Are Vaccine-Induced Sarcomas in Cats Responsive to Chemo?


Dr. Barchas: My 16 year old persian cat has fibrosarcoma. My vet tried to get all of the tumor on the right hip but could not. I was told this was due to vacine site injection. Since he is so old – diabetic – and has a heart murmur do you think I should put him through chemo? How long do you think he has? The tumor was removed several months ago. He is eating and still grooming.

Dallas, TX

Vaccine-induced sarcomas are cancers that occur rarely (experts estimate a rate of 1 in 10,000 to 1 in 3,000) in cats that receive rabies or leukemia vaccines. Other vaccines have not been linked to the cancers.

Vaccine-induced sarcomas occur at the site of injection. In the olden days, all feline vaccines were administered between the shoulder blades because that’s the easiest spot on a cat to administer an injection.

Although vaccine-induced sarcomas almost never spread throughout the body, they are very locally invasive. This means that they must be very aggressively removed. It is essentially impossible to remove a vaccine-induced sarcoma growing on the trunk of the body–as is the case when the tumor is between the shoulders. They almost always grow back. They are not very responsive to chemotherapy or radiation therapy. They usually lead to complications that require euthanasia.

No definitive solution to the problem has emerged. Some people–especially representatives of certain vaccine manufacturers–recommend vaccination with products that don’t contain adjuvant (material designed to stimulate the immune system). I have yet to see solid evidence that these vaccines are safer (if you know otherwise and can link to a peer-reviewed study that backs this up please do so).

Veterinary experts have recommended two ways to reduce the incidence and fatality of vaccine-induced sarcomas. First, decrease the use of leukemia and rabies vaccines. Second, stop giving the vaccines between the shoulders.

Guidelines issued by several reputable organizations state that rabies and leukemia vaccines should be administered in the rear legs as close to the feet as possible. If tumors grow far down on the leg, they can be eliminated by amputating the affected leg. This solution is gory and very far from perfect, but most people will agree that amputation beats euthanasia.

Here’s the problem: a lot of vets aren’t doing it right. They know that they’re supposed to give the vaccines in the rear legs, but they don’t aim for the foot. Instead they aim for the hip. When tumors grow near the hip they can’t be removed any more easily than they can when they grow between the shoulders.

Let’s not mince words. Jan, at some point somebody administered a vaccine to your cat in the wrong place. The worst part about this is that there’s basically nothing you can do. I am very sorry to say it. It’s worth talking to an oncologist, but it isn’t likely that chemo will make much of a difference for your cat.

For everyone else, there is a lesson to be learned from this sad story. Talk to your vet about the appropriateness of any vaccines he or she plans to administer to your pet. If you decide together that rabies or leukemia vaccines is in order, talk about where the injections will be administered. If the answer is (as it should be) on a rear leg, your next question needs to be where on the leg. Guidelines specifically state that the vaccines should be administered as far down as possible. No vet should ever aim above the ankle.

It is a feature of medicine (human and veterinary) that patients (or their owners) must participate in all decisions. No good vet should ever object to discussing vaccines, vaccine-induced sarcomas, or planned sites of injection with any client.

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