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A new cancer concern for cat owners is the link between the development of fibrosarcomas at injection sites.  The problem concerning how to safely immunize against such diseases as Feline Leukemia, Distemper, Viral Rhinotracheitis, Calicivirus and Pneumonitis to owners and breeders of cats that develop cancer at injection sites is the cause of current research.

It has been discovered that vaccine connected fibrosarcomas have a rather low incidence rate from one in 10,000 to one in 1,000 cats vaccinated.  However, the injection site tumors do spread fast and are poorly defined, which complicates surgery to completely remove the cancer.  These tumors have been reported to occur from two month to 10 years after vaccination, leaving owners with the difficulty of connecting a lump to a long past injection sight.

Commonly used treatment options include surgery, radiation and new chemotherapy drugs.

These injection site tumors are malignant cancers that manifest themselves in connective tissues, such as muscle.  They may be movable lumps under the skin or growths that deeply attach to under lying tissue.  A few cats develop multiple tumors at separate vaccination sites.  Owners should be aware of the risks in order to identify lesions early, when help can be given to eradicate the tumor.

Recommendations of sites to administer injections have been changed.  It is suggested that Rabies is to be placed in the bottom part of the right hind limb, feline leukemia in the bottom part of the left hind limb, and all others in the right shoulder.

The actual vaccine can cause inflammatory or immunologic reactions at the vaccine site.  The components of vaccines may stimulate immune cells to produce growth factors that consequently stimulate cancer cells, which make up connective tissue.  Cancerous tumors form in areas of inflammation produced by vaccines that contain enhancers that help to improve immune response to the vaccine.  The glitch some with the idea that many vaccines would be ineffective at immunizing a cat. A solution could be the use of vaccines without adjuvant. 

Treatment for cats that develop lumps or growths under their skin is given by Veterinary Cancer Specialists, who can give aggressive, multiple treatment procedures.  Sophisticated diagnostic imaging for treatment planning is important in helping to identify the tumor and define its margins. Any mass located at a vaccine site that persists for more than three months after vaccination should be surgically removed.  Some of these may be vaccine reactions rather than tumors.  A biopsy is used to determine whether a cancerous growth is malignant or a vaccine reaction. 

Removal surgery seldom is a total cure and may lead to a cancer recurrence and even more difficult second surgery.  The results are a 30 to 70 percent recurrence rate.  Surgery is a vital chance for a cure, but even in combination with radiation, the cure rate is low. 

Radiation therapy alone is not recommended, but it can be used to reduce the tumor size and increase patient comfort.  Combining surgery and radiation has proved helpful in increasing the control rate of the tumor. Chemotherapy helps to reduce the size of the tumor, and helps with surgery.

New on the scene of the battle against feline cancer is a chemotherapy drug called ifosfamide, which is effective in humans.  It has been proven to help cats with injection site cancer. However, tests are still on going to determine the best way to use it, resulting in evidence that it could make a major advance in the fight against feline vaccine site fibrosarcomas.    Side effects include the suppression of bone marrow formation.  However, recovery is usually very fast and the chance of illness is low. Other side effects are loss of appetite and vomiting.


In humans complications from ifosfamide are kidney and bladder toxicity.  For cats fluids are given to protect the kidneys and a drug is used to prevent bladder damage.  So far in the study no bladder damage has been seen in any cats.



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by John & Diana Fineran - Aug 1999- 2022.  
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