Other animals in the household that do go outside (like dogs and people!) can bring in different parasites that can affect the “indoor” cat -- from www.CatHospitalofChicago.com
Even if your indoor cat never goes outdoors, it is not a bad idea to implement a flea, heartworm and intestinal parasite prevention program. This is not just my opinion, but that of the Companion Animal Parasite Council (CAPC). The parasite control recommendations from the CAPC website www.petsandparasitescom state the following: “the use of year-round heartworm and broad spectrum flea, heartworm and intestinal parasite medications, as well as appropriate flea and/or tick products, is the foundation of an effective parasite control program for your cat”. -- at http://www.petparents.com/show.aspx/question/do-inside-cats-need-flea/tick-medicine.
So, I ordered the monthly preventative and am psyching myself up to apply it to the feline trio. But, for now, Micky doesn't have fleas, nor do I think he has had them in the ten years that he has lived with me, but he has lost 2 pounds since January. The diagnosis is not yet totally complete, but the broad, general thought is that he is old and things are changing -- including my opinion regarding flea and tick prevention for indoor cats.