Acetylcholinesterase is found in mammals, amphibians, fish, reptiles, and birds. 12 In these organisms, the binding of AChE with malathion allows the accumulation of ACh at the nerve junction. This accumulation of ACh leads to overstimulation of glandular cells, autonomic ganglia, the central nervous system, and both smooth and skeletal muscles. 9
Uptake and metabolism of organophosphates such as malathion are similar in insects and mammals. 13
Mammals and birds have greater carboxylesterase activity relative to levels in insects. This enables birds and mammals to degrade malathion more quickly than it is oxidized to the malaoxon form. Higher vertebrates therefore detoxify and excrete malathion more readily than do insects. This accounts for the relatively low toxicity of malathion to mammals and birds. 10,14,15
Greater carboxylesterase production with consequent increased detoxification of malathion appears to be the underlying mechanism in resistant insect pests. 15
Microorganisms such as bacteria may use malathion as a source of carbon and phosphorus. 15
Plants metabolize malathion to malaoxon although this appears to be a minor pathway, and maloaxon is rapidly eliminated. 5,15 Malathion is not expected to be toxic to plants or aquatic algae because its mode of action targets nervous systems. 1
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