The Humane Society of the United States
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Feline Vaccine-Associated Sarcoma

Watch the lump that appears after your cat is vaccinated—it could be cancer

cat resting


Few people think twice about having their cat vaccinated. The wide use of pet vaccinations in this country has made diseases such as distemper and rabies rare in U.S. cats.

But vaccinations come with their own set of risks. For cats, one of the most serious is fibrosarcoma, a type of cancer that appears at the injection site and spreads rapidly.

Fortunately, fibrosarcoma is uncommon, occurring once in approximately 10,000 vaccinations. But it's often fatal for any cat who develops it.

How fibrosarcoma develops after your cat is vaccinated

A lump may appear at the injection site after your cat receives a vaccination. This is a result of your cat's immune system reacting to the vaccine. The lump should disappear within a few days or a few weeks after the shot.

Sometimes the lump doesn't go away and a tumor appears. At first, small tumors can be successfully removed with surgery, but they tend to recur and become larger each time.

For decades, vaccinations were injected in the loose skin between a cat's shoulder blades. A tumor in this location would eventually become too large to take out because of the amount of skin and tissue that would have to be removed with it. By that time, the cancer had usually spread to the cat's other organs, and the prognosis was grim.

What causes fibrosarcoma?

It's not clear why these tumors form, but studies suggest that it's the adjuvant in the vaccine, not the virus itself. An adjuvant is a substance added to a killed-virus vaccine to enhance the immune system's reaction. In the case of cat vaccines, the adjuvant was aluminum salts, and aluminum has been found in excised tumors.

There are also studies indicating that the frequency of vaccination could contribute to the development of fibrosarcoma.

How the vaccine makers and veterinarians are working to protect your cat

As a result of all this research, there have been changes in the way vaccines are made and administered. Some manufacturers have stopped putting aluminum salts in vaccines, and veterinarians now give injections in cats' upper legs. If a sarcoma does occur, the entire leg can be amputated to prevent and limit the recurrence of the cancer.

Vaccination schedules have also changed. In the past, veterinarians prescribed annual vaccines. Now veterinarians may recommend that they be given every three years to cats at low risk. Researchers have found that vaccines often provide immunity far beyond one year, reducing the need for frequent revaccinations. Rabies vaccinations at certain intervals may be required by law.

How you can protect your cat from fibrosarcoma

  • Since outdoor cats are most at risk for being exposed to rabies, feline leukemia, and distemper, they must be vaccinated regularly. The simplest way to avoid revaccinating is to keep your cat inside, eliminating the need for certain shots.
  • When your cat needs to be revaccinated, ask your veterinarian about the type of vaccine used. Is it a killed or live virus vaccine, is it for one or three years, and does it contain the aluminum salt adjuvant?
  • Cats may be required by law to get rabies vaccinations, even if they're always kept indoors. Your veterinarian must follow the law. However, you can ask your veterinarian about doing a blood titer test to measure your cat's rabies antibodies. If the titer level is high enough, your veterinarian should be able to exempt your cat from being revaccinated at that time.
  • If your veterinarian insists on giving optional vaccines against your wishes, find another veterinarian.