Head Louse Resistance to Pediculicides: A
Note: This article expands on a
presentation given by the author in Anchorage in 2004 at the invitation of
the Environmental Protection Agency to the Commission for Environmental
Cooperation North American Task Force on Lindane.
Head lice are perfectly evolved human
predators. They establish and maintain residence on the human head. They
copulate and reproduce on the host. The host’s blood provides their
sustenance. Their success in this ecological niche exceeds perhaps any
other macroorganism that preys on humans. Their prevalence far exceeds
other parasites such as pinworm or hookworm. Sales of nonprescription
products for head lice and school-related expenses cost U.S. citizens an
estimated $350 million yearly.1
Potential Treatment Modalities
Potential therapies for pediculosis can be separated into four broad
groups: topical pesticides (also known as pediculicides), physical
methods, suffocating methods, and miscellaneous approaches. These
potential therapies must be examined closely in light of an emerging and
little-appreciated phenomenon: an alarming trend for head lice to develop
resistance to pesticides.
The Phenomenon of Resistance
Pharmacists are well aware of the phenomenon of resistance. Pathogenic
microorganisms have become resistant to many antibiotics, forcing
manufacturers to develop novel chemical structures in the hope that
resistance will not develop. It should come as no surprise that rapidly
reproducing insects such as head lice will inevitably develop resistance
to the pesticides used to combat them.2,3 The problem of
resistance has been characterized as “a growing problem” that is
“daunting.”4,5 One of the dangers is that patients will resort
to multiple treatments with pesticides, re-exposing children needlessly to
potentially toxic chemicals.
Resistance to Lindane. Lindane users have experienced seizures, lethargy,
slurred speech, and neck and extremity stiffness.6,7 Reports of
resistance have accelerated since the 1970s.8,9 The mechanism
for development of resistance is hypothesized to be alterations in amino
acids located at the nerve sheath sodium channel; this phenomenon may also
confer resistance to pyrethrins and permethrin.1
Resistance to Malathion
Malathion is an
odorous, flammable pesticide that must remain on hair until it is dry,
perhaps 8-12 hours later. It can sting and irritate skin. Although the
product marketing claims to have a residual effect, the manufacturer
suggests that patients undergo a second exposure if lice remain after 7-9
days. Malathion has had a checkered marketing history in the U.S., having
been known as Prioderm and Ovide. It was discontinued, but re-marketed in
1999.6 Treatment failures were reported as early as 1990, with
early reports emanating from Australia and England, where it is a
nonprescription product.10,11,12 Early reports cited a failure
rate of 8%, but reports from 1999 gave the figure as 64%.13,14,15
In 2001, a Cochrane Review concluded that resistance was widespread in the
United Kingdom; resistance in the United States has not been widely
reported, but may eventually develop with continued use, given its
emergence in other countries.8,16,17
Resistance to Synergized Pyrethrins
Pyrethrins are chrysanthemum derivatives that must be synergized with piperonyl butoxide, a petroleum derivative, to retain efficacy. They
cannot be used in patients allergic to ragweed. Reports of resistance to
this nonprescription pesticide began in 1986, growing in scope until in
1997 the Medical Letter reported that treatment failures had become
common.18,19,20 In 1999, researchers reported that synergized
pyrethrin’s efficacy was questionable in light of the accelerating number
of anecdotal reports of resistance.21 As case reports of
resistance continued to accumulate, dermatologists suggested that multiple
resistance had appeared, and that those patterns would undoubtedly be seen
throughout the world.22,23
Resistance to Permethrin. Permethrin was a prescription product (Nix) in
the U.S. from 1986-1990, when it became an OTC. Permethrin also has a
residual effect. The first reports of resistance to permethrin emerged in
1990, with reports coming from Israel, the Czech Republic, and Britain in
1995.10,19,24-26 In 1997, the Medical Letter stated that
resistance was increasing.20 It was reported in Washington
State in 1998.27 In 1999, it was reported to be virtually
useless in the U.K.; even very high concentrations had no effect on
resistant lice.14,27,28 Allegations of resistance continued to
accumulate; resistance figures as high as 87% were quoted for the U.K.16,17,22,29-31
A 1999 study indicated possible resistance in the United States; this was
demonstrated again in 2003.21,32 Resistance of head lice to
permethrin has prompted researchers to explore other chemicals.33
Permethrin resistance may develop through several mechanisms. If the kdr
(knock-down resistance) gene is involved, no strength of permethrin will
be effective, which eliminates consideration of the 5% prescription
concentration (e.g., Elimite).
