Arthropodrelated problems in poor urban and rural settings

The rapid growth of densely populated, low-income settlements - mainly in the Third World but even in the North - has come to constitute one of the most serious threats to health (WHO 1988a). The urban poor can be regarded as the interface between underdevelopment and industrialization, and their disease pattern often reflects the problems of both. From the first they receive a number of infectious diseases and malnutrition, and from the second a wide range of chronic and social diseases (WHO 1988a). The importance of water-associated diseases can be seen in many urban areas where, what were previously primarily rural diseases, for example, malaria and dengue, are now becoming endemic in the cities (WHO 1988a). A great number of infectious diseases are transmitted, or directly caused, by insects, mites, and other arthropods to humans in urban areas.

Most vector-borne human diseases are, or can be transmitted in the home environment which offers two key features: (i) food from man, his animals and products; (ii) shelter from extremes of climate. Apart from most flies and some mosquitoes, nearly all medically important arthropods in the domestic environment are active mainly at night. During daytime these insects, ticks, and mites are resting in dark crevices and similar protected shelters. Thus, reduction of dark undisturbed hiding-places and restricted access to sources of blood or other food at night can decrease the prevalence of these arthropods. Schofield and White (1984) and Schofield et al. (1991) have written informative overviews on house design and domestic ectoparasites and vectors of disease, from which the following information has been extracted.

The floor in many types of primitive houses is of beaten earth. Cracked or even mud floors may provide refuges for flea larvae, bed-bugs, soft ticks (vectors of relapsing fever), and house dust mites.

Replacement of mud floor with cement reduces the infestation with these arthropods. Houses raised on stilts to avoid flooding have a space underneath, often used for storage of firewood etc., which may become infested with rodents, snakes, spiders, cockroaches, bugs, etc.

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The wall is usually made of locally available material, for example, corrugated sheets, split logs, timber planks, mud and sticks, or woven thatch. Plywood, hardboard, or even card-board from old boxes is often used in slum areas where better material cannot be afforded. Most types of walls having cracks or crevices or spaces between planks, stones, or logs provide excellent undisturbed refuges for a number of ectoparasites, vectors, and pests. The few, if any, windows ensure a dark, poorly ventilated, usually humid environment where the arthropods will thrive.

The roof: Thin roofs exposed to direct sunlight become very hot during sunny summer days which discourage insects. The tendency of many insects to fly upwards when they encounter an obstruction such as a house wall, means that the eaves under a roof are the main points of entry of house-entering insects such as endophagic flies and mosquitoes. If the eaves cannot be blocked or screened an alternative solution is to fit a ceiling. However, the roof-space above a ceiling may become a favoured hiding place for blood-sucking bugs, fleas and mites, and for the ectoparasites' hosts, such as rodents, bats, and birds.

Restriction of food sources for rodents, birds, bats, and arthropods is important since these animals can serve as reservoirs and/or vectors for infections. Protection of the food may be achieved by using screened food stores. Restriction of other food sources for rats, flies, cockroaches, ants, etc. include the provision of closed refuse containers and well-maintained toilets. Domestic populations of bloodsucking bugs, fleas, and mites can be controlled or eradicated by the removal of domestic animals (dogs, cats, pigs, goats, sheep, cattle, etc.) to suitable outhouses or enclosures. In Sweden and other parts of northern and western Europe, improved housing and the construction of animal shelters separate from the domestic habitations was probably the single most important factor in diverting the malaria mosquitoes (Anopheles spp.) from feeding on humans to feeding on animals. In this way, a gradual decline and eventual disappearance of malaria took place ( Jaenson 1983). The use of mosquito nets over beds at night gives relatively effective protection, not only against mosquitoes and other night-flying insects, but also gives some protection against fleas and bed-bugs. The protective effect is considerably enhanced if the netting is treated with an appropriate insecticide, for example, permethrin or deltamethrin at the recommended dosages. However, an unimpregnated poorly maintained or incorrectly used net does not provide satisfactory protection. Bed-nets which have not been treated with an insecticide will hardly provide any protection against blood-sucking bugs, fleas, ticks, and mites which actually inhabit the bed (WHO 1989a).

Breeding sites: Exposed standing water is likely to serve as a breeding site for dipterous insects (flies, mosquitoes, biting midges, etc). Thus, it is unwise to locate houses within the flight range (usually about 2km) of these insects, unless the houses are adequately screened or the breeding sites controlled. On-site sanitation systems, such as pit-latrines or septic tanks and cess pits, are much cheaper than a system of sewers but have the drawback that under warm weather conditions they may provide suitable breeding of the nuisance mosquito Culex pipiens and a number of potentially myiasis-causing flies, for example, many blow-fly species (Calliphoridae). If cess pits are carefully constructed, kept free of cracks and apertures and screened, mosquito and fly problems can be avoided, but enforcement of such standards in low-income areas may be difficult (Curtis 1984). Almost all gravid blow-flies attempting to enter a pit latrine do so via the vent pipe, so a screen could effectively minimize the input of blow-fly eggs. This is less reliable because some enter via the latrine hole (Curtis 1984).

Flies of several families, including many species of facultatively myiasis-causing flies, have their primary breeding sites in animal and human wastes. Human faeces is among the most

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dangerous substances with which people can come into contact; it is the principal source of the pathogenic organisms of many communicable diseases, particularly infections of the intestinal tract (WHO 1988a,b). The house fly Musca domestica and several other flies in the families Muscidae, Calliphoridae, and Sarcophagidae are very efficient 'mechanical' vectors of pathogens (viruses, bacteria, protozoa, and helminth eggs) from human faeces to human food. These flies are often most serious pests in warm weather when they can reach high population densities. They breed in decaying food and garbage and are attracted to human food, human faeces, dead animals, garbage, rotting vegetation, and other organic material. Removal, destruction or protection of potential fly breeding sites is likely to effectively reduce the incidence of a number of diseases such as cholera, hepatitis A, and amoebiasis and other intestinal parasites.

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