What is a sarcoma in cats?

An Internet search for the above term produces a long list of articles on vaccine-associated feline sarcomas. That tells us immediately that most sarcomas in cats are caused by vaccines or at least that was the case as at 2008. However, strictly speaking, a sarcoma is a cancer that arises from muscle, bone or other connective tissue.

Injection site sarcomas are a worry
Injection site sarcomas are a worry. Photo: Shutterstock.
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Therefore, they arise from a variety of causes. However, in 1991 veterinarians began to notice a higher-than-expected number of sarcomas and they were occurring in places where vaccines were injected into the cat. This resulted in the conclusion that there was an association between vaccines and sarcoma development. FeLV and rabies virus vaccines have apparently been more frequently implicated in sarcoma development than other vaccines.

Both subcutaneous (under the skin) and intramuscular (inside the muscle) sites have been affected and injection from other vaccines have also been implicated.

It is believed that the increase in sarcomas at vaccine sites coincided with a change from using a modified-live rabies virus vaccine to an adjuvanted killed virus vaccine. It is believed that an aluminium-adjuvanted FeLV vaccine was the cause as it was introduced at the same time as an increase in sarcomas.

Adjuvants are added to vaccines to increase the immune response. It is believed that the culprit is aluminium adjuvants in particular. It is believed that the vaccines cause an inflammation at the vaccine site which is associated with sarcoma development but a link between the two had not been proven as at 2008. I’m sure that more modern research could add to this page.

As a consequence, vaccine manufacturers have been and are continuing to develop vaccines that do not use adjuvant and which cause less inflammation at the vaccine site. Further, the site of the injection has been modified to minimise the chance of injury i.e. the site is more distal to the cat’s body. In addition new vaccine guidelines try to minimise the number of injections given over a cat’s lifetime. This may have been a reaction to a perceived overvaccination by some vets in providing boosters that were unecessary.

Sarcomas linked to vaccines are still very rare and the current rate varies from one in 1000 to 1 in 10,000. The difference is caused by variation in genetic predisposition to this problem in certain cats. For example, some geographic areas show an increased rate of sarcomas. Sometimes a cancer starts months and perhaps years after vaccination.

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