When our cats used to go outside I used Brewer’s Yeast on their foods to repel insects. The B vitamins in it provide protection. Even my dad who used to be loved by mosquitoes was no longer bothered by them when he began taking a high potency B Complex.
You didn’t mention what diet you had them on, but regular store-bought foods are so “dead” that they create a lot of toxicity in the body during the breakdown of what’s in it. If you don’t know what’s in pet foods, read this article:
http://www.healthypetcorner.com/petdiet.html
A switch to totally natural and human-grade ingredients back in the late 70’s has made all the difference in our pets. We never have vet bills and their coat and eyes are so bright and glossy. You truly get what you pay for and in the end it costs less.