CONDITION
Haemangiosarcoma
Haemangiosarcoma is a cancer that arises from the cells lining blood vessels. It tends to form tumours that are fragile and prone to bleeding, and it can develop in many parts of the body, most often the spleen, heart or skin, and less often the liver. Because the tumours are blood-filled and can rupture without warning, the first sign may be sudden collapse, weakness, or pale gums, rather than a gradual decline. Many owners arrive on this page after their dog has experienced an unexplained episode of weakness or collapse, or after a mass has been found on imaging or during an examination for something else. The condition can also present as a visible lump on the skin, particularly in areas exposed to sunlight. The internal form is more common in older dogs, and certain breeds, including Golden Retrievers, German Shepherds, and Labrador Retrievers, are seen more frequently. This page explores what an owner may observe, what is happening inside the body, how haemangiosarcoma is investigated and confirmed, and the range of approaches that exist once a diagnosis is made. It also considers the factors that shape outlook and the conversations that often follow.
Why this matters now
Haemangiosarcoma is an aggressive cancer of blood vessel cells, arising most often in the spleen, the right atrium of the heart and the skin in dogs. German Shepherds, Golden Retrievers and Labrador Retrievers are over-represented, though any breed can be affected. The internal form often grows silently, so diagnosis frequently comes at the point of a bleed rather than during routine care. Weakness or collapse with pale gums, a swelling abdomen or fast breathing in a middle-aged or older large-breed dog fits this pattern, and the interval between a small bleed and a larger one can be short.
Haemangiosarcoma typically develops as blood-filled masses within organs. These tumours are fragile and prone to rupture, causing internal bleeding. Early bleeds may be small and pause on their own, so a dog can collapse and then seem close to normal within hours, and a larger bleed can follow hours or days later. A large bleed into the abdomen can cause shock within minutes to hours, and bleeding into the sac around the heart can stop the heart filling properly just as quickly. How much blood has been lost, and whether bleeding has stopped, cannot be judged from how the dog looks at home; in the first hours even a blood count can look near normal. The cancer spreads early to other organs, particularly the liver, lungs and heart, so the outlook remains guarded even with treatment.
Signals & patterns
Early signals
Episodes of weakness or collapse that resolve
Small bleeds may cause weakness or collapse followed by apparent recovery as bleeding pauses and some blood is reabsorbed. Recovery does not mean the tumour has stopped bleeding: a further, larger bleed can follow within hours or days, and whether bleeding has stopped cannot be judged at home.
Lethargy and reduced stamina
Vague tiredness that may be attributed to age can reflect chronic low-grade blood loss.
Pale gums
Slow blood loss can cause gradual paleness of the gums. Pale or white gums alongside weakness or collapse point to significant blood loss or shock rather than an early change.
Distended abdomen
A large splenic mass can slowly enlarge the abdomen. Swelling that appears over hours, particularly with weakness or pale gums, is more consistent with blood collecting in the abdomen.
Later signals
Sudden collapse
A large bleed into the abdomen or around the heart can take a dog from apparently normal to unable to stand, with pale gums, cold legs and rapid breathing, within minutes to hours. Some dogs die from a single large bleed.
Rapid, weak pulse
Shock from blood loss causes cardiovascular signs.
Breathing difficulty
Pericardial effusion (blood around the heart) causes respiratory distress. Lung metastases may also affect breathing.
Irregular heart rhythm
Cardiac involvement can cause arrhythmias.
Click to read about the biological mechanisms
How this is usually investigated
Investigation often begins when a dog arrives weak or collapsed from internal bleeding but may also follow incidental findings. Staging assesses the extent of disease.
Abdominal ultrasound
Chest radiographs
Echocardiography
Blood tests
Histopathology
Options & trade-offs
Treatment decisions depend on tumour location, extent of disease, and the dog's overall condition. Options range from stabilising a dog that is actively bleeding to planned surgery and chemotherapy.
Stabilisation of active bleeding
Intravenous fluids, blood transfusion, and supportive care for dogs presenting in crisis.
Trade-offs: May be life-saving in the short term. Provides time for diagnostic evaluation and decision-making. Some dogs stabilise well enough for surgery.
Splenectomy
Surgical removal of the spleen when it contains the primary tumour.
Trade-offs: Stops active bleeding. Dogs can live normally without a spleen. Provides tissue for diagnosis. Median survival after surgery alone for splenic haemangiosarcoma is typically around one to three months because of metastatic disease.
