Cats are overly sensitive to ivermectin. You need to measure in syringe the correct amount. I don't know that amount but it would be absolutely tiny. Other antiparasitics are a better choice. If you are treating external parasites then revolution, advantage, frontline, capstar, and sargeant's are all good options. I think the last 2 you can find at petstores.
Frontline has losts it's effectiveness in several areas of the country (including where I live). I would ask around before buying it. The fleas have become immune to it