digidoll
I’ll ditto lenamegan on the possibility of food allergies. And, I’ll add that my itchy/chewy/hotspot-prone Aussie is much less so now that I’ve switched his protein source to fish rather than chicken. Same brand (and grain-free), but different protein formula. It’s been about a month since I switched, and I rarely catch him worrying at himself anymore.
Also ditto on the tea-tree oil (or any essential oil) danger to cats, but not from personal experience.
As for Diatomaceous Earth, that stuff is great, both internally and externally, and I’ve used it successfully for treating fleas and tapeworms in both dogs and cats, as well as aphids on my rosebuds. However, do be careful about inhaling it, as it irritates soft tissue – I am very sensitive to it, and found out the hard way by almost sending myself to the ER with breathing problems after dusting it around once. Animals can be just as sensitive.
Good luck! It sucks when our critters are obviously uncomfortable…