A blood blister on a dog typically looks like a raised, fluid-filled bump under the skin that appears dark red, purple, or bruised. It can range from a small, soft swelling to a large, puffy mass depending on how much blood has pooled beneath the surface. Most owners notice them suddenly and wonder whether what they’re seeing is harmless or something more serious.
How Blood Blisters Look on Dogs
Blood blisters form when small blood vessels burst and blood collects under the skin. On a dog, this creates a soft, squishy swelling that may look red, dark purple, or bluish. The skin over the blister can appear stretched and shiny, and the surrounding area often shows bruising or discoloration. Some blood blisters stay small, while others grow large enough to distort the shape of the skin around them.
Unlike a regular bump or cyst, a blood blister often feels warm to the touch and may be tender when you press on it. If it ruptures, you’ll see bleeding from the surface. The color is the most telling feature: while ordinary cysts or fatty lumps tend to match your dog’s skin tone, blood blisters have that distinctive dark, bruise-like appearance because they’re filled with trapped blood rather than fluid or fat.
Where They Show Up Most Often
One of the most common spots is the ear flap. Called an aural hematoma, this happens when blood pools between the layers of cartilage in the ear, making it swell up like a pillow. The ear may droop, feel warm, and look red. It can be a small soft swelling or a large one that covers the entire ear flap. Dogs that scratch their ears aggressively or shake their heads hard, often because of an ear infection or allergies, are especially prone to these.
Blood blisters also appear on the body, legs, belly, and in skin folds like the armpit or groin. Blisters in areas where limbs move, such as between the legs, can cause noticeable pain or limping. On the belly or trunk, they may be harder to spot under fur until they grow large enough to feel during petting or grooming.
Common Causes
The simplest cause is trauma. A bump, a bite from another animal, or rough play can rupture small blood vessels under the skin and create a pocket of blood. Chronic scratching from allergies, flea infestations, or skin infections can do the same thing, especially on the ears. Dogs with thin skin or those on medications that affect blood clotting are more vulnerable.
Some blood blisters also develop without any obvious injury. In these cases, the swelling may be related to abnormal blood vessel growth in the skin. Benign vascular tumors called hemangiomas can look exactly like blood blisters. They may ulcerate or rupture within the skin and cause bleeding, but they don’t spread to other parts of the body. These are more common in dogs with light-colored or sparsely furred skin, particularly on sun-exposed areas.
When a Blood Blister Could Be Something Serious
This is the part worth paying close attention to. A type of cancer called hemangiosarcoma can look almost identical to a harmless blood blister. According to Cornell University’s College of Veterinary Medicine, skin hemangiosarcoma may appear as bruising, what looks like a “blood blister,” or a more well-defined raised mass that can rupture and bleed. If the mass sits deeper, under the skin and in the muscle, you might not see external bleeding, but the area will look more swollen or bruised than a typical surface blister.
The challenge is that you cannot reliably tell the difference between a benign blood blister and a cancerous mass just by looking at it. Both can be dark, raised, and prone to bleeding. There are a few patterns that should raise your concern:
- Rapid growth. A blister that gets noticeably bigger over days or weeks rather than staying the same size or shrinking.
- Recurring bleeding. Blisters that rupture, seem to heal, then fill with blood again.
- Multiple blisters. Several blood-blister-like spots appearing in different locations.
- Systemic signs. Weakness, reluctance to walk, pale gums, or collapse alongside the skin changes. These can indicate internal bleeding from a more advanced vascular tumor.
Any new lump or blood-filled swelling on your dog that doesn’t resolve within a few days warrants a veterinary exam. A vet can use a fine-needle sample or biopsy to determine whether the cells are normal blood-filled tissue or something that needs treatment. This is especially important for older dogs, since vascular tumors become more common with age.
What to Do When You Find One
If the blister is small and your dog isn’t bothered by it, keep it clean and watch it closely for changes in size, color, or texture. Don’t try to pop or drain it at home. Puncturing a blood blister introduces bacteria and can lead to infection, and if the underlying cause is a blood vessel abnormality, the bleeding may not stop easily.
For ear hematomas, treatment usually involves draining the collected blood and addressing whatever caused the scratching or head-shaking in the first place. Without treatment, the ear can heal on its own, but it often scars into a thickened, crinkled shape sometimes called “cauliflower ear.” If your dog’s ear flap suddenly balloons up, getting it treated early gives the best cosmetic and comfort outcome.
For blood blisters on the body, your vet will want to determine the cause before deciding on a plan. If it’s trauma-related, it may resolve on its own with rest. If biopsy results show a benign vascular tumor, surgical removal is typically straightforward and curative. If the results point to something malignant, your vet will discuss staging and next steps based on how deep the tumor extends.