Vector Control - Methods for Use by Individuals and Communities. © 1997, WHO.

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Lice


Biology
Public health importance
Control measures


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) (Fig. 4.15). 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.

Fig. 4.14. The egg sac of the sand flea can be removed with a sharp object.

Fig. 4.15. Human sucking lice are flat wingless insects with legs adapted for grasping hairs (infested man © L. Robertson; Lice © WHO).

Biology


Body lice
Head lice
Crab or pubic lice


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 (Fig. 4.16). 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.

Fig. 4.16. Life cycle of the louse (© WHO).

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.

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 (Figs. 4.17 and 4.18). Because the hairs grow about a centimetre 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.

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.

Fig. 4.17. Inspection of the hair for head lice. Girls tend to have heavier infestations than boys.

Fig. 4.18. Close-up of hair infested with lice and eggs (by courtesy of the Natural History Museum, London).

Public health importance


Nuisance
Louse-borne typhus fever
Louse-borne relapsing fever
Trench fever


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


Head lice
Crab or pubic lice
Body lice


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 (Fig. 4.19). 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 (20-26). They can be in the form of shampoos, lotions, emulsions or powders (Fig. 4.20; see also Table 4.3). 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 (27, 28). 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 (see box, p. 261).

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.

Fig. 4.19. A louse comb has very closely set fine teeth and is effective in removing head lice and their eggs.

Fig. 4.20. Hair can be treated with an anti-louse shampoo or lotion.

Table 4.3 Insecticides and formulations commonly used to control lice

Insecticide

Formulation and concentration (%)

bioallethrin

lotion

0.3-0.4

shampoo

0.3-0.4

aerosol

0.6

carbaryl

dust

5.0

DDT

dust

10.0

lotion

2.0

deltamethrin

lotion

0.03

shampoo

0.03

jodfenphos

dust

5.0

lindane

dust

1.0

lotion

1.0

malathion

dust

1.0

lotion

0.5

permethrin

dust

0.5

lotion

1.0

shampoo

1.0

(+)-phenothrin

shampoo

0.2-0.4

dust

0.3-0.4

propoxur

dust

1.0

temephos

dust

2.0


Insecticidal soap

The insecticidal soap bar is a recently developed inexpensive formulation of permethrin (1%) which is effective in killing head lice. It can also be used against the scabies mite (see p. 282).

How to use

The bar can be used as a shampoo. Apply to wet hair, work it into a lather and thoroughly massage into the scalp. Allow to remain on the head for 10 minutes, then rinse and dry the hair. Dead lice can be combed out over a towel. Repeat the procedure after three days. The hair will remain free of reinfestation for at least several weeks.

How to make

The bar, which is commercially available, can be produced locally for non-commercial purposes.

Ingredients

%

Crude raw coconut oil

57.0

Antioxidant

0.14

Permethrin

1.00

Mineral oil

8.86

Caustic soda solution

32.0

Natural clay

1.00


Premix the permethrin with the mineral oil at room temperature and add the mixture to the coconut oil in which the antioxidant has been dissolved. To this blend, add the caustic soda solution at ambient temperature, with rapid stirring. When all the caustic soda has been added, sprinkle the clay in and pour the emulsion into moulds, where the reaction continues for 12 hours.

The following day, cut the blocks into 40-g bars. If the bars are wrapped in polypropylene film and placed in an airtight box, the product will retain its effectiveness for more than two years. If they are packaged in a small plastic sandwich bag, or placed unwrapped in an airtight box, the shelf life is one year. If the product will be used up within a few weeks of manufacture, the lower-cost packaging is sufficient.


Impregnated mosquito nets

Head louse infestations disappear from people sleeping under mosquito nets impregnated with a long-lasting pyrethroid insecticide (5) (see Chapter 1 and p. 240).

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.

Body lice

Individual treatment

Regular washing and changing of clothes usually prevents body louse infestations. In areas where water is scarce, washing facilities are lacking and people own only a single piece of clothing, this may be impractical. Another solution is to wash clothing and bedding with soap containing 7% DDT.

Soap and cold water are not sufficient to eliminate lice from clothing. Clothing must be washed in water hotter than 60 °C and should then be ironed if possible.

Group or mass treatment for disease control

The preferred method for mass treatment is the blowing of insecticidal powder between the body and underclothes. A suitable powder consists of talcum powder mixed with permethrin (0.5%), DDT (10%), lindane (1%) or another insecticide. Alternative insecticidal dusts, as shown in Table 4.3, can be used in the case of resistance. Because the dusts come into close contact with the body, it is important that the insecticides have a low toxicity to people and do not cause irritation.

An advantage of dusting powder is that it is easily transported and stored. Application can be made by any type of dusting apparatus, such as compressed-air dusters, plunger-type dusters and puff dusters (Fig. 4.21) (see p. 250), or by hand. It is important to explain the purpose of dusting to the people to be treated because the powder leaves clearly visible traces on clothing.

For individual treatment, about 30g of powder can be applied evenly from a sifter-top container over the surfaces of clothing that are in close contact with the body. Special attention should be given to the seams of underwear and other garments. To treat large groups of people about 50g of powder per person is needed. The powder is blown into the clothing through the neck openings, up the sleeves and from all sides of the loosened waist (Fig. 4.22). Socks, headwear and bedding should also be treated. One treatment should be sufficient but retreatment may be needed at intervals of 8-10 days if infestations persist.

The impregnation of clothing with a pyrethroid emulsion may provide long-lasting protection (29), the insecticide possibly remaining effective after 6-8 launderings.

Fig. 4.21. Insecticidal dust can be applied to clothing with a hand-operated puff-duster (© WHO).


WHO 40160

Fig. 4.22. Treating an individual with insecticidal dust using a plunger-type duster. (Reproduced from Insect and rodent control. Washington, DC, Departments of the Air Force, the Army and the Navy, 1956.)


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