Report
Oxford, UK
International
IJD
Blackwell
1365-4632
45
Publishing,
Publishing
Journal Ltd,
of
Ltd.
Dermatology
2004
Transmission potential of the human head louse,
Pediculus capitis (Anoplura: Pediculidae)
Head louse transmission
Takano-Lee
et al.
Miwa Takano-Lee, PhD, John D. Edman, PhD, Bradley A. Mullens, PhD, and John M. Clark, PhD
From the Center for Vector-borne Diseases,
University of California, Davis, CA,
Department of Entomology, University of
California, Riverside, CA, and Department of
Veterinary and Animal Science, University of
Massachusetts, Amherst, MA
Correspondence
John Edman
Center for Vector-borne Diseases
University of California
Davis
CA 95616
E-mail: jdedman@ucdavis.edu
Abstract
Background Millions of people are infested by head lice every year. However, louse transfer
between hosts is not well-understood. Our goals were to determine: (1) which stages were most
likely to disperse and why, (2) the likelihood of fomites transmission, and (3) if host blood gender
affects louse development.
Methods Various life stages of lice at differing densities were permitted to cross over a 15-cm
hair bridge placed between two artificial blood-feeding arenas. Louse transfer caused by hot air
movements, combing, toweling, and passive transfer to fabric was investigated. The ability of
lice to oviposit on different foreign substrates and the hatching potential of eggs intermittently
incubated for 8 h /night on a host were likewise investigated. Louse in vitro development
following feeding on human female or male donor blood was compared.
Results Adult lice were the most likely to disperse. Neither population density nor hunger
significantly affected dispersal tendencies. Lice were dislodged by air movement, combs and
towels, and passively transferred to fabric within 5 min. Females oviposited on a variety of
substrates and 59% of eggs incubated for 8 h /night hatched after 14 –16 days. There was no
survivorship difference between lice artificially fed on female vs. male blood.
Conclusions Adult lice are the most mobile, indicating that they are most likely to initiate new
infestations. Although head-to-head contact may be the primary route of transmission, less
direct routes involving fomites may play a role and need further evaluation. Blood-borne factors
do not appear to cause any gender-biased host preference.
Introduction
Pediculosis, the infestation by head lice (Pediculus capitis
De Geer), affects 6–12 million people each year in the United
States; most cases occur among elementary school children.1
Louse infestation is annoying and leads to pruritis, sleeplessness, and in extreme cases, anemia.2 Social stigma still surrounds
those with head lice even though lice do not discriminate by
socioeconomic status. It remains unknown how lice are transferred from one host to another.
There are two major mechanisms by which louse transmission is assumed to occur: direct host-to-host contact or
via inanimate objects, also known as fomites.3 Burkhart and
Burkhart4 proposed that transmission also might occur via
(1) fallen hairs containing lice or nits; (2) wind movement;
(3) static movement; or (4) contact with dislodged lice crawling on the floor. Greater control of louse transmission could
be achieved if it were known which stages dispersed and why,
and how long various stages can survive away from a host.
Epidemiological studies of louse outbreaks have been
documented around the world,3,5–15 but the conclusions made
© 2004 The International Society of Dermatology
as to why this happens vary widely. These differences may be
attributable to the skill of the louse screener, ages of the
children involved, intensity of infestations, cultural differences, etc. Most people intuitively conclude that lice most likely
transfer when direct contact between hosts occurs.2,3,16,17
More controversial is whether transmission by fomites occurs
to a significant extent.2,3,5,11,13,16–19 The role of transmission by
fomites has not been experimentally quantified, although the
likelihood of transfer from hair to hair was recently investigated in the laboratory.17
We maintained several in vivo colonies to provide sufficient
numbers of lice. Use of an in vitro bioassay system, which
consists of a human hair tuft maintained on an artificial
membrane-covered blood-feeding arena, permitted us to study
louse mobility, behavior, and development in a more realistic
and reproducible setting20,21. The goals of the present research
were to determine which stages are most likely to disperse and
why, to investigate whether transmission via fomites presents
a legitimate risk to uninfested individuals, and to perform a
preliminary evaluation of the impact of host gender on louse
development.
