Tea tree oil is great externally! That should help, too.
I've used a lot of herbal treatments for pets, but I have been studying herbs for years and living with and healing sick or injured pets all my life - I even trained as a vet assistant many many many moons ago. That said, and understanding I'm no doctor and I certainly believe a good vet is your animals best friend, I've never had a pet react badly to echinacea. Just remember, they are NOT little humans and their systems are MUCH smaller and faster than ours. If your tincture or remedy calls for 2 drops in a glass of water for you, it is too strong to attempt to give a cat. OTOH, brewing up some echinacea tea as usual then mixing 1/3 tea to 2/3 water and giving them a little (5 to 10cc's) every other hour or so, or 2 or 3 times a day, For A Few Days should be fine.
Oh, and do NOT give your kitty Tylenol or Advil!!! Acetaminophen(Tylenol) is toxic to cats and ibuprofen(Advil) is not much better! If you have a feverfew plant you can let them chew it IF They Want, but don't try to force feverfew into them as it can cause side effects that your cat can't 'tell' you about.