PET PEEVES: Mast cell tumors common in dogs

Dara Johns

I can honestly say that no two Mast cell tumors are alike. Mast cell tumors are the most common tumor of the skin seen in dogs. They get their name because, well, if you biopsy them and look at a slide under the microscope, they are full of mast cells.

Mast cells are a white blood cell that is created in the bone marrow and then released into the blood stream. A normal mast cell travels to a site in the body where there has been some form of allergic insult and mediates the inflammatory response by releasing histamines. The trigger could be anything from pollen exposure or a bee sting. If a lot of mast cells release their histamines in large amounts at the same time, it can trigger a whole-body reaction such as hives or even anaphylaxis.

While most mast cell tumors form on the skin, they can also be found in the spleen, liver, intestines or bone marrow. Mast cell tumors that grow on internal organs generally have more serious effects and more dire consequences.

Boston Terrier is one of the breeds most commonly plagued with mast cell tumors.

Tumors found on the skin can look like many different things. They can grow very quickly after being present for a year or more without changing. Sometimes they just suddenly appear and grow quickly. They can be ulcerated or change in size, growing larger and then smaller and then larger again.

Sometimes they cause the surrounding tissue to swell and get red, almost like a bee sting. Usually if this happens it is because the mast cells in the tumor are releasing histamines and other compounds. This is called “degranulation." Degranulation is the release of histamines and chemicals from the mast cells. These chemicals can go to other parts of the body and trigger allergic reactions, hives, and stomach ulcers.

These tumors are diagnosed by fine needle aspirate in your veterinarian’s office. Remember, they can look like anything- a wart, a benign tumor, a fatty tumor, or a bee sting. Any unusual lump should be checked out. Especially if your dog is one of the breeds that are prone to these tumors. Breeds most commonly plagued with mast cell tumors include Shar-Peis, Boxers, Labrador Retrievers, French Bull Dogs, Golden Retrievers, Boston Terriers and American Staffordshire Terriers.

Until recently, the treatment of choice for removing these has been surgery. The surgeon has to be sure and get wide margins, because these tumors spread very easily. As of 2020, the FDA has approved an injection that can be used on superficial skin tumors anywhere on the body and tumors that are under the skin on the legs as long as they are below the elbow or the hock. 

The injection is called Stelfonta. It is created from a chemical called tigilanol tiglate. Discovered by the Australian company OBiotics, it comes from the seed of the Australian native Blushwood plant. It was isolated in 2009, and after much study in the lab, clinical field trials began in 2016. One injection of Stelfonta will destroy 75% of mast cell tumors by day 28. With a second injection, 87% of tumors are destroyed. If you have a pet that has a mast cell tumor and you want to know more about the product, you can go to the website vet-us.virbac.com/stelfonta or you can ask your veterinarian for more details and see if this would be a good alternative to surgery for your pet.

Have a question for Dr. Johns? E-mail her at JohnsDVM@aol.com. Write to Pet Peeves, P.O. Box 2949, Fort Walton Beach, FL 32549. Johns is a Niceville veterinarian.