Permethrin Poisoning (Dog Flea Treatment)

Permethrin poisoning is the classic example of a drug that is perfectly safe in one species and dangerous in another. It is a synthetic pyrethroid insecticide, closely related to the natural pyrethrins in chrysanthemums, and it works brilliantly against fleas and ticks on dogs. On a cat, the same chemical at dog-strength concentrations can be devastating.

The reason is metabolic. Cats are notoriously poor at a liver process called glucuronidation — the same deficiency that makes paracetamol so dangerous to them. Where a dog rapidly breaks permethrin down and excretes it, a cat cannot, so the compound lingers and repeatedly fires the nerve cell sodium channels it acts on. The result is an over-excited nervous system: fine muscle tremors that build to whole-body shaking, unsteadiness, hypersensitivity to touch and sound, and in serious cases full seizures. Constant tremoring also generates heat, so affected cats often become dangerously hyperthermic.

Most cases are entirely accidental. An owner treats the dog and the cat, reaching for whatever is in the cupboard; or a cat curls up next to a freshly treated dog and absorbs the product from its coat, then swallows more while grooming. Signs usually appear within a few hours, though they can be delayed.

The good news is that with prompt veterinary treatment the outlook is genuinely good. Vets will bathe the cat to remove any remaining product, use medication to stop the tremors and seizures, support the cat with fluids and careful temperature control, and may give an intravenous lipid emulsion that acts as a sink for the fat-soluble drug. Cats treated early very often make a complete recovery.

Prevention, though, is trivially easy and worth repeating to every cat owner: read the label, never use a dog spot-on on a cat, and keep treated dogs apart from cats until the product has dried.