International Journal of Dermatology 2005, 44, 811– 816
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Takano-Lee et al.
infested tuft). Hair tufts were washed with shampoo, rinsed with
water, and air-dried between experiments. Hair bridges and feeding
arenas were discarded after use.
Data were analyzed by comparing arcsine (square-root)transformed proportions of lice dispersing over the hair bridge by
one-way ANOVA. A separation of means test also was performed.
Significance was determined at P < 0.05.
Figure 1 Photograph of two in vitro louse-rearing feeders,
connected by a 15-cm hair bridge
Materials and Methods
Louse maintenance
Four separate head louse colonies from California (CA),
Ecuador (EC), Florida (FL), and Panama (PA) were maintained as
previously reported.20 Head lice were gently grasped by tweezers
and placed on human hair tufts in modified 15-mL centrifuge tubes
and secured to the legs of the investigator (M. T.-L.).
“Head-to-Head” transmission
Two in vitro feeding arenas that mimic human “mini-heads”,20
located 6.25 cm apart, were connected by a 15-cm hair bridge
(∼15 human hairs) (Fig. 1). Each artificial membrane-covered
feeding arena was suspended within a reservoir containing
warmed human blood (31 °C). The blood meal was composed of
a 1 : 1 mixture of red blood cells (RBCs) : plasma (matching A +
blood from the same individual) and was amended with 200 µL of
penicillin/streptomycin (Sigma, St. Louis, MO) antibiotic/mL blood.
Blood components were purchased from a local blood bank and
contained citrate dextrose-phosphate, a common anticoagulant
that dilutes the blood by 10-fold. Our artificial feeding apparatus
effectively eliminated the need for a live host, but permitted normal
louse life functions to occur.20
The following experimental factors were evaluated: (1) density
(n = 5, 15, or 30 lice/arena); (2) hunger (freshly blood fed or starved
for 10–12 h); (3) life stage (1st, 2nd, and 3rd instars, males and
females); and (4) time (dispersal after 5 min or 1 h). Groups of lice
were introduced onto hair tufts (1.5 cm) and permitted to rest within
the in vitro feeding arena (infested “mini-head”) for 5 min before a
second uninfested hair tuft was placed into a separate in vitro
feeder (uninfested “mini-head”) 10 cm away, and the 15-cm hair
bridge added to connect the two feeders for a defined period of
time. The distribution of lice was observed by counting individuals
and documenting their position at 5 min and 1 h after the hair
bridge was placed into direct contact with the hair tufts in the two
feeding arenas. Lice were considered to have dispersed if they
were on the distal half of the hair bridge (i.e. furthest from the
International Journal of Dermatology 2005, 44, 811– 816
Vertical walking speed
Lice (EC) of varying life stages and nutritional levels were placed
on the bottom end of a single strand of human hair (25 cm). Each
hair strand was attached to a Plexiglas frame (25 × 25 cm) and
maintained at room temperature (22 – 23 °C). The upward (vertical)
walking speed of each louse was recorded with a stopwatch. Different
life stages were compared by one-way ANOVA, followed by a separation
of means test, where significance was determined at P < 0.05.
Data for blood fed vs. starved lice were compared by life stage
using two-sample t-tests and significance determined at P < 0.05.
Fomites transmission
Four possible methods of louse transmission were examined to
investigate the potential of lice to be transferred by hot air movements,
combs, towels, or contiguous fabric. The ability of lice to be removed
by five passes of a hand-held vacuum (Dustbuster 200, Black and
Decker, Hampstead, MD) or one pass of a convertible vacuum
cleaner (RS, The Hoover Company, North Canton, OH) was evaluated.
A small number of hair strands (∼15 strands, 25 cm in length)
were glued at one end and loose at the other end. Groups of five
lice (PA), composed of either all females or a random mixture of
2nd and 3rd instars (six replications of each group),were placed
onto the group of hair strands and exposed to one of the following
treatments: (1) 1-min exposure to low (4 m/s at 31 °C) and high
(8.9 m/s at 38 °C) cycles of a typical hair dryer (Model SBPC30,
Sunbeam, El Paso, TX) at a distance of 15 cm; (2) combing (five
strokes) with the wide-toothed portion of a comb, followed by the
narrow-toothed portion; (3) agitation in water for 30 s before toweldrying (gently patting the strands between two towel layers in a
downward fashion) for 5 s; or (4) placement of a wristlet of human
hairs securely around the investigator’s wrist and then loosely
covered by a piece of dark blue corduroy (3 × 24 cm) for 5 min. The
number of lice remaining on the hair strands was recorded. Data
were analyzed by comparing arcsine (square-root)-transformed
proportions of lice removed by treatment using one-way ANOVA.
