LICE
Lice are small bloodsucking insects that live on the skin of
mammals and birds.
Three species of lice have adapted themselves to humans: the
head louse (Pediculus
humanus capitis), the body louse (Pediculus humanus) and the
crab or pubic louse
(Pthirus pubis) . All three species occur worldwide. Lice infestations
can
cause severe irritation and itching. In addition the body
louse can transmit typhus fever, relapsing fever and trench fever. Outbreaks of
louse-borne typhus fever,
sometimes claiming thousands of lives, have occurred in
colder areas where people
live in poor, crowded conditions, especially in some
highland areas of Africa, Asia
and Latin America.
Biology
The three species live only on humans (not normally on
animals) and feed on
human blood; the life cycle has three stages: egg, nymph and
adult .
Development from egg to adult takes about two weeks. The
white eggs (called
nits) are glued to a hair or, in the case of the body louse,
to fine threads on clothes.
The nymphs are similar to the adults but much smaller. Fully
grown lice are up to
4.5 mm long and feed by sucking blood. Feeding occurs
several times a day. Lice
can only develop in a warm environment close to human skin,
and die within a
few days if they lose contact with the human body. They are
normally spread
by contact, e.g. in overcrowded sleeping quarters and other
crowded living
conditions.
The three species of human lice are found on different parts
of the body:
• the head louse occurs on the scalp and is most common in
children on the
back of the head and behind the ears;
• the pubic louse or crab louse is mainly found on hair in
the pubic region but
it may spread to other hairy areas of the body and, rarely,
the head;
• the body louse occurs in clothing where it makes direct
contact with the
body; it is similar to the head louse but slightly bigger.
Body lice
Body
lice are most commonly found in clothing, especially where it is in direct
contact
with the body, as in underwear, the crotch or fork of trousers, armpits,
waistline,
collar and shoulders. They attach themselves to body hair only when
feeding.
The eggs are attached to thin threads of clothing. Body lice are most
common
in colder areas where people do not frequently wash or change clothes.
Body
lice are spread by close contact between people. They are most commonly
found,
therefore, on people living in overcrowded, unhygienic conditions,
as in
poorly maintained jails, refugee camps and in trenches during war. They also
spread
by direct contact between people in crowded transport vehicles and markets.
Body
louse infestations may also be acquired through sharing bedding, towels
and
clothing or by sitting on infested seats, chair covers or cushions. Body lice (Pediculus humanus
corporis) are normally associated with crowded and unhygienic living
conditions but are also transmitted through any
close contact with an affected person.
Body lice are similar to head lice but the region of
occurrence is important in their
identification. They are found mainly in clothing,
often in the seams, but also on the body surface,
especially the armpits and around the waist. In
Australia, they are less common than headlice or
pubic lice.
Body lice are flat wingless insects with six legs
ending in a claw. They may be greyish or brown in
colour and range in size from 2 – 5 mm. They feed on
human blood and their bites can cause
irritation. Secondary infections can occur if louse
faeces are introduced into the skin by the affected
person scratching.
Transmission
Body lice can be transmitted in clothing
and bedding as well as by close physical contact.
Head lice
The head
louse is the most common louse species in humans. It lives only in the
hair on
the head and is most often found on children. The eggs (or nits) are firmly
glued to
the base of hairs of the head, especially on the back of the head and behind
the
ears. Because the hairs grow about a centimeter a month
it is
possible to estimate the duration of an infestation by taking the distance
between
the scalp and the furthest egg on a hair. Infested persons usually harbour
10–20
adult head lice. The females lay 6–8 eggs per day. Head lice are spread by
close
contact between people, such as children at play or sleeping in the same bed.
Head
lice are also spread by the use of other people’s combs that carry hairs with
eggs or lice
attached. Headlice (Pediculus
humanus capitis) are a social pest and are rarely a threat to health
however their
presence can have social, economic, psychological
and educational repercussions. Correct
information, treatment and monitoring should ensure
they are controlled with minimum impact on
individuals and communities.
Transmission
Headlice are mainly transmitted by
head-to-head contact with an infested person.
Children are more susceptible because
they are more likely to have close contact with other children
and adults in homes, playgrounds,
schools, kindergartens and childcare centres. Adults and older
children may become desensitized to
headlice bites, thereby delaying treatment and becoming a
source of infestation to others.
Headlice will dehydrate quickly once
removed from the head. In humid conditions they may survive
for more than 24 hours off the head,
however they are unlikely to be able to reinfest another person. In
the early stages of dehydration, lice
will continue to move or crawl but are unable to feed or lay eggs
(nits). For this reason, items such as
brushes, headgear, bedclothes and towels are less likely to
transmit headlice than close contact
although transmission may occur if these items are used
immediately following use by an infested
person.
Crab or pubic lice
Crab
lice, also called pubic lice, are greyish-white and crab-like in appearance.
They are
most often found on hair in the pubic region, and eggs are laid at the base of
the pubic hair. Heavy infestations may spread to other hairy areas of the body,
such as
the chest, thighs, armpits, eyelashes, eyebrows and beard. Crab lice are
mainly
spread through sexual or other close personal contact, and are most
common
in young, sexually active adults. Pubic lice (Phthirus pubis) are not associated with the
spread of disease but their bites can cause
irritation. Scratching of the irritation
may lead to localized infection.
