CAS RN: 121-75-5

Toxicity Summary

IDENTIFICATION AND USE: Malathion is a clear colorless liquid when pure. It has been employed for control of citrus orchard-destructive Mediterranean fruit flies and mosquitoes. Malathion is used for the topical treatment of head lice infestation. The drug also has been used for the topical treatment of pubic lice infestation, body lice infestation, and scabies (mite infestation). HUMAN EXPOSURE AND TOXICITY: Manifestations of acute intoxication may include a mix of muscarinic, nicotinic, and CNS effects. Following ingestion of malathion, symptoms may appear rapidly or may be delayed up to 12 hours. Initial signs and symptoms of malathion poisoning may be largely due to excessive muscarinic effects, which may predominate in milder cases; such effects may include nausea, vomiting, abdominal cramps, diarrhea, urinary and/or fecal incontinence, hyperhidrosis, sialorrhea, miosis (pinpoint pupils), bradycardia, lacrimation, and increased nasal, pharyngeal, and bronchial secretions. Nicotinic effects, including muscle fasciculation, muscle weakness, tachycardia, weakness or paralysis of respiratory muscles, and hypotonia, may occur in moderate and severe intoxications. CNS effects may include anxiety, restlessness, and headache. In more severe cases, tremors, confusion, dizziness, drowsiness, a reduction or loss of deep tendon reflexes, seizures, bradycardia, and coma also have been reported; death may occur. Respiratory failure may result from a combination of muscarinic, nicotinic, and CNS effects. In children, the signs and symptoms of poisoning may be predominantly related to the CNS (e.g., seizures, alterations in mental status including lethargy and coma). Hypotonia, muscle weakness, miosis, and excessive salivation also have occurred in children, while some of the typical cholinergic effects (e.g., bradycardia, muscular fasciculation, excessive lacrimation, sweating, bronchial secretion) may be observed less frequently than in adults. Concentrations of up to 400 ug/mL of 95% malathion failed to increase chromosomal aberrations in human hematopoietic cell cultures; however, others reported a positive result in human lymphocytes with 99% pure malathion. A significant increase in chromosomal aberrations was found in the lymphocytes of 14 people intoxicated with a commercial formulation of malathion, as compared with that in healthy controls. Aberrations observed included chromatid breaks, chromatid isobreaks, chromatid exchanges and unstable chromosomal and structural aberrations. There is limited evidence of malathion carcinogenicity in humans for non-Hodgkin lymphoma and prostate cancer. ANIMAL STUDIES: Undiluted malathion dropped on rabbit's eye caused slight immediate irritation. A daily dose of 46 mg/kg malathion ip for fifteen days affected the activity of the adrenal gland and liver glycogen in rats. Neurotoxicity, reflected by the occurrence of leg weakness in atropinized chickens given single, subcutaneous doses of 100 mg/kg malathion. A bioassay of malathion for possible carcinogenicity was conducted in rats. Groups of 49 or 50 rats of each sex were fed diets containing 2,000 or 4,000 ppm malathion for 103 weeks. All surviving rats were killed at 105 or 106 weeks. Malathion was not carcinogenic in male or female rats. Zebrafish larvae were used in order to determine the effects of malathion, on zebrafish behavior and AChE activity. Embryos and larvae were exposed to malathion during different time points in development and then tested at 5 days post-fertilization for behavioral, neurodevelopmental and AChE abnormalities. Malathion altered behaviors in the larvae such as swim speed and rest. Larvae treated with malathion also had significantly smaller forebrain and hindbrain regions compared to controls by 5 days post-fertilization. Malathion was ineffective in inducing sex linked recessive lethal mutations in Drosophila melanogaster. The clastogenic effect of malathion was studied in mice. At 230 mg/kg, increasing the frequencies of abnormal metaphases and chromosomal aberrations were noted in animals killed 6 or 24 hr after injection. Mice injected with 460 mg/kg, exhibited significant increments of abnormal metaphases, gaps, breaks, and chromatid exchanges in relation to controls. Malathion was tested by the plate incorporation assay with Salmonella typhimurium strains TA1535, TA1537, TA1538, TA98 and TA100 as well as with Escherichia coli strain WP2 uvrA-. No increases in revertants with any strain were reported. ECOTOXICITY STUDIES: Moribund mullet, Mugil cephalus, in an estuary sprayed with malathion (3 oz/acre) during a large scale mosquito control operation had about 98% inhibition of brain acetylcholinesterase. Inhibition of acetylcholinesterase and mortality were noted in pinfish 24, 48, and 72 hours at measured concentrations of 142, 92, and 58 ug/L, respectively. A concentration of 31 ug/L caused 34 percent acetylcholinesterase inhibition in pinfish but no deaths in 72 hours. Growth of oyster, Crassostrea virginica, was reduced 32% by 96 hr exposure to 1 mg/L. Bullfrogs (Rana catesbeiana) were exposed to malathion in water in a 28-day static renewal test. Survival was decreased at the level of 2,500 ug/L and higher. Development of tadpoles was significantly delayed by malathion exposure.
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