Bioresmethrin is a synthetic pyrethroid (type 1) insecticide formerly used as a grain protectant. Bioresmethrin is one of the most effective broad spectrum insecticides currently available. It exhibits a high order of insecticidal activity, which when coupled with its excellent toxicological properties, makes it potentially one of the safest and most useful insecticides now being produced. A pyrethroid is a synthetic chemical compound similar to the natural chemical pyrethrins produced by the flowers of pyrethrums (Chrysanthemum cinerariaefolium and C. coccineum). Pyrethroids are common in commercial products such as household insecticides and insect repellents. In the concentrations used in such products, they are generally harmless to human beings but can harm sensitive individuals. They are usually broken apart by sunlight and the atmosphere in one or two days, and do not significantly affect groundwater quality except for being toxic to fish. Insects with certain mutations in their sodium channel gene may be resistant to pyrethroid insecticides. (3, 2, 4, 6)
Pyrethroids exert their effect by prolonging the open phase of the sodium channel gates when a nerve cell is excited. They appear to bind to the membrane lipid phase in the immediate vicinity of the sodium channel, thus modifying the channel kinetics. This blocks the closing of the sodium gates in the nerves, and thus prolongs the return of the membrane potential to its resting state. The repetitive (sensory, motor) neuronal discharge and a prolonged negative afterpotential produces effects quite similar to those produced by DDT, leading to hyperactivity of the nervous system which can result in paralysis and/or death. Other mechanisms of action of pyrethroids include antagonism of gamma-aminobutyric acid (GABA)-mediated inhibition, modulation of nicotinic cholinergic transmission, enhancement of noradrenaline release, and actions on calcium ions. (7, 4)
Metabolism
The biodegradation of bioresmethrin is a complex phenomenon with several reactions occurring simultaneously and taking place at various positions in the molecule. The initial step in the metabolism is cleavage at the ester linkage, a reaction found to be catalyzed by esterases localized in the liver microsome. Transisomerization was reported with bioresmethrin but was apparently limited to isomerization of degradation products, not observed with the parent molecule and seen only when bioresmethrin was administered at low levels. Bioresmethrin is degraded by ester cleavage and the alcohol moiety is oxidized to 5-benzyl-3-furylmethanol (BFA), 5-benzyl-3-furoic acid (BFCA), 4'-hydroxy BFCA and alpha-hydroxy BFCA (alpha-OH-BFCA). The chrysanthemate (acid) moiety undergoes oxidation from trans-chrysanthemic acid (tE-CHA) to 2,2-dimethyl3-(2'hydroxymethyl-1'-propenyl)cyclopropanecarboxylic acid (tE-CHA) (oxidative metabolism at the methyl group of the isobutenyl side chain trans (E) to the cyclopropane). This is further oxidized through the formyl derivative (CAA) to the dicarboxylic acid isomers (tE-CDA and cE-CDA). It is at the CAA oxidation stage where isomerization may occur through the proposed aldehyde (cE-CAA) intermediate to (cE-CDA) the cis-dicarboxylic acid (Ueda et al., 1975b). This metabolic sequence may also account for the consideration of Verschoyle and Barnes (1972) that as a delay in signs of poisoning was evident following iv administration, bioresmethrin may be converted in vivo to a toxic metabolite. The presence of (+)-trans-CA,-BFA and BFCA as metabolites, which are more toxic than bioresmethrin, may account for their observation and conclusions. The metabolic sequence is very similar qualitatively to that observed with resmethrin although much less complicated because of the lack of isomeric products and because of the specificity of certain isomers to enzymatic degradation by selected routes as mentioned above. (5)
As for every type I pyrethroid, bioresmethrin effects typically include rapid onset of aggressive behavior and increased sensitivity to external stimuli, followed by fine tremor, prostration with coarse whole body tremor, elevated body temperature, coma, and death. (4)
Symptoms
Following dermal exposure to bioresmethrin, feelings of numbness, itching, burning, stinging, tingling, or warmth may occur, that could last for a few hours. Dizziness, headache, nausea, muscle twitching, reduced energy, and changes in awareness can result from inhalation or ingestion of large amounts of bioresmethrin. Paralysis can occur after exposure. (4)
Treatment
Following oral exposure, the treatment is symptomatic and supportive and includes monitoring for the development of hypersensitivity reactions with respiratory distress. Provide adequate airway management when needed. Gastric decontamination is usually not required unless the pyrethrin product is combined with a hydrocarbon. Following inhalation exposure, move patient to fresh air. monitor for respiratory distress. If cough or difficulty breathing develops, evaluate for respiratory tract irritation, bronchitis, or pneumonitis. Administer oxygen and assist ventilation as required. Treat bronchospasm with inhaled beta2 agonist and oral or parenteral corticosteroids. In case of eye exposure, irrigate exposed eyes with copious amounts of room temperature water for at least 15 minutes. If irritation, pain, swelling, lacrimation, or photophobia persist, the patient should be seen in a health care facility. If the contamination occurs through dermal exposure, remove contaminated clothing and wash exposed area thoroughly with soap and water. A physician may need to examine the area if irritation or pain persists. Vitamin E topical application is highly effective in relieving parenthesis. (8)
