Feline Injection-Site Sarcomas in Cats

There is an association between using certain vaccines and injection-site cancer in cats

In cats, a rare form of cancer can develop at former injection sites. They were once called Vaccine-Associated Sarcomas (VAS), but are now known as Feline Injection-Site Sarcomas (FISS). These cancers are aggressive and deadly. They spread locally and sometimes metastasize to other parts of the body. It is estimated in various sources that somewhere between 1 in 1,000 and 1 in 10,000 vaccinated cats will develop FISS.

FISS History

Previous to the mid 1980's, these cancers were a very rare thing. They were almost unheard-of, and were so rare that they didn't even have their own name. The first reports of these cancers occurring at sites traditionally used for vaccination appeared in 1991 and were based on a series of cases seen by veterinary pathologists over the preceding 5 years in the northeastern United States (1)

Soon such cancers were being reported from all over North America and Europe, stimulating a great deal of debate, speculation, recrimination, and investigation over the ensuing decades.

So what happened in the mid 1980's?

It is very significant that in 1985 a change in legislation in the United States occurerd, requiring the use of killed vaccine rather than modified live rabies vaccine in cats. While killed vaccines may sound safer, they contain certain chemicals called adjuvants that are added to increase their effectiveness. Adjuvants stimulate the immune system to respond better to the vaccine. Vaccines with adjuvants are called "adjuvanted vaccines."

It is postulated that the adjuvants in vaccines can cause chronic inflammation which leads to cancer at the injection sites.

Most traditional vaccines contain adjuvants.

Most vaccines need adjuvants to make them work properly. There are a few exceptions:

All of the traditional FVRCP, or distemper vaccines for cats, do not contain adjuvants. Never did, never will.
There is a non-adjuvanted rabies vaccine for cats called Purevax^®
There is also a non-adjuvanted feline leukemia vaccine also called Purevax^®

That's it. All the rest have adjuvants.

The link between adjuvants and cancer formation is strong, but is not 100%

Like many things in life, this issue is not as simple as it might seem.

For one, other injections besides adjuvanted vaccines have also been show to cause FISS formation in cats.
For another, these types of sarcomas can also form spontaneously at non-vaccination sites.
For a third, some of the more recent studies do not show as clear an association as some of the older studies.

So it's not a slam dunk.

There are some highly respected people in the field who do not believe adjuvants are responsible for FISS tumors. There are some highly respected people who do.

However, a few points are quite clear:

Although other things can cause these cancers, the only proven cause for the FISS injection site sarcomas in cats is prior administration of a killed, adjuvanted vaccine. Rabies and leukemia vaccines are the only vaccines with solid causal associations (4,8).
These cancers were almost unheard-of until the mandate to use adjuvanted vaccines. Then suddenly the incidence skyrocketed a few years later, which is how long it can take for the FISS cancers to develop.
The adjuvanted rabies and leukemia vaccines are 10 times as likely to cause FISS formation than non-adjuvanted rabies and leukemia vaccines. (6)

The evidence is compelling but not overwhelming

In science, nothing should ever be beyond doubt. There is always evidence for and evidence against any theory. Not everyone believes that adjuvanted vaccines can cause problems in cats. That said, there is plenty of compelling evidence that adjuvanted vaccines are not a great idea for cats. This evidence has lead the following organizations to issue the following statements:

2016 World Small Animal Veterinary Association Vaccination Guidelines: "Non-adjuvanted vaccines should be administered to cats whenever possible." (2)
European Advisory Board on Cat Diseases, Feline injection-site sarcoma: ABCD guidelines on prevention and management: "Non-adjuvanted, modified-live or recombinant vaccines should be selected in preference to adjuvanted vaccines." (3)
2013 American Association of Feline Practitioners: "...some Advisory Panel members consider that, on balance, risk might be mitigated by the use of modified-live vaccines." "...some authors recommend considering non-adjuvanted vaccines to try to reduce local inflammation." (5)

Bias Alert

It is interesting to note that while The American Association of Feline Practitioners guidelines contains the weakest language in regards to adjuvanted versus non-adjuvanted vaccines, that panel also was financially sponsored by, wait for it..... Boehringer Ingelheim, a huge multi-national vaccine manufacturer who at that time did not make non-adjuvanted rabies and leukemia vaccines! Since that time, 2017 to be exact, Boehringer Ingelheim bought out the Merial Corporation, which was the original manufacturer of the non-adjuvanted vaccines at the time the paper was written, and now they are produced under the BI label.

The bottom line

The bottom line is that there is compelling evidence that adjuvanted vaccines may cause cancer in cats. This evidence is not 100%, and there is evidence to the contrary as well.

There are highly respected practitioners who continue to use adjuvanted vaccines in cats and highly-repsected practitioners who would never use adjuvanted vaccines in cats.

At Sunnyside Veterinary Clinic

We have weighed the evidence and have determined that we will use non-adjuvanted vaccines in cats. For the feline leukemia vaccine, there is no significant drawback. In the case of the non-adjuvanted rabies vaccine, it must be boosted annually instead of every three years. We feel that this is a small price to pay for the safety and peace-of-mind that non-adjuvanted vaccines provide.