Pyrethroid toxicity in felines: prognosis good to guarded
The pyrethroids are synthesized chemicals used widely as insecticides both in agriculture and in households.
Table 1 shows the "family members" of the pyrethroids. Different formulations including spot-ons, sprays, dips, shampoos and aerosols are used to control fleas, ticks and other insect infestations both outside and inside the house. Home pest control companies are replacing organophosphates and carbamates with pyrethroid compounds.
What are they?
Pyrethroids are lipophilic substances rapidly absorbed after oral or dermal exposures. The liver and plasma esterases hydrolyze the ester linkages of these insecticides decreasing their toxicity. The metabolites are then excreted in the urine.
In the May 2003 issue of Chemosphere (Kakko, T. Toimela, H. Tahti, pages 475-480) report that membrane ATPases are one target of the neurotoxic effect of pyrethroid compounds.
The effects on the nervous system are expressed in the axonal membrane by altering the activity of sodium ion channels. The sodium conductance is prolonged, inducing depolarizing afterpotentials. This effect results in repetitive nerve firing, and thus the neurologic signs of this toxicity.
Most brands of permethrin "spot-on" products are labeled for "use in dogs only." These permethrin compounds may be obtained over-the-counter in grocery stores or pet stores. Inappropriate use of such products in cats may lead to toxic signs and even death. The clinical signs occur a few minutes to hours after exposure and consist of depression, salivation, vomiting, muscle tremor, hyperexcitability, seizures, ataxia, dyspnea, anorexia, weakness, prostration and death.
Cats may exhibit ear flicking, contractions of the superficial cutaneous muscles and paw shaking. Cats or dogs with liver dysfunction or portosystemic shunts are at greater risk for signs of toxicity due to decreased hydrolysis of the pyrethroid ester linkages.
The diagnosis of pyrethroid toxicosis is mainly based on the history, the clinical signs and evaluation of differential diagnoses. The clinical signs of pyrethroid toxicity may be confused with exposure to organophosphates or carbamate intoxication.
Photo 1 shows a cat where an over-the-counter topical formulation has been applied by the owners. Cholinesterase enzyme activity is normal in case of pyrethroid exposure, but may be decreased with organophosphate or carbamate poisoning.
Attempted measurement of pyrethroids in blood or tissue samples is usually futile. Plasma and tissue esterases hydrolyze the compounds before assays can detect them.
Most cats affected by pyrethroid poisoning are presented as an emergency. The patient should be stabilized if hypothermia or hyperthermia is present.
There is no specific antidote for this toxicity. Seizure activity may develop and should be treated with the intravenous administration of diazepam (0.5- 1.0 mg/kg), propofol (initial dose of 1 mg/kg, then constant rate infusion of 0.1 mg/kg/minute for one to two days), or phenobarbital (1.0 - 5.0 with intravenous methocarbamol at a dose of 44 to 220 mg/kg). Half the dose is given rapidly and the remaining dosage is given to effect (Richardson JA. Permethrin Spot-On Toxicoses in Cats. J. Vet. Emerg. Critical Care, 10(2), June 2000, 103-106).
Once tremors and seizures are controlled, follow up with oral methocarbamol at 61 to 132 mg/kg/day divided every eight to 12 hours may be considered.
Gas anesthesia is another option for controlling refractory seizures.
The hair and skin should be washed with soap or detergent after dermal exposure. The use of an emetic is indicated shortly (within one hour) after oral ingestion of pyrethroid insecticide. The gastrointestinal absorption can be decreased with the oral administration of activated charcoal (2 mg/kg), preferably in association with a cathartic such as magnesium sulfate (250 mg/kg), or 70 percent sorbitol at a dosage of 3 ml/kg body weight.
Hypersalivation may be controlled with atropine at a dosage of 0.02 to 0.04 mg/kg body weight given intramuscularly or subcutaneously. Care should be taken to avoid excessive administration of atropine sulfate, which can result in central nervous system stimulation and tachycardia.
The prognosis of pyrethroid poisoning in cats is guarded to good. The best way to avoid serious problems in cats is by educating cat owners. Products that are labeled "for dogs only" should not be used on cats.