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My Portuguese Water Dog, Fin, was still an active dog at 12 years old. We took 2-mile walks daily, visited a local elementary school each week where children read to him, and made monthly visits to memory care patients. One day, very suddenly, Fin became unresponsive and confused. After a few minutes, he was back to normal until the same thing happened again several days later. What was going on with him?

Fin’s veterinarian took a blood sample, which showed an unusual pancreatic enzyme count. An abdominal X-ray revealed a suspicious shadow. The next step was an ultrasound that showed a mass on Fin’s spleen, a nodule on his liver, and evidence of hemorrhaging in his abdomen. Within a few hours of our morning walk around the lake, we had received a hemangiosarcoma diagnosis. What was it, and what did it mean for Fin?

What Is Hemangiosarcoma in Dogs?

Hemangiosarcoma (HSA) is a highly invasive canine cancer. This cancer causes blood vessels to branch, fragment, become leaky, and ultimately rupture. It’s the cause of about two-thirds of heart and splenic tumors, with metastasis (secondary malignant growths) affecting the liver, lungs, lymph nodes, and bones. A less common form of hemangiosarcoma affects the skin.

Hemangiosarcoma is known as a “silent killer” because dogs often show no symptoms until the tumor has grown so large that it ruptures and spreads—too late to save the dog’s life. Hemangiosarcoma is the cause of approximately 300,000 dog deaths in the U.S. annually, representing between 5% and 7% of the approximately 6 million canine cancers newly diagnosed each year.

These episodes of weakness are often so brief that they’re easy to miss. In an HSA report for the American Kennel Club Canine Health Foundation (CHF), Dr. Michelle Ritt notes that even large hemangiosarcoma tumors may show no signs of the life-threatening disease. The clinical signs are recurrent, but they also are subtle enough to go unnoticed for some time.

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“Dogs with the most common splenic form of HSA will usually arrive at an emergency clinic having suddenly collapsed and with pale gums due to internal bleeding from a previously undetected and now ruptured tumor,” says Kelly Sams, lab manager at the Coonrod Lab at Cornell College of Veterinary Medicine’s Baker Institute for Animal Health. Internal bleeding sometimes results in sudden death.

By the time a dog shows any symptoms of this cancer, according to Sams, the disease has progressed and usually spread. Symptoms include loss of appetite, ataxia (instability), fatigue, weakness, and pale gums. There can be little or no sign of the disease before the dog may suddenly collapse or even die.

Are Some Dogs More Susceptible?

The cause of HSA is not yet known, but there may be a genetic component since it is more commonly diagnosed in certain breeds. Gene studies from dogs with hemangiosarcoma are identifying mutations, with one study showing the most significantly mutated gene to be tumor suppressor TP53, which may appear in certain breeds. According to Dr. Jaime Modiano at the University of Minnesota College of Veterinary Medicine, breeds like Golden Retrievers, Portuguese Water Dogs, Boxers, and German Shepherd Dogs are considered to be at higher risk. “Any dog of any breed, including mixed breeds, and of either sex, regardless of whether it is intact or neutered, can develop hemangiosarcoma,” says Dr. Modiano.

German Shepherd Dog feeling tired laying down indoors.
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Although HSA is much more common in senior dogs aged between 8 and 12 years, Cornell’s Baker Institute found dogs as young as 2 and as old as 15 were being diagnosed with HSA. “Why dogs are particularly susceptible is unknown, but hemangiosarcoma of the internal organs is very rarely seen in livestock like horses, cows, and goats,” says Sams. On rare occasions, HSA can occur in cats.

Hemangiosarcoma in Dogs: Diagnosis, Treatment, and Prognosis

There are no easy lab tests that can diagnose hemangiosarcoma. X-rays and ultrasounds will show the size and location of a mass but won’t definitively tell you if your dog has cancer. “Right now,” says Sams, “the only way to truly diagnose HSA is to surgically remove the affected tissue and send it to a pathologist.” But doing that must be done by a veterinary surgeon, as it is complicated and possibly dangerous.

After Fin’s ultrasound, the veterinary radiologist explained that since the mass he saw in the spleen had bled internally and spread to the liver, the cause was almost certainly hemangiosarcoma. But he couldn’t say for sure. “Dogs with no identifiable metastases at the time of diagnosis that are treated aggressively with chemotherapy and radiation may live up to six months longer. The costs of these treatments can easily surpass $6,000 to $10,000,” says Sams. Since Fin’s cancer had already metastasized, chemo was not a good option for him and won’t be for every dog with HSA.

When a dog is diagnosed with hemangiosarcoma, and it’s too late to treat it, studies show the dog is most likely to live only a few more weeks. “Survival times usually do not exceed one year, even with surgical and chemotherapeutic treatments,” says Sams. I lost Fin within three weeks.

Research for Early Detection

There are no known preventatives or screening tests available for canine hemangiosarcoma. New, effective diagnostic tests are urgently needed to catch HSA at an earlier stage and allow for better treatment. But research is being done to learn more about HSA.

Golden Retriever getting comforted while lying on a table at the vet.
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  • Cornell’s Coonrod Lab received a grant from the AKC Canine Health Foundation to develop ways to identify HSA and differentiate it from non-life-threatening conditions. This could make earlier detection possible and help dog owners make more informed decisions about what treatments to try based on the dog’s chance for long-term survival. Screening of “at-risk” breeds at annual wellness checkups could detect HSA in the early stages, allowing surgery on the spleen while the dog is stable (and before the cancer has metastasized).
  • Tufts Cummings School of Veterinary Medicine is researching a blood test for early detection and working to identify which genetic changes may drive resistance to therapy.
  • A clinical trial at the University of Pennsylvania School of Veterinary Medicine will examine the safety and efficacy of a chemotherapy drug called Copanlisib (used to slow the growth and spread of human cancers).
  • The Shine On Project at the University of Minnesota College of Veterinary Medicine is conducting research to develop a simple and reliable test that can be used to predict the risk of dogs developing hemangiosarcoma, as well as other types of cancer. They are also examining a possible way to intervene in the development of HSA to prevent or delay its onset.
  • The Morris Animal Foundation is conducting a lifetime study of Golden Retrievers to investigate risk factors for cancer and determine the frequency of diagnosis of four types of cancer, including hemangiosarcoma, lymphoma, osteosarcoma, and high-grade mast cell tumors. After 10 years of research, data shows that 75% of study dogs’ deaths are cancer-related. Of those cancer deaths, almost 70% of the canine patients had hemangiosarcoma.

This article is intended solely as general guidance, and does not constitute health or other professional advice. Individual situations and applicable laws vary by jurisdiction, and you are encouraged to obtain appropriate advice from qualified professionals in the applicable jurisdictions. We make no representations or warranties concerning any course of action taken by any person following or otherwise using the information offered or provided in this article, including any such information associated with and provided in connection with third-party products, and we will not be liable for any direct, indirect, consequential, special, exemplary or other damages that may result, including but not limited to economic loss, injury, illness or death.