through weaning to yearling age. During this time growth rates can easily be boosted above average by a combination of overfeeding and inadequate exercise.
A genetic predisposition to DOD has been shown in humans, dogs, pigs and horses. Certain stallions have been shown to produce progeny with a higher incidence of DOD problems, even though the stallion himself may not be affected. As well as a direct genetic predisposition to DOD there can also be an indirect predisposition to the disorder in certain families. These families may have the genetic potential to grow rapidly to heavy body weights, or they may have fine bones or conformation faults, all of which increase stresses or pressure on the growth cartilage.
Trauma to the Growth Plates
Exercise is beneficial for the optimum development of the muscloskeletal system. However, too much exercise can also contribute to the development of DOD. This is particularly so when there is a sudden increase in exercise leading to stress on the developing bone. If a foal is confined to a stable or small yard for any reason, the bone formed during the period of confinement may be weaker than bone formed when adequate exercise is available. The weaker bone may not be able to withstand the stress of normal, exuberant foal exercise when the foal is turned out into a larger area. Even confinement for a few weeks can lead to production of weaker bone.
After any period of confinement greater than a week or so, the re-introduction of exercise should be done gradually. It is not unusual to see foals that move stiffly, tremble at the knees when standing and lie down more than normal when they are adapting to exercise in a larger paddock. Heavier, faster growing young horses will place greater stress on their developing bones than their leaner, slower growing paddock mates.
· Too much energy
High-energy diets lead to faster growth rates and larger body sizes. High-energy diets fed to weanlings have been shown to consistently produce DOD lesions. Feeding excessive amounts of grain to weanlings or allowing access to unlimited amounts of lucerne are two feeding practices that can result in too much energy in the diet. Apart from being high in energy grain is also low in calcium and high in phosphorus which can lead to imbalances in these important nutrients in addition to high growth rates.
· Calcium and Phosphorus Imbalance
Adequate amounts of calcium and phosphorus are needed for cartilage to be converted to bone. Without adequate amounts the cartilage can become thickened, bone density and growth can decrease and DOD problems can develop. Young horses can tolerate high levels of calcium provided that phosphorus and energy levels are not excessive. However low levels of calcium, particularly when there are high levels of phosphorus and energy in the diet (eg. from too much grain), are a recipe for DOD.
· Copper and Zinc
A copper deficiency is known to increase the risk of DOD as copper is involved in stabilising bone development. Foals fed low copper levels had three to four times more DOD lesions than those fed adequate levels of copper. It is also important to note that foals from mares that were fed low levels of copper during pregnancy also have a higher incidence of DOD. It is therefore important to supplement mares during pregnancy, at least during the last 3 months
Excess zinc has been shown to decrease calcium absorption leading to a calcium deficiency in foals grazing pastures contaminated with residue from nearby zinc smelters. Adequate but not excessive levels of zinc should be included in the diet of growing horses.
MANAGEMENT MEASURES TO AVOID DOD IN GROWING FOALS
1. Aim for steady growth patterns - avoid sudden growth spurts and "catch up" growth periods, particularly after periods of confinement when exercise has been reduced
2. Ensure young foals, weanlings and yearlings receive balanced mineral nutrition with an adequate and balanced intake of calcium, phosphorus, copper, zinc, magnesium, selenium, iodine and the vitamins A, D and E. A calcium and trace mineral supplement such as CAL-PLUS can be added to growing horse rations to supplement inadequate calcium levels in grain or cereal hay based diets. Additionally a balanced vitamin and mineral supplement such as Feramo-H will provide over 50% of the recommended daily intake of a whole range of other vitamins and trace minerals including copper, to top-up the natural levels the horse can obtain from its basic ration. This will minimise the chance that there is an underlying trace nutrient deficiency in the diet
3. Do not forget the mare's nutrition - particularly during the last 3 months of pregnancy. Deficiencies in mineral intake during this time can lead to deficiencies in the unborn foal. CAL-PLUS and Feramo-H are both suitable as supplements for the pregnant and lactating mare.
4. Avoid high-energy diets in any form, whether the energy comes from grain, lucerne or fat. It is better to aim for slow, steady growth rates rather than to feed high energy diets to achieve overgrown young horses.
6. Avoid breeding from horses with a family history of DOD
7. If a young horse shows signs of DOD it is recommended that the growth rate be slowed immediately by decreasing the dietary energy and protein intake. Exercise should also be restricted to limit trauma and stresses on the growth plates and the joint cartilage - your vet can advise on the degree of confinement that is appropriate for the particular form of DOD. Feed a good quality grass or cereal hay to reduce energy and protein intake and slow the growth rate. Calcium, phosphorus and other trace minerals (eg CAL-PLUS) should be supplemented in a small amount of palatable feed to bolster inadequate levels in the grass hay. Many horses will show improvements within 4-6 weeks of diet and exercise restriction. Some DOD lesions may require surgical correction so your vet should always be consulted.
Document Contents © Leverton & Co. 2007