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A seroma is a pocket of clear fluid that collects under the skin, almost always underneath a surgical incision. It feels like a soft, squishy lump, it usually isn't sore, and most of them settle on their own within a couple of weeks with rest and warm compresses. The one thing I'd ask you not to do is drain it, at home or in the exam room, unless there's a real reason to think it's infected.
Key Takeaways
Call us the same day if: the lump is hot, red or sore to touch, there's cloudy or bloody discharge from the incision, the incision edges are opening, or your dog is off their food, lethargic or feverish. Those are signs of infection, not a simple seroma, and they need to be seen. Call (519) 896-0532 or book an appointment.
A seroma is a collection of serous fluid under the skin. Serous fluid is the clear, straw-coloured part of blood, the part left over once you take away the red cells and the clotting factors. It's slightly sticky between your fingers.
During surgery, small blood and lymphatic vessels get cut. They leak that fluid into whatever space is available. If there's a gap under the skin for it to pool in, it pools, and you get a soft swelling that wasn't there yesterday.
Most owners find it four or five days after surgery, when they're checking the incision and something feels different. It can be the size of a grape or the size of an orange, and the size on its own doesn't tell you much.
The one you have to rule out is an infected incision, because the two can look similar from a distance and are managed completely differently. Seromas are typically non-painful, soft and dependent, meaning they sit toward the low point of the incision. Infection adds heat, redness, pain on palpation and discharge.1
| What you're feeling | Likely | What I'd do |
|---|---|---|
| Soft, squishy, not sore, no heat, clear fluid if anything leaks | Seroma | Rest and warm compresses. Recheck if it's still there in two weeks |
| Firm, hot, red, painful, cloudy or smelly discharge, dog is off | Surgical site infection | Same-day exam. This is where sampling and antibiotics come in |
| Bruised, purple or dark red, appeared within a day or two of surgery | Hematoma (blood, not serum) | Have it looked at. Usually resolves, but I want to know it isn't still bleeding |
| Soft bulge at the incision that gets bigger when the dog stands or strains | Incisional hernia, the same idea as an umbilical hernia but at the surgical site | Needs assessing promptly. This one can need repair |
| Firm lump not related to any surgery, has been there a while | Something else entirely, such as a sebaceous cyst or a salivary mucocele | Book a routine exam so we can sample it properly |
If you're not sure which one you're looking at, that's a completely reasonable reason to send us a photo or book a recheck. Telling these apart is what the recheck appointment is for.
Two things drive it: dead space and movement.
Dead space is the gap left between tissue layers after surgery. With a primary closure, the layers should be closed individually to minimize the dead space that contributes to seroma formation.2 When that gap is left open, fluid fills it, and that fluid is a good growth medium for bacteria, which is why we'd rather not leave it sitting there.3
Movement is the other half. Every time the skin slides over the tissue underneath, it disrupts the healing surfaces and keeps fluid weeping into the gap. That's why seromas show up most often over mobile areas, and why the dogs who get them are so often the ones who felt fine on day three and got the zoomies.
Big mass removals are the other one. Take a large lump out and you're left with a large cavity. In those cases I'd either place a drain or use a closure technique that tacks the layers down and takes the dead space out, because eliminating dead space is one of the main reasons drains exist.3
For an uncomplicated seroma, the treatment is rest and warmth, and that's genuinely it.
Restrict your dog's activity strictly. Apply a warm compress to the area for a few minutes, a few times a day. The warmth causes local vasodilation, which helps the fluid reabsorb back into the vessels it came from. Recommended management of a post-surgical seroma is exactly this: limiting activity, sedating the patient if needed to keep them quiet, and warm compresses to reduce the swelling.1 Most resolve over a couple of weeks.
I would not drain it, and I'd ask your vet not to either unless there's a specific reason. The published guidance is that seromas should not be sampled or drained unless infection is strongly suspected.1 Sticking a needle into a sterile fluid pocket creates a route for bacteria to get in, and an abscess is a much worse problem than the seroma you started with. Aspiration also doesn't fix the dead space underneath, so the pocket refills.
Where drainage does have a role is diagnostic. If I'm worried the swelling is infected rather than a simple seroma, then sampling it tells me what I'm dealing with and whether antibiotics are needed. That's a different decision made for a different reason.
Most of the prevention that matters happens in the two weeks after you bring your dog home.
On the surgical side, we're managing dead space with layered closure and, on larger removals, drains.2,3 Careful tissue handling and shorter surgical times also lower the rate of incisional problems overall.4
Call us if the lump is painful, hot or red, if there's discharge that isn't clear, if the incision is opening, or if your dog seems unwell in themselves.
Also call if a clear seroma is still sitting there after two weeks of proper rest, or if it's getting bigger rather than smaller. Neither is an emergency, but both are worth a recheck so we can confirm it's still just fluid.
And if you simply aren't sure what you're feeling, call. I would much rather look at a normal post-op lump and tell you it's fine than have you sit on an infected incision for a week because you didn't want to bother us. If your dog is recovering from surgery in Kitchener, Waterloo or Cambridge, you can book a recheck or reach us at (519) 896-0532.
Spays and large mass removals are the two I see it after most. A spay is usually done on a young dog, and young dogs are close to impossible to keep quiet for the full 10 to 14 days, so the incision keeps getting moved. Large lump removals are a different problem: taking out a big mass leaves a big cavity behind, and that empty space fills with fluid. Any surgery over a mobile area, like the knee, has the same tendency for the same reason.
TPLO stands for tibial plateau levelling osteotomy, and it's the operation we do most often for a torn cranial cruciate ligament, the dog's equivalent of the human ACL. Some swelling around a TPLO incision is very common in the first week. A true seroma recorded as a complication is less common than most owners expect. In a series of 769 TPLOs, seroma was recorded in 1.2% of cases, against an overall complication rate of 19.2% and a surgical site infection rate of 8.5%. So if there's swelling around a TPLO incision, the question worth answering is whether it's a seroma or an infection. Infection is the more likely of the two, and it's the one that needs treating.
Not really. If your dog's ear flap is swollen and squishy, the far more likely answer is an aural hematoma, which is a collection of blood between the ear cartilage and the skin rather than serum under a surgical incision. It usually follows head shaking or scratching from an ear infection or an allergy. It also needs treating, unlike most seromas, so an ear that's ballooned up should be seen.
Most settle within about two weeks with strict rest and warm compresses. If it's still the same size after two weeks of genuine activity restriction, or if it's getting bigger rather than smaller, book a recheck. That's not an emergency, but it's worth confirming that what you're feeling is still just fluid.
No, and I'd ask you not to. The guidance for post-surgical seromas is that they shouldn't be sampled or drained unless infection is strongly suspected. A seroma is a sterile pocket of fluid, and putting a needle into it gives bacteria a direct route in. Draining also does nothing about the space underneath, so it refills. Rest and warm compresses are the treatment.
Go by how it feels rather than how big it is. A seroma is soft, squishy, not sore, has no heat in it, and sits toward the low end of the incision. An infected incision is firm, hot, red and painful when you touch it, often with cloudy or bloody discharge, and the dog is usually a bit off in themselves as well. If it's hot, sore or discharging, treat that as a same-day problem.
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