Pyrethrins are insecticidal substances produced from the flower head of the pyrethrum plant, Chrysanthemum cineraiaefolium. Pyrethrins rapidly knock down, paralyze and kill insects by disrupting nerve function.

Pyrethroids are synthetic analogues and derivatives of pyrethrins, which have undergone extensive chemical modifications to make them more toxic to insects, with a greater knockdown effect than the plant pyrethrins.

What is the toxicity of pyrethrins and pyrethroids?

The toxicity among the various pyrethrins and pyrethroids varies greatly, which is reflected in the wide range of LD50 concentrations— LD50 is the dose at which 50 per cent mortality is observed in exposed laboratory animals.

Generally, pyrethrins are less toxic to mammals than pyrethroids and the toxicity of pyrethroids varies depending on the type of pyrethroids (isomer ratio), as well as factors such as the type of formulation.

The primary target of acute pyrethrin, or pyrethroid, induced toxicity appears to be the nervous system. In cats exposed to high concentrations of permethrin (a third-generation pyrethroid), clinical signs such as hyperexciteability, depression, vomiting, excessive salivation, muscle tremors, not eating, seizures, convulsions, lack of coordination and death have been observed within minutes or days after treatment.

The clinical signs associated with high doses of pyrethrins or pyrethroids in cats are generally not seen at lower doses.

The line-on or spot-on flea treatment products registered for use in dogs but not cats generally contain high concentrations of permethrin (400 to 650 g/L) and as a result, are considered highly toxic to cats.

The pyrethrin-containing products formulated as sprays or powders contain a much lower concentration of active constituent (1.8 g/L for spray products and between 1.0 and 2.5 g/kg for powder products) which means the potential for exposure to toxic amounts of pyrethrins is very low.

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