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Hypertrophic osteopathy is a bone disease secondary to disease in the lungs. It is characterized by new bone formation on the outside of the diaphyses of long bones of the limbs, without destruction of cortical bone. Symptoms include stiffness and warm, firm swelling of the legs, and signs of lung disease such as coughing and difficulty breathing. Hypertrophic osteopathy differs from the condition in humans in that in dogs it is usually caused by lung tumors or infections such as Mycobacterium fortuitum or Corynebacterium. The most common cause in humans is congenital cyanotic heart disease. Other potential causes in dogs include heartworm disease, heart disease, and pulmonary abscesses. It has also been associated with nonpulmonary diseases such as renal tumors and rhabdomyosarcoma of the bladder. At least once it has been caused by congenital megaesophagus in a six year old dog. Hypertrophic osteopathy is rare in cats.One theory is that hypertrophic osteopathy is caused by increased blood flow to the ends of the legs, overgrowth of connective tissue, and then new bone formation surrounding the bones. This is secondary to nerve stimulation by the lung disease. The condition may reverse if the lung mass is removed or if the vagus nerve is cut on the affected side.Interestingly, similar bone lesions are seen in dogs that develop hepatozoonosis, an infectious disease carried by ticks and caused by Hepatozoon americanum.
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