In my cat's case, she has severe allergies and, if left untreated, she would eventually lose all her hair. I'd have the vet check it out because, if it is an allergy, the itching is maddening for them. It compares to a human having poison ivy. It might be a food that's causing a reaction and, if so, you'll have to do a process of elimination. If it's environmental, then you'll have to figure out what s/he might be allergic to. Also, years ago, back in the dark ages, I bought a flea collar for a cat of mine and it caused her to lose all her hair where the collar was. And, lastly, if your cats go outdoors, it could be from fleas or bugs or allergies or a host of other things. Play it safe and have your vet look at them.