Malathion

Malathion is an organophosphate parasympathomimetic which binds irreversibly to cholinesterase. Malathion is an insecticide of relatively low human toxicity.

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

Risks
Malathion breaks down into malaoxon, which is 60 times more toxic than malathion. For this reason, if malathion is used in or somehow enters an indoor environment, as it breaks down into malaoxon, it can seriously and chronically poison the occupants living or working in this environment. 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 1976, numerous malaria workers in Pakistan were poisoned by isomalathion, a common impurity in malathion, which is capable of inhibiting carboxyesterase enzymes in those exposed to it; the original toxicity evaluation for malathion had not anticipated isomalathion coexposure.

Clinical uses
Malathion is used as a treatment for head lice, body lice and scabies. It effectively kills both the eggs and the adult lice.

Instances of use
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 San Bernardino 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 malathion. Use of the insecticide has been blamed for large lobster die-off in Long Island Sound.

Manitoba, Province of Canada, 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 regular basis during summer months to kill nuisance mosquitos, 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 to fog an area where there are plenty 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.