Hemangiosarcoma is a malignant cancer of the cells that form blood vessels. Because these tumors start in blood vessels, they are frequently filled with blood. Consequently, when a blood-filled tumor ruptures, it can cause problems with internal or external bleeding. Hemangiosarcoma is considered to be a very aggressive tumor and can spread rapidly to other organs.Hemangiosarcoma is more common in dogs than in cats. It usually occurs in middle-aged to older dogs 9 to 11 years of age, and German shepherds appear to be predisposed to developing this cancer.The most common primary location of this cancer in dogs is the spleen. Other primary locations include the heart, liver, skin, and bone; however, it can start in any location where blood vessels are present. These tumors usually spread to the lungs, liver, spleen and heart.

Signs of this disease are usually the result of the tumor rupturing, which causes bleeding. This may occur without any warning, and the symptoms will depend upon where the tumor is located.

• When the tumor is located in the spleen or liver, the signs are usually due to rupture of the tumor and subsequent bleeding into the abdomen. This causes anemia, weakness and if the bleeding is severe, collapse. The gums may appear to be pale or white. In relatively few animals the diagnosis is made before the tumor ruptures.
• When this type of tumor occurs in the heart, it can cause symptoms, such as weakness, collapse, difficulty breathing, exercise intolerance and fluid build-up in the abdomen. This is usually due to the development of fluid around the heart, called pericardial effusion. A thin sack, called the pericardium, normally surrounds the heart. In pericardial effusion the sack fills up with fluid and prevents the heart from beating effectively. With hemangiosarcoma, the pericardium fills up with blood due to rupture of the tumor.
• When the tumor occurs in the skin, a mass or lump can usually be felt in or under the skin. The mass may become ulcerated and bleed. When the tumor occurs in a bone, it can cause pain and discomfort. In some locations, such as a rib, the tumor can be felt as a firm swelling in the bone. What to Watch For
• Anemia, weakness and collapse
• Pale or white gums
• Difficulty breathing
• Exercise intolerance
• Fluid build-up of the abdomen
• Mass or lump under the skin
• Swelling in the bone
• Pain in the bones