Vaccine Associated Sarcoma

Vaccine Site-Associated Sarcomas in Cats

Is this something new?

Sarcoma's are not new forms of cancer, but in 1991 veterinarians began to notice a higher number of sarcoma's occurring on cats bodies in places where a vaccine is commonly injected. An association between vaccine administration and sarcoma development has been established, though most sarcomas are not associated with vaccines and those that are associated occur infrequently.

I found a lump where my cat was vaccinated. What happens now?

It is quite common for a small, firm, painless swelling to form under the skin at the site where a vaccine was injected. The lump almost always disappears after a few weeks. However, in some cases, the swelling may progress to a sarcoma. If you detect a lump, you should contact your veterinarian as soon as possible. Small samples of the lump will be sent to a laboratory for diagnosis if any of the following are true: the lump persists for more than three months after vaccination; it is larger than two centimeters in diameter; or it is increasing in size one month after vaccination. If the lump is found to be a sarcoma, you veterinarian may refer you to an oncologist for management.

If vaccines are causing problems, why use them at all?

Disturbing as the issue may be, there is great concern that cat owners, attempting to keep their cats from harm, may forego vaccinations entirely. These owners may be placing their cats at far greater risk of acquiring a fatal infection than any risk the vaccine poses.

So what's being done?

Even though vaccine-associated sarcomas are uncommon the problem is receiving attention from veterinarians and feline vaccine procedures. The Vaccine-Associated Feline Sarcoma Task Force is a coalition of national veterinary organizations dedicated to resolving this dilemma. This group has been set up to determine the true scope of the problem, the cause, and the most effective treatment of vaccine-associated sarcomas.

How should I respond?

Until this problem is solved, the best response is to discuss the issue with your veterinarian. In the vast majority of situations, vaccines are much more beneficial than harmful, and they continue to help protect cats from serious infection and disease. But one way to reduce the chance of sarcoma development is not to vaccinate unnecessarily. Veterinarians are being urged to evaluate each individual cat's risk of infection to guide in deciding which vaccines should be given. After considering both the vaccine and your cat's situation, your veterinarian will assist you in designing a vaccination program that not only protects against infectious disease but is as safe as possible

At the hospital we have been monitoring this situation since about 1990 when we first heard about the problem. We have never recommended feline leukemia vaccine for indoor cats that can be kept from direct contact with outdoor cats. In general at Foothills Animal Hospital we are recommending the following:

We recommend that cat be kept indoors and kept separate from untested cats to avoid the need for Feline Leukemia vaccine
If cats do go outside we recommend that they be vaccinated subcutaneously in the left rear leg once yearly and that the vaccine site be observed for reaction. The vaccine is not 100% effective but the incidence of the disease in the outdoor population is significant.
We recommend that a 3 year vaccine for rabies be administered every 3 years (after the first annual booster) subcutaneously in the right rear leg and that the vaccine site be observed for reaction.
We will continue closely following the literature for changes and new recommendations regarding feline vaccines.

Understanding Your Pet's Medical Diagnosis

What are vaccine site-associated sarcomas in cats?

Sarcoma is a connective tissue cancer (malignant tumor), arising from bone, cartilage, muscle, or skin. The first vaccine site-associated sarcomas were identified in locations where vaccinations were administered commonly; therefore, the tumors were called "vaccine site-associated sarcomas." The injection of vaccines can induce sarcomas in cats; however, it is now believed that the injection of other medications may lead to the formation of sarcomas. The time between vaccination or injection and tumor development may be as short as several months or may be as long as several years. This condition is recognized only in cats. The incidence of developing sarcomas is low.

What causes vaccine site-associated sarcoma?

The cause of vaccine site-associated sarcomas is unknown. The sarcomas develop following vaccination or injection. Many research studies are ongoing to attempt to identify the cause of these tumors. Aluminum hydroxide, an ingredient in many vaccines, has been suggested as a possible cause. The disease has been seen following administration of a variety of vaccines, such as rabies vaccine and feline leukemia virus (FeLV) vaccine. No particular vaccine has been identified that is more likely to cause the development of the sarcomas. Other possible causes are being investigated. These causes include growth factors and cancer-related genes in the cat itself.

What are the signs of vaccine site-associated sarcoma?

The signs of vaccine site-associated sarcomas depend on the location of the vaccination or injection and the awareness of the pet guardian. Generally, a firm, painless swelling will be identified under the skin at a previous vaccination or injection site. These tumors may be quite large.

How is vaccine site-associated sarcoma diagnosed?

Vaccine site-associated sarcoma is diagnosed by history (especially related to previous vaccinations or injections) and physical examination. Other causes of skin swelling, such as abscesses, cysts, or other types of cancer, need to be ruled out as the cause for the swelling or mass. Radiographs (X-rays) may be taken of the swelling and of the lungs to determine the extent of disease and the likelihood of any spreading (metastasis) to the lungs. The mass may be aspirated using a syringe and needle to provide cells for microscopic evaluation (cytology). A biopsy (tissue from the site is removed and examined microscopically) may be performed. Biopsy is necessary for a definitive diagnosis. Additional imaging studies, such as a magnetic resonance imaging (MRI), may be performed to evaluate the true size of the tumor. Vaccine site-associated sarcomas frequently have tentacle-like growth and the tumor visible under the skin may only be part of the actual tumor.

How is vaccine site-associated sarcoma treated?

Vaccine-site associated sarcoma must be treated aggressively. The tumor and surrounding tissue must be removed surgically. If complete removal is not possible, radiation therapy is indicated. In some cases where the tumor cannot be removed, chemotherapy may provide some relief. Following treatment, the cat needs to be seen by the veterinarian monthly for the first 3 months and then every 3 months for the remainder of the first year for follow-up evaluations.

The incidence of vaccine-site associated sarcoma may be minimized if vaccines and other injections are administered at different sites on the cat's body. Vaccines and injections should be given in sites that can be more easily removed surgically (such as low in the rear leg). These injection sites do not lower the likelihood of developing the sarcomas, but may make the tumor more accessible to treatment.

What is the prognosis for cats with vaccine site-associated sarcomas?

The prognosis (outcome) for cats with vaccine site-associated sarcomas is guarded. The prognosis depends on the rapidity of diagnosis and treatment. Vaccine site-associated sarcomas tend to be aggressive tumors that do not respond well to conventional treatment.