Resistance as a Cumulative Phenomenon
Resistance is the development of mechanisms to survive potentially deadly
onslaughts. Once these successful mutations are incorporated into the DNA
of the living being, they will continue to be passed to succeeding
generations. The numbers of resistant organisms can only stabilize or grow
in the face of continual challenge by the provocative agent. Thus, any
figure for the percentage of resistant members of a population is
outdated; the actual percentage of resistant louse strains can only
stabilize or increase. Given the dynamic nature of organism adaptation,
increase is the only logical conclusion, meaning that any figure for
percentage is of necessity lower than reality.
Survey of Pharmacists
A survey of
pharmacists discovered that 81.7% of respondents had encountered patients
with apparent treatment failures after use of synergized pyrethrins and
78.6% following use of permethrin.34 Treatment failures
occurred once or twice weekly in 58.1% of pharmacists’ practices, often
causing patients to treat themselves more frequently or in higher doses
Manufacturer Explanations of
The manufacturers of topical pesticides for lice state
that they have reports of efficacy. However, lab-bred lice are different
from free-living lice in the U.S. at large. Data from specially-bred lab
lice populations cannot necessarily be extrapolated to the hardier lice
typically found on human heads.35
Manufacturers also blame the victim of
resistance through suggesting the victim failed to apply the pesticide for
a second time, that too little product was used, that the victim was
re-exposed, by stating that the victim did not remove nits, or by stating
that the victim did not clean the environment.
What is the Future of Pesticides?
At this time, no pesticide is proven to be resistance-free. To the
contrary, each available pesticide has been implicated in causing
resistance, which is widespread in some cases. Continued use of pesticides
will undoubtedly select for resistance lice, just as continual use of
antibiotics selects for resistant strains of organisms.
of head louse treatment have the greatest potential for safely and
effectively halting an infestation. However, the proper techniques and
products must be chosen to realize the full benefit. One of the
possibilities is individual louse removal. This involves use of a bright
light, magnifying glass, and blunt scissors to clip away any hair that has
a nit. Live lice are removed and placed in a bag. The method is
time-consuming and may miss lice and/or nits. A better method of diagnosis
is the use of a nit detection/removal comb. Combing has the potential to
compensate for ineffective pesticides through removal of nits.34,36
However, in the opinion of one expert, “Unfortunately, the free plastic
combs included in many product packs are as ineffective as the products.”
Preliminary research has demonstrated that the LiceMeister Comb (National
Pediculosis Association) is superior to other plastic and metal combs due
to its unique construction, having 32 closely spaced, rigid, stainless
steel teeth.29 In contrast to the use of pesticides, combing
with the LiceMeister presents no risk of pesticide absorption or toxicity,
presents no danger to the environment, and can simultaneously detect and
treat an infestation.
desperation, frustrated parents and caregivers may turn to alternate louse
control methods that are of unknown safety and/or efficacy. Some are
unpleasant for victims. They include covering the head with olive oil,
petrolatum (left on overnight under a shower cap), full-fat mayonnaise
(also left on overnight), hair styling gel, salad dressings (overnight
covered with plastic wrap), baby oil, or mineral oil. Removal of the
normal amount of oil from one’s hair with shampoo is fairly simple;
removal of a jar of petrolatum is infinitely harder. Washing the hair
several times with harsh shampoos may leave the scalp more irritated than
the louse infestation itself.
