Thor610 - Thanks for that link! I'd read something unspecific about 'possible" issues and a few of the counter arguments from advocates who've never had problems before, but had not actually heard of those particular issues - maybe I've just been lucky with cats. A few drops mixed in a cup or more of hydrogen peroxide & water applied Externally as a wash has never harmed any of my them. Then again, I've never applied even 1/10 of the dose that manufactuers consider 'safe' so maybe that's why. I do much prefer adding a few drops of Grapefruit-Seed-Extract which was my first suggestion.
However, many people try to give tea tree oil internally to cats or to add it to vaporizers, both of which I would (and have) Strongly warn against as I know tea tree oil is hard for humans internally, even if you get the stuff specifically for internal use. And cats, well, basically all our furry friends, have Much more developed noses than we do and what we might think smells good can burn their more sensitive tissues. Ever notice how your cat really doesn't like being around your cleaning supplies long? Cologne and air fresheners can be irritating to them, so imagine how especially potent the stronger distilled essential oils must be. (Hmm, which seems to be what that site is actually about, I may need to read the whole site beyond just that link...)
One of the biggest mistakes people make when home treating animals is assuming that a safe ratio for them (say, 1 part per 10 per pound) is directly comparable to animals. No. First, call and ask the vet if there are any known toxicity issues (like acetaminophen and cats.) If not, figure out the dilution as if it were comparable, then use no more than 1/10 of that, or even less if the original ratio seems 'strong'. And, what is safe for dogs is not necessarily safe for cats, etc and so on with all the animals.
Just remember, as I stated earlier, a good vet is your pet's best friend and most are happy to answer quick toxcity questions over the phone. All that said, if you're not sure or feel uneasy, don't do it! =)
And, from here on out, I won't advocate any use of tea tree oil around kitties as there are numerous other products with less controversy.