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Post-Vaccination Sarcoma in Cats

By Rania Gollakner, BS DVM; Ernest Ward, DVM

Care & Wellness, Medical Conditions, Pet Services

What is a sarcoma?

A sarcoma is a term for any cancer of mesenchymal tissues. Mesenchymal tissues include connective tissues such as skin and muscles, bone, cartilage, peritoneum, and blood vessels. Post-vaccination sarcomas usually involve the fibrous connective tissue under the skin and are often fibrosarcomas.

Why are they called post-vaccination or vaccine associated?

Sarcomas affecting various body tissues have always been seen in cats, but since the early 1990's, the occurrence of sarcomas at sites commonly used for vaccination, such as the shoulder, lumbar region (lower back), flank (side of body), and upper hind leg, led researchers to see if there was a direct association with vaccination.

"...sarcomas have been associated with injections of other medications in cats..."

It is important to realize that sarcomas have been associated with injections of other medications in cats, so the condition is more correctly called injection-site sarcoma.cat_sarcoma_location_2018-01

Is there an association with vaccination?

The evidence is still not clear, but in a few genetically susceptible cats it seems that some components of certain killed vaccines may trigger a prolonged and intense inflammatory reaction that can eventually become a sarcoma. Based on research in North America and Great Britain, it appears that the problem is related to adjuvanted vaccines, or vaccines that contain a substance designed to enhance the immune response.

Recent genetic studies in the United States have demonstrated mutations in a specific gene (called the tumor suppressive gene) in cats that developed sarcomas. There is a considerable amount of research being conducted to determine what role the vaccine plays in the development of sarcomas. The association is still controversial and the most recent research continues to question the exact mechanism of action for these tumors.

How common is the condition?

In relation to the numbers of cats vaccinated, the incidence of sarcomas is extremely small.

"The likelihood that sarcomas are the culmination of multiple factors is thought to be the most likely explanation."

While the exact incidence is unknown, it has been reported to be somewhere between 6 and 30 per 100,000 cats vaccinated. The likelihood that sarcomas are caused by something other than a vaccine or is the combination of multiple factors is thought to be the most likely explanation.

How can it be recognized? 

These sarcomas are usually seen as single, firm lumps under the skin commonly over the flank or shoulder blade. They usually do not develop sooner than three months after a vaccination. Any lumps that develop in the first 3 months after vaccination are likely part of a transient local response to the vaccine, and these usually resolve by three months.

How are sarcomas diagnosed?

A biopsy of the lump and histopathology (microscopic examination) of the abnormal tissue will confirm that it is a sarcoma as opposed to inflammation or infection.

What is the treatment?

"A generous amount of tissue around the margins of the tumor must be removed to reduce likelihood of recurrence."

The tumor tissue must be surgically removed. A generous amount of tissue around the edges of the tumor must be removed to reduce the likelihood of recurrence. Depending on the size and position of the tumor, amputation of a limb or other tissues may be necessary.

Is treatment usually successful?

These sarcomas do tend to recur at the original site. Metastasis or spread to other sites occurs less frequently. The use of radical surgery, combined in some cases with radiation or chemotherapy, increases survival time.

In view of these sarcomas, is it safe to vaccinate my cat?

Your veterinarian will discuss all aspects of the risks and benefits of vaccination with you. Vaccine manufacturers are actively investigating this concern, and most of feline vaccines have adjuvant free options. In general, the risk of your cat becoming infected with a serious disease is far greater than the very small risk of developing a sarcoma.

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