Noncore vaccines: which ones and how often? Part 2 – cats (Proceedings)

Nov 01, 2009

Classification of feline vaccines

As with canine vaccines, feline products are now categorized as core, noncore, and not generally recommended. As the names imply, vaccines that all cats should receive are core, while noncore are optional based on lifestyle, local disease prevalence, and risk/benefit ratios. These categories are from the 2006 AAFP guidelines (see Proceedings on core vaccines).

Core vaccines include panleukopenia, herpes, calici, and rabies. Vaccines classified as noncore and not generally recommended are discussed below.

Noncore vaccines

Leukemia virus (FeLV)

There is ongoing debate as to which cats should be vaccinated for FeLV, and if so when and how often. It can be difficult to assess an individual cat's risk of being exposed. Certainly cats that spend time outdoors are more likely to be infected than indoor cats, but lifestyles and confinement can change through a cat's lifetime. Owners may believe they will keep new them indoors for life, but cats may have other ideas.

Kittens are much more susceptible to FeLV infection than adults. Cats older than 1 year of age can be exposed but are less likely to become persistently infected. The minimum age that kittens can be vaccinated (according to label recommendations) is 8, 9, or 10 weeks, depending on the brand, so FeLV vaccine is typically not given to kittens presented at 6-8 weeks of age.

After two vaccinations 3-4 weeks apart (for either kittens or adults), a booster is given 1 year later. Afterward, annual revaccination is recommended as extended DOI studies have not been performed. A 1-year challenge study is available for one vaccine (Leukocell 2, Pfizer). Kittens were vaccinated at approximately 9 and 12 weeks of age and challenged oronasally one year later. 14/18 vaccinates (78%) were protected. However, only 9/15 (60%) of controls (nonvaccinates) became infected, so the preventable fraction was 63%. The likely explanation for the 6/15 young adult cats that were not infected by the challenge is age-related resistance. The 4/18 cats not protected despite vaccination indicates that FeLV does not always induce immunity. A relative lack of efficacy was observed in an outdoor colony where 3 of 5 FIV+ cats became infected with FeLV even after annual vaccination (Leukocell 2). All 19 FIV- cats were protected from FeLV up to 5 years with annual vaccination.

Efficacy studies of FeLV vaccines are available but difficult to compare, as different models of vaccination, challenges, and outcomes were used. An older study of one product showed that immunity lasted for 3 years, and based on knowledge of immune system memory, it is reasonable to expect that annual revaccination is not absolutely necessary. Another study found that cats vaccinated and then challenged with FeLV did not have viremia (ELISA negative) but still tested positive for proviral RNA by quantitative and real-time RT-PCR testing. Therefore, FeLV vaccination is unlikely to provide "sterilizing" immunity but does seem to protect against viremia, shedding, and clinical disease.

The safety of FeLV vaccines has been a concern ever since the recognition of injection-site (or vaccine-associated) sarcomas. Some studies suggest that both rabies vaccine (RV) and FeLV are associated with an increased risk of post-vaccination chronic inflammation, most likely due to adjuvants. Some practitioners now limit FeLV vaccines to cats at risk of exposure, while others recommend routine vaccination to most or all cats. The current AAFP recommendation is to vaccinate all kittens due to their higher risk of infection and unknown indoor/outdoor status later in life. For adult cats, more selective use is appropriate, considering that inflammation and neoplasia (while rare) can be a significant problem.

Another strategy to minimize the risk of FeLV vaccine-associated sarcomas is to use a nonadjuvanted product. Currently, only one type is available in the U.S. that is produced with recombinant technology and delivered transdermally with a needle-free injection device. Studies and clinical experience to date suggest that post-vaccination inflammation does not occur, which indicates that neoplastic transformation is unlikely. This vaccine stimulates both humoral and cell-mediated immunity because of delivery directly to dendritic (antigen-presenting cells) in the skin. This is in contrast with other vaccines that are injected into the subcutaneous space and are not in direct contact with the immune system.