A Lump Appeared on My Cat Overnight: What Can I Do?

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  • It’s always concerning when a new lump pops up on your cat. Especially when it happens overnight. Some lumps grow over a long period, while others grow fast. In my practice, bumps caused by allergic reactions, infections, and cancer are the most common types of lumps or bumps that appear suddenly. However, other lumps can pop up seemingly overnight, so it’s important not to jump to conclusios. In this article, we will take a look at lumps in cats that can show up suddenly. We will also discuss diagnosis, treatment, and prognosis of each.

    All new lumps should be examined by a veterinarian right away. While all tumors are rare in cats, cancerous tumors make up the majority of all lumps found in cats. In fact, 1 in 5 cats will be diagnosed with cancer at some point in their lifetime.

    But cancerous lumps aren’t the only bumps that show up suddenly in cats. Lumps caused by infection and allergic reactions are common and can also be very serious – sometimes life-threatening.

    When to be concerned about a new lump appearing overnight

    Overall, you should be concerned if your cat’s lump is accompanied by any of the following symptoms:

    • Rapid growth
    • Warm skin on or around the lump
    • Pain
    • Ulcers
    • Bleeding
    • Irregular shape
    • Loss of appetite
    • Vomiting
    • Diarrhea
    • Trouble breathing

     Types of Lumps or Bumps that Can Suddenly Appear on a Cat

    1. Abscess

    An abscess appears when a scratch or bite on your cat’s skin becomes infected. Typically, bacteria from another animal’s nails or mouth enter the wound and cause infection. Abscesses occur most commonly in unneutered male cats, as they are more likely to become territorial with other cats. An abscess is the most common lump that appears overnight in my clinic.

    Bite wound abscess on a cat's neck

    Learn more about bite wound abscesses in cats.


    • Fluid- filled lump or swelling
    • Lump that is warm to touch
    • Fever
    • Smelly discharge
    • Limping (if the abscess is on the leg)

    Treatment & Prognosis:

    Treatment for an abscess depends on the severity of the infection. Most open abscesses can be drained and flushed without surgery, while closed abscesses will need to be cut open and drained. Most abscesses will heal within two weeks after proper drainage by a veterinarian. However, those that are not drained and flushed by a vet can become Infected and spread throughout the body. Learn more about abscesses in cats.

    2. Insect bites

    Insect bites typically cause a mild reaction in most cats. However, some cats have severe reaction and need medical treatment. Insect bites do not usually cause large lumps. However, in severe cases they can cause large swellings on the skin. Let’s take a look at the difference between a mild reaction and a severe reaction.

    Image: Hypersensitivity reaction from insect bites on a cat’s ears


    Signs of a mild insect bite reaction in cats:

    • Small lumps with crusts
    • Ulcers
    • Bumps on ears and nose

    Signs of a severe insect bite reaction in cats:

    • Severe swelling around the head and neck
    • Hives
    • Difficulty breathing
    • Excessive drooling
    • Agitation
    • Vomiting
    • Diarrhea
    • Dizziness
    • Disorientation
    • Seizure

    Treatment & Prognosis for Insect Bites

    The treatment for an insect bite depends on the severity of the reaction. Common treatments for cats with insect bites include:

    • Topical creams
    • Allergy medication
    • Steroids
    • Epinephrine (in severe cases)
    • IV fluids (in severe cases)
    • Hospitalization (in severe cases)
    • Oxygen therapy (in severe cases)

    A mild reaction to an insect bite may resolve within a few hours. However, more severe reactions can cause breathing issues and these cats may require overnight hospitalization and supportive care. Hospitalization for cats with severe reactions may include oxygen, fluids, and medications. However, if your cat is stabilized quickly it will be back to normal within a few days. For more information on insect bites, review this helpful PDF from Village Veterinarian.

    3. Mild, soft tissue trauma

    Soft tissue trauma occurs when a cat is injured and can cause large swellings to appear suddenly. Lumps are caused by fluid build up under the skin (hematoma). Soft tissue injuries usually happen in one of the following ways:

    • Jumping off furniture
    • Getting hit by a car
    • Falling off furniture
    • Abuse- being hit by a human
    • Animal fights
    • Strain from overuse of muscles during exercise


    Cats with a mild trauma injury may have one or more of the following symptoms:

    • Bruising, (bleeding under the skin)
    • Limping, lameness
    • Inflammation
    • Swelling
    • Trouble putting weight on legs when walking
    • Stiff joints
    • Rapid breathing
    • Pain in the area
    • Crying out
    • No appetite
    • Not acting normal
    • Licking the injury

    Treatment & prognosis:

    Treatment for soft tissue injuries usually includes on or more of the following:

    • X-rays of the area
    • Rest
    • Pain medication
    • A splint
    • A bandage

    Most soft tissue injuries will heal themselves with rest and pain medication. The average healing time is two weeks.

