About FISS

Feline Injection Site Sarcoma (FISS), also known as Vaccine-Associated Sarcoma, is a rare form of cancer seen in cats. It is estimated that between approximately 1 in 30,000 and 1 in 10,000 cats are impacted by this disease. However, some experts claim the occurrence could be as high as 1 in 1,000 cats due to underreporting by pet owners and/or veterinarians.

Little is known about how or why FISS forms, but it is generally considered to be a rare and severe adverse reaction to an injection/vaccine caused by inflammatory processes that triggers neoplastic transformation (transformation of normal cells into cancer cells) in the surrounding tissue. The rabies and feline leukemia vaccines are most commonly associated with FISS; however, FISS has been linked to many other forms of injections and vaccinations. Some believe FISS may also appear spontaneously without apparent cause. Adjuvanted vaccines (vaccines which contain an ingredient that promotes a stronger immune response), especially those with an aluminum or killed-disease adjuvant, are thought to cause more inflammation in cats and therefore increase the risk of FISS. However, the link between vaccine adjuvants and FISS has not been well researched.

Approximately 80% of tumors associated with FISS are identified as fibrosacromas/soft tissue sarcomas. These are mesenchymal tumors (tumors arising from the fibrous connective tissue, blood vessels, lymphatics, nerves, adipose tissue, and/or smooth muscle) that are locally invasive and have a low to moderate chance of metastasizing (spreading to other areas of the body). Metastasis rates in cats with FISS are reported around 25% and the most common area of metastasis is the lungs. Other tumor types reported in connection with FISS include fibrous histiocytoma, osteosarcoma, rhabdomyosarcoma, liposarcoma, chondrosarcoma, and undifferentiated sarcoma.

Depiction of an FISS tumor and its far-reaching growths.

Cats appear to be far more susceptible to FISS than other commonly-vaccinated species. The reason behind this is ultimately unknown, but the limited research available indicates cats may be genetically predisposed to FISS. Mutations in the TP53 gene (the “cancer suppressor gene”) of some cats with FISS have been detected in studies conducted at the University of Minnesota. Ultimately, the pathogenesis of FISS is believed to consist of the interaction between intrinsic factors, such as individual genetics and nature and degree of inflammatory response following injection, and extrinsic factors, such as type of vaccine administered, frequency of administration, and number of vaccines administered at a site.

What makes the treatment of FISS so challenging is the unique behavior and structure of the tumors. FISS generally consists of a large, palpable tumor near the skin’s surface. However, in addition to this, these tumors are known for their far-reaching, microscopic tentacle-like growths which grow deep into the tissues surrounding the tumor. This makes complete excision of the tumor very difficult. For this reason, veterinarians are advised to administer injections in the cat’s extremities (legs, tail) so in the case that FISS forms, the limb or tail can be amputated for the best chance of complete tumor removal. FISS is also commonly seen in the shoulder area where complete excision is very difficult.

FISS is known for its high reoccurrence rate and locally-invasive nature. Because of it’s tentacle-like growths, even with complete excision of macroscopic disease it is common for microscopic disease to remain deep in the surrounding tissues. Local reoccurrence rates of FISS range between 14 and 69% and depending on a number of factors including surgical margins, tumor size, tumor necrosis, whether the surgery was performed by a primary or referral veterinarian, the location of the tumor, mitotic cell count, and many others. Reoccurring tumors are generally considered more aggressive and difficult to treat, which makes aggressive treatment upon discovery of the initial tumor imperative for your cat’s prognosis.

Photomicrographs of feline injection-site sarcoma.

a. An inflammatory response (arrow) in the peripheral part of the tumur

b. A multinucleated giant cell (arrow)