unproven methods have been marketed, including homeopathic products
containing diluted salt water (Licefreee!), one containing dimethicone
(Rid Pure Alternative) and others containing herbs such as rue, tea tree
oil, paw paw, sage, rosemary, thyme, pennyroyal, and essential oils. One
product claims to make nits visible by means of a “neon” aerosol, which is
noxious to breathe when sprayed. Desperate patients may resort to
potentially deadly alternatives such as gasoline, kerosene, lantern oil,
and industrial strength diazinon. The first three often explode, killing
and maiming users. The latter caused irreversible brain damage when
applied to the head of a child. Pharmacists must advise against unproven
and unsafe methods whenever they are asked about their use.
Advice to Patients: What To Do About Head
Head lice on your children—it’s more than
embarrassing. Control of these parasites can be difficult, time-consuming
and frustrating, especially if you use the wrong products.
What to Avoid
You should avoid
most products commonly sold for head lice. First are the shampoos. Some
are prescription, and you can buy others in any pharmacy. Head lice in
certain areas have become resistant to them. In other words, some head
lice have developed the ability to live through the shampoos. This means
that they cannot work, even if you use them exactly as recommended. Never
use them in higher doses or more often than recommended, as they may be
dangerous to your children. You should avoid them altogether.
There are also sprays to treat the
environment, pillows, beds, and mattresses. Any head lice away from the
head will die shortly since they cannot get the blood they need to
survive. You can vacuum thoroughly if you wish, but you should not use
sprays. Your child might breathe them in all night if you cover the
mattress and pillows with them. They are of unknown safety.
Also avoid products that promise to kill
or remove lice, but contain nothing more than diluted table salt in water
(labeled as natrum muriaticum), neon paint, dimethicone and others
containing herbs such as rue, tea tree oil, paw paw, sage, rosemary,
thyme, pennyroyal, and essential oils. None of these is yet proven safe
Avoid so-called “suffocating treatments”
that force you to cover the head with olive oil, salad dressing, full-fat
mayonnaise, petrolatum, and other thick, gooey or oily substances. You may
be forced to leave them on overnight underneath several layers of plastic
wrap. Removal of these greasy oils can involve several courses of regular
shampoo, which can irritate the scalp. These methods have never been
proven to kill head lice in any case.
Never resort to potentially deadly
treatments such as gasoline, kerosene and lantern oil. These have all
exploded in flash fires in people’s homes, killing them and causing severe
burns. Never use any type of garden or pet insecticide, and never use any
industrial strength chemicals.
What to Use
that combing can safely and effectively stop head lice. However, most
plastic or metal combs do not do the job as promised. Existing evidence
points to one comb that is superior to others because of its construction.
Known as the LiceMeister Comb, it is the only comb which has a set of
metal teeth in a permanently sealed handle. It is inexpensive, and can be
re-used, since it can be boiled between uses. When you comb through a
child’s (or adult’s) hair according to the directions provided, this comb
can treat lice by removing the live lice and their eggs. It is available
from your pharmacy and by contacting
www.headlice.org/licemeister/. This is the address of the National
Pediculosis Association, the only nonprofit health and information
organization dedicated to stopping the use and abuse of dangerous
chemicals on children’s heads.
“What About Public Lice?”
Pharmacists are often asked to help treat
pubic lice. It is vital to remember that nonprescription permethrin (Nix)
has not been found effective for pubic lice, and is not labeled for this.
The only other nonprescription alternative is synergized pyrethrins.
However, as one would expect, resistance is also an issue, as evidence by
one case report. A 43-year-old male with adult lice and nits in the pubic
area was treated with synergized pyrethrins, but the infestation
persisted.37 While 5% permethrin cream was eventually successful in this
case, resistance may eventually emerge to this product also.
W. Steven Pray, Ph.D., D.Ph.
Bernhardt Professor of Nonprescription Drugs and Devices
Pharmacy Southwestern Oklahoma State University
Weatherford, OK 73096
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