Vaccines and sarcomas in cats

Vaccines are important to pet health and wellbeing. They help protect our furry companions from highly contagious and/or deadly diseases, and have several other benefits as well. Still, like other types of medical treatment, vaccines carry some risk of side effects. Although these side effects typically are temporary and minor, cats are susceptible to a rare but serious problem known as feline injection-site sarcoma.

Injection-site sarcomas are cancerous tumors that can arise several weeks or more after injections—most commonly vaccinations. Although we’ve known about these types of sarcomas for decades, their exact cause remains unclear. What we do know is that cats are more susceptible to injection-site sarcomas than other pets, like dogs or ferrets. Although injection-site sarcomas have been found in other animal species, cats seem to have a more aggressive body reaction for reasons not yet fully understood. Scientists have not found genetics to play a direct role, but research in this area continues.

The good news is that improvements in vaccines and vaccination techniques have greatly reduced the risk of injection-site sarcomas. The chance of this happening is quite low, occurring in about 1 cat per 10,000 to 30,000 vaccinations, but it’s good to be aware of the possibility in case you notice new lumps on your cat that seem to be getting larger.

If your cat is one of the few that does develop a sarcoma, you can help give them the best chance for a good outcome by knowing the signs and seeking veterinary care as soon as you notice a suspicious lump. 

Why do kittens and cats need to be vaccinated?

Vaccination is one of the most important things you can do to protect your cat from common or potentially fatal infectious diseases. Plus, when it comes to rabies, we need to follow local and state laws that may require vaccination to protect animal and public health.

Vaccines teach your cat’s immune system to recognize and fight off infections from disease-causing agents. They work by stimulating the production of antibodies that identify and destroy these agents. 

The benefits of vaccination are many, especially when vaccines are given as recommended. 

• Vaccination prevents or lessens the severity of many cat illnesses, and, in turn, can improve cats’ quality of life.
• Vaccination can help avoid costly treatments for preventable diseases.
• Vaccination prevents diseases that can be passed between cats and from cats to people.
• Unvaccinated cats are at risk of life-threatening diseases found in wildlife, such as rabies.
• Reputable pet boarding services require that cats be vaccinated against certain diseases to protect all boarded animals.
• Many local and state laws require that cats—including indoor-only cats—be vaccinated against certain diseases.

Vaccines typically need to be given in a series—at appropriate time points—to ensure adequate protection throughout a cat’s life. After a cat has been fully vaccinated, only rarely will their immune system fail to fight off the disease. It’s important to follow the vaccination schedule recommended by your veterinarian to reduce the possibility of a gap in protection. 

Your veterinarian can help you decide which vaccines are most important based on your cat’s age, health status, lifestyle, and other factors, including whether your cat had an injection-site sarcoma in the past. 

Note that nowadays most cats are vaccinated at body sites that would allow easier tumor removal should one develop, such as low on the legs or on the tail. In addition, some vaccines can be given via the nose, avoiding injection altogether. Talk to your veterinarian about the pros and cons of available vaccine options for your cat.

Learn more about pet vaccinations

How are sarcomas diagnosed?

After vaccination, a small, firm swelling may develop under the skin at the site of a recent vaccination. This swelling, caused by inflammation, is typically harmless and should disappear within a couple of weeks. If the swelling lasts for more than three weeks, or if it appears to grow larger in size, it could indicate something more is going on.

The time interval between vaccination and sarcoma development can vary tremendously among cats. Overall, these tumors have been found anywhere between 2 months and 10 years after vaccination.

To determine whether a persistent or growing lump is a sarcoma or something else, your veterinarian likely will start by collecting some cells or tissue from the lump and examining them under a microscope. Depending on the situation, your veterinarian may suggest microscopic examination of cells from nearby lymph nodes and x-rays of the lungs to check for signs the tumor is spreading. Your veterinarian also may recommend laboratory tests like bloodwork and urinalysis to assess your cat’s overall health, but such tests alone can’t show whether or not a cat has cancer. 

Because injection-site sarcomas can grow aggressively, advanced diagnostic imaging such as computed tomography (CT) or magnetic resonance imaging (MRI) is recommended when feasible to determine the extent of tumor spread and develop a tailor-made treatment plan. 

How are sarcomas treated?

There are various treatment options, including surgery, radiation therapy, and chemotherapy. Your veterinarian will work with you to develop an individualized treatment plan that meets your and your cat’s needs. Unfortunately, injection-site sarcomas often return after treatment, but the chance of this happening can be significantly reduced with a quick and aggressive approach.

Depending on the cat, radical resection of the tumor by a veterinary surgical specialist usually has the best chance of success. This means removing the tumor along with a margin of healthy surrounding tissue and structures (like bone) to help ensure no cancer cells are left behind and reduce the risk of the tumor returning. The procedure may involve amputation of affected body parts. Surgery often is combined with radiation therapy of the affected area, with the aim of shrinking the tumor before surgery and/or reducing the chance of the tumor returning afterward. 

Chemotherapy may be used in combination with surgery and radiation therapy to help shrink the tumor or slow its growth or spread, but neither therapy is likely to fully eliminate the tumor when used alone. When the tumor has spread to other parts of the body, the addition of chemotherapy may increase a cat’s survival time. 

Newer treatments like immunotherapy and electrochemotherapy are being investigated for their potential to further reduce sarcoma recurrence rates, when combined with other treatment approaches. Be sure to check the AVMA Veterinary Clinical Trials Registry for any trials that your cat may qualify for.

Besides these options, your veterinarian may recommend other therapies, such as medications to manage pain or nausea, and specialized diets or nutritional supplements to support your cat’s overall health and immune function. 

What’s the usual outcome for cats with injection-site sarcomas?

The outcome for cats with injection-site sarcomas depends on many factors, including the size and location of the tumor, the tumor’s behavior, whether the tumor has spread to other parts of the body, and the impact of treatment. As with most cancers, early detection and early treatment offer the best chance of success. 

Regular checkups allow your veterinarian to find tumors early on, before they become large and difficult to remove. You can help by noticing unusual lumps or bumps in your cat, and immediately visiting the veterinarian if they seem to be growing.

For cats with a history of injection-site sarcoma, some veterinarians suggest that further vaccination be minimized as much as possible, and that adjuvant-free or non-injectable vaccines be used when revaccination is necessary (such as rabies for legal reasons). If injectable vaccines are necessary, they can be given at sites that would make tumor removal easier if another one were to develop, such as low on the legs or on the tail.

What can we do to prevent injection-site sarcomas from happening in cats?

Because we don’t know for sure what causes injection-site sarcomas, it’s difficult to know how to prevent them. What’s not a good idea is avoiding vaccinations altogether, as this could do your cat more harm than good. Your veterinarian can work with you to develop a vaccination plan that protects your cat’s health and meets legal requirements, while avoiding unnecessary injections.