False Cysts in Cats and Dogs

False cysts are sacs of fluid that develop after a forceful knock or a blow to the body.

Dogs and cats can get false cysts. By: jeffreyw
Dogs and cats can get false cysts. By: jeffreyw

The definition of a false cyst is “a cyst without a secretory lining.” Accurate, but not particularly helpful.

I can see you scratching your head and asking what the definition of a cyst is. Well, a cyst is a “membranous sac containing fluid secreted from the lining.” This means a false cyst is a sac of fluid caused by a knock or a blow rather than a cyst secreted by the lining of the sac.

An example of a false cyst is a skin hematoma, or blood blister, resulting from a heavy object landing, say, on a foot (like when I dropped a brick anchoring the roof of the guinea pig run on my big toe).

False cysts are usually not serious and may even resolve of their own accord. The exception is if the swelling is so severe that it causes the surrounding tissue to die away, or if the swelling is filled with pus (an abscess).


When you stroke your pet, you may notice a bump on the surface of the skin.

When you feel more carefully, the lump may be round and slightly raised. If you give it a gentle squeeze, you may be able to surmise that it’s filled with fluid. Because false cysts develop from trauma, such as a knock or a blow, the swellings can come up suddenly and be tender and bruised, and the pet may feel discomfort when you investigate it.

If the false cyst contains blood, it can appear darkly pigmented. For some skin lumps, dark pigment is a warning sign of skin cancer. So if you see a dark lump, get it checked by a veterinarian just to be on the safe side. 


False cysts are most commonly filled with either blood or serum (the technical terms for these are hematoma and seroma, respectively).

When tissue is damaged, small blood vessels may rupture and leak blood, which then accumulates in a blister or larger sac.

Seromas are a similar sort of false cyst but often develop as a result of trauma to fat, which tends to seep or exude serum.


Your veterinarian may do a fine needle aspirate on the lump. This involves inserting a fine needle into the center of the swelling and sucking back to see if fluid or solid tissue is present.

In the case of a hematoma or seroma, a sample of blood or serum will be drawn off. This can give useful information as to whether it is safe to leave the lump alone and see if it resolves of its own accord or if a biopsy is necessary.

The only way to be 100% certain that a false cyst is nothing serious is to surgically remove it and send the sample for histopathology. A pathologist will examine stained sections of the tissue to confirm what cells are there and if the lump is cancerous or not. This is important to do if there is any question as to whether this is a false cyst or a true lump.


Surgical removal is curative for each individual false cyst; however, it is not always necessary to take action. Just as the lump came up suddenly because of seepage under the skin, the fluid can be slowly reabsorbed — again, the lump may disappear of its own accord.

Some false cysts stubbornly refuse to disperse. If they are inconveniently placed, such as under the dog’s collar, removal may be best. Another option is for repeated aspiration of the fluid from the lump, but this meets mixed results as more fluid often seeps back into the cavity to occupy the dead space.



Prevention is about being careful and avoiding unnecessary trauma — such as dropping bricks on your foot.


This pet health content was written by a veterinarian, Dr. Pippa Elliott, BVMS, MRCVS. It was last reviewed Dec. 17, 2018.