Resmethrin is a pyrethroid insecticide with many uses, including control of the adult mosquito population. It is also used for fabric protection, pet sprays and shampoos, and it is applied to horses or in horse stables. Technical resmethrin is a mixture of its two main isomers (molecules with the same chemical formula but slightly different configurations); a typical blend is 20 to 30% of the (1RS)-cis-isomer and 70 to 80% of the (1RS)-trans-isomer. A commercial trade names for products that contain resmethrin are Chrysron, Crossfire, Pynosect, Raid Flying Insect Killer, Scourge, Sun-Bugger #4, SPB-1382, Synthrin, Syntox, Vectrin and Whitmire PT-110. 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. (6, 8, 9)
Both type I and type II 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. They also inhibit calium channels and Ca2+, Mg2+-ATPase. (11, 12, 7)
Metabolism
Following ingestion, pyrethriods are hydrolysed by various digestive enzymes in the gastro-intestinal tract. However, a small portion of the insecticidally active compounds or its derivatives are absorbed, as shown by their toxicity and their effect on the liver. Pyrethriods may also be absorbed following inhalation or dermal contact. They are rapidly distributed to most tissues, particularly to those with a high lipid content, and are concentrated in central and peripheral nervous tissues. Pyrethriods or their metabolites are not known to be stored in the body or to be excreted in the milk, but no study of the matter has employed modern methods. The major metabolic pathways for pyrethriods are hydrolysis of the central ester bond, oxidative attacks at several sites, and conjugation reactions, to produce a complex array of primary and secondary water-soluble metabolites that undergo urinary excretion. Metabolism is believed to involve nonspecific microsomal carboxyesterases and microsomal mixed function oxidases, which are located in nearly all tissue types, with particularly high activities in the liver. Metabolites are excreted in the urine and faeces. (7, 10)
Pyrethroid 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. Paresthesia, severe corneal damage, hypotension and tachycardia, associated with anaphylaxis, can also occur following pyrethriod poisoning. (7)
Symptoms
Following oral exposure, severe fine tremor, marked reflex hyperexcitability, sympathetic activation can occur. Nausea, vomiting and abdominal pain commonly occur and develop following ingestion. Sudden bronchospasm, swelling of oral and laryngeal mucous membranes, and anaphylactoid reactions have been reported after inhalation. Hypersensitivity reactions characterized by pneumonitis, cough, dyspnea, wheezing, chest pain, irritability to sound and touch, and bronchospasm may occur too. Dermatitis is the main effect of a dermal exposure resmethrin. (13)
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. (5)
