Feline Injection Site Sarcoma in Cats

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  • What is feline injection site sarcoma?

    Feline injection-site sarcoma, (vaccine-associated sarcoma, vaxosarcoma or vaccine site sarcoma) is a rare but aggressive type of cancer with links to vaccinations, especially rabies, and feline leukemia vaccines.

    Feline injection-site sarcoma (FISS) was first noted in the late 1980s and a link was made between certain vaccines and VAS in 1991. Dr Mattie Hendrick, a veterinary pathologist at the University of Pennsylvania School of Veterinary Medicine, observed sarcomas appearing at the site of vaccinations. Before this, in 1985 several changes to vaccinations had occurred.

    • The killed rabies vaccine replaced the modified live virus with the addition of adjuvants to stimulate a stronger immune response.
    • Introduction of the FeLV vaccine.
    • Pennsylvania made rabies vaccination compulsory in 1987.
    • In 1987 the rabies vaccine became compulsory in Pennsylvania.

    The exact incidence is now thought to be less than 1 per 10,000 per doses of vaccine. Rabies and FeLV are the most common vaccines associated with injection site sarcoma; however, any vaccine, and in fact injection (such as a microchip, or long-term antibiotics or corticosteroids) can cause an injection site sarcoma.

    The cause is still not fully understood, the adjuvant holds the antigens at the vaccine site which is released over a period of time, to stimulate an immune response and it is speculated that this can cause inflammation, which in turn develops into cancer. DNA damage can promote resistance to chemotherapy drugs.

    The most common sarcoma is fibrosarcoma, although there are reports of other types of sarcoma.


    The most common symptom is a firm subcutaneous swelling at the site of the injection; there may also be ulceration. If you notice a lump at the site of a recent injection, see your veterinarian.


    Your veterinarian will perform a complete physical examination and obtain a medical history from you.

    Veterinarians have created the 3, 2, 1 rule as follows:

    • 3: Present in any form for three months after vaccination
    • 2: Larger or equal to 2 cm across
    • 1: Present for one month after vaccination and fast-growing

    If any of the above criteria are met, the veterinarian will perform a wedge biopsy, which will be evaluated by a specialist laboratory (histopathology).

    Other diagnostic workups:

    • CT scan, x-ray and ultrasound: Where indicated, diagnostic imaging can help the veterinarian to determine the extent of the lesion. It is also useful to perform chest and abdominal imaging to determine if there is any sign of metastasis (the tumour spreading).
    • Baseline tests: Biochemical profile, complete blood count, and urinalysis to evaluate the overall health of the cat.


    Vaccine-associated sarcomas are fast-growing and very invasive. Removal involves a wide margin of 5 cm or amputation of the limb.

    Radiation therapy will be a follow-up.


    As injections are the cause, and all cats will need vaccines and injections at some point in time, it is not possible to prevent injection site sarcoma. But, there are ways to reduce risks:

    Follow the latest vaccination protocols set out by the American Association of Feline Practitioners.

    Avoid administration of vaccines in the back of the neck. Safer options include the limbs and distal (furthest away from the body) part of the tail.

    Inject cats only when necessary, opt for oral medications where available.

    Seek veterinary attention if you notice any lumps or bumps on the cat after a recent injection or microchip.

    Key points

    Feline injection-site sarcoma is a serious issue, but we must remember that vaccinations have saved millions of lives. Vaccinations are still an important aspect of cat health. Your veterinarian is the best person to advise you on vaccinations, and you should speak to him about any concerns you may have.

    Current AAFP guidelines state:

    The veterinarian should undertake a clinical risk/benefit assessment for each animal and discuss recommended vaccination schedules with the owner so that they can make an informed choice. The assessment should include discussion on the likelihood of exposure, the health and lifestyle of the animal, and the risks related to vaccination.

    Additional reading:

    Musings on feline injection-site sarcomas

    2013 AAFP Feline Advisory Panel Report


    • Julia Wilson, 'Cat World' Founder

      Julia Wilson is the founder of Cat-World, and has researched and written over 1,000 articles about cats. She is a cat expert with over 20 years of experience writing about a wide range of cat topics, with a special interest in cat health, welfare and preventative care. Julia lives in Sydney with her family, four cats and two dogs. Full author bio