Ectoparasiticides

ECTOPARASITICIDES

These are drugs used to kill parasites that live on body surface. Lice (Pediculus sp.—wingless insects) and mites (Sarcoptes/Acarus scabiei— arachnids) are minute arthropods infesting human skin and hair.

Scabies It is highly contagious; the mite burrows through the epidermis, laying eggs which form papules that itch intensely. Lesions may get secondarily infected requiring systemic antimicrobial therapy. The finger webs are the preferred sites of entry, but may soon spread to forearms, trunk, genitals and lower legs. Other members of the patient’s family should be treated concurrently; garments and bed linen should be washed in hot water and put in sun to prevent cross infection and reinfection.

Pediculosis The lice thrive on head (P. capitis), body (P. corporis) or pubic region (P. pubis). They cause itching, suck blood and transmit typhus and relapsing fever. The eggs called nits get attached to the hair and clothing by a chitin like cement.

Drugs used are:

Permethrin          

Sulfur

Lindane (BHC)    

Dicophane (DDT)

Benzyl benzoate  

Ivermectin

Crotamiton

Permethrin

This broad-spectrum and potent pyrethoid insecticide is currently the most efficacious and most convenient drug for both scabies and lice. It causes neurological paralysis in insects, probably by delaying depolarization. Toxicity of permethrin in humans is very low; apparently 40–400 times lower than that of lindane. After application, permethrin persists on the skin for days; systemic absorption is minimal. Nearly 100% cure rates have been obtained in scabies and pediculosis; comparative studies have found it to be more effective than lindane, benzyl benzoate and crotamiton. Single application is needed in most cases. Resistance to permethrin is very rare and it is effective in lindane nonresponsive cases. Few patients may experience mild and transient burning, itching, tingling, erythema or rash.

For scabies:

PERMITE, OMITE, NOMITE 5% cream; apply all over the body except face and head; wash after 8–12 hours. SCABERID 5% cream, 1% soap; SCABPER 5% lotion.

For head lice:

PERLICE, KERALICE 1% cream rinse, ELICE 5% lotion, SCALTIX 1% lotion; massage about 30 g into the scalp, washoff after 10 min.

Thus, permethrin is now the 1st choice drug for scabies and pediculosis

Lindane (Gamma benzene hexachloride, BHC) 

Another broadspectrum insecticide which kills lice and mites by penetrating through their chitinous cover and affecting the nervous system. Lindane is highly effective in treating headlice (67–92% cure) and scabies (84–92% cure) by single treatment. However, efficacy is lower than permethrin. Both lice and mites can develop resistance to lindane. Combining it with benzyl benzoate precludes resistance and improves cure rate to nearly 100%.

GAB 1% lotion, oinment; GAMADERM, SCABOMA 1% lotion; GAMASCAB 1% lotion, cream; ASCABIOL 1% emulsion with cetrimide 0.1%; BENZO 1% lotion, 1% soap.

For pediculosis:

Apply to scalp and hair (taking care not to enter eyes), leave for 12–24 hr. (a shower cap may be used for long hair) and then wash off. If lice are still present, repeat treatment after 1 week.

For Scabies:

The lotion/cream is rubbed over the body (below neck) and a scrub bath taken 12–24 hr later. Single treatment suffices in most patients; can be repeated only after a week, if the mite is still present.

The disadvantages of lindane are:

Being highly lipid soluble it can be absorbed through the skin (especially from oily vehicles and in small children)— can produce systemic toxicity —CNS stimulation, vertigo, convulsions (especially in children) and cardiac arrhythmias.

Absorbed lindane is widely distributed in the body, especially in fat; is metabolised and eliminated with a t½ ~24 hr. It can induce CYP isoenzymes in liver and affect metabolism of many drugs.

Though well tolerated by most patients if instructions are followed, it is less favoured for treatment of scabies—because application over large body surface is required—possibility of systemic absorption is more. It should be avoided in infants, young children and during pregnancy. Skin irritation is not prominent.

Benzyl Benzoate

It is an oily liquid with faint aromatic smell; has been popular for treatment of scabies. The emulsion is applied all over except face and neck after a cleansing bath.  A second coat is applied next day which is washed after 24 hours. The treatment is convenient and does not interfere with routine activities. It has achieved 76–100% cure in scabies. Benzyl benzoate is minimally absorbed through the skin; systemic toxicity is low, but neurological symptoms have occurred in children—contraindicated in them. Skin irritation is common, especially in children. Contact dermatitis is possible.

BENZYLBENZOATE APPLICATION 25% lotion; DERMIN 25% lotion; SCABINDON 25% oint with DDT 1% and benzocaine 2%

For pediculosis, it can be applied to the scalp, taking care not to enter eyes, and is washed off after 24 hours. Benzyl benzoate is now a 2nd choice drug for scabies and seldom used for pediculosis. Its combination with lindane is highly effective.

Crotamiton

It is an effective scabicide, pediculocide and antipruritic, but has produced lower cure rates (60–88%) in scabies. Better results have been obtained by extended 5 day application in children. It is less prone to cause skin irritation and has low systemic toxicity despite absorption through the skin—may be preferred for children. It is applied twice at 24 hr interval and washed off day after that.

CROTORAX, CROTON 10% cream, lotion

Because of lower efficacy and need for repeat application, it is a second choice drug for scabies and pediculosis.

Sulfur

It is the oldest scabicide and weak pediculocide, antiseptic, fungicide and keratolytic. Applied to skin it is slowly reduced to H₂S and oxidized to SO₂ and pentathionic acid. These, especially the latter, dissolve the cuticle of itch mite and kill it. The reactions are carried out by epidermal cells and the arthropods themselves.

Sublimed sulfur or precipitated sulfur is used as a 10% ointment. After a warm scrubbing bath (to open the burrows) the ointment is massaged over the entire body (below the neck) for 3 consecutive days, followed by soap water bath on the fourth day. It is cheap but has disadvantages:

·      Treatment is messy.

·   Produces bad odour—socially unacceptable —may interfere with patient’s vocation.

·      Repeated applications are required.

·      Sulfur has been superseded by better drugs.

Dicophane (DDT)

It has been a popular insecticide for mosquitoes, flies and other pests. For this purpose, it is used in the dust or watery suspension form, which is poorly absorbed through skin. For pediculosis and scabies a 1–2% lotion or ointment is applied and washed off after 12–24 hours. It penetrates through the exoskeleton and acts as a neurotoxin for the arthropods. When oily vehicles are used, significant amounts may be absorbed through the skin and cause rashes, muscle weakness, tremor. Very high doses produce BHC like convulsions. It gets stored in body fat and induces microsomal enzymes. Combination with benzyl benzoate (SCABINDON point) is more effective. It is rarely used.

Ivermectin 

This  anthelmintic  drug has been found highly effective in scabies and pediculosis as well. It is the only orally administered drug used for ectoparasitosis. A single 0.2 mg/kg (12 mg in adults) dose has cured upto 91–100% patients of scabies. AIDS patients with scabies also respond. Most cases of head/ body lice have been successfully treated.

Ivermectin is very well tolerated by scabies/ pediculosis patients, with few if any side effects. However, it is not to be given to children  5 yr, pregnant and lactating women. Though experience is still limited, ivermectin appears to have the potential of a first line drug for scabies and pediculosis.