DEAR DR. FOX: My 17-year-old cat Willie has been an “only child” for most of his life – up until last year, when I took in a stray kitten who’s taken up attacking Willie any chance he gets. I spray the kitten with a water bottle and sometimes shout him out of the room to give the older cat some peace. Then I feel bad because the kitten acts like he doesn’t know what he did wrong. On the other hand, poor Willie wonders what happened to his peace and quiet. It was his house first! Do you have any other ideas to get the kitten to stop this behavior? – L.J.B., Fort Plain, N.Y. DEAR L.J.B.: I would seriously recommend that you get a third cat, a little younger than your spirited adolescent. Three-cat families work out well because a “therapeutic” third cat can make for a harmonious triangle. Older cats do need to be protected from rambunctious younger cats that want to rough-play with them. Discipline does not help. Put the young cat in another room, and play games with it to give the old one a break. DEAR DR. FOX: For several years, foxes and their families have lived in our barn, and all has been well. But now two of them have gotten mites – one badly infected, one partially and the other not at all. I’m treating their food every other week with Ivomec. Was it the weather that brought the mites out in full force? – D. McC., Great Falls, Va. DEAR D. McC.: Mange mites come from other animals, some of which may be healthy carriers. These skin-living parasites have been evolving for millions of years and continue to do so as some species develop immunity to pesticides. A fox’s immunity declines during stressful times – cold, thirst, shortage of food and competition with other foxes for territory and mates. Send questions to Dr. Michael Fox, care of United Features Syndicate, 200 Madison Ave., New York, NY 10016