Surgery with chemotherapy
Combining splenectomy with systemic chemotherapy to address micrometastases.
Trade-offs: Extends median survival to four to six months, with some dogs living longer. Chemotherapy is generally well tolerated.
Pericardiocentesis
Draining blood from around the heart to relieve tamponade.
Trade-offs: Can be life-saving when bleeding from a heart tumour is compressing the heart. May need repeating. Provides temporary relief but does not treat the underlying tumour.
Palliative care
Supportive treatment focused on comfort when neither surgery nor chemotherapy is pursued.
Trade-offs: Appropriate for advanced disease or when other treatments are declined. Focuses on quality of remaining time.
Surgical removal of skin tumours
Excision of haemangiosarcoma arising in the skin. The form confined to the skin itself typically develops on thinly haired, lightly pigmented skin such as the belly in sun-exposed, short-coated breeds including Whippets, Greyhounds and Dalmatians.
Trade-offs: Tumours confined to the skin itself (the dermis) behave far less aggressively than the internal form, and removal alone can be curative, with reported median survival of around two years. Tumours extending into the tissue beneath the skin carry a more guarded outlook and may be staged and treated more like the internal form.
Common misconceptions
"A mass on the spleen means cancer"
The likelihood depends on whether the mass has bled. In studies of dogs with a splenic mass and bleeding into the abdomen, roughly 60 to 80 per cent of masses proved malignant, most often haemangiosarcoma. When a mass is found incidentally and has not bled, around two-thirds or more prove benign, such as haematomas or nodular hyperplasia. Imaging cannot reliably tell benign from malignant masses; histopathology after removal gives the diagnosis.
"Removing the spleen cures haemangiosarcoma"
Surgery removes the primary tumour but microscopic spread has usually already occurred. Chemotherapy helps extend survival but cure is rare.
"The collapse means earlier signs were missed"
Haemangiosarcoma often causes no obvious signs until it bleeds. This reflects the nature of the disease rather than missed opportunities. Some dogs do show subtle changes, such as tiredness, that seem related only in retrospect.
Related conditions
Splenic Mass
A splenic mass discovered on imaging or during examination may be a haemangiosarcoma, though other tumour types and benign conditions can also present as masses in the spleen. Because haemangiosarcoma of the spleen is prone to rupture and bleeding, the two pages often describe overlapping presentations, particularly sudden weakness or collapse, and similar diagnostic pathways.
Cardiac Tamponade
Haemangiosarcoma arising in or near the heart can rupture and bleed into the pericardial sac, leading to cardiac tamponade as fluid accumulates and compresses the heart. This is one of the ways haemangiosarcoma may first become apparent, presenting as sudden collapse or weakness rather than a visible mass.
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Pericardial Effusion
Right atrial haemangiosarcoma is the most common tumour behind bleeding into the sac around the heart in dogs. Weakness, pale gums and fast breathing can come from either abdominal or pericardial bleeding, and the two can be reliably told apart only with imaging.
Chemodectoma (Heart Base Tumour)
Chemodectoma is the other main tumour causing pericardial effusion in dogs, more common in brachycephalic breeds. Echocardiography helps distinguish it from cardiac haemangiosarcoma by the tumour's position, though not always with certainty.
Anticoagulant Rodenticide Toxicity
Anticoagulant rat poison can cause bleeding into the abdomen, chest or around the heart, with the same weakness, pale gums and collapse. Clotting tests and imaging usually separate the two, and possible access to rodenticide matters because this form of bleeding responds to vitamin K treatment.
Gastric Dilatation-Volvulus (GDV) in Dogs
In a large-breed dog, a swollen abdomen with weakness or collapse can reflect either bleeding from a splenic tumour or a twisted stomach. Unproductive retching and a tight, drum-like abdomen fit GDV more closely, but the two cannot be reliably told apart without examination and imaging.
A splenic mass found incidentally, without bleeding, is more often benign than malignant, but its nature cannot be determined from imaging, and a mass that has not yet bled can still rupture. Periodic abdominal ultrasound is sometimes suggested for older dogs of higher-risk breeds; it has not been shown to improve survival and can miss small tumours, so it is a question of trade-offs rather than settled practice. Weakness or collapse with pale gums, a swelling abdomen or rapid breathing marks the bleeding form of the disease, and a dog that recovers from such an episode can bleed again. While histopathology is awaited, outlook depends on whether the mass proves to be haemangiosarcoma, another cancer or a benign lesion.
Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS