Vector Control - Methods for Use by Individuals and Communities. © 1997, WHO.
House dust mite
Mites are very small, ranging from 0.5 to 2.0 mm in length; there are thousands of species, of which many live on animals. Like ticks, they have eight legs and a body with little or no segmentation. In most species there are egg, larval, nymphal and adult stages. The immature stages are similar to the adults but smaller.
Some mites are important vectors of rickettsial diseases, such as typhus fever due to Rickettsia tsutsugamushi (scrub typhus) and several viral diseases. Mites can present a serious biting nuisance to humans and animals. Many people show allergic reactions to mites or their bites. Certain mites cause a condition known as scabies. The major mite pests discussed here are:
- biting mites (vectors of scrub typhus);
- scabies mites;
- house dust mites.
Public health importance
Numerous species of mite are parasitic on mammals and birds and occasionally attack humans. Their bites can cause irritation and inflammation of the skin. One group, the trombiculid mites, transmits typhus fever due to R. tsutsugamushi in Asia and the Pacific. Only the trombiculid mites are described here, the biology and life cycle of other biting mites being similar.
Adult trombiculid mites are about 1-2 mm in length, bright red or reddish-brown in colour, and of velvety appearance. The nymph is similar but smaller. The larvae, also called chiggers, are very small, being only 0.15-0.3 mm in length (Fig. 4.30). Neither the adults nor the nymphs bite animals or humans; they live in the soil and feed on other mites, small insects and their eggs. The larvae, however, feed on skin tissue.
Fig. 4.30. The biting mite (Trombicula species). Reproduced from reference 49 with the permission of the publisher. Copyright Macmillan Publishing Company.
After emerging from the eggs the larvae crawl onto grasses or low-lying vegetation and leaf litter to wait for an animal or human host. They attach themselves to the skin of reptiles, birds, mammals and humans walking or resting in the habitat. On humans they seek out areas where clothing is tight against the skin, the waist and ankles being the parts most commonly attacked.
The larvae remain attached to the skin of the host for between two days and a month, depending on the species. They then drop to the ground and enter the soil to develop into the harmless nymphal and adult stages.
Mites have a very patchy distribution over small areas because of their special requirements. The nymphs and adults need certain soil conditions for their survival and development while the larvae require host animals, such as wild rats, other small rodents and birds. Suitable habitats are found in grassy fields, shrubby areas, forests, abandoned rice fields and cleared forests. The mites are also found in parks, gardens, lawns and moist areas alongside lakes and streams.
The larvae wait on leaves or dry grass stems until an animal or human passes by. People usually become infested after walking or standing in mite-infested areas. Bamboo bushes are favoured by the mites in the tropics and subtropics.
The bites can cause severe itching, irritation and inflammation of the skin (scrub itch). They usually occur on the legs. At the site of a bite the skin swells slightly and turns red. In the centre a red point indicates the location of the chigger. Because chiggers are invisible to the naked eye, most people are not aware of their presence until bites appear.
Biting mites can transmit a number of rickettsial and viral diseases to humans but only the most important one, scrub typhus, is discussed here. It is caused by Rickettsia tsutsugamushi and causes an acute fever, severe headache and lymphadenopathy.
At the site of attachment of the infected mite a primary skin lesion consisting of a punched-out ulcer covered by an eschar commonly develops before the onset of the fever attack. Depending on a number of factors the mortality rate is in the range 1-60%.
Distribution and transmission
Scrub typhus occurs mostly in low-lying rural areas of Asia and Australia (Fig. 4.31). It was very common in troops during the Second World War. The disease occurs most frequently in people visiting or working in mite-infested areas in scrub, overgrown terrain, forest clearings, reforested areas, new settlements and newly irrigated desert regions.
Fig. 4.31. Areas in south-east Asia and the western Pacific where scrub typhus occurs, 1996 (© WHO).
Treatment, prevention and control
Infected persons can be treated with tetracycline or its derivatives. Prevention is possible by avoiding contact with mites. The chiggers can be controlled by spraying of residual insecticides in woodland or bush areas, although this is expensive.
Prevention of bites
Biting can be prevented by avoiding infested terrain and applying repellents to skin and clothing. Openings in clothing can be treated by hand or spray. A band of 1-3 cm is normally sufficient. Benzyl benzoate, dimethyl phthalate, deet, dimethyl carbamate and ethyl hexanediol are effective repellents. Under conditions of frequent exposure the best protection is given by impregnated clothing and by tucking trousers inside socks. Where vegetation is low it is sufficient to treat socks and the bottoms of trouser legs. The clothing can be treated with one or a combination of the above repellents or with a pyrethroid insecticide (see Chapter 2) providing more long-lasting protection, even after one or two washes. Deet and dimethyl phthalate have been shown to be the most effective repellent compounds against some mite species (50, 51).
Removal of vegetation
The control of mites by killing them in their habitats is very difficult because of the patchy distribution of their populations. If it is possible to identify the patches of vegetation that harbour large numbers of larval mites (mite islands), it may be advantageous to remove them by burning or cutting and then to scrape or plough the top-soil. Mowing grass or weeds in these areas also helps. Such measures are recommended in the vicinity of camp sites and buildings.
Residual spraying of vegetation
Where the removal of vegetation is not possible, mite islands can be sprayed with residual insecticide. The spraying of vegetation up to a height of 20 cm around houses, hospitals and camp sites is effective against grass mites in Europe. The insecticides can be applied as fogs with ultra-low-volume spray equipment. Some suitable compounds are diazinon, fenthion, malathion, propoxur and permethrin (52).
