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.
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.
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