Where do Vaccine-Associated Tumors Develop in Cats?


Hi Dr. Barchas,
I know that the feline rabies vaccine has been associated with fibrosarcomas at the site of injection. Is it possible for the vaccine to cause tumors in other locations? Someone told me that the three-year rabies vaccine was more likely to cause cancer – is this true?

My cats received the three-year vaccine several years ago. One of them developed an aggressive tumor in her abdomen a couple years later. As surgery would not improve her quality of health, we elected to put her to sleep. Although a couple of years have passed, I remain worried about my other cat (her sister) and if my choice to have them vaccinated with the three-year shot contributed to my beloved cat’s death. Thank you for any insight.

Columbus, OH

You have asked some very good questions. I’ll do my best to answer each of them.

Vaccine-associated fibrosarcomas are cancers that develop rarely at the site of rabies or feline leukemia vaccines. They appear to occur only in cats. The cause of the tumors is very poorly understood.

The cancers develop at the site of injection. I have not seen any reports of vaccines causing tumors in distant locations. Therefore, it is extremely unlikely that your cat’s abdominal tumor was linked to a vaccine. Fibrosarcomas can develop naturally, and if your cat suffered from a fibrosarcoma in her abdomen it likely occurred spontaneously. I sincerely doubt that your decision to vaccinate your cat contributed to her death.

Three-year rabies vaccines usually contain a component called adjuvant that is designed to help stimulate the immune system. One-year vaccines generally do not contain this component. Some people have hypothesized that adjuvant contributes to fibrosaroma development. These people therefore speculate that the one-year vaccine may be less likely to cause cancer than the three-year vaccine. I have not seen conclusive proof of this.

For instance, some studies have suggested that merely inserting a needle into the skin may trigger a cascade of inflammation that ultimately leads to cancer. This implies that the adjuvant is not to blame.

Also, remember that one-year vaccines must be given three times as often as three-year vaccines. The relevant question therefore is whether one-year vaccines are three times less likely to lead to cancer.

A few years ago I attended a lecture by a representative of a company that produces one-year, non-adjuvanted feline rabies vaccines. After the lecture I cornered her (literally–she was trying to get away and I backed her into a corner) and asked the following question. I remember it verbatim.

Can you provide quantitative evidence that three-year vaccines are three times more likely to cause cancer as your one-year, non-adjuvanted vaccine?

Her answer was no. To this day I have not seen any study that proves that three one-year vaccines are safer than one three-year vaccine (if anyone is aware of such a study, please let me know).

It is unlikely that your remaining cat will develop a fibrosarcoma from her rabies vaccine. Fibrosarcomas develop in approximately one out of every 3000 – 10,000 cats that receive the vaccine. I have no reason to believe that your cat is at increased risk.

Although vaccine-associated fibrosarcomas are not common, I still recommend that you never accept a one-size-fits-all vaccination plan for your pet. Good vets always are willing to talk about the risks and benefits of vaccination. Make an informed decision.

Photo: Myriah after removal of a vaccine-associated fibrosarcoma.

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