Description

Overview

Malathion is a topical organophosphate insecticide commonly used to treat head lice (Pediculosis capitis) and scabies (caused by Sarcoptes scabiei). It is commercially available as a 0.5% lotion (Ovide) for human use and is also widely employed in agricultural and pest control applications. Malathion works as a pediculicide, meaning it kills lice and their eggs, offering an effective solution for resistant lice infestations. It is typically applied to dry hair and left on for a recommended period before being rinsed off.

Mechanism of Action

Malathion is an acetylcholinesterase inhibitor. It functions by irreversibly binding to the enzyme acetylcholinesterase, preventing the breakdown of acetylcholine at nerve synapses. This leads to the accumulation of acetylcholine, causing excessive nerve stimulation, paralysis, and eventual death of lice.

In humans, malathion is rapidly metabolized into non-toxic byproducts, making it relatively safe when used as directed. However, excessive exposure can result in toxicity due to systemic absorption, especially in cases of misuse or ingestion.

Benefits and Indications

Malathion is primarily indicated for the treatment of:

1. Head Lice (Pediculosis capitis):

• Malathion 0.5% lotion is FDA-approved for head lice treatment in patients aged 6 years and older.
• It effectively kills both adult lice and their ova (eggs), reducing the need for multiple treatments.
• It is particularly useful in cases where lice have developed resistance to pyrethroids like permethrin.

2. Off-Label Use for Scabies:

• Although not FDA-approved for scabies, malathion has been used successfully in some cases where resistance to standard treatments (permethrin, ivermectin) occurs.

Possible Side Effects

While malathion is generally safe when used topically and as directed, some potential side effects include:

• Local Reactions:
• Scalp irritation
• Itching or burning sensation
• Dryness or redness of the scalp and skin
• Allergic Reactions:
• Rash
• Swelling
• Hypersensitivity reactions
• Systemic Toxicity (Rare, usually due to ingestion or excessive exposure):
• Nausea, vomiting, diarrhea
• Dizziness or headache
• Muscle weakness or tremors (cholinergic toxicity)
• Excessive salivation and sweating
• In severe cases, respiratory depression or seizures

Drug Interactions

Malathion has minimal known drug interactions when used topically, as systemic absorption is generally low. However, individuals taking medications that affect cholinergic pathways (e.g., acetylcholinesterase inhibitors used for Alzheimer’s disease, myasthenia gravis, or pesticides) may be at an increased risk of toxicity if malathion is absorbed in significant amounts.

Precautions and Considerations

• Children and Elderly: Safe for use in children over six years old, but caution is advised in younger children and elderly individuals due to increased sensitivity to toxicity.
• Pregnancy and Breastfeeding: Considered a pregnancy category B medication, meaning no known harm has been observed in animal studies, but human data is limited. Should be used with caution in breastfeeding mothers.
• Flammability: Malathion lotion contains alcohol and is highly flammable. Avoid exposure to heat sources, flames, or smoking during and immediately after application.