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5 Facts You Need to Know About Injection Site Sarcoma

They're associated with more than just vaccines; they're hard to predict and hard to prevent.

Lisa Richman  |  Oct 8th 2015


Injection site sarcomas are just what they sound like — high-grade, life-threatening cancers that occur in cats at the sites of injections, whether vaccines or other types of shots. Veterinarians know little about what causes injection site sarcomas, so prevention strategies are difficult to determine. Here are five facts about injection site sarcomas that cat owners should know.

1. Renaming the phenomenon was not a marketing ploy

Some of you might know that injection site sarcomas were once known as VAS, or vaccine associated sarcomas. When the name was changed after a few years of unsuccessful research into finding a cause — and thus prevention — some speculated this was done to deflect attention away from the disease. Untrue.

Instead, researchers began to find fibrosarcomas at other injection sites, too. These injections weren’t vaccines; they were corticosteroid injections often used to help manage arthritis pain. There has even been one documented case of a sarcoma found at the injection site for a microchip.

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Orange moggie waiting for his vaccine. Photo by Shutterstock.

2. Cats who have adverse reactions to vaccines are more at risk

This is not as simple as it looks. For a vaccine to work, it needs to “activate” the immune system. This means that if your cat runs a slight fever, there’s no need to panic. It’s her body’s immune system kicking in and making that vaccine useful.

On the other hand, if your cat’s reaction is alarming — high fever, excessive malaise for an extended period — it’s worth noting and being watchful.

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Vaccine in vial with syringe, by Shutterstock.

3. It’s caused by adjuvants in the vaccine

While some adjuvants, such as aluminum and mercury (thimerosol), have been linked to diseases including Alzheimer’s and have been shown to suppress the immune system, there is not enough evidence to declare it an outright hazard.

In fact, there was a small increase in the number of sarcomas reported after veterinarians began actively using non-adjuvanted vaccines, so adjuvants are not the only cause.

At least one study suggests that genetics may play a larger role than we once thought. If true, and we can identify the gene, we hope researchers could create a screening test given prior to administering any injections.

4. It can strike any cat at any time

According to a recently published study — the longest of its kind and one that has followed injection site sarcomas in cats for 20 years — cats are most susceptible between ages 9 and 14.

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Kitten getting his vaccines, by Shutterstock.

5. Avoiding vaccination carries its own risks

More veterinarians believe we might be overvaccinating our pets, especially as cats reach adulthood and the all-important kitten immunizations have been given. But be aware of two things:

Diseases such as feline distemper (panleukopenia), calicivirus and rhinotracheitis — the Big Three disorders that kittens are inoculated against — are highly contagious. If you do not vaccinate, you risk your cat catching:

  • A disease that has a maximum mortality rate of 90 percent (distemper)
  • A disease for which there is no cure (rhinotracheitis)
  • A disease that is highly communicable (calicivirus)

There is no cure for rabies. Yet. Although a study released in January looks promising. Aside from that, there are local laws to consider.

What if your cat were to bite a neighbor and authorities asked you to prove your cat was up-to-date on vaccinations? In some cities, an unvaccinated cat might simply be quarantined for a period of time. In others? It depends on the municipality.

I believe we are overvaccinating our adult cats. But, as mentioned above, the sad fact is that vaccines are not the only thing that lead to injection site sarcomas. We’re also increasingly microchipping our cats, and as pet owners become more savvy and attuned to the needs of our cats as they age, we see more pet parents pursuing therapies such as corticosteroid injections to ease pain.

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Cat receiving injection, by Shutterstock.

We just don’t know yet what causes injection site sarcomas. Nor do we know how to prevent them.

You could ask your veterinarian to titrate your cat’s blood, to confirm there are enough antibodies to equal the immune system of a vaccinated cat. But if you choose to do so, be aware that may not exempt you from legal requirements wherever you live.

Do the risks outweigh the benefits?

Common thought is that somewhere between 1 in 1,000 and 1 in 10,000 cats will develop an injection site sarcoma. Yet the U.S. Centers for Disease Control and Prevention has found rabies in cats is on the rise. And consider this: If you or any family member chooses to volunteer at a shelter — a highly rewarding experience and a role that cats need — do you dare risk bringing home feline distemper or calicivirus to your non-vaccinated cat when the mortality rate for these can approach 100 percent? I wouldn’t.

This is a highly controversial topic, so please, tell me in comments where you stand on the risks associated with vaccination.

Read more cool cat science stuff by Lisa Richman:

About Lisa Richman: Writer, director, pilot, foodie, cat person. When she’s not on set, this director of film and video can usually be found taking photos of cats (and food) with her trusty Nikon, or cruising aloft at 3,000 feet. She’s cat mom to an opinionated Tonkinese, a hearing-impaired Siamese, and a feline fashionista. She’s also the owner of a recently launched humor blog, and the Cat Writer’s 2014 Entertainment Blog, A Tonk’s Tail.