Injection-Site Sarcomas in Cats

“Cats are not small dogs,” is a saying used in veterinary medicine to capture the uniqueness of the feline species. It helps us remember that treatments used for dogs may not be suitable for cats. One unique feature of cats is how they react to anything placed under their skin. When medications, nonabsorbable suture and microchips are placed into the subcutaneous tissue of cats, an inflammatory reaction occurs. In some cats, “It is hypothesized that post-injection inflammatory reactions, with the release of cytokines and growth factors, result in uncontrolled fibroblast and myofibroblast proliferation, leading to malignant transformation.” (Axiak, S) This means chronic inflammation may lead to cancer.

In my experience, fibrosarcomas are the most common type of tumor to occur after an injection in cats although osteosarcomas, chondrosarcomas, malignant fibrous histiocytomas, rhabdomyosarcomas and undifferentiated sarcomas have been documented. They usually occur after vaccines although they can also occur after injection of long-acting medications like the steroid triamcinolone. The malignant transformation may occur within weeks to years of the injection. I have seen two cats over my career who developed cancer from a rabies injection. Historically, fibrosarcomas are more likely to occur after rabies and FELV vaccination in products that are made from ‘killed’ virus or contain adjuvants (aluminum).

To decrease the occurrence of fibrosarcomas after vaccination, the Feline Injection Site Sarcoma Guidelines recommend:

  1. Vaccinate as infrequently as possible to prevent adverse reactions.
  2. Give vaccines subcutaneously, not intramuscular.
  3. Bring vaccines to room temperature prior to injection.

A fourth recommendation, that I offer and adhere to in my own practice, is to use non-adjuvant vaccines containing as few antigens as possible. Adjuvants are added to vaccines to amplify the immune response.  A study by Dr. Day found that vaccines that did not contain adjuvants created less inflammation at the injection site. Since chronic inflammation increases the chances of malignant changes in fibroblasts, I try to avoid adjuvants in cats. So far, there are no well-designed scientific studies which document a lower rate of sarcoma between vaccines with adjuvants versus those without.  However, it seems commonsense to me to use those without adjuvants as they may be superior – so in case it is true, I believe we should give our cats every effort we can to prevent sarcomas.

Once an injection-induced sarcoma has developed, the prognosis is poor. Even with aggressive surgical removal, recurrence is common because these tumors have indistinct boundaries. Advanced imaging with CT often demonstrates the tumor extends far beyond the lump at the injection site. Radiation and chemotherapy have been tried with varying degrees of success. In my experience, it is rare for a cat to live more than 2 years without recurrence even with radiation. The tumor invades surrounding tissues including bone causing pain. Most cats are humanely euthanized to prevent further suffering.

If a lump develops at the site of a vaccination, the Vaccine-Associated Feline Sarcoma Task Force recommends the 3-2-1 rule. A biopsy should be performed if the lump has been present for more than 3 months, the lump is > 2 cm in size or if the lump increases in size after 1 month. All injection sites should be monitored closely for formation of a lump.


-Axiak, Sandra. “Feline Injection-Site Sarcoma: You have asked…What is the ideal way to identify and approach feline injection-site sarcoma?” Ask the Expert/NAVC Clinician’s Brief/February 2012.

-Day, MJ, et al. “A kinetic study of histopathological changes in the subcutis of cats injected with non-adjuvanted and adjuvanted multi-component vaccines.” VACCINE. 2007 May 16;25(20):4073-84. Epub 2007 Mar 7.

-Hartman, Katrin. ‘Feline Injection Site Sarcoma’ Update Sept 24, 2015. First published J. of Feline Medicine and Surgery(2015)17:606-614.



Published by kristennelsondvm

Dr. Kristen Nelson grew up on a farm in Watertown, Minn., where she developed a deep love for animals of all kinds. She received a Doctor of Veterinary Medicine degree from the University of Minnesota, College of Veterinary Medicine. Kris then completed a small-animal internship at the prestigious Animal Medical Center in New York City. In addition to writing and speaking, she cares for small and exotic animals in Scottsdale, Az. Dr. Nelson is widely quoted in the media. Her credits include Ladies’ Home Journal, USA TODAY, the Los Angeles Times and numerous radio and television interviews. Dr. Nelson has written two books, Coated With Fur: A Vet’s Life and Coated With Fur: A Blind Cat’s Love. Kris and her husband Steve share their home with rescued cats, birds and a dog.