When Leanne Smith felt the golf-ball-sized lump between her cat Tardy’s shoulder blades, she was filled with dread.

“I knew what it was immediately,” said Smith, a vet tech at Georgia Veterinary Specialists in Sandy Springs. “I was really upset.”

Tardy had a rare but dangerous tumor, called an injection-site sarcoma. Years ago, Smith said, that’s where the 11-year-old cat had been given vaccinations.

An estimated three of 10,000 vaccinated cats will develop cancerous tumors in the spots where they’ve received routine shots, such as those that protect against rabies and feline leukemia.

Veterinary experts aren’t sure why this happens. But since 1996, they have recommended limiting the number and frequency of shots that cats receive because of the tumor risk. They also have recommended that cats get injections as low as possible on their legs to increase their odds of survival and allow for amputation of the limb if needed to remove a tumor.

Yet a high proportion of tumors are still developing between cats’ shoulder blades, indicating that some veterinarians aren’t following the guidelines, according to a study published last month in the Journal of the American Veterinary Medical Association. Cat owners need to take an active role in their pets’ medical care to make sure the shots the animals are receiving are appropriate, experts said.

Researchers from the University of California-Davis examined the cases of 392 cats treated at the university’s veterinary hospital. From 1990 to 1996, before the recommendations on vaccination locations, 53 percent of tumors developed in the shoulder-blade region. After the recommendations were issued, that dropped to about 39 percent during the 1997-2006 period, the study found.

“You can tell there’s been some change, but we’re still not finding that everything has changed,” said Michael Kent, a veterinarian and assistant professor of radiation oncology who was a co-author of the study.

“The fact that our one institution has seen over 400 of these, it’s sad. And probably what makes it worse is that this is a disease caused out of our desire to prevent a disease,” Kent said. Injection-site sarcomas are almost exclusively a problem for cats and, on rare occasions, for ferrets, Kent said. Dogs don’t appear to develop the tumors, he said.

A possible link between vaccinations and the development of the tumors was first recognized in the early 1990s. Some experts believe that a component in many vaccines designed to spur an immune response, called an adjuvant, is the likely culprit. Vaccines against rabies and feline leukemia viruses have been frequently implicated, but injections of nonvaccine medications also have been associated with the tumors.

In 1996, a task force of feline veterinary experts issued several recommendations, including to administer shots as low as possible on cats’ legs.

The American Association of Feline Practitioners’ current vaccine guidelines group shots into three categories: Core vaccines — such as rabies — that all cats should get; noncore vaccines that only some cats need depending on their individual circumstances; and a few shots that aren’t recommended at all.

“One of the things we try to do with these recommendations is to not vaccinate cats more often than they need to be,” said veterinarian Fred Scott, interim director of the Cornell Feline Health Center in Ithaca, N.Y.

“Each time you give a vaccine, you do increase the risk of producing fibrosarcomas,” Scott said. “Unfortunately when it does occur, it’s a nasty condition.”

The cancerous tumors tend to be aggressive and spread rapidly.

Finding tumors early improves a cat’s chances of survival, experts said. From 50 percent to 80 percent of cases can be cured with surgery and radiation, depending on the tumor and its location, said Terrance Hamilton, a board-certified medical oncologist at Georgia Veterinary Specialists.

“This is a very treatable disease if moved on early,” Hamilton said. “That’s the biggest mistake pet owners tend to do: They see a lump and assume it’s going to go away.” Any lump, particularly one that develops in areas where a cat has received a shot, should be evaluated immediately by a veterinarian, he said.

Tumors on cats’ limbs are easier to treat successfully, he said. If the tumor is between the shoulder blades, it’s more difficult to deliver radiation but still possible.

Treatment isn’t cheap: Surgery alone can cost about $2,000; if surgery and radiation are needed, the bill could be around $4,000, Hamilton said. Pet insurance, if the owner has it, will often cover the costs. And owners can also contact the veterinary drug company that made their cat’s vaccine and Hamilton said they may cover all or part of the treatment.

Smith is hopeful that Tardy will be a survivor. In January, Tardy underwent 18 days of radiation treatments and on Feb. 10 had surgery to remove the tumor.

“She actually has a really great prognosis at this point,” Smith said.

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WHICH RABIES VACCINE IS BEST?

While experts advise decreasing the frequency of injections for cats, there is disagreement about which type of rabies vaccine is safest.

Some vets prefer giving cats a rabies vaccine that lasts for three years — believing it is safest because it limits how often cats get the shot. Others think it’s safer to give cats an adjuvant-free rabies vaccine called Purevax, even though it requires a booster shot every year. The American Association of Feline Practitioners has not taken a stance on which is best.

“There is significant controversy with what exactly causes the injection-site sarcomas,” said association President Roberta Lillich, a Kansas veterinarian. “So it boils down to what practitioners feel more comfortable with.”

Lillich personally uses the three-year rabies vaccine in her practice because she believes it’s most important to limit the frequency of shots. “I feel very strongly that there is some subpopulation of cats that are genetically predisposed to develop injection-site sarcomas,” she said.

West Hamryka, a veterinarian at Sugar Hill Animal Hospital in Gwinnett County, is among the vets who prefer to reduce cats’ exposure to adjuvants and annually give the Purevax rabies vaccine. Hamryka, a past president of the Georgia Veterinary Medical Association, said most cat owners are unaware of the sarcoma issue and just trust their vets to give the best vaccine.

Georgia law requires rabies vaccinations for cats. And while state law allows use of either the three-year or one-year vaccines, some local jurisdictions still require annual vaccination, said Dana Cole, public health veterinarian with the Georgia Division of Public Health.

“We tend to support the idea that cats should be offered the three-year vaccination,” Cole said, adding that it makes it less likely that a cat will be past-due and require isolation and observation if it is bitten by a wild animal or if it bites someone else.

Whichever vaccine is chosen, it should be based on the best medical decision for the cat, not a desire to force owners to bring cats in to the vet annually, Lillich said.

Vaccinations are important, she said, but owners should realize they are not the main reason their cats need yearly checkups. The visits are critical in identifying early signs of thyroid or kidney problems, dental disease, parasites and other conditions that can shorten a cat’s life if left untreated.

Compared with dogs, cats receive significantly less veterinary care. About 36 percent of the nation’s 81.7 million cats did not visit a veterinarian in 2006, compared with just 17 percent of dogs, according to the American Veterinary Medical Association.