It was with concern that I learned of an urgent recall by the Veterinary Medicines Directorate (VMD) of a flea product intended for tick and flea treatment in dogs that has, in unknown quantities been incorrectly packaged for cats and kittens. The product is manufactured by Armitage Pet Care and the details can be found on the VMD website below:
Working in busy general practice, every year I see several cats that have been poisoned by incorrectly applied over the counter flea products. The majority have been treated by their owners in error with spot-on products intended only for use in dogs.
Anyone who owns pets will understand the nuisance and distress an infestation of fleas can cause. In fact the most common species of flea found on both cats and dogs is Ctenocephalides felis felis, commonly known as the cat flea. The term ‘cat flea’ can however, cause confusion, as the name refers to the genus and species and not to fleas only isolated from cats. Not surprisingly, in my experience owners tend to be in a hurry to treat an infestation once it has been discovered. On the surface, over the counter products can appear to offer a cheap, more readily accessible alternative to the somewhat more expensive licensed products offered by vets.
So if dogs and cats can share the same fleas, why can’t they share the same treatment?
In the majority of cases, such as with the current product recall, the toxic component to cats is permethrin, a synthetic pyrethroid. Permethrin originates from a modified extract of the chrysanthemum flower. It is an insecticide, commonly used in spot-on preparations used control fleas, ticks, lice and ear mites. Insects are very sensitive to permethrin, in addition their lower body temperature increases the toxic effect making permethrin an effective insecticide. Cats can tolerate up to 0.1 per cent solutions well but the higher concentrations of 2 % or more found in some preparations for dogs are extremely toxic.
One theory behind why cats are so sensitive to permethrin is that as obligate carnivores they have no evolutionary requirement to metabolise plant based toxins. In herbivores and omnivores such as the dog for example an enzyme called glucoronyl transferase metabolises permethrin in the liver via a process called glucoronidation but in cats gene expression for the enzyme is believed to be faulty. It is the lack of this enzyme that also makes drugs like paracetamol so toxic to cats. Other chemicals added to flea products to improve efficacy may also block alternative metabolic pathways for permethrin in the cat. In addition permethrins are absorbed via the skin, however cats are fastidious about grooming and exposure is significantly increased by oral ingestion.
Permethrin is lipophilic (fat soluble) allowing it to distribute into fat and also cross the blood brain barrier, it accumulates in nervous tissue. Permethrin has a reversible effect on voltage gated sodium channels present in cell membranes. In nerve cells premethrin causes earlier depolarisation of the cell membrane generating repetitive firing of impulses known as ‘action potentials’ that travel to other nerves or stimulate muscle fibre contraction.
Over ninety percent of cats exposed to permethrin spot-on develop clinical signs after exposure. Onset of symptoms occur within 1-3 hrs, they include tremors, twitching, seizures and hyperthermia (as a result of increased muscular activity). Recovery can take 2- 3 days and in some cases longer. Breathing difficulties may develop secondary to muscle weakness. The effects can be fatal in 10% of cases.
Treatment consists of removal of the chemical and controlling the effects on the nervous system with muscle relaxants and anti-seizure medication whilst continuing supportive therapy like intravenous fluids to prevent dehydration. More recently however intralipid therapy has shown promise as a potential method of detoxification. Intralipids are sterile fat emulsions, traditionally used in human medicine to provide fat and energy intravenously to patients who are unable to eat for prolonged periods. In a toxicological context Intralipids were first used in people to reverse the toxic effects of local anaesthetics on the heart. Recently veterinarians have been using intralipid to treat lipohilic toxins like permethrin. Intralipids are thought to form a lipid sink for lipohilic drugs reducing the distribution of the drug within fats and nervous tissue and expediating recovery.
Having witnessed the dramatic and prolonged effects of permethrin toxicity first hand I would always urge pet owners to use flea products recommended by their vet.
J Feline Med Surg. 2007 Aug;9(4):335-9. Epub 2007 Jul 12. Clinical effects and outcome of feline permethrin spot-on poisonings reported to the Veterinary Poisons Information Service (VPIS), London.Sutton NM1, Bates N, Campbell A.
Journal of Veterinary Emergency and Critical Care 22(6) 2012, pp 697–702 Use of intravenous lipid therapy in three cases of feline permethrin toxicosis Mark D. Haworth and Lisa Smart
Court MH & Greenblatt DJ (2000) Molecular genetic basis for deficient acetaminophen glucuronidation by cats: UGT1A6 is a pseudogene, and evidence for reduced diversity of expressed hepatic UGT1A isoforms. Pharmacogenetics 10(4):355-369
Veterinary Clinics of North America November 2009 Volume 39, Issue 6, p993-1212 Small Animal Parasites: Biology and Control Edited by David S. Lindsay, Anne M. Zajac. Biology, Treatment and Control of Flea and Tick Infestations p1173-1200 B Blagburn, M Dryden
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Small animal Critical Care Medicine edited by Deborah C Silverstein, Kate Hopper published by Saunders Elsevier. Pyrethrins. Manuel Boller Chapter 94 p394-398
Kirk’s Current Veterinary Therapy XV edited by John D Bonagura, David C Twedt. Over-the-Counter Drug Toxicosis Chapter 27 p115-120 D. Dorman.
Color atlas of physiology, 4th Edition by Agaemnon Despopulos and Stefan Silbergnagl published by Thieme