Hemangiosarcoma
Also known as: HSA, Canine hemangiosarcoma
Also known as: HSA, Canine hemangiosarcoma
In short
Hemangiosarcoma is a highly aggressive, malignant cancer of the blood vessels that commonly affects the spleen, heart, or skin in dogs and cats. Recognizing the signs of internal bleeding and understanding your diagnostic and treatment options can help you make informed decisions for your pet.

TL;DR. Hemangiosarcoma is a rapid, highly aggressive cancer of the blood vessels that primarily affects dogs, causing internal bleeding, sudden weakness, and requiring urgent veterinary intervention.

Hemangiosarcoma often presents with subtle signs of lethargy or weakness before a sudden crisis occurs.
Hemangiosarcoma (HSA) is a highly aggressive, malignant cancer that originates in the endothelial cells, which are the cells that line the inside of blood vessels. Because blood vessels are present throughout the entire body, this cancer can theoretically develop anywhere. However, it has a strong preference for specific organs that are rich in blood supply.
When these cancerous cells multiply, they form abnormal, fragile blood vessels that do not hold blood properly. These malformed vessels are highly prone to leaking or rupturing entirely. This leads to sudden, life-threatening internal bleeding. The spleen is the most common site for this tumor to develop, but it frequently affects other vital organs as well.
According to a leading veterinary internal medicine reference:
"Approximately 50% of the tumors originate in the spleen, 25% in the right atrium, 13% in subcutaneous tissue, 5% in the liver, 5% in the liver-spleen–right atrium, and 1% to 2% simultaneously in other organs (i. e. , kidney, urinary bladder, bone, tongue, prostate)."
While hemangiosarcoma is most frequently diagnosed in dogs, cats can also develop this disease. In both species, the cancer is characterized by its ability to metastasize (spread) to other parts of the body very early in the course of the disease, often before any external signs of illness are visible.
The exact underlying cause of internal hemangiosarcoma remains unknown. It is widely considered a multi-factorial disease where genetics play a primary role. There is no evidence that lifestyle, diet, or routine vaccinations influence the development of internal hemangiosarcoma.
However, for the cutaneous (skin) form of hemangiosarcoma, environmental factors play a clear role. Ultraviolet (UV) radiation from sun exposure is a major risk factor for dogs with light-colored skin and short coats. As noted in a standard veterinary dermatology guide:
"Solar damage may be involved in the development of tumors on the ventral glabrous skin in short-coated, lightly pigmented dogs (especially Whippets, Italian Greyhounds, Beagles, Great Danes, and English bulldogs), and on the head and ears"
Age is another significant risk factor. Hemangiosarcoma is principally diagnosed in middle-aged to older pets, with the risk increasing as they age.
The symptoms of hemangiosarcoma can vary dramatically depending on where the tumor is located and whether it has ruptured. Because the tumor grows silently inside the body, many pets show no signs of illness until the tumor ruptures and causes internal bleeding.

Pale or white gums are a hallmark sign of severe anemia and internal bleeding, requiring immediate veterinary care.
One classic hallmark of internal hemangiosarcoma is a "waxing and waning" pattern of weakness. A pet may suddenly collapse or become very weak, only to recover and act normally a few hours or a day later. This happens when a small rupture occurs, causing minor internal bleeding that stops as the body's clotting mechanisms temporarily seal the leak. The body then reabsorbs the blood, and the pet feels better—until the next, often larger, rupture occurs.
Diagnosing hemangiosarcoma requires a combination of blood tests, advanced imaging, and tissue analysis. Because the disease is highly aggressive, your vet will work quickly to determine the extent of the tumor and whether it has spread.
Your vet will start with a thorough physical exam and a complete blood count (CBC). A blood smear will be evaluated under a microscope to look for specific cellular changes. A leading veterinary internal medicine reference highlights these common blood findings:
"Most dogs with HSAs (83%) evaluated at the author's clinic are anemic; more than one half had RBC fragmentation and acanthocytosis. The pretreatment hemostasis profiles are normal in less than 20% of the dogs; most dogs (75%) have thrombocytopenia. Approximately one half of the hemostasis profiles meet three or more criteria for diagnosis of DIC."
These findings mean that the majority of affected dogs have damaged, fragmented red blood cells (schistocytes) and abnormally spiked red blood cells (acanthocytes) due to the physical trauma of squeezing through the tumor's malformed blood vessels. Furthermore, many dogs develop Disseminated Intravascular Coagulation (DIC), a catastrophic condition where the body's clotting factors are entirely consumed, leading to uncontrollable bleeding.
To locate the tumor and check for metastasis, your vet will utilize several imaging tools:

