Insecticides and the skin

What are insecticides used for in dermatology?

Insecticides used in dermatology are products designed to kill insects such as lice (head lice, pubic lice, body lice), mites (scabies, bird mites), ticks, mosquitoes and fleas. Most of these insecticides are topical preparations (such as creams, lotions, shampoos, foams or sprays). Ivermectin is a systemic medication used to treat selected cases of scabies as well as parasitic intestinal worms.

What makes an ideal insecticide?

• Proven effectiveness against the target insect
• Capable of killing the eggs (ovicidal) as well as the adult parasites
• Non irritating to the skin and no systemic toxicity
• Easy to use and inexpensive

Commonly used insecticide treatments

For insecticides to work properly it is important to meticulously follow the directions, including directions related to product application, laundering of clothes and linen, treatment of contacts and repeat application of treatment. Topical agents are meant for external use only. Avoid all contact with the mucous membranes (such as the nose and mouth) and avoid contact with the eyes.

Combing of wet hair using a regular conditioner and a fine-tooth louse comb is the treatment of choice for head lice in children younger than 2 years of age.

Suffocants

• Herbal based formulations used for head lice
• May be as or more effective than synthetic insecticides
• Do not require combing
• Non-toxic

Pyrethrum/pyrethrin/phenothrin (can be combined with piperonyl butoxide)

• Naturally occurring insecticide found in the chrysanthemum flower used for head lice, pubic and body lice
• Not ovicidal
• Can be toxic to humans in high doses
• Seek medical advice before using in children under 2 years of age

Permethrin

• A synthetic form of pyrethrin used for head lice, pubic and body lice, and scabies
• Effective against adult lice, and has some ovicidal activity
• Can be sprayed onto clothes, tents, and mosquito nets to act as an insect repellent
• Effective against ticks and fleas
• Has rarely been associated with toxic effects at high doses, including eye and skin irritation, tremors, loss of coordination, hyperactivity and paralysis
• Seek medical advice before using in children under 2 years of age

Organophosphates (malathion, also known as maldison)

• Used topically to treat head lice, pubic and body lice, and scabies
• Effective against adult lice and is ovicidal
• Seek medical advice before using in children under 2 years of age
• Contains flammable alcohol, so do not expose lotion or wet hair to open flame or electric heat, such as a hair dryer

Benzyl benzoate

• Hydrocarbon used topically to treat scabies
• Seek medical advice before using in children under 2 years of age

Ivermectin

• Used topically for resistant head lice
• Used orally to treat crusted scabies and other cases of scabies where topical therapy is impractical or has failed
• Potential neurotoxicity
• Should not be used in children less than 6 years of age

Lindane (gamma benzene hexachloride) is rarely used in humans, especially young children, because it is neurotoxic (causing seizures and death).

Insecticide surface sprays and dusts

A variety of insecticide surface sprays and dusts are available for domestic or commercial control of fleas, bed bug and other biting insects. These products contain insecticides such as:

• Synthetic pyrethroids e.g. permethrin
• Organophosphates (e.g. malathion)
• Benzyl benzoate
• Carbamates (e.g. propoxur, carbaryl)

Cutaneous adverse effects of insecticides

• People who are allergic to chrysanthemums may experience contact allergic dermatitis to pyrethrin or permethrin, although this is rare. As a precaution, these products should be avoided if chrysanthemum or ragweed allergy is present. Patients who are allergic to turpentine should also avoid pyrethrin-based products.
• The most frequent side effect of permethrin is itching (likely related to the condition being treated, such as scabies or head lice, but itch may be heightened after treatment). Oral antihistamines and/or topical corticosteroids can help relieve itchiness.
• Permethrin at 5% concentration (a high dose used for treating scabies) can cause a mild burning or stinging sensation, mild itchiness, tingling, numbness, redness, swelling or rash. These preparations also contain a small amount of formaldehyde, which can rarely cause allergy or contact irritant dermatitis.
• Rarely malathion may cause skin irritation, rash, redness and allergic reactions.
• An insecticide-containing aerosol spray for head lice contains flammable hydrocarbons and thermal burns have been reported.
• Benzyl benzoate may cause skin irritation. Apply to a small area of skin, if tingling or pain is experienced, try an alternative product.
• Ivermectin may cause a transient exacerbation of itch at the beginning of treatment. Very rarely, Stevens-Johnson syndrome / toxic epidermal necrolysis or other hypersensitivity reactions have been reported.

Caution

Permethrin is poisonous to cats. Contact or ingestion may result in skin irritation, poor appetite, vomiting, diarrhoea, tremors and seizures. It may also cause adverse reactions in small dogs (bichon frise, chihuahua, maltese, shih tzu and yorkshire terrier).

Seek medical advice before using in children under 2 years of age particularly in infants under 6 months.

Insecticide resistance

Insecticide resistance is a particular problem in head lice in some regions. There is no information on the resistance patterns of lice in New Zealand. In the USA, head lice resistant to permethrin have been found. In the UK, head lice resistant to both permethrin and malathion have also been detected and are a difficult problem to manage.

There is controversy as to whether failed clinical response to topical permethrin represents true resistance, or “pseudoresistance” if it is due to poor therapy application, or reinfestation from untreated contacts. Compelling evidence however comes from the identification of knockdown mutations of the voltage selective sodium channels that have been identified in other parasites that have acquired permethrin resistance.

If a course of treatment fails to cure, a different insecticide should be used for the next course.

Most insecticides currently used to treat head lice are toxic to the parasite's nervous system (neurotoxic). New agents are in development, which work by clogging the respiratory spiracles and asphyxiating the lice. Due to their mode of action, development of resistance is not expected.

Benzyl alcohol lotion is one such agent and has recently been approved by the US Food and Drug Administration as a prescription medication, for use in patients 6 months of age and older. Common side effects of benzyl alcohol lotion include irritation of the skin, scalp, and eyes, and numbness at the site of application. Use in premature infants could lead to serious respiratory, heart- or brain-related adverse events such as seizure, coma, or death.