The pubic (crab) louse (refer to Figure
3) is a small 1-2 mm, light brown, round, flat, six-legged insect
that clings to the pubic hairs, sucks
blood for nourishment, and fixes its eggs (nits) to the pubic hairs.
As its name suggests it looks very much
like a tiny crab. The life cycle lasts about two to three weeks.
Small red sores and itching may occur as
a reaction to the injection of saliva from the louse. They are
mainly found in the pubic hair but may
also be found on other hairy parts of the body e.g. eyebrows or
armpits.
Transmission
Pubic lice are transmitted mainly by
close body contact, including sexual activity and, occasionally,
by clothing, bedding or towels recently used by an infested
person.
Public health importance
Only the
body louse is a vector of human diseases. It transmits typhus fever,
relapsing
fever and trench fever.
Nuisance
Lice feed
several times a day and heavy infestations can cause intense irritation and
severe
itching. Toxic reactions to the saliva injected into the skin may lead to
weariness
and a general feeling of illness.
Louse-borne typhus
fever
This
disease is caused by a microorganism, Rickettsia
prowazekii, and is an acute,
highly
infectious disease with headache, chills, fever and general pains as symptoms.
It may
be fatal in 10–40% of untreated cases.
The
disease has occurred on all continents except Australia. It is prevalent in
cool
areas where heavy clothing is worn and where the vector is most common. In
the past
the disease was most common during war and famine. Today, foci of
transmission
are found in mountainous regions of South America, in Central and
East Africa
and in the Himalayas.
Transmission
Body
lice take the disease organisms up with the blood of an infected person
and then
expel it with their faeces. Since louse faeces dry to form a fine black
powder
they are easily blown about. The powder can infect small wounds, such as
those
caused by scratching, or the mucous membranes of the nose and mouth.
Because
the disease organism can remain alive for at least two months in dried
louse
faeces, it is dangerous to handle the clothing or bedding of patients with
typhus.
Treatment
Effective
treatment is possible with tetracycline, doxycycline or chloramphenicol.
Prevention and control
A
vaccine has been prepared but is not yet commercially available. Infection can
be
prevented by controlling the body lice. Epidemic outbreaks are controlled by
the
application of a residual insecticide to the clothing of all persons in
affected
areas.
Louse-borne
relapsing fever
This
disease is caused by a microorganism, Borrelia
recurrentis. Infected people
suffer
periods of fever lasting 2–9 days which alternate with periods of 2–4 days
without
fever. Usually, about 2–10% of untreated persons die but the mortality
rate may
be as high as 50% during epidemics. The disease occurs in limited areas
of
Africa, Asia and South America.
Transmission
Louse-borne
relapsing fever occurs under similar conditions to those of typhus
fever
and the two diseases may appear together. Humans become infected by
crushing
infected body lice between the fingernails or the teeth. The disease
organisms
are thus released and can enter the body through abrasions, wounds or
the
mucous membranes of the mouth.
Treatment
Treatment
is possible with tetracycline.
Prevention and control
Prevention
and control are as described for typhus fever; no vaccine is available.
Trench fever
This
bacterial disease, caused by Rochalimaea
quintana, involves intermittent fever,
aches
and pains all over the body, and many relapses. Infection rarely results in
death.
The
disease can probably be found wherever the human body louse exists.
Cases
have been detected in Bolivia, Burundi, Ethiopia, Mexico, Poland, the
former
USSR and North Africa. Epidemics occurred during the First and Second
World
Wars among troops and prisoners living in crowded and dirty conditions,
hence
the name “trench fever”.
Transmission
Transmission
occurs through contact with infected louse faeces, as for typhus
fever.
Treatment
Tetracycline,
chloramphenicol and doxycycline are probably effective but, as the
disease
is rather mild, they have not been adequately tested.
Prevention and control
Prevention and control are
as for typhus fever; no vaccine is available.
Control measures
The
control methods used depend on the importance of the health problem.
Individual
or group treatment may be carried out where lice are merely a nuisance.
Large-scale
campaigns are recommended for the control of epidemic outbreaks of
disease.
Head lice
Hygienic measures
Regular
washing with soap and warm water and regular combing may reduce the
numbers
of nymphs and adults. However, washing will not remove the eggs, which
are
firmly attached to the hair. A special louse comb with very closely set fine
teeth
is
effective in removing both adults and eggs . Shaving the head is
effective
and this measure is sometimes adopted with young boys; however, it is
often
objected to and should not be insisted on.
Insecticides
Insecticide
applications to the hair give the most effective control . They
can be
in the form of shampoos, lotions, emulsions or powders .
Some
pyrethroids are the most recommended products, since
they do
not cause the burning sensation of the scalp or other side-effects sometimes
associated
with other insecticides, such as lindane . Powder or dust
formulations
are usually less effective and less acceptable for use than lotions or
emulsions.
A soap formulation containing 1% permethrin can be applied as a
shampoo
.
How to make insecticidal dusts, shampoos and lotions
An
insecticidal dust can be made by adding insecticide powder (wettable powder)
to
talcum powder to obtain the recommended dosage of active ingredient (in
grams).
An insecticidal shampoo is made similarly by adding insecticide powder or
emulsifiable
concentrate to hair shampoo with a neutral pH. An insecticidal lotion
is made by mixing an
emulsifiable concentrate with water or alcohol.
Crab or pubic lice
Shaving
the infested pubic hairs from the body has been replaced by the application
of
insecticidal formulations, as described for head louse control. In heavy
infestations all hairy
areas of the body below the neck should be treated.
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