Abdominal ultrasound is a key diagnostic tool used to visualize masses on the spleen or liver.
While imaging and blood tests can strongly point to hemangiosarcoma, the gold standard for diagnosis is histopathology (biopsy). This involves sending the surgically removed tumor or a tissue sample to a veterinary pathologist for microscopic analysis. Fine-needle aspiration (FNA) and cytology (looking at cells collected via a needle) are often non-diagnostic because these tumors are highly vascular, meaning the sample is usually diluted with too much blood to find actual cancer cells.
Treatment for hemangiosarcoma is focused on stabilizing the pet, controlling bleeding, and slowing the spread of the cancer. Because this disease is highly aggressive, a multi-modal approach combining surgery and chemotherapy is typically recommended.
For tumors located on the spleen or skin, surgical removal is the first line of defense. A splenectomy (complete removal of the spleen) is performed to eliminate the primary tumor and prevent further life-threatening internal bleeding. However, because micro-metastasis (the spread of microscopic cancer cells) has almost always occurred by the time of diagnosis, surgery alone is rarely curative for internal tumors.
To combat the rapid spread of the cancer, postoperative chemotherapy is highly recommended. The primary chemotherapy drugs used for hemangiosarcoma include:
These treatments are designed to delay the progression of the disease while maintaining an excellent quality of life. Most dogs tolerate chemotherapy very well, with far fewer side effects than humans experience.
Pets presenting in crisis often require immediate stabilization before any long-term treatment can be considered. This includes intravenous fluid therapy, blood transfusions (to treat severe anemia), and medications to manage pain and control ventricular arrhythmias.
Unfortunately, the long-term prognosis for pets with internal hemangiosarcoma is poor. The tumor is highly aggressive and spreads exceptionally fast.
Without treatment, survival times are very short, typically ranging from 20 to 60 days from the time of diagnosis, with death usually resulting from a catastrophic rupture and internal bleeding.
If surgery is performed to remove the primary tumor and is followed by a complete course of postoperative chemotherapy, survival times can be prolonged to approximately 140 to 202 days. While some pets do exceed these averages, owners should prepare themselves for a limited timeline and focus on maximizing their pet's quality of life.
For cats, hemangiosarcoma is less common, and long-term prognosis data in this species is limited. However, the disease behaves similarly aggressively in felines, though cutaneous forms that are completely excised early can sometimes carry a more favorable outlook.
There is no known way to prevent internal hemangiosarcoma. Because the genetic factors are highly complex, there are currently no definitive genetic screening tests available for breeding dogs.
For the cutaneous (skin) form of the disease, prevention focuses on sun protection. You can protect high-risk dogs by:
For senior pets of highly predisposed breeds, routine screening—such as annual or bi-annual abdominal ultrasounds—can help detect splenic masses before they have a chance to rupture, allowing for controlled, elective surgery rather than an emergency crisis.
Hemangiosarcoma can cause sudden, catastrophic emergencies. You should seek immediate emergency veterinary care if your pet exhibits any of the following signs:
If your pet has already been diagnosed with hemangiosarcoma and is undergoing palliative care, these signs indicate a likely tumor rupture and require urgent veterinary assessment to ensure your pet's comfort.
While any dog can develop hemangiosarcoma, certain breeds carry a significantly higher risk.
German Shepherd Dogs, Golden Retrievers, and Labrador Retrievers are highly predisposed to the abdominal form of the disease. As noted in a leading veterinary reference:
"Abdominal hemangiosarcoma is principally found in older dogs, especially German Shepherd Dogs and Golden Retrievers."
Other breeds with suspected predispositions to internal hemangiosarcoma include English Cocker Spaniels, English Setters, Afghan Hounds, Siberian Huskies, Irish Setters, and Great Danes.
Greyhounds have a unique presentation of this disease, frequently developing tumors within the muscles of their hind limbs. As described in veterinary literature:
"Greyhounds with intramuscular HSA typically present with a swollen and bruised rear limb; the tumor is frequently in the biceps femoris or quadriceps."
For short-coated, lightly pigmented breeds such as Whippets, Italian Greyhounds, Beagles, Great Danes, and English Bulldogs, the risk is highest for the cutaneous (skin) form of the disease due to solar damage on their sparsely haired bellies and ears.
Treatment must be prescribed by a licensed veterinarian based on your pet. Specific drug doses are intentionally not shown here.
Hemangiosarcoma is a highly aggressive, malignant cancer of the blood vessels that commonly affects the spleen, heart, or skin in dogs and cats. Recognizing the signs of internal bleeding and understanding your diagnostic and treatment options can help you make informed decisions for your pet.
Anemia、Acute collapse、Hemoabdomen、Spontaneous bleeding、Thrombocytopenia、Weakness、Abdominal distention、Alopecia
Histopathology / Biopsy、3-view chest radiographs、Abdominal Ultrasound (AUS)、Blood smear、Echocardiography、Fine-needle aspiration (FNA) / Cytology
Treatment must be prescribed by a licensed veterinarian based on your pet. Specific drug doses are intentionally not shown here.
This article is for general education and is not a substitute for professional veterinary advice. If your pet is unwell, please consult a veterinarian.
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