A comparison of means test (Tukey) was performed to determine
significance at the P < 0.05 level.
Oviposition substrates
A variety of different oviposition substrates (human hair, denim,
charmeuse, felt, or faux fur) were offered to female lice (EC).
Females were starved for 15 h before receiving an in vivo blood
meal. Lice were placed in a 3.5-cm diameter Petri dish on an
oviposition substrate (2.5 × 2.5 cm) (n = 3 females/dish; five
replicates), transferred to an incubator (31 °C), and maintained
© 2004 The International Society of Dermatology
Takano-Lee et al.
Head louse transmission Report
until hatching had ended. The number of eggs laid per female was
compared between substrates using a one-way ANOVA, with
significance determined at P < 0.05. A separation of means test
(Tukey) also was performed to determine significance. The
proportion of viable eggs was arcsine (square-root) transformed,
analyzed by one-way ANOVA, and significance determined at
P < 0.05.
Night-time exposure of eggs to a human
Fresh eggs (CA), laid within a 4-h period on a human hair tuft
(1.5 cm), were attached to a human host for 8 h per night until
hatching occured or egg inviability was confirmed (egg shriveled,
no eyespot formation, or no embryo movement). Eggs on the hair
tuft were retained within an in vivo rearing chamber20 at all times.
The chamber was secured to the host (M. T.-L.) for the 8-h period.
When not exposed to the host, the rearing chamber was
maintained at room temperature (∼20 °C) and ambient relative
humidity. Tufts were examined daily to record egg hatch. This
experiment was designed to mimic the incubation condition of
eggs on unattached hairs shed in the bed or that may have been
laid on fomites (i.e. stuffed animals) in close contact with hosts
during the night-time hours.
Blood-borne factors influencing development
The development time of our four louse colonies was documented
over a 5-month period on our automated in vitro louse feeder.21
The blood of each donor (three females, two males) was used
exclusively for approximately 4–6 weeks and survivorship and
development recorded daily. These blood meals consisted of
RBCs reconstituted with plasma at a ratio of 1.25 : 1.0
(RBC : plasma) with 200 µL of penicillin/streptomycin antibiotic/
mL blood. Blood products were purchased from a local blood
bank and contained citrate phosphate dextrose as an
anticoagulant.
In a separate study, the mean ratio of RBC : plasma was
calculated for RBCs and plasma units of 14 donors (eight female,
six male) purchased from a blood bank over a period of 17 months.
The proportions of RBC : plasma were arcsine (square-root)transformed and compared between male and female donors by a
two-sample t-test. Significance was determined at P < 0.05.
Figure 2 Mean proportion (± SEM) of lice found closer to
the uninfested arena (destination feeder) within (a) 1 h and
(b) 5 min. Means containing the same letter are not significantly
different (P > 0.05)
Results
“Head-to-Head” transmission
Louse dispersal increased with age (Fig. 2a). There were no
significant differences between dispersal tendencies of adult
males and females, so they were pooled; likewise, 2nd and
3rd instars were pooled. Males and females were much more
likely than immatures to disperse within just 5 min (Fig. 2b).
Adults were more likely to cross the hair bridge after 1 h than
2nd instars and 3rd instars, which were more likely to disperse
than 1st instars (Fig. 2b).
Vertical walking speed of lice
Both males and females consistently moved the most rapidly,
followed by 3rd instars, 2nd instars, and 1st instars, which
was the slowest stage (Table 1). Climbing speed of unfed
Table 1 Mean traveling speed (cm /min) for various in vivo-reared lice (EC) to vertically climb upwards
Speed (cm/min) ± SEM
Teneral 1st instars*
Starved 1st instars
Starved 2nd instars
Starved 3rd instars
Starved males
Starved females
2.3 ± 0.1a
5.3 ± 0.2b
6.1 ± 0.3bc
8.0 ± 0.4cd
9.3 ± 0.5d
9.5 ± 1.0d
Speed (cm/min) ± SEM
n
26
30
28
30
25
30
–
Fed 1st instars
Fed 2nd instars
Fed 3rd instars
Fed males
Fed females
–
5.9 ± 0.2a
6.5 ± 0.4a
10.3 ± 0.7b
11.1 ± 0.7b
13.2 ± 0.6b
n
–
25
25
25
27
25
Column means containing the same letter are not significantly different (P > 0.05).
