Vaccination Risk Assessment in Cats

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Veterinarians and their teams strive to abide by the mantra “do no harm.” Although this is generally a beneficial principle, excessive caution can sometimes interfere with good medicine. This paradox can be seen when concerns over injection site sarcomas lead veterinarians to avoid recommending the noncore yet highly effective feline leukemia (FeLV) vaccine, even for cats at risk for exposure.¹ In an attempt to minimize risk, veterinarians should ask themselves whether the reluctance to vaccinate against FeLV is truly serving our patients.

Injection Site Sarcoma Risk

The risk for injection site sarcoma in cats is relatively low. The reported incidence of in vaccinated cats varies between studies but has been estimated to be 1 to 4 cases in every 10,000 vaccinated cats.^2-4 Injection site sarcoma has been associated with a variety of injection types and not linked to specific vaccines or adjuvants. Although these sarcoma cases are challenging, the condition is rare. Both pet owners and veterinary team members may perceive this risk as being higher than it is, which may interfere with accurate risk:benefit assessments for feline patients.

Feline Leukemia Risk

Although vaccines can prevent several infectious diseases, FeLV is one of the most important and concerning vaccine-preventable infectious diseases in cats. Unfortunately, FeLV infection is relatively common, even in client-owned cats. A study of >62,000 cats tested at North American veterinary clinics and in animal shelters found that 3.1% of the cats were positive for FeLV antigen.⁵ The incidence of FeLV was even higher in client-owned cats than shelter cats, likely because they were being tested due to illness instead of as part of a standard intake procedure.

An effective vaccine is available for cats that are determined to be at risk for exposure to FeLV. The American Association of Feline Practitioners (AAFP) vaccination guidelines recommend vaccinating all kittens for FeLV and administering a booster at 1 year of age.¹ Although this vaccine is considered noncore after the first adult booster and is typically only recommended for at-risk adult cats, it is important to ensure that all at-risk cats receive this vaccination.

Putting It All Together: To Vaccinate or Not to Vaccinate?

The risk for injection site sarcoma is not as high as many clients and veterinary team members might believe. Furthermore, clients and veterinary team members may underestimate the risk for infectious disease. Many cats never have their FeLV status assessed, even when it is clearly indicated. In fact, a 2008 study showed that only 10% cats with bite wounds had been tested for FeLV/FIV, and only 54% of veterinarians recommended follow-up testing, despite the obvious possibility of exposure.⁶

When clients and veterinary team members overestimate the risk for injection site sarcoma while underestimating the risk for FeLV, they may not be following evidenced-based medicine guidelines and may be avoiding effective vaccination strategies. In addition to FeLV, there are other highly contagious infectious diseases that can also be prevented through vaccination, including rhinotracheitis, calicivirus, panleukopenia, and rabies.

Although no vaccine is entirely risk-free, the risks associated with infectious disease far exceed the risk for injection site sarcoma. All cats should receive the recommended core vaccines, and noncore vaccines should be administered to any cat that is deemed to be at risk for infection—not just to cats that are thought be at an exceptionally high risk.

If the veterinary team understands the minimal risk associated with injection site sarcoma, combined with the greater risk posed by infectious disease, patients may have a greater likelihood of receiving necessary vaccines and being adequately protected against infectious disease.

Talking to Clients About Vaccination

The first step in feline vaccination is to perform an individual risk assessment. During this process, a member of the veterinary team should ask about the lifestyle of the cat to determine which noncore vaccines may be appropriate. This lifestyle assessment is essential to determine whether to continue FeLV vaccination in adult cats that have received their initial kitten series and 1-year adult booster.

Merely asking whether the cat is an “indoor” or “outdoor” cat is often inadequate in determining a cat’s risk. Supposedly “indoor” cats may interact with other cats through the screen door or even go outside in their owner’s backyard. Indoor cats may also escape and return after having been exposed to infected cats, even without obvious wounds. Finally, owners of indoor cats may not consider the vaccination status of their current cat when adopting a new cat or pet-sitting for a friend. Instead of thinking of FeLV vaccines as recommended only for outdoor cats, it is important to consider all possible exposure risks, and team members should ask questions that better evaluate whether an indoor cat is truly indoors (eg, “does your cat ever visit other cats or go on kitty play dates?”, “do you board your cat or use a cat-sitter?”). In addition, not all pet owners respond the same way to questions. Phrasing questions differently to see how clients respond could be a helpful exercise in determining how best to get informative responses and can keep the team involved in making sure all cats at risk are protected.

When discussing vaccines, it is important to dispel client myths and misconceptions. Clients who are concerned about injection site sarcoma may need to be reminded that the risk is very low, especially as compared with the risk for infectious disease.

Conclusion

Conveying an accurate understanding of vaccination risks and benefits to all clients is essential to protect cats from infectious disease. To do so effectively, it is important that the entire veterinary team understand the risks and benefits of feline vaccination.