Vaccine Related Sarcoma in Cats

It’s just a routine. Every year you bring your cat in for vaccines and getting vaccines is a simple procedure. But if you really think about it, when your cat gets its vaccines, it is not simply ‘going in for its needles’. A vaccination is a medical procedure, meaning that it has risks and benefits just like a surgery or a blood test. Some cat owners may be aware of one of the potential risks of vaccine- sarcomas. Sarcomas are a type of cancer that may be indirectly caused by a vaccination.

It is highly recommended that every cat be vaccinated. Veterinarians have unanimously agreed that there are ‘core’ and ‘non-core’ vaccines. Core vaccines protect against rabies, panleukopenia, rhinotracheitis, and calicivirus. Government laws require the rabies vaccination. The other three diseases are highly contagious and severe. Non-core vaccines are only needed for certain, higher risk cats such as those cats that spend a lot of time outside.

Why do veterinarians recommend vaccines at all? Won’t our cats be healthy and safe if they wander around without vaccines? The purpose of vaccinating our cats is to expose them to a harmless form of a disease. Once exposed to a harmless form of the disease, the cat can create a defense system to this disease without actually getting the disease (because the vaccine is a harmless form). It’s like a sport team studying information about their next opponent: they haven’t actually played the game, but they know more about the team and are more prepared for the game when it starts.

If an animal is exposed to a disease without being vaccinated first, the pet does not have the advantage of an already prepared defense system. An unvaccinated cat will take longer to create a defense system than a vaccinated cat. This delay in fighting the disease gives the disease a huge advantage to cause problems. By the time an unvaccinated cat has created a defense, the disease might have already caused severe damage to the body. This is not so much of a problem if a certain disease causes mild diarrhea, but is a very big problem if the disease causes kidney failure or potentially fatal pneumonia.

So why wouldn’t we vaccinate against every possible disease? There are a few reasons. The first reason is that many diseases are hard to create vaccines for. But also, even though vaccines can protect the cat against disease, vaccines can have side-effects. People who get the flu shot are probably very familiar with side-effects. There can be swelling and pain at the site of the injection. The cat can develop a fever or be lethargic. It is almost never seen, but some animals can go into an allergic shock after receiving a vaccination. But something different in cats is that (very rarely) they can develop cancer at the site of injection.

Before we understand the cancer, we must understand how vaccines cause side-effects. In order to help the body create a defense system, a vaccine must excite and recruit the immune system. The body is tricked into believing that this harmless vaccine is actually the real disease. Therefore, the immune system responds by creating a defense system against the vaccine (and ultimately against the disease itself). Whenever the immune system is active (whenever disease is present) there is the chance for swelling and pain (locally), and fever and lethargy (throughout the body).

It is not known exactly how a vaccine may cause cancer, but it is suspected that the active immune system sets up a good environment for the cancer to develop. This does not appear to happen in species other than the cat. For some reason the cat is predisposed to developing these sarcomas at the site of the vaccine injection.

The cancer that develops at the injection site is called a sarcoma. Sarcoma is simply a type of cancer, and not all sarcomas are caused by vaccines. A vaccine-related sarcoma is a lump of tissue at the site of a previous vaccine injection. Remember, not all skin lumps are sarcomas. Skin lumps can be many things, including infection, insect bites, and non-cancerous fatty lumps. Sarcomas can be treated with surgery and/or chemotherapy or radiation. Surgery is performed with the intention to totally remove the sarcoma, but sometimes some cancer cells remain in the body after the surgery. Owners may choose to use chemotherapy or radiation to kill those remaining cancer cells. Sarcomas are more successfully removed with surgery when they are small. This is because as sarcomas grow, they become very aggressive and invade the surrounding body tissue. For this reason, large sarcomas do not have a very good prognosis.

Sarcomas have never been common, but they are even less common now than they ever were. No one is exactly sure why they are less common. Part of that reason may be that veterinarians are actively trying to reduce the amount of side effects. They are recommending only the necessary vaccines for your cat’s lifestyle. Some future vaccines will only need to be given every 3 years, instead of every year. By avoiding ‘over-vaccinating’, veterinarians are making sure that your cat is protected from disease, but at the same time are trying to decrease the chance of side effects.

If you remember one thing, remember this: vaccine-related sarcomas are extremely rare! There is about a 1/10,000 chance that your cat will develop a vaccine-related sarcoma. This risk is far outweighed by the benefits your cat will have from receiving the vaccination. Vaccinated cats have a very tiny chance of developing a sarcoma, but unvaccinated cats have a very large chance of developing a severe, potentially fatal disease such as rabies or panleukopenia. Vaccine-related sarcomas are not a good reason to stop getting your cat vaccinated, but you should understand that it is a small, but possible side-effect. Always be aware of lumps that remain more than a month at the site of injection, and notify your veterinarian if you have any concerns.