Malathion

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.^[6] In the US, it is the most commonly used organophosphate insecticide.^[7]

A malathion mixture with corn syrup was used in the 1980s in Australia to combat the Mediterranean fruit fly.^[8] In Canada and the US starting in the early 2000s, malathion was sprayed in many cities to combat west Nile virus.^[9]

In the United Kingdom, malathion was withdrawn from sale in 2002.^[10]

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.^[11]

Malathion is produced by the addition of dimethyl dithiophosphoric acid to diethyl maleate or diethyl fumarate in the presence of catalytic amounts of triethylamine and hydroquinone at elevated temperature:

(CH₃O)₂PS₂H + C₈H₁₂O₄ → C₁₀H₁₉O₆PS₂

This process produces the S enantiomer.^[12]

Malathion in low doses (0.5% preparations) is used as a treatment for head lice and body lice infection (pediculosis) in the US, where it is approved by the US Food and Drug Administration.^[13][14] In some areas in the UK, head lice had developed a resistance to malathion as of 1999^[update], and it was ineffective against 64% of cases. This is believed to be caused by development in the lice of an enzyme-mediated malathion specific esterase, able to destroy malathion bound to the acetylcholine receptor.^[15]

It is also used for the treatment of scabies.^{[16][page needed]}

Preparations include Derbac-M, Prioderm, Quellada-M and Ovide.^[17][18]

Different organizations and governments have set different drinking water quality standards for malathion.

The World Health Organization has determined that while malathion can last in water for months and years, it usually does not last in water for longer than one to two weeks; and since the amount of malathion found in drinking water is usually lower than a level that would lead to concerns about health that there is no need to set a drinking water guideline for the level of malathion allowed in water.^[25]

In contrast, the Canadian government has set a maximum acceptable concentration of 0.29 mg/L of malathion in drinking water, and requires that drinking water should be monitored for the chemical when there is a reason to suspect it is in the water and might need to be removed.^[26]

Australia has a much lower maximum level of 0.07 mg/L of malathion in drinking water.^[27]

The European Union has set their level still lower at 0.10 μg/L for any one pesticide in drinking water, including malathion.^[28]

The United States does not have an official maximum contaminant level for malathion, but proposes a lifetime health advisory of 0.5 mg/L.^[29]

• Isomalathion
• Pesticide toxicity to bees