If I am allowed to post an email address for WormGuard Plus you can email them and ask about their product. It is food grade Diatomaceous Earth that you can add to the cats' food to rid them of parasites. As TokyoParrot said, it can be put down around the bowls of food.
Email address is: SCDP@Videotron.ca
I have been using this product for quite some time now with no ill effect.