Researchers probe vaccine-associated feline sarcoma

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In a symposium on fibrosarcomas in cats at this year's AVMA Annual Convention, Dr. Dennis Macy, a professor of internal medicine at Colorado State University, asked attendees if they had seen a vaccine-associated feline sarcoma in the past two years. Roughly 80 percent raised their hands. Only a smattering went up when asked if they had successfully treated the cancer.

Vaccine-associated feline sarcomas were first recognized in 1991. The prevalence of the disease has stabilized, and the incidence ranges somewhere between one in 1,000 and one and 10,000 cats. Veterinarians are focusing on trying to improve treatment, but what about getting to the root of the problem? 

"We know the prevalence has not changed," said Dr. Richard Ford, a professor of small animal internal medicine at North Carolina State University College of Veterinary Medicine. "We have basically washed our hands and said, 'I guess we are going to have fibrosarcoma.' I don't think we can afford to continue to say that." 

Dr. Macy agreed. During his presentation, which followed Dr. Ford's, he discussed recent research he has been conducting. 

In one study, he evaluated whether all currently used FeLV and rabies vaccines induce inflammation at vaccination sites. Research has indicated that vaccine adjuvants may be associated with local, postvaccination inflammation, and adjuvants are used in many inactivated feline vaccines. 

"Inflammation and the injury associated with inflammation at the level of DNA does cause mutation and mutagenesis," Dr. Ford explained. 

Dr. Macy focused on six inactivated vaccines (three for rabies and three for FeLV) intended for use in cats. Each vaccine was administered to six cats. Twenty-one days later, researchers evaluated the cats for evidence of inflammation near the injection site. 

Among cats vaccinated with inactivated rabies vaccine, 80 percent to 100 percent had local inflammatory reactions, depending on the vaccine brand. The degree of local inflammation was similar for all rabies vaccines, even if the adjuvant contained aluminum. The inflamed lesions associated with the rabies vaccinations were roughly twice the size of the lesions associated with any of the FeLV vaccination sites. 

Of the three FeLV vaccines tested, the aluminum-containing vaccine most consistently induced local inflammation at the vaccine site, followed by nonaluminum-adjuvanted FeLV vaccines, which caused measurable reactions in only one of six vaccinated cats. No measurable local inflammation was evident at the nonadjuvanted FeLV vaccination sites. 

Dr. Macy also investigated whether administering vaccines intramuscularly or massaging the vaccination site after administration impacted inflammation. Some veterinarians have suggested that vaccines administered intramuscularly do not induce tumors and that massage immediately after a vaccination may prevent subsequent tumor development. Dr. Macy says he did not observe a significant reduction in inflammation when a vaccine was administered intramuscularly. He also found that massaging a site only changed the shape but not the volume of the inflamed area. 

Dr. Macy says vaccine-associated feline sarcoma may have a genetic component. Vaccines cause localized inflammation in nearly all the vaccinated cats, but tumors obviously don't develop in all cats. He says some cats develop sarcomas at every vaccination site, and many related cats are affected with vaccine-associated sarcomas. An inherited susceptibility or a genetic defect may play a role in pathogenesis of the tumors. 

Clearly, more research is needed.