Feline Mast Cell Tumors: What They Are and How They’re Treated

Feline mast cell tumors are a type of cancer made up of mast cells (immune cells involved in inflammation and allergies) that most often affects a cat’s skin or internal organs such as the spleen and intestines.  

In this article, you’ll learn what feline mast cell tumors look like, common symptoms, how vets diagnose and treat them, what prognosis can depend on, and when a new lump, swelling, vomiting, or lethargy means it’s time to call your veterinarian. 

What Are Feline Mast Cell Tumors?

Feline mast cell tumors are a type of white blood cell present throughout the body, including the skin, respiratory tract, and digestive tract.  

These cells contain histamine, a chemical compound that plays an important role in the immune system’s defense against perceived allergens. 

When a mast cell is exposed to an allergen, it goes through a process called degranulation and releases histamine to remove the allergen from the body. This causes allergy symptoms such as sneezing, itching, and a runny nose or eyes.  

When histamine is released in excessive amounts, however, it can cause anaphylaxis—a life-threatening allergic reaction. 

A mast cell tumor (MCT) is a type of tumor that arises from the rapid replication and division of mast cells in the tissue. Mast cell tumors can grow slowly over time or they can develop very rapidly, seemingly overnight. 

Mast cell tumors in cats are found on the skin (cutaneous MCTs) in most cases, typically appearing on the head and neck—but can appear anywhere on the body. These are the second most common type of skin tumor to develop in cats.  

Mast cell tumors can also develop on the spleen (visceral or splenic MCTs) and within the intestinal tract (intestinal MCTs), typically the small intestines. 

Mast cell tumors in cats are not always a medical emergency, but they can be if they cause life-threatening symptoms such as difficulty breathing, collapse, vomiting, or diarrhea. If your cat is experiencing any of these symptoms of anaphylaxis, see a veterinarian immediately. 

Types of Feline Mast Cell Tumors  

Tumors can be benign or malignant (cancerous): 

• A benign tumor or growth is non-cancerous and does not invade surrounding tissues or spread to other parts of the body. These lesions are typically slow-growing and have a favorable prognosis overall. 

• A malignant tumor or growth is cancerous and comprised of abnormal cells that tend to divide rapidly. These cells invade the surrounding tissues readily. The spread to other areas of the body can also occur via the bloodstream or lymphatic system. Malignant tumors tend to be aggressive and generally have a poor long-term prognosis. 

Mast cell tumors in cats can be benign or malignant depending on the characteristics of the cells present and where they are located. For example, mast cell tumors on the skin tend to be benign, while mast cell tumors affecting the internal organs are more likely to be malignant. 

Symptoms of Feline Mast Cell Tumors

Clinical signs and symptoms can vary depending on where the mast cell tumor is located and how aggressive (if malignant) the tumor is. Mast cell tumors on the spleen or within the intestinal tract will be more likely to cause systemic (whole-body) signs: 

Cutaneous (skin): 

• Hard, hairless, flattened bumps (plaques) located commonly on the head and neck, but can develop anywhere on the body 

• Small lumps (nodules) that feel very firm within the skin 

• Excessive itchiness and hair loss 

• Ulceration (an open sore) 

Visceral (within the spleen or liver): 

• Vomiting 

• Weight loss 

• Poor appetite 

• Diarrhea 

Intestinal (in the intestines): 

• Vomiting and diarrhea 

• Fresh blood in the stool 

• Black or tarry stool, indicating digested blood 

Causes of Feline Mast Cell Tumors

While the exact cause of mast cell tumors is not fully known, both genetics and environmental factors are believed to play a role.  

Environmental hazards, such as secondhand smoke and household cleaners, may increase the risk. Chronic inflammation and exposure to allergens could also play a role. 

Regarding genetics, Siamese cats, for example, tend to be prone to the development of cutaneous mast cell tumors. 

