Hypertrophic Osteodystrophy (HOD) is a bone disease that occurs most often in fast-growing large and giant breed dogs; however, it also affects medium breed animals like the Australian Shepherd. The disorder is sometimes referred to as metaphyseal osteopathy, and typically first presents between the ages of 2 and 7 months. HOD is characterized by decreased blood flow to the metaphysis (the part of the bone adjacent to the joint) leading to a failure of ossification (bone formation) and necrosis and inflammation of cancellous bone. The disease is usually bilateral in the limb bones, especially the distal radius, ulna, and tibia. The Weimaraner, Irish Setter, Boxer, German Shepherd, and Great Dane breeds are heavily represented in case reports of HOD in the veterinary literature, but the severity of symptoms and possible etiology may be different across the breeds. For example, familial clustering of the disease has been documented in the Weimaraner, but not in other breeds. The disease in the Weimaraner and Irish Setter can be particularly severe, with significant mortality observed in untreated dogs. The classical age of onset is typically 8 to 16 weeks of age, with males and females equally affected. Speculated causes of HOD Causes have been speculated to include decreased Vitamin C uptake, increased vitamin (other than C) and mineral uptake, and infection with canine distemper virus(CDV). Decreased Vitamin C uptake has been dismissed as a cause, but excessive calcium supplementation remains a possibility. There is no evidence over-feeding is a significant cause. In Weimaraners, recent vaccination with a modified live vaccine has been a suspected cause, partly because HOD often presents immediately after a vaccination, and partly because of the autoimmune nature of the disorder. The canine distemper vaccination in particular has been a suspected causal factor due to the significant number of overlapping symptoms observed between systemically affected HOD puppies and dogs suffering from distemper, but to-date, no definitive linkage has been demonstrated. The cause of canine HOD largely remains unknown. However, because of the familial clustering, HOD in the Weimaraner is suspected to have a genetic, or partly genetic, origin. Clinical Features A primary characteristics of the condition is lameness. This is generally due to the swelling of the metaphysis of the long bones that is observed. Other bones may be affected, particularly the ribs, the metacarpal bones, the mandible, and the scapula. Lameness may present as mild limping or more severely as