Whether to vaccinate against feline leukemia (FeLV) is a good question for many kitten and cat owners. The answer depends on several factors.

The first question concerns the potential for exposure your cat or kitten will have to an infected animal. FeLV is spread by direct contact with an infected cat. Obviously, cats that are allowed to roam outside unsupervised are at greatest risk.

Many people assume that if they keep their cat indoors they don't need to consider vaccination against FeLV. That's true up to a point.

FeLV vaccine is not considered a core vaccine for most cats. For kittens though the American Association of Feline Practitioners does recommend FeLV vaccine.

Why is it considered core for kittens and not for cats?

Two reasons really. The first is that FeLV is primarily a disease of kittens and young cats. As they age they become much less susceptible to infection. Virtually 100% of kittens exposed to the FeLV at 6 weeks of age will become infected for life. That drops to 30% at six months and only 5-10% at one year of age.

The second reason is that things change. You might be certain that you will keep your new kitten indoors when you first bring her home. Statistics indicate that this perception is subject to revision.

Many indoor only kittens end up spending time outdoors, and despite the fact that adults are more resistant to infection, any healthy susceptible adult can become infected if they come into contact with a carrier cat.

If I was bringing home a new kitten I'd get the core series; feline rhinotraecheitis (herpes), calici virus and panleukopenia (distemper), and I'd get a series of FeLV vaccines and rabies of course. I would booster the core series at one year and then no more than every three years. Unless my indoor kitten had become an indoor/outdoor cat I'd forego subsequent FeLV vaccines.

For more on the general topic of Feline Leukemia take a look at this article from the Veterinary College at Cornell University.