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Cats can develop cancerous tumors called fibrosarcomas, or sarcomas, at the locations where they have been vaccinated. These aggressive tumors can appear just months after vaccination or many years after the fact.
While these tumors are very serious, they are not very common. It’s estimated that approximately one to two out of every 10,000 vaccinated cats develop this condition.
Vaccines help protect cats from dangerous infectious viruses by stimulating an immune response (forming antibodies) against the virus. Vaccines generally contain very tiny amounts of the target virus, or protein particles derived from the virus. When this material is introduced into the body in a vaccine, the body’s immune system responds through a series of steps that include making antibodies and modifying other cells that will recognize the target organism later. These changes constitute an immune response. When the vaccinated individual encounters the “real” organism later, the body recognizes the organism and reacts to protect the vaccinated individual from becoming sick.
Live viruses can create an immune response, but they also have the potential to infect the animal. To prevent this, the viruses in vaccines are modified or killed to make the vaccines safer. In many cases, killed viruses cannot stimulate an immune response as effectively as modified or live viruses. A substance called an adjuvant is added to these vaccines to help the animal mount a more effective immune response over a longer period of time.
While no one is exactly sure what causes vaccine-associated sarcomas, it has been suggested that the adjuvant, combined with local inflammation, may be a contributing factor. Although these tumors have not been linked to a single brand of vaccines, they are more commonly associated with feline leukemia virus (FeLV) and rabies vaccines.
If your cat develops a swelling in the skin after a vaccination, don’t panic. Some vaccines can cause a mild reaction, which usually resolves in a few weeks. However, you should monitor the area and take your cat to the veterinarian if the swelling:
Vaccine-associated sarcomas can be locally aggressive (meaning they can start just under the skin but quickly invade deeper structures, like muscle) and grow relatively fast. They can metastasize (spread) to other locations in the body, such as the lungs. Over time, the tumors can become large, unmovable, and ulcerated.
The best way to diagnose these tumors is by obtaining a surgical biopsy (tissue sample) and submitting it to a laboratory for analysis. At the same time, your veterinarian may recommend radiographs (X-rays) to determine if other areas of the body are affected.
These tumors must be removed surgically, while your cat is under anesthesia. Before surgery, it may be necessary for your cat to undergo a computed tomography (CT) scan to help the surgeon determine how much tissue to remove. It is extremely important that the entire tumor is removed, or it may return in a more aggressive form.
Surgical removal is often followed with radiation therapy to eliminate any microscopic cancerous cells that may still be in the tissues. Some cats may require chemotherapy as well.
The chance of your cat contracting a serious disease is much higher than the chance of your cat developing a vaccine-associated sarcoma. So it’s generally recommended to keep vaccines current.
At the same time, veterinarians are taking every precaution to help reduce the risks associated with vaccines, including:
It’s a good idea to discuss the risks and benefits of vaccination with your veterinarian to determine which vaccines are right for your cat. This article has been reviewed by a Veterinarian.
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