In a longhaired cat it can be hard to find and as I mentioned can start off just looking like a bit of dandruff, the pic you have shown is once it has gotten hold and spread, that cat also looks like it has been clipped. Have a very careful comb through the coats and see if you can see any specks that look like dandruff.

Common sense but wash your hands every time you have handled the cat or you could end up catching it again if your cats have it