The two most common infections by external parasites are scabies and lice.

Scabies ("the Itch") is a dermatological infection of Sarcoptes scabiei.  It is most often spread by skin-to-skin contact.  It may be spread by sexual contact but is rarely spread by clothing or bedclothes.  The pregnant female burrows into the stratum corneum, laying her eggs in the burrow.  Following hatching, the larvæ congregate around hair follicles.  Initial signs of infection include fine, wavy, dark lines that represent the burrows.  These are often soon obscured by signs of inflammation or scratching.  Areas of inflammation most often include the webs of fingers, wrists, elbows, areola of females and genitals of males, belt lines and the lower buttocks.  A positive diagnosis is made by taking skin scrapings and observing for mites.

Pediculosis is infestation by lice on the head (Pediculus capitis), pubic area (Prthitus pubis) or the body (Pediculus corporis).  Head lice are most often transfered by shared hats, caps, or combs.  Body lice is relatively rare under normal hygeinic conditions.  Genital infections most often occur with sexual contact.  In each, the nits or eggs of the louse may be found attached to hair.  The skin may exhibit small red areas indicating bite marks of the louse.  Itching is common for all three infections.  The louse may not be observed in the hair, but the eggs may be seen.

Lindane (gamma benzene hexachloride)
Mechanism of Action -- Lindane is a GABA antagonist.  It blocks the inhibitory actions of GABA, resulting in neuronal excitation in both the parasite and mammalian systems.

Therapeutics -- Lindane may be used for the treatment of lice or scabies.

Side Effects -- Lindane may cause local irritation and eczema

Toxicity -- Excess absorption may cause CNS excitation and seizures.  Hepatotoxicity may also occur.

Contraindications -- The use of lindane is contraindicated in neonates and patients with seizure disorders.

Malathion
Mechanism of Action -- Malathion is an orgranophosphate insecticide.  It irreversibly binds to and inhibits acetylcholinesterase, causing toxic cholinergic activity in the insect.  It is selectively metabolised by insects to the active metabolite (metabolism in mammals is to an inactive metabolite).

Therapeutics -- Malathion is used in the treatment of lice infections.  It exerts both lousicidal and ovicidal activity.

Side Effects -- Local irritation may occur.  Other side effects are rare.

Permethrin
Mechanism of Action -- Permethrin is a pyrethroid insecticide, derived from naturally occurring products found in chrysanthemums.  It prolongs sodium channel opening, resulting in excess sodium influx and delayed repolarisation.  It is metabolised relatively rapidly by mammalian systems but relatively slowly by insects and arthropods.  The addition of piperonyl butoxide to pyrethroid preparations serves a two-fold purpose.  It enhances parasitic absorption of the insecticide and inhibits its insect metabolism by inhibiting the parasite's P₄₅₀ system.

Therapeutics -- Permethrin is used in the treatment of lice and scabies.

Side Effects -- Since permethrin is derived from a natural product, allergic sensitivity may occur. The primary side effects are local irritation.  Some burning/tingling may occur.  Excess absorption may cause seizures, but the incidence is much lower, relative to lindane.

Crotamiton
Mechanism of Action -- The mechanism of action of crotamiton is not known.

Therapeutics -- Crotamiton is used primarily in the treatment of scabies.

Side Effects -- Allergic sensitivity and local irritation.

Lindane (Kwell® Cream or Lotion)
Application Instructions -- Avoid contact with eyes, open wounds.  Wear gloves when applying.  Sexual contacts should be treated at the same time.  Apply to the whole body (approximately 1 fl. oz. for an adult) from the neck down in a thin, even manner.  Leave on for 8-12 hr, wash thoroughly.  One treatment is usually effective.  Repeat treatments should not be performed, due to risk of toxicity.  However, if mites are observed one week after the treatment, a second dose may be applied.  Note that itching may indicate a side effect of the drug and not continued infestation.  Confirmation of mite presence should be made with a skin scraping.

Permethrin (Elimite® Cream or Lotion)
Application Instructions -- Apply to whole body (1 fl. oz./adult) from neck down.  Leave on for 8-14 hr, then wash thoroughly.  One application is usually curative.
 
Crotamiton (Eurax® Cream or Lotion)
Application Instructions -- Apply to whole body, leave on for 24 hr.  Apply a second coating for an additional treatment.  Wash thoroughly 48 hr AFTER the LAST application.

Clinical Treatment of Lice

Lindane (Kwell® Cream, Lotion, or Shampoo)
Application Instructions -- Lindane should be applied to DRY hair (1 fl. oz. for short hair, 2 fl. oz. for long hair).  After 4 min, apply water and slowly lather.  Rinse the hair, towel dry and use a nit comb to remove dead lice and eggs.  A repeat treatment should be performed 7 days following the initial treatment.
Permethrin (Nix® Liquid)
Application Instructions -- Permethrin should be applied to WET hair.  Wash the hair and towel dry.  Apply the permethrin and leave in for 10 min.  Rinse, towel dry, and comb as directed above.  Permethrin is in a vehicle that may actually let it persist in the hair.  One treatment is usually 97-99% curative.  A second treatment is usually not necessary.
Malathion (Ovide® Lotion)
Application Instructions -- Sprinkle the lotion into DRY hair.  Rub in gently until the hair is thoroughly moistened.  Allow the hair to dry naturally (do not use a hair dryer -- this may increase absorption of the malathion and, since it is in an alcohol base, may result in combustion). Leave in for 8-12 hours and then wash the hair with non-medicated shampoo.  Rinse and comb as indicated above.  A second treatment may be required in 7-9 days.

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