Malathion

Malathion is a widely used organophosphate insecticide. It belongs to a diverse family of chemicals known as acetylcholinesterase inhibitors. This means it interferes with how certain nervous systems function. People use malathion in many different ways to control pests. It is applied in large-scale agriculture to protect crops. It is also used in residential landscaping and public recreation areas. In the United States, it is the most common organophosphate insecticide used. Public health programs often use it for mosquito eradication.

The chemical works through a specific biological mechanism. Once a target organism takes up the malathion, it interacts with enzymes. It binds irreversibly to a serine residue. This residue is located in the active catalytic site of the cholinesterase enzyme. This binding creates a phosphoester group that stays strongly attached to the enzyme. This process irreversibly deactivates the cholinesterase. As a result, there is a rapid build-up of acetylcholine at the synapse. The synapse is the space where nerve cells communicate.

Malathion has several different applications depending on the setting. In agriculture, it helps manage insect populations that threaten food supplies. In public health, it is used to combat viruses like West Nile virus. In the early 2000s, many cities in Canada and the US sprayed malathion for this reason. In Australia, a mixture of malathion and corn syrup was used in the 1980s. This specific mixture helped combat the Mediterranean fruit fly.

Medical uses for malathion are also quite common. In the United States, the Food and Drug Administration approves low doses for health. Specifically, 0.5% preparations are used to treat head lice and body lice infections. This condition is known as pediculosis. Malathion is also used to treat scabies. Some common medical preparations include Derbac-M, Prioderm, Quellada-M, and Ovide. However, some lice in the United Kingdom have developed resistance. In those cases, the malathion was ineffective against 64% of the lice. This resistance is caused by a specific enzyme called malathion specific esterase. This enzyme destroys the malathion before it can work.

There are important details regarding the toxicity and metabolism of the chemical. In arthropods, malathion is metabolized into a substance called malaoxon. Malaoxon is 61 times more toxic than malathion. In studies involving rats, malaoxon was 1,000 times more potent at inhibiting acetylcholinesterase. Humans also metabolize the chemical into malaoxon upon absorption or ingestion. The body typically clears the chemical within three to five days. In 1981, B. T. Collins, the director of the California Conservation Corps, swallowed a dilute solution. He did this to prove the chemical was safe. He survived and lived for another twelve years.

Scientists also study the potential long-term health risks of malathion. The IARC classifies malathion as a probable carcinogen, which is Group 2A. The US EPA states there is suggestive evidence of carcinogenicity. This classification comes from observing liver tumors in mice and female rats. These tumors appeared after exposure to very large doses. Exposure to organophosphates is also associated with non-Hodgkin's lymphoma. However, the Agricultural Health Study found no clear association with cancer between 1993 and 1997. Furthermore, using malathion as a fumigant was not associated with increased cancer risk.

Finally, different governments regulate how much malathion can be in our environment. The World Health Organization notes it usually lasts in water for one to two weeks. Because levels in drinking water are usually low, they do not set specific guidelines. In contrast, Canada sets a maximum acceptable concentration for drinking water. Australia and the European Union set even lower limits for pesticides in water. The United States proposes a lifetime health advisory level. We must also consider the impact on wildlife. For example, malathion is known to be toxic to leopard frog tadpoles.