Pet Columns, Office of Public Engagement, College of Veterinary Medicine at the University of Illinois at Urbana-Champaign

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Cat Owners Must Weigh Vaccination Risks With Benefits

Pet Column for the week of April 2, 2001

Office of Public Engagement
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907
By Carrie Gustavson
Information Specialist

Millions of animals have benefited from the widespread availability of low-cost, highly effective vaccines, but recently the cause-and-effect relationship between vaccination and vaccine-induced disease, especially fibrosarcomas in cats, has become a concern of veterinarians and owners alike.

About 10 years ago a veterinarian in Pennsylvania noticed several cats with a similar aggressive fibrosarcoma tumor between the shoulder blades-a common vaccination site. A fibrosarcoma is a type of cancer that sometimes occurs at an injection site.

"Later, this trend was correlated with a new rabies law that required a shift from a modified-live vaccine to a killed vaccine," says Dr. Tim Fan, veterinary oncology specialist at the University of Illinois Veterinary Teaching Hospital in Urbana. "Killed vaccines are typically mixed in a adjuvant containing aluminum hydroxide. Evidence suggests that the adjuvant may cause excessive inflammation, which may lead to tumor formation."

Recently the American Association of Feline Practitioners (AAFP) issued new recommendations for feline vaccinations based on research into the effectiveness of certain vaccines. For feline parvovirus, feline herpes virus-1, and feline calicivirus-usually lumped together in one shot called "FVRCP"-AAFP recommends that for certain cats with minimal risk, vaccinating no more frequently than once every three years is acceptable. Keep in mind that this recommendation goes against the labeled instructions for the vaccine and that kittens, outdoor cats, and cats with unknown vaccination history run a higher risk of exposure to these diseases. Even indoor cats can run a high risk of exposure to all three of these diseases, and the vaccine is very beneficial. Vaccination for these three viruses has not been known to cause tumor formation, so it is very safe and will likely help keep your cat healthy.

Though the rabies vaccine containing the adjuvant has been more frequently associated with vaccine-associated sarcomas, it is highly recommended by the AAFP. Rabies is a lethal disease, and infected cats can serve as a source of infection in people. In addition, some cities and states legally require a rabies vaccination for cats. Except for one recently approved recombinant vaccine, all rabies vaccines currently available contain the suspect tumor-inducing adjuvant. However, because of its newness, it is unknown whether the recombinant vaccine will be less likely than other rabies vaccines to induce fibrosarcomas.

The AAFP recommends vaccination for feline leukemia virus (FeLV) based on risk of exposure: outdoor cats, stray cats, and feral cats should be vaccinated. FeLV vaccines have been more frequently associated with vaccine-associated sarcomas.

To determine which vaccinations are the leading culprits in causing fibrosarcomas, U.S. veterinarians have agreed to put different shots in different parts of a cat's body. "FVRCP goes in the right shoulder, rabies goes in the right rear leg, and FeLV goes in the left rear limb," says Dr. Fan. "In ten years we'll be able to pinpoint whether there is an increase in prevalence of fibrosarcomas in one vaccine over another."

In the meantime, cat owners should be especially alert for any lumps or bumps after the vaccination. "Be on the lookout for a mass that persists more than three weeks, is larger than two centimeters in diameter, and increases in size after a month," says Dr. Fan.

"The time it takes for a tumor to develop can vary from less than three weeks to three years," says Dr. Fan. "Fibrosarcomas tend to remain local, but can send out branches and affect multiple muscle groups. That can make them hard to remove completely with surgery. If just a little bit is missed, regrowth occurs." With this type of aggressive tumor, it is really important to catch it early.

It is estimated that tumors will occur in only about 1 in 1,000 to 1 in 10,000 cats, or about 2,200 cases annually in the entire country. Many practitioners note that the risk of acquiring many of these diseases is probably higher than the risk of developing a vaccine-associated fibrosarcoma. Your veterinarian will evaluate each cat on an individual basis and can advise you of the risks and benefits of vaccination in order to provide the most effective and safest protection possible for your cat. In the end, the decision of whether or not to vaccinate a cat lies with you, the owner.

Even if you decide not to vaccinate your cat, a yearly or semi-yearly visit to the veterinarian is important to address dental care, behavior concerns, proper nutrition, disease testing, parasite control, and control of zoonotic diseases.

For more information on the AAFP's vaccination recommendations or vaccine-induced fibrosarcomas, consult your local small animal veterinarian or the AAFP Web site at