Vaccine-Associated Sarcoma in Cats

A sarcoma is a cancer of the connective and soft tissues. Sarcomas are not a new form of cancer in cats. But in 1991, veterinarians began to notice a higher than expected number of sarcomas occurring in places where vaccines are commonly injected.

Subsequently, an association between vaccine administration and sarcoma development has been established. FeLV and rabies virus vaccines have more frequently been implicated in sarcoma development than have other vaccines. Both subcutaneous and intramuscular sites have been affected. Injections other than vaccines may also be implicated.

The increased appearance of these sarcomas roughly coincided with the change from using a modified-live rabies virus vaccine to an adjuvanted killed virus vaccine. At about the same time, an aluminum-adjuvanted FeLV vaccine was introduced. Adjuvants are added to vaccines to increase the immune response-especially in vaccines that use killed versions of a virus. Adjuvants in general, and aluminum adjuvants in particular, were therefore thought to be the culprit. However, researchers are no longer certain this is the case. It is believed that these vaccines cause some kind of inflammation at the vaccination site that, in some cases, is associated with sarcoma development, but an exact link has not been proven.

Nonetheless, vaccine manufacturers are developing recombinant vaccines that do not use adjuvant and that cause less inflammation at the vaccination site. Many modified-live virus vaccines are available for other viral diseases and some of them do not contain adjuvant. New vaccination guidelines try to minimize the number of injections given over a cat’s lifetime, as well, and also recommend specific sites on the body for injections to be given.

It’s important to remember that vaccine-associated sarcoma is still a very rare form of cancer. The occurrence rate varies from 1 in 1,000 to 1 in 10,000. The wide range seems to be associated with a genetic predisposition to this problem in certain cats and lines of cats. For instance, some geographic areas show an increased rate.

These cancers may show up months or even years after a vaccination. Although a fair number of cats have a small lump after getting a vaccination, the lump should be gone within a month. If it is not, have the cat examined by a veterinarian.

Because so much is still unknown, the Vaccine-Associated Feline Sarcoma Task Force was formed as a joint effort of the American Association of Feline Practitioners, American Animal Hospital Association, American Veterinary Medical Association, and Veterinary Cancer Society. This group is working to determine the true scope of the problem, the cause, and the most effective treatment for vaccine-associated sarcomas.

Treatment: This is an aggressive cancer that tends to spread in and between muscle layers, making it very difficult to remove all of the cancerous cells surgically. Surgery, with radiation done either before or after the surgery, seems to be the most successful treatment plan, but most of these cancers recur, nonetheless.