In dogs, there is a mutation present in a specific gene, referred to as KIT, that’s associated with the development of mast cell tumors. The KIT protein plays a role in cell replication and division. A mutation in the KIT has also been detected in cats with mast cell tumors. 

How Veterinarians Diagnose Feline Mast Cell Tumors

Feline mast cell tumors can be diagnosed with a complete physical exam and laboratory testing. Your veterinarian will want to know what symptoms are present and when they began. If a mast cell tumor is suspected, the following tests may be recommended: 

• Blood work: A sample of blood can be used to check your cat’s overall health, including blood cell counts and organ function.  

• Urinalysis: Also used to check your cat’s overall health, abnormalities can help a veterinarian better assess extent of illness and treatment options. 

• Imaging: Ultrasound of the abdomen or X-rays of the chest or abdomen can help the veterinarian visualize the internal organs for abnormalities.  

• Fine needle aspirate (FNA): A small needle is inserted into the concerning lump to remove a sample of cells that can be stained and viewed under the microscope. This is usually highly effective for diagnosing mast cell tumors.   

• Biopsy: In the event an FNA did not provide a diagnosis, a small sample of the lump can be removed surgically and submitted to a pathologist for special testing and diagnosis. This may be done with local or general anesthesia. 

Treatment of Feline Mast Cell Tumors

The recommended treatment for feline mast cell tumors will depend on where the tumor is located and how invasive the tumor is.  

The choice of treatment for cutaneous mast cell tumors is surgical removal. 

Complete surgical removal of splenic and intestinal mast cell tumors may not be doable, and cancerous cells can be left behind.  

While rare, cutaneous mast cell tumors in cats can spread to other areas of the body. Alternative treatment options may be recommended in these cases, such as chemotherapy or radiation. This will likely require a referral to a veterinary oncologist. 

When pierced or ruptured (as by aspiration or surgery), all feline mast cell tumors can cause systemic symptoms in your pet due to the release of histamine, and therefore your veterinarian will likely prescribe medications to avoid any negative side effects during treatment.  

Antihistamines, antacids, anti-nausea, and pain medications are common examples. Recommended medications may include: 

• Diphenhydramine 

• Famotidine  

• Maropitant 

• Buprenorphine 

Recovery and Management of Feline Mast Cell Tumors

Most cats with cutaneous mast cell tumors will go on to live healthy, normal lives.  

Recurrence of cutaneous mast cell tumors can happen, but this is rare and generally occurs when some tumor cells were left behind during surgery.  

Cats that develop splenic, liver, or intestinal mast cell tumors typically have a poor long-term prognosis since these types tend to be malignant and spread to other areas of the body more readily.  

Daily medications to help manage systemic illness and improve quality of life may also be recommended in these cases. These may include the following: 

• Anti-inflammatories, such as prednisolone, to slow tumor growth and reduce inflammation 

• Antacids, such as famotidine, to reduce nausea  

• Antihistamines, such as cetirizine, to block histamine release 

If your pet is diagnosed with a mast cell tumor, surgery will likely be recommended as a treatment option, and it is important to follow your veterinarian’s instructions once your pet is discharged.  

A recovery cone will likely be needed to prevent any self-trauma or injury to the surgical site while your pet is healing post-operatively. 

It’s also important to keep the surgical site clean and dry, and to limit your pet’s activity for at least 10-14 days to prevent a reopening of the wound.  

If you have any concerns while your pet is recovering at home, promptly contact your veterinary clinic. 

Prevention of Feline Mast Cell Tumors

It’s difficult to fully prevent mast tumors in cats, but pet parents can take the following steps to reduce their cat’s risk:  

• Ensure your cat receives regular veterinary wellness exams to identify any skin lumps or signs of illness early.  

• Regularly feel your cat’s skin for new lumps that may warrant veterinary attention. 

• Reduce your cat’s stress in the home environment to support a healthy immune system. You can add a Feliway diffuser to your cat’s resting area.