Malathion is a pesticide that is widely used in agriculture, residential landscaping, public recreation areas, and in public health pest control programs such as mosquito eradication. In the US, it is the most commonly used organophosphate insecticide. A malathion mixture with corn syrup was used in the 1980s in Australia and California to combat the Mediterranean fruit fly. In Canada and the US, malathion was sprayed in many cities to combat west Nile virus. Malathion was used over the last couple of decades on a regular basis during summer months to kill mosquitoes, but homeowners were allowed to exempt their properties if they chose. Today, Winnipeg is the only major city in Canada with an ongoing malathion adult-mosquito-control program.
Mechanism of action
Malathion is an acetylcholinesterase inhibitor, a diverse family of chemicals. Upon uptake into the target organism, it binds irreversibly to the serine residue in the active catalytic site of the cholinesterase enzyme. The resultant phosphoester group is strongly bound to the cholinesterase, and irreversibly deactivates the enzyme which leads to rapid build-up of acetylcholine at the synapse.
Malathion in low doses is used as a treatment for:
Head lice and body lice. Malathion is approved by the US Food and Drug Administration for treatment of pediculosis. It is claimed to effectively kill both the eggs and the adult lice, but in fact has been shown in UK studies to be only 36% effective on head lice, and less so on their eggs. This low efficiency was noted when malathion was applied to lice found on schoolchildren in the Bristol area in the UK, and it is assumed to be caused by the lice having developed resistance against malathion.
Preparations include Derbac-M, Prioderm, Quellada-M and Ovide.
Risks
General
Malathion is of low toxicity; however, absorption or ingestion into the human body readily results in its metabolism to malaoxon, which is substantially more toxic. In studies of the effects of long-term exposure to oral ingestion of malaoxon in rats, malaoxon has been shown to be 61 times more toxic than malathion, and malaoxon is 1,000 times more potent than malathion in terms of its acetylcholinesterase inhibition. Indoor spillage of malathion can thus be more poisonous than expected, as malathion breaks down in a confined space into the more toxic malaoxon. It is cleared from the body quickly, in three to five days. According to the United States Environmental Protection Agency there is currently no reliable information on adverse health effects of chronic exposure to malathion. Acute exposure to extremely high levels of malathion will cause body-wide symptoms whose intensity will be dependent on the severity of exposure. Symptoms include skin and eye irritation, cramps, nausea, diarrhea, excessive sweating, seizures and even death. Most symptoms tend to resolve within several weeks. Malathion present in untreated water is converted to malaoxon during the chlorination phase of water treatment, so malathion should not be used in waters that may be used as a source for drinking water, or any upstream waters. In jurisdictions which spray malathion for pest control, it is often recommended to keep windows closed and air conditioners turned off while spraying is taking place, in an attempt to minimize entry of malathion into the closed environment of residential homes. In 1981, B. T. Collins, director of the California Conservation Corps, publicly swallowed a mouthful of dilute malathion solution to demonstrate its safety following an outbreak of Mediterranean fruit flies in California. Malathion was sprayed over a area to control the flies. In 1976, numerous malaria workers in Pakistan were poisoned by isomalathion, a contaminant that may be present in some preparations of malathion. It is capable of inhibiting carboxyesterase enzymes in those exposed to it. It was discovered that poor work practices had resulted in excessive direct skin contact with isomalathion contained in the malathion solutions. Implementation of good work practices, and the cessation of use of malathion contaminated with isomalathion led to the cessation of poisoning cases. Malathion is flammable. Patients using this agent should not be exposed to heat sources when it is applied, such as blow dryers, cigarettes, or cooking flames.
Carcinogenicity
Malathion is classified by the IARC as probable carcinogen. Malathion is classified by US EPA as having "suggestive evidence of carcinogenicity". This classification was based on the occurrence of liver tumors at excessive doses in mice and female rats and the presence of rare oral and nasal tumors in rats that occurred following exposure to very large doses. Researchers conducted a study involving participants from six Canadian provinces and found that exposure to organophosphates as a group and malathion alone was associated with non-Hodgkin's lymphoma. Malathion used as a fumigant was not associated with increased cancer risk. Between 1993 and 1997, as part of the Agricultural Health Study, researchers surveyed 19,717 pesticide applicators about their past pesticide exposures and health histories and no clear association between malathion exposure and cancer was reported.
Attention-deficit/hyperactivity disorder
A May 2010 study found that in a representative sample of US children, those with higher levels of organophosphate pesticide metabolites in their urine were more likely to have attention-deficit/hyperactivity disorder, but no causal relationship was established. Each 10-fold increase in urinary concentration of organophosphate metabolites was associated with a 55% to 72% increase in the odds of ADHD. The study was the first investigation on children's neurodevelopment to be conducted in a group with no particular pesticide exposure.
Amphibians
Although current EPA regulations do not require amphibian testing, a 2008 study done by the University of Pittsburgh found that "cocktails of contaminants", which are frequently found in nature, were lethal to leopard frog tadpoles. They found that a combination of five widely used insecticides in concentrations far below the limits set by the EPA killed 99% of leopard frog tadpoles.
