A Message from the Vet - November 2010
Vaccine Reactions and the Curious Injection Site Sarcoma in Cats

Vaccines have played a crucial role in the annual examination and health care of cats for decades. They have been tested and proven to be safe and effective for the vast majority of cats; but in rare cases, some cats display unwanted reactions of concern: lethargy and fever, or a lump at the injection site.

Since the mid 1980's, a phenomenon called injection site sarcoma was identified in veterinary medicine. It appeared to correlate with the widespread use of killed rabies vaccines and feline leukemia vaccines. Here in Hawaii, we did see some vaccine-induced sarcoma lesions, but not to the extent that our mainland colleagues witnessed. This was likely due to the decreased rabies vaccination numbers in our unique rabies-free environment.

In this phenomenon, some cats (3 in 10,000) developed a severe reaction at the site of vaccines. These were a group of cancers called sarcomas, which turned out to be very aggressive. If caught and treated early, surgery was able to remove the cancer in many cats, but certainly not all. In order to safeguard against harm, veterinarians began to reassess the wisdom of annual vaccinations for cats. The exact source of the reactions was not known initially, so veterinarians began to standardize where vaccines were given to help identify the problematic vaccines. We also reduced the number and frequency of vaccinations, depending on each cat's lifestyle and exposure risk.

Researchers found that an additive to the vaccines caused some of the problems. They also found a particular genetic family of cats that reacted the most, and within that cat group, any kind of injection (antibiotics, cortisone, worming medications, vaccines, and even cat bite punctures) could stimulate the cat's body to develop a sarcoma.

Fortunately, many steps have been taken to increase the safety of feline vaccinations. Veterinarians have modified their patients' vaccination protocols by tailoring them to each individual's risk. A few vaccines have been modified to replace the possibly causative chemicals contained. And cat owners were educated to recognize sarcoma lesions early, yielding a greater chance of treatment success. With these safety protocols in place, we do recommend vaccinating your cat to protect against the particular deadly viruses they are at risk for. Please speak with us about any concerns you have regarding vaccine-induced sarcomas at your cat's next visit.

John Haddock, DVM


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