Malathion

Malathion is an organophosphate parasympathomimetic which binds irreversibly to cholinesterase. Malathion is an insecticide of relatively low human toxicity; however, a 2010 study has shown that children with higher levels of organophosphate pesticide metabolites in their urine are more likely to have attention deficit hyperactivity disorder.

In the former USSR it was known as carbophos, in New Zealand and Australia as maldison and in South Africa as mercaptothion.

Pesticide use
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.

Malathion was used in the 1980s in California to combat the Mediterranean Fruit Fly. This was accomplished on a wide scale by the near weekly aerial spraying of suburban communities for a period of several months. Formations of three or four agricultural helicopters would overfly suburban portions of Alameda County, San Bernardino County, San Mateo County, Santa Clara County, San Joaquin County, Stanislaus County, and Merced County releasing a mixture of malathion and corn syrup, the corn syrup being a bait for the fruit flies. Malathion has also been used to combat the Mediterranean fruit fly in Australia.

Malathion was sprayed in many cities to combat West Nile virus. In the Fall of 1999 and the Spring of 2000, Long Island and the five boroughs of New York City were sprayed with several pesticides, one of which was malathion. While it was claimed by some anti-pesticide groups that use of these pesticides caused a lobster die-off in Long Island Sound, there is as of yet no conclusive evidence to support this.

Manitoba ordered the city of Winnipeg, Manitoba to be sprayed in July 2005 as part of the West Nile virus campaign. Prior to this, malathion was used over the last couple of decades on a regular basis during summer months to kill nuisance 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 nuisance-adult-mosquito-control program.

Malathion is also used in conjunction with diesel fuel to fog an area where there is an infestation of mosquitoes. By diluting the mixture, it becomes much weaker. It is possible to dilute the mixture to the point where mosquitoes are not killed, but become more resistant to the mixture, making it less effective in subsequent foggings.

Medical use
Malathion in low doses (0.5% preparations) is used as a treatment for:


 * head lice and body lice. Malathion and lindane are the only two agents approved by the FDA 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  found 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.


 * scabies.

Preparations include Derbac-M, Prioderm, Quellada-M and Ovide.

General
Malathion itself is of low toxicity; however, absorption or ingestion into the human body readily results in its metabolism to malaoxon, which is substantially more toxic. It is cleared from the body quickly, in three to five days. Chronic exposure to low levels of malathion have been hypothesized to impair memory, but this is disputed. 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. Possible 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 1981, the late B. T. Collins, Director of the California Conservation Corps, publicly swallowed and survived a mouthful of dilute Malathion solution. This was an attempt to demonstrate Malathion's safety following an outbreak of Mediterranean fruit flies in California. Malathion was sprayed over a 1400 sqmi. 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 breaks down into Malaoxon. 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.

If malathion is used in an indoor, or other poorly ventilated environment, it can seriously poison the occupants living or working in this environment. A possible concern is that malathion being used in an outdoor environment, could enter a house or other building; however, studies by the EPA have conservatively estimated that possible exposure by this route is well below the toxic dose of malathion. Regardless of this fact, 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.

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 (carbaryl, chlorpyrifos, diazinon, endosulfan, and malathion) in concentrations far below the limits set by the EPA killed 99% of leopard frog tadpoles.

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. 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

Carcinogenicity
Malathion is classified by US EPA as having “suggestive evidence of carcinogenicity but not sufficient to assess human carcinogenic potential.” This rating implies that insufficient evidence is available to either rule out malathion as a carcinogen, or to state that it is a carcinogen. No studies on carcinogenicity have been performed in humans; however, studies in rats and mice have yielded conflicting results. Liver tumours were found to be induced in rats, but only at excessive doses. On the other hand, malaoxon, a structurally related chemical, was found not to induce tumour formation in rats. A review of the classification of malathion as 'suggestive' was carried out in 2000, by the FIFRA Scientific Advisory Panel. The conclusion of this panel was that there was still insufficient evidence to either declare malathion as non-carcinogenic, or to declare it a carcinogen.