Vaccine Associated Fibrosarcomas

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Vaccine Associated Fibrosarcomas

Since the late 1980's there has been increasing evidence that a type of cancer (called a fibrosarcoma) is caused by vaccinations in cats. Other types of sarcomas have also been linked to vaccines but in much lower numbers. Vaccines became implicated when aluminum was found within biopsy sites of these tumors. Aluminum is used as an adjuvant in vaccines. This means it modulates the body's immune response to the vaccine. Both the rabies and FeLV vaccines contain aluminum adjuvant. The current thinking is that the fibrosarcomas arise from an excessive inflammatory or immunologic reaction to the vaccine components leading to an overproliferation of certain cell types. The incidence of these tumors ranges from 1 in 1,000 to 1 in 10,000 vaccinated cats. There is some discrepancy because many experts feel that the actual number of vaccine-associated tumors is under-reported.

Any time a firm nodule on the skin is found on your cat it should be looked at by your veterinarian. This is especially true if the lump is at the site of a vaccine. When a lump is noted, it can be aspirated and have the cells examined microscopically or it can be biopsied. The latter yields much better results. If a diagnosis of cancer is made after biopsy/aspirate results are returned, further staging may need to be done to assess the extent of the tumor. Fibrosarcomas rarely metastasize (spread) but a chest x-ray and abdominal ultrasound can help assess this clinically.

Unfortunately, fibrosarcomas associated with vaccine sites are very aggressive. In one study, 62% of these tumors recurred following removal. These typically recurred within 6 months. Furthermore, when they recur, they are typically more invasive and are faster growing. Depending on the size and location of the tumor, it is sometimes best to have the tumor removed at a veterinary surgical specialist since the tumors are so invasive. Occasionally, parts of ribs or other body parts may need to be removed in order to completely eliminate the tumor from the skin. Research is being done on whether or not radiation therapy is helpful in shrinking these tumors but no conclusive evidence has been published. Chemotherapy has not been shown to be effective in treating these fibrosarcomas but research is being done in this area as well.

To help categorize the incidence of these tumors, veterinarians now routinely give vaccines to cats at different sites on the cats' body (rabies in the right rear leg, FVRCP on the right forearm, and FeLV in the left rear leg). This will help pinpoint which vaccine is causing the problem and can also help with removal of the tumor. Research is currently being done to create vaccines that are less likely to cause these tumors. Some experts are advocating vaccinating cats every three years instead of yearly. It is not certain if there is a greater risk of getting a vaccine-induced fibrosarcoma or of the cat getting a disease for which it is routinely vaccinated. It is a very important topic in veterinary medicine and it is being studied extensively on many levels.