Dogs get cysts when skin cells, hair follicles, or oil glands become blocked or damaged, trapping material beneath the skin’s surface. It’s one of the most common skin issues in dogs, especially as they age, and most cysts are completely benign. Understanding what causes them helps you know which lumps to watch and which ones need veterinary attention.
How Cysts Form Under the Skin
A cyst is essentially a small pocket that fills with fluid, oil, or solid material when something disrupts the normal flow of substances out of the skin. The most common trigger is a blocked hair follicle. When a follicle gets clogged or damaged, dead skin cells and a protein called keratin accumulate inside it, forming a firm lump. These keratin-filled cysts contain a gray, brown, or yellowish “cheesy” material and account for the majority of cysts found in dogs.
Oil-producing glands in the skin can also malfunction. When these glands overproduce or their ducts become obstructed, oily secretions build up and create a fluid-filled sac. This is the classic “sebaceous cyst” that many dog owners notice as a smooth, round bump that moves freely under the skin. In older dogs, the oil glands themselves can overgrow, producing small lumps rarely larger than one centimeter across, often with a shiny surface.
If a cyst ruptures internally (beneath the skin rather than outward), the trapped material leaks into surrounding tissue and triggers inflammation and pain. This is why some cysts that seemed harmless for months suddenly become red, swollen, and sore.
Types of Cysts in Dogs
Not all cysts are the same, and the type your dog develops depends on where it forms and what it contains.
Follicular cysts are the most common variety. They develop from malformed hair follicles and come in several subtypes, but they all share the same basic trait: a pocket of keratin and dead cells trapped inside a follicle structure. These can appear anywhere on the body and range from pea-sized to golf ball-sized.
Sebaceous cysts arise from oil glands and tend to feel softer because they contain liquid or semi-liquid oil rather than solid keratin. They’re frequently found on the head, neck, and trunk. Some dogs develop multiple sebaceous cysts over time, particularly certain breeds.
Dermoid cysts are different from the others because they’re congenital, meaning a dog is born with them. These unusual cysts can contain fully formed hair shafts inside them and are often multiple. They result from a developmental quirk where skin tissue gets trapped in the wrong layer during embryonic growth.
Interdigital cysts (technically called interdigital furunculosis) form between the toes and have a distinct set of causes, covered below. They’re among the most painful type because of their location.
Breed and Body Type Risks
Some dogs are genetically predisposed to cysts based on their breed and physical structure. Short-coated breeds with stiff, bristly hair are especially prone to follicular cysts because their short hair shafts are easily forced backward into the follicle during normal movement. This “traumatic implantation” of hair creates a foreign body reaction that leads to inflammation and cyst formation.
Breeds with wide paw conformations and greater distance between their digital pads, including Labrador Retrievers, English Bulldogs, German Shepherd Dogs, and Pekingese, are more likely to develop interdigital cysts. These dogs bear weight on the haired skin between their pads rather than solely on the pads themselves, which creates friction and drives hair shafts into the tissue. Chinese Shar Peis are also predisposed because of their prominent interdigital webbing.
Overweight dogs face higher risk for interdigital cysts because excess body weight puts more pressure on the spaces between toes. Dogs with lameness or abnormal gait also shift weight in ways that increase friction on vulnerable skin.
Environmental and Physical Triggers
Beyond genetics, everyday environmental factors play a significant role in cyst development. Dogs confined to wire cages, concrete runs, or rough ground experience repeated surface trauma to their paw pads and skin, which damages hair follicles and sets the stage for cyst formation.
Foreign bodies are another common trigger. A grass awn, thorn, or splinter that penetrates the skin can create a localized reaction that develops into a cyst. These foreign-body cysts are usually solitary (just one lump) and tend to appear on a front foot. Once the foreign material is embedded, the body walls it off with inflammatory tissue, and the resulting lump persists until the object is removed.
Allergies also contribute. Dogs with environmental allergies, food sensitivities, or contact dermatitis often lick their paws compulsively. This repetitive licking traumatizes the skin between the toes and creates the moisture and tissue damage that allows cysts to develop. The licking itself becomes a self-perpetuating cycle: inflammation leads to licking, which leads to more inflammation.
When a Cyst Becomes a Problem
Most cysts sit quietly under the skin for weeks or months without causing trouble. They become a concern when they grow, change appearance, or get infected. An infected cyst typically becomes red, warm, swollen, and painful. It may ooze discharge or bleed if the dog scratches or bites at it. Rubbing, licking, and chewing at a cyst are the main behaviors that push a harmless lump toward infection, so anything that keeps your dog from bothering the area helps.
The trickier concern is distinguishing a benign cyst from something more serious. Sebaceous gland tumors, for example, can look nearly identical to simple cysts. Most are benign growths, but a rare malignant form (sebaceous adenocarcinoma) can spread to lymph nodes. Oil gland growths that ulcerate, extend deep into the skin, or grow rapidly warrant closer examination.
How Vets Identify Cysts
A veterinarian can often identify a simple cyst by feel and appearance alone, but when there’s any uncertainty, a fine needle aspirate is the standard first step. This involves inserting a small needle into the lump and extracting a sample of cells for examination under a microscope. It’s quick, inexpensive, and doesn’t require sedation in most cases.
Simple cysts and those containing only debris typically don’t even need this step. The general veterinary guideline is that any skin lump larger than one centimeter that has been present for a month should be aspirated if there’s diagnostic uncertainty. If the aspirate doesn’t provide a clear answer, a biopsy (removing a small piece of tissue for laboratory analysis) is the next step. This is particularly important for lumps that could be infiltrative or malignant, since those are difficult to diagnose from needle samples alone.
Treatment: Watch or Remove
The decision to remove a cyst or simply monitor it depends on what the cyst is doing. Benign cysts that aren’t growing, painful, or infected often don’t need surgical removal. Your vet may recommend a watch-and-wait approach with checkups every three to six months to measure the lump and track any changes.
Surgery is recommended when a cyst causes pain, irritation, bleeding, or recurrent infection. It’s also recommended when a cyst is in a location where future growth could make removal more complicated, such as near a joint or on a leg where there isn’t much loose skin for wound closure. For cysts in tricky locations or in dogs with other health issues, you and your vet may decide together that the risks of surgery outweigh the benefits of removal.
Between vet visits, checking your dog monthly for new or changing lumps is genuinely useful. Some veterinarians suggest creating a simple body map where you note the location and size of each lump you find. This gives you an objective record over time and makes it easy to spot anything new.
Why You Shouldn’t Squeeze a Cyst
It’s tempting to treat a dog’s cyst like a pimple, but squeezing it can rupture the cyst wall internally. When the contents leak into surrounding tissue, the body treats that material as a foreign invader and mounts an inflammatory response. This can lead to a painful foreign body reaction or a secondary infection that’s harder to treat than the original cyst. If a cyst is draining on its own, keep the area clean and let your vet handle any further intervention.
Reducing Cyst Recurrence
For dogs prone to cysts on the body, regular grooming helps keep hair follicles clear and allows you to catch new lumps early. Brushing removes dead skin cells and distributes natural oils, which can reduce the kind of follicle blockages that lead to cyst formation.
For dogs with interdigital cysts, addressing the underlying cause is essential. If allergies are driving the problem, managing the allergic condition reduces the paw-licking that traumatizes tissue. Keeping paw hair trimmed, drying paws after walks on wet ground, and avoiding rough surfaces all reduce the friction and moisture that contribute to cysts between the toes. Weight management matters too: bringing an overweight dog to a healthy weight reduces pressure on the interdigital spaces and can significantly decrease recurrence.