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Vaccine Induced Sarcomas in Cats

Updated on January 15, 2009

What cat owners should know


As more and more cats develop severe vaccine reactions or worse vaccine induced sarcomas, owners and veterinarians begin to question and outweigh the pros and cons of vaccinating pets. While it is a known fact that cats need to be vaccinated in order to prevent potentially deadly diseases, studies are currently underway on establishing if vaccinations should be given more sparingly as new vaccination protocols are being considered.

There are two vaccinations that have been associated with the VAS (vaccine associated sarcoma) in cats: the feline Leukemia vaccination and the Rabies vaccination. While the FELV vaccination is imperative in cats that are exposed to the outdoors and other cats, the Rabies vaccination is mandated by law in most States. In many scenarios, therefore, bypassing these vaccinations can be out of question.

The culprit of such vaccine induced sarcomas seems to not be the vaccination itself but the aluminum adjuvant used in order to administer such vaccines. Such aluminum adjuvants are added to killed vaccines in order to stimulate a strong immune response, however it yet is to be determined if other types of adjuvants are to blame.

What to Watch For:

While the incidence of such sarcomas may be relatively uncommon (it is estimated that 1 out of 1000 vaccinated cats will develop VAS) it is a good practice to keep an eye at the injection site for the weeks following vaccination. Typically signs to watch for according to are:

1) A firm mass that increases in size after 1 month

2) A firm mass that is over 2 centimeters in diameter

3) A firm mass that does not go away witihin 3 months

It is imperative to distinguish between a vaccine induced sarcoma and a granuloma. The formation of a granuloma can be pretty common in cats after being vaccinated. Typically a small swollen area that appears painless upon palpation may form and disappear within a few weeks. However, the small lump should be checked out if it does not disappear after three months. In some cases, however, the development of sarcoma may not be observed for months and even years.

Fortunately, in some cases when caught  at its earliest, metastasis (spread of the cancer) may have not occurred, therefore the sarcoma is treated locally via surgery and radiotherapy if necessary. However, there may be chances of relapses in the months or years to come.

Because of the incidences of vaccine induced sarcomas, it has become veterinary protocol to give the Rabies vaccination in the right thigh and and Felv in the left thigh. The reason behind this is the fact that should a tumor appear, as a last resort the leg can be amputated to prevent the potentially malignant tumor to spread.

It would be a wise choice for owners to periodically run their hands through the vaccine injection sites and palpate for possible abnormal lumps or swellings. In some cases, tumors may appear as ulcerated lumps and hairless spots. Owners should promptly report any abnormalities to their veterinarian and follow his/her advice for follow-ups.


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      AliceSacco 7 years ago

      7 months ago my cat had VAS. Cat owners need to be informed about risk of vaccine.