Cat fleas can be killed by using an external anthelmintic specially designed for cats. The anthelmintic needs to be dripped on the back of the cat’s neck. This can prevent the cat from being poisoned by licking the anthelmintic. If you are worried, you can also give it to the cat. Elizabethan Circle. It is best to do external deworming for cats once a month. You should also pay attention to the hygiene of your home, clean and disinfect it regularly, and try not to take your cat into the grass.