    4. Sebaceous Cyst

    These cysts form when a hair follicle or pore gets blocked by dirt, debris, or scar tissue. They can grow quickly and show up over night. However, they aren’t very big. Sebaceous cysts can resolve without treatment. However, most cysts will need to be removed with surgery.

    cyst sebaceous on pet skin

    Below is a Siamese cat with a sebaceous cyst:

    cat with cyst above the eye


    Your cat may have a sebaceous cyst if they have a lump with:

    • Oozing
    • White, grey, creamy discharge
    • Itching
    • Pain
    • Redness
    • Warmth

    Treatment & prognosis:

    Cysts can be drained with a needle or removed surgically. After surgery, medicated shampoos can be used to prevent more sebaceous cysts from forming. Most sebaceous cysts will completely resolve with surgery. Cysts that aren’t removed may grow, itch, bleed, and cause pain. For more information on sebaceous cysts see our article here: Sebaceous Cysts in Cats – Cat-World

    5. Mast cell tumors

    Mast cell tumors are caused by an over-replication of cells that regulate your cat’s immune response. They can appear in cats of any age and can grow large overnight.

    cat with several mast cell tumors on head


    Some of the signs of mast cell tumors in cats are:

    • A Lump
    • A Lump that is any color
    • A Lump that is any size
    • A lump that grows larger and smaller but never goes away completely.
    • Itchy skin
    • Red inflamed skin
    • A lump that is the same size for a while and suddenly grows quickly.


    Treatment for mast cell tumors depends on the grade, stage, and location of the tumor. Common treatments for mast cell tumors are:

    • Surgical removal of the tumor
    • Steroid medication
    • Allergy medication
    • Nausea medication
    • Chemotherapy
    • Radiation
    • Palliative care


    The prognosis for mast cell tumors in cats is guarded, and depends on the grade, stage, and location of the tumor. Low-grade tumors have a good prognosis and high-grade tumors have a poorer prognosis. According to this article, Cats that are eating decently at the time they are first brought to the veterinarian have a median survival of 19 months, while cats that are not eating have a median survival of 8 weeks. Learn more about mast cell tumors in cats.

    6. Swollen lymph nodes

    Pictured below is a very large swollen lymph node on a tabby cat:

    very large swollen lymph nodes on cat

    Many owners will notice swollen lymph nodes when they’re petting their cat. Lumps are usually felt under the arms, jaw, or in the groin. Lymph nodes can swell suddenly and when they do it’s  caused by one of the following conditions in cats:

    • Infection
    • Cancer
    • Viruses
    • Fungal infection


    Some of the common signs seen in cats with swollen lymph nodes  are:

    • Bumps under the jaw, armpits, or in the groin
    • Excessive drooling, swallowing,
    • Cough
    • Poor appetite
    • Limping


    Treatment for lymphadenitis depends on the cause. It will include one or more of the following:

    • Exam
    • Bloodwork
    • X-ray, MRI, or CT scan
    • Surgery
    • Antiobiotics
    • Chemo
    • Radiation


    The prognosis depends on the diagnosis. Cats with cancer may have a poorer prognosis while those with the infection will recover with medication and supportive care.

    7. Mammary Gland tumors

    Mammary gland tumors are among the most common cancerous tumor in cats. These tumors appear in the mammary gland under your cat’s nipples and can grow very quickly. They occur most in cats that are 10 to 12 years and 85% of them are cancerous.

    Mammary cancer in cats


    • A lump or swelling in one or more of your cat’s breasts
    • Sores or ulcers on the nipples
    • Soft or hard lump


    Treatment for mammary gland tumors depends on the grade, stage, and location. The most common treatment for these cats includes:

    • Fine needle aspirate of the lymph nodes
    • Surgery
    • Chemotherapy
    • Lymph node removal
    • Biopsy


    Prognosis depends on the size of the tumor. Life expectancy based on tumor size and is as follows:

    • Larger tumors less than 3cm around, expected survival is 4-6 months.
    • Medium sized tumors, 2-3cm around, expected survival is 2 years.
    • Small tumors, less than 2cm around, expected survival is over 3 years.

    For more information, see our article about mammary gland tumors in cats.

    8. Feline acne

    Feline acne is a disease of the skin. It appears when the hair follicles on the chin produce too much keratin and become plugged. Feline acne can appear overnight. However, it does not usually cause a large lump.

    Feline acne


    • Red pustules (bumps)
    • Dirty looking chin
    • Hair-loss on the chin


    The treatment for feline acne depends on the severity. Some cats will do fine with no treatment, while other cats will need antibiotics and supplements. Here are some of the most common treatments for feline acne:

    • Leave bumps alone, do not pick or pop pustules (pimples)
    • Antibiotics (to help get rid of the bacteria)
    • Topical shampoo, wipes, or gels (to reduce bacteria)
    • Clean the cat’s face regularly do not use plastic bowls, instead use ceramic, stainless steel, or glass.
    • Omega 3 fatty acids (Amazon link)


    Feline acne may disappear without treatment. For cats that get treatment, symptoms usually improve in 4-6 weeks.

