A seroma is a collection of fluid in "empty" spaces beneath your dog's skin, often occurring at an incision or trauma site. Though the swelling looks alarming and painful, it's generally not a serious condition. In most cases it clears up on its own, but If it doesn't, the vet can remove the fluid to put your pup back in good order.
PetMD defines a seroma as an accumulation of serum outside the blood vessels. A seroma doesn't contain red blood cells, in contrast to a hematoma, and can occur anyplace on your dog's body, usually under the skin, known as a subdermal seroma. Seromas also can occur on the ear, in the head or in the brain or other organs, though they're commonly found in high motion areas, such as the shoulder. Typically, seromas develop after surgery if the surgeon leaves too much space, called "dead space," in the fatty layer between a dog's skin and abdominal wall muscles.
The classic sign of a subdermal seroma is fluid-filled swelling under the skin. If your dog recently had surgery, you may notice what looks like a buildup of fluid around the incision site. The seroma may develop between muscle layers, though it's most often found in the higher subcutaneous layer. Generally, a seroma doesn't hurt or cause lameness, though it may become infected, becoming red with a discharge.
Some swelling and redness is normal after trauma or surgery as the body heals. According to Pet Place, it takes up to two weeks for a noninfected incision to heal. In the first few days, a bit of swelling and redness is normal, as the body is accumulating cells and fluid to heal the wound. Bruising and even a bit of blood-tinged fluid seeping out of the wound is also normal. However, excessive swelling and dripping or bleeding from the wound are signs of trouble.
In most cases a seroma will clear up on its own, as the dog's body reabsorbs the fluid. However, if your dog's swelling is not decreasing, or it's showing signs of infection, you should contact your vet. The vet may collect fluid to determine if there's an abscess developing and may decide to aspirate the sac of fluid to help make your dog more comfortable. Occasionally a drain is necessary for a seroma that won't reduce on its own.
You can minimize the chance of developing a seroma by keeping your dog indoors and confined during recovery. Activity increases the burden placed on any sutures and wound site, leading to swelling and greater risk of seroma. If your dog is licking the site excessively, he should wear an Elizabethan collar.