Public health importance
The scabies mite, Sarcoptes scabiei, causes an itching condition of the skin known as scabies. Infestations with scabies are common worldwide.
The mites are between 0.2 and 0.4 mm long and virtually invisible to the naked eye (Fig. 4.32). Practically the whole life cycle is spent on and in the skin of humans. In order to feed and lay eggs, fertilized females burrow winding tunnels in the surface of the skin. The tunnels are extended by 1-5 mm a day and can be seen on the skin as very thin twisting lines a few millimetres to several centimetres long.
Fig. 4.32. The scabies mite. With a length of 0.2-0.4 mm it is hardly visible to the naked eye (by courtesy of the Natural History Museum, London).
Development from egg to adult may take as little as two weeks. The females may live on people for 1-2 months. Away from the host they survive for only a few days.
Scabies mites are commonly found where the skin is thin and wrinkled, for instance between the fingers, on the sides of the feet and hands (Fig. 4.33), the bends of the knee and elbow, the penis, the breasts and the shoulder blades. In young children they may also be found on the face and other areas.
Scabies is usually transmitted by close personal contact, as between people sleeping together, and during sexual intercourse. Dispersal mostly takes place within families and if one family member becomes infested it is likely that all the others will follow suit. The mites are unlikely to be acquired by someone sleeping in a bed previously used by an infested person, but may be passed on in underclothes.
Scabies occurs throughout the world in persons of all ages and social groups. In some developing countries up to a quarter of the population may be affected. It is most common in young children. Outbreaks of scabies are frequently reported from places where people live in overcrowded, unhygienic conditions (e.g. refugee camps) and where there is poor hygiene, such as in poorly maintained prisons and nurseries.
Fig. 4.33. A heavy infestation of scabies mites in the skin of the wrist (53).
Initially a small, slightly elevated, reddish track appears, which itches intensely. This is followed by the formation and eventual rupture of papulae and tiny vesicles on the surfaces of the skin. Scratching causes bleeding and leads to the spread of the infestation. Vigorous and constant scratching often results in secondary infections, giving rise to boils, pustules and eczema.
A typical scabies rash can develop in areas of the body not infested with mites. This occurs mainly on the buttocks, around the waist and on the shoulders, and is an allergic reaction.
In newly infested persons the itching and rash do not appear until about 4-6 weeks after infestation but in previously infested individuals the rash develops in a few days.
A rare form of the disease is Norwegian scabies, which is associated with an immense number of mites and with marked scales and crusts, particularly on the palms and soles. It appears to occur more frequently among people with immuno-deficiency disorders (especially HIV infection) than among immunocompetent patients (54-56).
Scabies infection can be confirmed by scraping the affected skin with a knife, transferring the material to a glass slide, and examining it for mites under a microscope. The application of mineral oil facilitates the collection and examination of scrapings. Another method involves applying ink to infested skin areas and then washing it off, thus revealing the burrows.
It has recently been discovered that ivermectin, which is used in the treatment of onchocerciasis and lymphatic filariasis, is also suitable for the treatment of scabies infections. It is administered in a single oral dose of 100-200 mg per kg of body weight (57-59).
Conventional treatment methods aim to kill the mites with insecticide (see Table 4.5). Information on how to make and apply the formulations is provided on pp. 259-261. After successful treatment, itching continues for some time but eventually it disappears completely. Treatment of all family members is necessary to prevent reinfestation.
Most treatments provide a complete cure but sometimes a second application within 2-7 days is needed. Overtreatment should be avoided because of the toxicity of some of the compounds.
Commonly used insecticides are lindane (10% lotion), benzyl benzoate (10% lotion), crotamiton (10% cream) and permethrin (5% cream). The latter is now considered the treatment of choice because of its high efficacy and the low risk of associated side-effects (55, 60-62).
Table 4.5 Formulations of insecticides which can be applied as creams, lotions or aqueous emulsions for use against scabies
in oily liquid
1% cream or lotion
1% aqueous emulsion
1% soap bar or 5% cream
The formulation must be applied to all parts of the body below the neck, not only to the places where itching is felt. It should not be washed off until the next day. Treated persons can dress after the application has been allowed to dry for about 15 minutes.
Prevention and control
House dust mites (Dermatophagoides complex) have a worldwide distribution (Fig. 4.34). They are very small (0.3 mm) and live in furniture, beds, pillows and carpets where they feed on organic debris, such as discarded skin scales and scurf. The inhalation of house dust laden with mites, mite faeces, and other debris and fungi associated with them produces allergic reactions in many people, such as asthma and inflammation of the nasal mucous membrane. Large numbers of allergens produced by house dust mites may be in the air after bed-making.
In temperate climates, mites occur throughout the year mainly in beds and carpets. Mites living on living room floors show a seasonal peak in density in late summer and early autumn.
Some other mites causing similar reactions in humans live among stored products, grains and animal feeds.
The density of house dust mite allergens can be assessed by a test which measures the concentration of mite excreta (guanine) in dust (63).
Mites and associated fungi can be controlled by decreasing the humidity in rooms, improving ventilation and removing dust. Bedrooms and living rooms should be aired regularly, or other measures should be taken to reduce dampness. The shaking of bedclothes and frequent washing of sheets and blankets reduces the availability of food and therefore the number of mites. Vacuum cleaning of beds, carpets and furniture is also effective. General insecticides used for pest control are not effective but a special product containing benzyl benzoate is available, which destroys mites when applied to mattresses, carpets and upholstery (63, 64).
Fig. 4.34. The house dust mite (Dermatophagoides pteronyssinus) (by courtesy of the Natural History Museum, London).