    9. Basal cell tumor

    These tumors are the most common cancerous tumors in cats, making up 20-28% of all tumors. They can occur in any cat, but they are mostly found in Siamese, Persian, and Himalayan cats. These tumors do not usually show up overnight. The exact cause of these tumors is not known but they are believed to be caused by:

    • Sun exposure
    • Genes


    Cats with these tumors may have lumps that are:

    • Hard
    • Hairless
    • 2-10cm around.
    • Ulcerated
    • Infected


    The treatment for your cat will depend on the size, and severity of the tumor. Common treatments for basal cell tumors include:

    • Fine needle aspirate of the lump and lymph nodes
    • Surgery
    • Cryosurgery (freezing of the lump)
    • Radiation, if the tumor is not removed completely with surgery.


    Treatment for these tumors depends on the severity of the tumor. Most cats will make a complete recovery within 2 weeks following surgery. However, cats with tumors that have spread (metastatic tumors) may require repeat treatment and radiation. For more information, read our article about Basal Cell Tumors in Cats.

    Learn more on Cat World: Lumps and Bumps on Cats

    Is my cat in pain? How can I tell?

    It is hard to tell when cats are in pain because instincts tell them to hide pain to avoid predators. However, there are ways to tell if a cat is in pain. If your cat is showing any of the following symptoms there is a good chance they are in pain:

    Physical signs of pain in cats:

    • Rapid heartbeat
    • Fast breathing
    • Panting
    • Slow moving
    • Crouching

    Behavioral signs of pain in cats:

    • Poor appetite
    • Not wanting to walk or get up from laying down
    • Hiding under furniture
    • Irritability
    • Licking, biting, or rubbing the area of pain

    Is there anything I can do at home to help my cat?

    All lumps need to be examined by a vet. However, there are some things you can do at home while waiting for a vet appointment. The most helpful thing you can do for your pet is to keep them comfortable while protecting the lump from further damage or inflammation.

    1. Put an e-collar on your cat: 

    E-collars are made to prevent your cat from licking or scratching wounds. They can be purchased at your local veterinary clinic, pet supply store, or online. Follow the instructions below for a perfect fit:

    • Bring your cat to your vet or a local pet supply store and ask them if you can try a collar out.
    • Make sure the collar fits around your cat’s neck with 1-2 fingers space between the collar and neck. This way the collar will not dig into your cat’s skin.
    • The collar should also only extend slightly beyond your cats nose so they can eat and drink comfortably.
    • If your cat refuses to eat with the collar on you can remove it during feeding as long as you are watching him.

    2. Clean the lump with a mild salt solution or antibacterial solution:

    You should never try to squeeze or drain any lump on your own. However, you can prevent infection by cleaning around the lump 2-3 times a day. Follow the steps below to safely clean your cat’s lump: 

    • Mix a small amount of chlorhexidine solution with distilled water (I use 2% chlorhexidine diluted at a 1/30 ratio.) This means that if you have 1 Tbs. of 2% chlorhexidine solution, you will need to mix it with 30 Tbs. of distilled water. Note: ) Chlorhexidine can be purchased online or at a pet supply store)
    • After mixing the anti-bacterial solution, use either gauze squares or a clean towel to gently clean the outside of the lump.

    Do lumps sometimes go away on their own?

    Yes and no. This depends on the type of lump your cat has. Lumps like allergic reactions, abscesses, and cysts may resolve without treatment. However, leaving a lump to heal without treatment is risky. For instance, abscesses can go away without antibiotics, but the infection that causes them may make your cat very sick before the abscess goes away. Also, lumps like mast cell tumors can appear to be improving (they can get larger and smaller day to day) but they may never heal completely. Overall, quick treatment leads to the best outcome for you and your cat. Waiting could cause a severe infection or it could allow cancer to spread deeper into your cat’s body.

    When should I call my vet if a new lump just appeared overnight?

    If your cat has a new lump you should call your vet immediately. While most lumps are treatable, lumps that are accompanied by cancer, infection, and severe allergic reaction are the most worrisome. Your vet can prevent most lumps from becoming emergencies. For instance, if your cat has an infection from a bite wound, a quick prescription of antibiotics could stop the infection when given within 24 hours of the bite. And cancerous lumps are always better off when they are caught and treated early.

    What should I know before my vet visit? 

    Often when we visit the vet nerves are high. We might get so anxious that we forget to ask important questions. To prevent this, you should write down your answers to the following questions, and write down any questions you have for the vet. Additionally, you should know that the basic diagnosis process for any lump usually includes:

    Note: Depending on the type of lump your cat has there may be more treatments added on.

    Questions the vet may ask

    1. Any change in your cat’s behavior?
    2. Is it smaller or larger than it was when you first noticed it?
    3. Has your cat been through any minor or major injuries?
    4. Are there other lumps?
    5. Is your cat eating normally?
    6. Any vomiting or diarrhea?

    Learn more on Cat World:

    Lumps and Bumps on Cats