Hyper (meaning excessive) and trophy or tropic (refers to growth), so the name describes an abnormal and excessive growth of bone (os) in specific locations.
There is swelling and thickening at the ends of long bone shafts. There is also swelling of the surrounding soft tissue. It is accompanied at times with fever, loss of appetite, inability or reluctance to move, severe pain. In fact the pain may occur in all 4 limbs at the same time making the dog reluctant to move at all. There is hyperextension of joints and sometimes angular deformities in the bones of lower limbs, such as bowing of the foreleg below the elbow, turned out front feet (east/west). There may even be depression and sometimes pain in the jaw as well. Diarrhea may also precede any episode. The dog may display partial paralysis in the rear, very weak (slanted) pasterns with splayed feet and swollen wrists or foreleg joints. When the dog stands, he/she may stand with back arched and all 4 limbs tucked under the body. Symptoms may recur throughout growth cycles.
Age of onset is usually between 2 to 7 months. It can affect both males and females, but is more common in males.
There is no known cause although there are several thoughts of what may have a contributing factor : Canine distemper virus is associated with this disease, excessive mineralization takes place which results in calcium deposits at the affected sites. It appears that like panosteitis, early rapid growth rate is also a factor just as it is in hip dysplasia. Nutritional imbalance is also considered to be a factor in this disease.
Correct diagnosis should be made with x-rays. As young bones grow, the end sections are continually changing in composition between cartilage and bone. A short distance from the end of the bone is a transverse line of cells known as a growth plate. In order to make the bone grow in length, cartilage near the end of the bone shaft is replaced with bone cells while bone in the epiphysis (end) is transformed to cartilage at the growth plate (metaphyseal). Cartilage on the far end of the epiphysis (end of bone) ossifies, and is added to by simple cell-division growth. The greatest growth changes occur in the lower ends of the long bones of lower legs where growth is more rapid. The periosteum is the tough, smooth, elastic white covering of bones and it serves as a point of attachment for other connective tissue such as ligaments, tendons and cartilage and muscles to attach to.
Treatment should consist of controlling the pain foremost. Secondly, correcting any nutritional imbalance is of utmost importance. Do not supplement puppies with calcium as this only makes the situation worse or can bring on this disease.
Those that have studied HOD have concluded that it is probably a metabolic disorder. An imbalance of minerals, protein, and vitamins interferes with normal deposition of calcium phosphate (bone) and leads to the physical and visual changes causing excess calcium deposits. It is also thought that because the livers of young pups become stressed by the vaccines against distemper, hepatitis, and other viral disorders, and it is the liver where most of the dog’s Vitamin C comes from, the shots or vaccines temporarily disrupt the natural synthesis of Vitamin C. The larger, faster growing breeds require more ascorbate/vitamin C than their livers can keep up to the demand of because of the enormous demand of collagen production needed. It would be very wise not to over feed or supplement with Vitamins A or D or calcium. Excessive calcium supplementation can exacerbate the pain and will make the HOD worse. Keep puppies on the thin side. Use a high quality, but not high energy (calorie) food. The dog with HOD may even have a copper deficiency, a diet too high in protein and calories, a microbial infection, a challenged immune system, or any combination. Since HOD seems to come in waves, it stands to reason that there may be a multiple cause of HOD.
Medical management of HOD should be to stop the diarrhea, relieve the pain, lower the fever, and possibly get rid of any parasites.
Like Panosteitis, HOD will correct itself over time and symptoms usually stop by the time the dog reaches 2 years of age. It is traumatic because of the pain it causes the puppy and the owner’s feeling of helplessness and frustration.
You should be aware that some dogs are left with permanently bowed forelegs because of the fact that the ulna has grown at a different rate than the radius. Some dogs are also cow-hocked for life. Most however, survive the effects of HOD without permanent damage.