*Unfed lice hatching within 24 h.
© 2004 The International Society of Dermatology
International Journal of Dermatology 2005, 44, 811– 816
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Takano-Lee et al.
Night-time exposure to a host
The majority of eggs exposed to a human host for just 8 h
each day hatched (59%), with a mean time to hatch of
15.2 ± 0.3 d. A minority (12%) lacked an eyespot and were
assumed to be unfertilized. An additional 29% of eggs were
fertile (eyespot present) but still failed to hatch. Eggs exposed
to a host for only 8 h each day required almost twice as long
to hatch compared with in vivo-reared lice (CA) (8.4 ± 0.0 d)
(15.2 vs. 8.4 d, d.f. = 442; t = −22.24; P < 0.0000) (data not
shown) and fewer eggs hatched (59% vs. 76%).20
Blood-borne factors influencing development
Figure 3 Mean proportion (± SEM) of lice (adults, 2nd and
3rd instars) separated from the original substrate (10 –15 hair
strands) after exposure to a hair dryer, combing, toweling, or
contact with fabric. Means containing the same letter are not
significantly different (P > 0.05)
lice declined as follows: adult females (9.5 cm /min), adult
males (9.3 cm/min), 3rd instars (8.0 cm /min), 2nd instars
(6.1 cm/min) and 1st instars (5.3 cm /min). Blood-fed lice
of all stages except 2nd instars moved significantly faster
(~10–20%) than starved lice ( Table 1). Teneral 1st instars
(< 24 h from hatch and unfed) represented the least mobile
stage (2.5 cm/min).
Fomites transmission
There was no significant difference between females or older
nymphs, so the transmission data were pooled. Head lice
were consistently dislodged by using a hair dryer set at either
low or high settings or by using a normal comb (Fig. 3) (Note:
lice either fell to the ground or remained on the comb.) Lice
also were easily transferred from wet hair to a towel (63%).
Approximately 27% of lice passively transferred from hair
wristlets to adjacent fabric within 5 min (Fig. 3). Hot air
currents and combing were more likely to dislodge lice than
toweling. To evaluate the benefit of vacuuming, we attempted
to remove lice with five passes of a hand-held vacuum, but
failed. All lice remained firmly attached to the fabric.
However, a single pass with a carpet vacuum removed all
lice from the carpet material.
Oviposition substrates
There was no significant difference between the proportions
of viable eggs laid on any of the five substrates (range = 0.27
– 0.67; d.f. = 4, 16; F = 0.57; P > 0.05); overall hatching
averaged 58%. Significantly more eggs/female were laid on
charmeuse (1.1 ± 0.1 eggs/female) than on felt (0.4 ± 0.1 eggs/
female) or denim (0.3 ± 0.2 eggs/female) (d.f. = 4, 20; F = 4.59;
P < 0.01). All eggs laid on the charmeuse were laid on the
frayed ends of the fabric.
International Journal of Dermatology 2005, 44, 811– 816
There were no consistent developmental trends among
lice from the four colonies when fed exclusively on genderspecific blood. The data could not be pooled because some
colonies displayed slight differences with female blood resulting in slightly longer development time (data not shown).
The RBC : plasma ratio varied significantly between
human males and females, a fact well-documented in medical
texts.22 The mean ratio for female blood was 1.16 ± 0.07
(range = 0.92–1.25) whereas the mean for males was 1.72 ±
0.23 (range = 1.25–2.8) (d.f. = 12, t = −3.04, P = 0.01). There
was no significant difference between 24-h survivorship of
lice feeding on exclusively male or female blood (d.f. = 146,
t = 1.48, P = 0.14) (data not shown).
