Overall Effect of Permethrin as Chemical for Pest Control
Permethrin is one of many synthetic pyrethroids developed for use as an insecticide based on the chemistry of pyrethrum flowers (certain Tanacetum species).
Permethrin is used in household insect foggers and sprays, flea dips and sprays for cats and dogs, ornamental garden and turf products, repellent/insecticide for clothing, mosquito abatement products, termite treatments, agricultural products, lice shampoos, body lotions for scabies control, and aircraft cabin insect control. Pyrethroid insecticides are some of the most widely used pesticides in the world because they are believed to be less harmful to humans than other pesticides.
Permethrin can be inhaled into the lungs, absorbed through the skin or ingested. The most dangerous route is inhalation, as this allows the chemical to move directly to target sites (e.g., the brain) without being metabolized by the liver and other organs. A more recent exposure route is through clothing impregnated for mosquito repellent.
Permethrin is comprised of two isomers, with the cis isomer being ten times more toxic than the trans isomer. The toxicity of permethrin varies with the trans/cis ratio, which is commonly 60/40 in agricultural use.
Permethrin exerts its primary toxic effects directly on the nervous system. It interferes with normal propagation of the nerve impulse along the axon (the action potential), by prolonging the open-time of sodium channels, thus provoking repetitive after-discharges. Studies in mice and rats show that sub-lethal intoxication leads to aggression, hypersensitivity to external stimulation, whole-body tremor, convulsions, and paralysis.
Central and Peripheral Nervous System Effects
The most common finding in studies of the nervous system was a decrease in acetylcholinesterase (AChE) activity. This occurred in the midbrain, brainstem, cerebellum, cerebral cortex, hippocampus, and the hypothalamus. AChE is an enzyme that breaks down acetylcholine at the synaptic cleft, allowing cholinergic neurons to return to their resting state after activation. Without AChE, post-synaptic neurons experience repetitive firing. Various effects on the dopaminergic system were also documented, primarily in the corpus striatum. Molecular/metabolic actions include effects on Ca+ and Na+ levels, and c-fos and BDNF gene expression in the brain.
Behavioral studies of rats and mice showed permethrin-induced impairments in balance, strength, and speed. Studies also found reduced response to conditioned behavior, enhanced startle response, and various sensorimotor effects. One study found behavioral effects in the offspring of mice given permethrin prior to mating (transgenerational effects).
Immune System Effects
Every one of the 15 studies that we found on the immune system showed significant effects. This included studies using rats, mice, and in vitro human studies. The most common findings included reduction or inhibited production of thymocytes, T-cells, and lymphocytes. Results indicated that permethrin led to the death and/or reduced production of blood cells necessary to fight bacteria and viruses, and remove waste products from the blood.
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