Amanda Boag MA VetMB DipACVIM DipACVECC FHEA MRCVS
Lecturer in Emergency and Critical Care, Royal Veterinary College, London

A number of permethrin based insecticides are available in the UK as spot-on formulations for the treatment of fleas in dogs. When used correctly in the dog, they have a wide margin of safety and present little poisoning risk. Cats are however particularly sensitive to these products and the clinical signs of intoxication can be devastating. A study conducted by the Veterinary Poisons Information Service (VPIS) and shortly to be published in the Journal of Feline Medicine and Surgery confirms the reality of this problem in the UK today.

Permethrin is a synthetic pyrethroid compound. Both its insecticidal and toxic properties relate to its effect on neuronal sodium channels, both insect and mammalian. The period of sodium conductance following nerve activation is prolonged leading to repetitive nerve firing. Cats are typically exposed either when their owner inadvertently uses a product labelled as for dog use only on their cat, or when cats come into contact with dogs that have been treated recently. The reason cats are so susceptible to permethrin toxicity is unknown but is thought to be related to differences in metabolism between species.

Clinical signs can develop within hours of exposure although may be delayed for up to 72 hours. Signs relate principally to the neurological system with muscular tremors, facial twitching, hyperaesthesia, ataxia and seizures being commonly reported. Hyperthermia may result from the excessive muscular activity. Other signs include vomiting and diarrhoea, depression and hypersalivation. Without prompt recognition and treatment, the seizures may progress to unconsciousness and death. Topical allergic reactions have also been reported but are rarely life-threatening.

Diagnosis is based on the presence of compatible clinical signs and recent confirmed or suspected exposure to a permethrin product. Differential diagnoses include metabolic abnormalities (hypoglycaemia, hypocalcaemia), intracranial disease and other toxicities (e.g. other insecticides, strychnine, metaldehyde, tremorigenic mycotoxins).
Treatment should be instituted at the earliest possible opportunity to give the best chance of a successful outcome. No specific antidote is available and the general principles of managing any poisoned patient should be employed. Further exposure should be reduced by bathing dermally exposed animals in a mild dishwashing detergent. In some long haired patients, clipping may also be helpful. Following decontamination the cats should be collared to prevent grooming and also isolated from other animals to prevent cross contamination. Care must be taken not to induce hypothermia, as the toxicity of permethrin is enhanced by low body temperature. As most exposures are dermal, gastrointestinal tract decontamination is not usually recommended except in the rare instances where oral exposure is documented.

Control of the seizures and/or muscle tremors may also be required. Although diazepam may be effective in mildly affected cases, it is rarely sufficient to control clinical signs in more severely affected patients. In this situation, the drug of choice is the centrally acting muscle relaxant methocarbamol. This drug is widely used in the US for this purpose when it is initially administered intravenously at a dose of 50-150mg/kg – the first half of this dose is given slowly over approximately 5-10 minutes with the second half of the dose being given as needed to effect. The dose can be repeated up to a maximum of 330mg/kg/day – in severely affected cases, the total daily dose can be calculated and given over 24 hours as a constant rate infusion. Unfortunately, intravenous methocarbamol is not available in the UK and other drugs may need to be employed. A constant rate infusion of propofol (0.1-0.4 mg/kg/min) may be used but the duration of infusion in cats is limited to approximately 12 hours due to its ability to promote red cell oxidative injury in this species. Phenobarbital is another drug that has been used; however it should be noted that even when given intravenously its onset of action is in the order of 20-30 minutes making it less useful for control in the patient with acute seizures. Whatever drug is chosen, it is not usually necessary to eliminate all muscle activity and a judgement must be made as to the balance between the improvement in clinical signs and potential side effects of the sedative drug.

Clinical signs may take 2-3 days to resolve and patients may need to be sedated throughout this time. Meticulous nursing care is a very important part of a successful outcome. Fluid therapy should be used (typically a balanced electrolyte solution such as Hartmanns) at 2-4ml/kg/hr to account for maintenance needs and any ongoing losses. Promoting good urine production is important to minimise the chances of any renal damage secondary to rhabdomyolysis. Patients should be turned regularly and kept clean and comfortable. Body temperature should be monitored as once muscle tremors are controlled these patients are prone to hypothermia which in itself may slow recovery. Nutritional support should be considered in severely affected patients. Dependent on the degree of sedation, the airway may also need to be protected by placement of an endotracheal tube.

For mildly affected cases the prognosis is good and there are unlikely to be any long term effects. For more severely affected cases, survival rates can also be good but only if patients receive early aggressive management; unfortunately some will be euthanased or die due to the severity of their signs. Although most cats can recover with veterinary care, treatment may need to be intensive and the problem is best avoided if possible. Educating owners as to the potential severity of signs that can follow exposure of cats to some canine flea products is an important part of helping achieve this goal.

Further reading
Hansen SR (2006) Pyrethrins and Pyrethroids. In: Small Animal Toxicology, eds ME Peterson and PA Talcott pp 1002-1009, Elsevier Saunders, St Louis.
Richardson JA (2000) Permethrin spot-on toxicoses in cats Journal of Veterinary Emergency and Critical Care 10 103-106.

Please also see:

'Clinical effects and outcome of feline permethrin spot-on poisonings reported to the Veterinary
Poisons Information Service (VPIS), London'

Tackling fleas on cats