June 11, 2026, 3:33 pm | Read time: 5 minutes
While petting the dog, a small lump suddenly becomes noticeable under the skin. Many owners initially think of an insect bite, a wart, or a harmless sign of aging. However, it could actually be a sebaceous cyst–a blocked sebaceous gland filled with skin material and secretion. PETBOOK explains how to recognize an atheroma, whether you can express it yourself, and when a visit to the vet is necessary.
How does a sebaceous cyst develop in dogs?
Sebaceous glands are located in the dog’s skin and produce an oily secretion called sebum. This keeps the skin and coat supple and protects against drying out.
In a sebaceous cyst, medically known as an atheroma, the normal drainage of this sebum is disrupted. As a result, the material accumulates in a closed capsule under the skin. Sebum, dead skin cells, and other tissue debris deposit there, causing the swelling to slowly increase. A sebaceous cyst often feels firm and elastic, can be easily moved, and is initially painless.1
Why do sebaceous glands get blocked in dogs?
A sebaceous cyst usually forms due to a blockage of sebaceous glands or hair follicles. Hair follicles are the small structures in the skin from which hair grows. If the drainage is blocked by sebum, skin cells, or other skin material, a cyst can form.
Possible risk factors include genetic predispositions, skin irritations, inflammations, or skin injuries. Some breeds, such as Cocker Spaniels and Basset Hounds, seem to be more frequently affected. In rare cases, an atheroma can be present at birth and only become visible later.
How can you recognize a sebaceous cyst?
Typically, it is a slowly growing, usually painless lump in or under the skin. It is often round and well-defined. Its size can range from a few millimeters to several centimeters. Many owners discover it by chance while brushing or petting.2
Not every dog shows health issues. However, some animals start licking, scratching, or nibbling at the affected area. This can lead to itching, redness, tenderness, or pain.
It becomes critical if the dog scratches the cyst open or if it bursts on its own. Bacteria can then enter and cause inflammation. In the worst case, pus, deeper skin damage, or a bloody, mushy discharge can develop. Even if the contents of an atheroma can be partially drained, the cyst should not be expressed by oneself. This often does not solve the problem permanently: Many sebaceous cysts refill after some time.
Why every lump should be checked by a vet
Even if a swelling seems harmless, it cannot be reliably assessed from the outside. Other skin changes, tumors, or abscesses can look similar.3
The vet will first thoroughly examine the dog and palpate the lump. They will pay attention to size, tenderness, mobility, and changes in the skin. Often, a fine needle aspiration is also performed. Cells or some content from the lump are taken and examined microscopically.
In some cases, a biopsy is necessary. A small tissue sample is taken to clearly determine the cause of the swelling.
Can you express a sebaceous cyst yourself?
Many dog owners wonder if they can express a sebaceous cyst themselves. In principle, the contents of a non-inflamed cyst can be drained. However, this usually does not solve the problem permanently, as the capsule under the skin remains and can refill.
There is also the risk of inflammation. If bacteria enter the opened area, pain, swelling, and infections can occur. If the cyst is red, painful, scratched, or already oozing, owners should not intervene themselves but have the dog examined by a vet.
Small, non-inflamed atheromas are often simply monitored. However, if a sebaceous cyst is inflamed, infected, painful, or located in an area exposed to constant pressure, treatment may be necessary.
Depending on the findings, the vet may clean the wound, use antibiotics for bacterial infections, or surgically remove the cyst. The capsule is removed as completely as possible. If parts of it remain, a sebaceous cyst can form again in the same place.
When should you take a sebaceous cyst to the vet?
Veterinary advice should be sought when:
- the lump grows quickly
- the area is red or warm
- pus or fluid is oozing
- the dog licks or scratches it
- pain occurs
- the area bleeds
- the lump affects movement or wearing a collar or harness
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How dangerous is a sebaceous cyst?
The prognosis is generally good. Atheromas are benign cysts and often cause little or no discomfort.
If a sebaceous cyst is completely removed, the likelihood of it recurring in the same spot is low. However, dogs that are prone to developing such cysts may later develop more atheromas in other areas of the body.4
Can you prevent sebaceous cysts?
There is no sure way to prevent them. Regular brushing, petting, and examining the dog can help detect skin changes early.
Good coat and skin care can also help remove excess sebum and dead skin cells. Additionally, a balanced diet and vet-recommended measures for skin health are suggested.