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Hypertrophic Osteodystrophy

Anatomy
The ends of bones of growing puppies have a zone called the growth plate. The bone adjacent to the growth plate is soft and very developmentally active.

Cause
The cause of hypertrophic osteodystrophy in dogs is largely unknown. Proposed causes include distemper virus infection, vaccination with distemper virus, bacterial infection and other viral infections. Vitamin C deficiency is unlikely to be a cause of this disease, as previously believed.

With HOD, a series of events take place at the microscopic level within the affected bones. First, the blood vessels near the growth plate become distended and bleed into the bone. Next, the bone in this region dies, gets resorbed and develops microfracturing due to weakening of the bone structure (arrow in photo). In response to this, new bone is laid on the surface of the bone (star in photo).

Signs and diagnosis
Clinical signs of HOD include fever, anorexia and depression. Lameness is always seen with this disease and may vary from mild to severe. With multiple limbs affected, the patient may be reluctant to stand or walk. Typically, the ends of the long bones become thickened. The radius, ulna and tibia are most commonly affected; however, the ribs, jawbone and the bones in the paw can also be affected. Other clinical signs may include diarrhea, discharge from the eyes, tonsillitis, thickening of the footpads, pneumonia, and abnormal development of the enamel of the teeth.

This condition is commonly seen in rapidly growing large and giant breed dogs from two to six months of age. Common breeds affected include great Danes, boxers, German shepherds, and Weimaraners.

The diagnosis of HOD is based on the presence of supporting clinical findings on X-rays. A line of lucency shows destruction of the bone adjacent to the growth plates. Sometimes new bone production is also seen on the outside of the bones.

Occasionally the growth plate adjacent to the HOD lesion will be damaged, which will subsequently result in a bowing of the bone.

Treatment
Treatment is only supportive for HOD. When a patient goes through an acute phase of the disease, intravenous fluids are usually required to keep the patient hydrated. Nutritional support is provided with a feeding tube if the patient refuses to eat for five or more days. Pain is controlled with narcotics and nonsteroidal anti-inflammatories. Antibiotics are indicated if the patient has signs of pneumonia or other bacterial infections. If the bones become twisted due to growth plate damage, corrective surgery may be indicated. Because the distemper vaccination has been implicated, inoculation should be delayed until the pet has been in remission for a couple of months.

Results
Hypertrophic osteodystrophy is a self-limiting disease that can last for a few weeks per episode. Recurrence of the condition is expected in most affected dogs until the pet is 8 to 10 months of age. Patients that are severely affected by the disease and are unresponsive to treatment should be euthanized. In severe cases of HOD, dwarfism is a common sequel.

For more information on this subject, speak to the veterinarian who is treating your pet.