Quoted:
We have dipped a cat in heavily diluted Pyrethrin with no ill effects. At the time, I did the math
to come up with what should have been a safe ratio when mixed with 30 gallons of water in a bath tub.
Natural repellent from Chrysanthemums: = pyrethrins
Synthetic copy of pyrethrins = permethrin
Natural pyrethrins are broken down by digestive juices, while synthetic pyrethrins (permethrin) are metabolized by the liver
Toxicity in cats occurs because their livers are incapable of properly metabolizing synthetic pyrethrins, AKA permethrin. Natural pyrethrins are generally considered safe for cats, although in some cases they can have severe reactions. Symptoms of synthetic pyrethrin toxicity are far more frequent and serious. In a British study, they reported that nearly 97 percent of 286 cats exposed to a spot flea treatment with the synthetic pyrethrin Permethrin developed symptoms of toxicity.
Permethrin isn't anywhere as toxic in dogs. In fact, Advantix II is permethrin based. So, if you have a cat, just be careful when using either the natural or synthetic versions. It is also highly toxic to fish.