Discussion
Dispersal was not evaluated continuously but still yielded a
relative estimate of louse movement, as it may vary by stage
or feeding status. All stages of lice are quite mobile, but 1st
instars demonstrated a greatly reduced tendency to wander
from a host. The increased mobility of older stages has been
observed in the literature, but never quantified or directly
compared by other researchers.17,23 Our data indicate that
adults are most likely responsible for initiating new infestations and suggests that louse control measures should focus
on this stage. The fact that females require multiple mating to
remain fecund20 means there are benefits if males disperse
along with females.
Adults, nymphs, and eggs could all transmit themselves to
a new host.25 Louse transmission could be further facilitated
under conditions of duress, such as that caused during
host grooming. In these stressful situations, lice have been
observed to exhibit a “flee response”24,25 or a rapid burst of
movement away from the disturbance area. Potentially, lice
could remove themselves from a host, or move directly onto
a new host.
The ability of adults to move rapidly (based on either
walking speed or dispersal from a feeding arena) suggests that
fomites transmission may be more significant than has been
assumed. We demonstrated that lice can readily be dislodged
by air movements, combing, toweling, or passive transfer to
© 2004 The International Society of Dermatology
Takano-Lee et al.
adjoining fabric. Likewise, head lice are capable of laying
viable eggs on a variety of substrates other than hair, although
the more hair-like substrates (charmeuse, hair, faux fur)
were preferred to smoother materials (felt and denim). It also
appears that dislodged gravid females may oviposit on any
available substrate. If that substrate happens to be bedding,
it is possible that the eggs will hatch in approximately 2 weeks
(if not properly laundered).
Our data also suggest that louse transmission by fomites
may occur more frequently than is commonly believed and at
close proximity may suffice to increase the likelihood of a new
infestation. Although passive transfer to adjacent fabric was
the least likely route of transfer, its occurence was sufficient to
provide a mechanism for significant louse movement. Passive
transfer suggested the possibility that lice could transfer to
hats, upholstery, headphones, etc. These results support
epidemiological data that infested persons are likely to have
infested family members.7–9,13,18 Thus, all family members
should be examined for head lice when a family member
has been diagnosed and treated for lice in order to avoid
home-based reinfestation.
Human blood differs between individuals based on gender
and age, although there is a wide normal range.22 Because the
composition of blood varies between people, it remains possible that immunological factors, Rh group, specific proteins,
or blood group type could affect the ability of lice to thrive on
some hosts more than others,2 or that fecundity may be adversely
affected, as Maunder27 suggests. We did not observe significant
or consistent developmental differences based on the gender
of the host blood fed to head lice. However, relatively minor
differences were noted in some louse strains. Such differences
may be related to packed cell volume and nutritional quality
of the blood, but further work is needed to explain such
variability. Lice seem to not prefer one gender of host to
another. If preferences exist, as certain epidemiological
studies have found, they are probably owing to gender-biased
grooming or social behavior.
Our study is among the first to quantify the transmission
potential by fomites, and to demonstrate that adults may be
naturally more likely to initiate new infestations regardless of
population density or hunger. By observing the rearing of lice
in vivo, we have witnessed head lice walking across linens,
clothing, upholstery, carpet, and even attaching to our hands
and arms without difficulty. Louse transmission via fomites
likely occurs more frequently than is typically assumed. This
finding suggests that louse control measures should include:
(1) use of a louse comb, instead of visual inspection, to
screen for louse presence; (2) screening of all individuals
within an infested person’s immediate circle of contact (e.g.
family members, classmates, and best friends); (3) laundering of everything within the infested individual’s bed or
temporary quarantining of such materials for ≥ 18 d; and
(4) thorough vacuuming of floors, carpets, and upholstery
© 2004 The International Society of Dermatology
Head louse transmission Report
with a standard vacuum cleaner, or otherwise cleaning (without pediculicides).
Acknowledgments
Funding for this project was provided by NIH R01AI45062.
Head louse specimens were generously contributed by
various sources: FL lice were obtained from Lidia Serrano (Lice
Sources, Inc., Plantation, FL); CA lice were obtained from
school nurses within the San Bernardino City Unified School
District; EC lice were collected by Dr David Taplin, Field
Epidemiology Survey Team (FEST), University of Miami,
School of Medicine, Miami, FL; and PA head lice were collected
by Terri Meinking, FEST, University of Miami, School of
Medicine, Miami, FL.
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© 2004 The International Society of Dermatology