Medical injury

Most orders of insects and other arthropods contain species that have medical importance, either because they bite, sting, suck blood, transmit parasites and pathogens, or because they induce allergies, delusional parasitosis, or entomophobia. No medically importantpesthas an exclusively urban distribution; all occur in urban and natural habitats, to a greater or lesser degree. However, when these pests occur in or around the living space or workplace, their importance increases and control actions are more common. Arthropods with the highest pest status are those that inflict a painful bite, sting, or suck blood (whether painfully or not). Although they may present only a limited health risk, their presence is not tolerated. The most common ofthese worldwide include head louse, scabies mite, bed bugs, and spiders.

Bites, stings, and blood-sucking arthropods Bed bugs, scabies, and lice occur naturally in the human population, and at all socioeconomic levels around the world. People differ in their reaction to these arthropods: some are little affected, but if feeding continues or populations increase, sensitization occurs. Theabundance ofscabies and lice appears to be cyclic in some industrialized countries, but is more common and less cyclic in developing countries. They are commonly found on elementary schoolchildren, and there is often a social stigma associated with their presence. Lice and scabies are also common during wartime and famine when there are large numbers ofrefugees, poor sanitary conditions, and crowded living conditions. Bed bugs are similarly linked to humans. These blood-feeding parasites are distributed worldwide, and periodically they become numerous and infestations increase in residential and commercial buildings. Favorable indoor conditions, rapid movement of people and materials around the world, and decreased insecticide use indoors may have contributed to the re-emergence of these domiciliary pests. Regardless of the conditions or the physiological response, people dislike these ectoparasites because of their presence, and their impact on the quality of life. The pest status of lice, scabies, and bed bugs may be based on the unsightly condition of the infected skin, and the itching and discomfort caused by their feeding.

The pest status of spiders is primarily aesthetic since the majority of those found indoors are not likely to bite or be a health threat. There are a few species that have a painful bite, sometimes with severe outcomes. Nearly all spiders are poisonous, at least with regard to their normal prey, but only about20 of the approximately 30 000 described species are dangerously poisonous for humans. The most important species are: the aggressive house spider (Tegenaria agrestis), which often bites people without provocation; yellow sac spiders (Cheiracan-thium spp.), which occur indoors around the world; and species of recluse (Loxosceles) and widow spiders (Latrodectus). The bite of these spiders is generally painful and the venom may be locally or systemically toxic.

Transmission of parasites and pathogens Mosquitoes, reduviids (conenose bugs), and ticks transmitthe major arthropod-borne diseases in the urban environment. Most of the vectors occur primarily in domestic and peri-domestic habitats, or readily move to these habitats from reservoir populations outside urban areas. Their success and worldwide distribution are based in part on their ability to adapt to new hosts or substitute their natural breeding sites for those available in or around human dwellings.

Species of Aedes, Anopheles, and Culex mosquitoes occur in urban habitats. Many salt marsh and floodwater species of Aedes, such as Ae. dorsalis, Ae. sollicitans, Ae. squamiger, Ae. tae-niorhynchus, and Ae. vexans, have flight distances from 6.4 to 64 km, which brings them within range of urban habitats. Worldwide distribution of Ae. aegypti is linked to its adaptation to human habitats, such as its ability to breed in artificial containers and to travel with humans. Around human dwellings are various containers that hold water and easily substitute for the ancestral tree hole conditions of this species. The ability of the eggs to survive desiccation provides for long-distance transport to new areas. This species survives best where there is open-water storage and artificial containers. Because ofits adaptation abilities and occurrence in urban environments around the world, ithas the potential of transmitting new arboviruses that may develop into regional epidemics. Ae. albopictus is another species that has substituted its natural breeding site of plant cavities for household containers and automobile tires in the urban environment. Ithas expanded its distribution out of Asia, and is now a major pest in urban and suburban areas, and an important urban vector ofdengue.

Culextarsalis,thevector for western equine encephalitis virus, and Cx. tritaeniorhynchus, the vector of Japanese encephalitis virus, are rural species but enter urban habitats after heavy rainfall and flooding. They breed in structures that hold water. Cx. pipiens is one of the most common nuisance species in urban environments, and it transmits several arboviruses. The subspecies Cx. pipiens quinquefasciatus (= Cx. pipiens fatigans) is the major mosquito vector in urban environments throughout the world. It breeds in ground pools and in water that collects in household containers, and readily enters houses. This species is well-adapted to urban and industrial conditions, and it is a dominant species in the septic fringe in developing countries. In the USA, Cx. pipiens quinquefasciatus breeds in pools at the ends ofculverts and street drain catch basins, and it is the vector of the urban cycle of St. Louis encephalitis virus. The decrease in Japanese encephalitis in urban Japan has been attributed to people staying indoors in air-conditioned houses in the evening and watching television, instead ofsitting outside exposed to urban mosquito vectors.

Anopheles stephensi feeds and rests indoors, and breeds in wells, cisterns, roof gutters, fountain basins, garden tanks, and discarded tins in India and the Middle East. In India, An. culicifacies normally breeds in natural waters, but will reproduce in flooded burrow pits and pools in urban areas. Flooding and heavy rainfall provide breeding sites for An. atroparvus, An. messae, An. sacharovi, and An. superpictus in cities in Europe. The flight range of these species, and their dispersal by the prevailing wind at the edge ofcities, has influenced the urban occurrence of these and other anopheline mosquitoes. Populations of An. gambiae occur in urban areas in sub-Saharan Africa, where it breeds in underground cisterns and catch basins of storm drains in cities.

Species of the reduviids Triatoma, Rhodnius, and Panstrongy-lus have adapted to urban habitats. Most species occur in the western hemisphere. Triatoma species are often associated with rodents in natural areas, but are attracted to lights and may enter houses in suburbs and shantytowns. Because ofhouses built in the chaparal on the edges of cities, there has been an increase in the occurrence of conenose bugs and Chagas disease, which is caused by a Trypanosoma transmitted by these bugs. Trypanosoma cruzi is maintained in the urban environment in the domestic and peridomestic populations of cats, dogs, opossums, armadillos, squirrels, and several species of rats and mice. The primary vectors are Triatoma spp., Rhodnius prolixus, and Panstrongylus megistus. The focus of this disease is the poor household conditions in rural areas and septic fringe ofcities.

Dermacentor, Rhipicephalus, and Ixodes ticks find suitable conditions and hosts in the greenspace and peridomestic habitats. In suburban areas (urban-ecosystem A) there are cases ofRocky Mountain spotted fever (RMSF) in eastern USA, and cases of Lyme disease in the USA and other countries. An increase in RMSF is associated with the success of Dermacentor variabilis in suburban vegetation and perhaps the abundance ofdomes-tic dogs and other host animals. The increased incidence of tick paralysis follows the abundance of D. variabilis in suburban areas in eastern USA, and with the abundance of Ixodes holocyclus in Australia. In the suburbs of Sydney, I. holocyclus is abundant because the mix of natural vegetation in peri-domestic habitats provides suitable conditions for the bandicoot (Parmeles spp.), the primary host for this tick. Lyme disease is one of the most common arthropod-borne diseases in suburban areas around the world. I. scapularis is the principal vector in northeastern USA. The immature stages of this tick feed on numerous birds, mammals, and humans; the white-footed mouse is the primary reservoir and vector to humans, and the white-tailed deer is the primary overwintering site and host for the adult tick. Adults do not move from host to host and do not transmit the disease. The distribution range of I. scapularis is expanding in suburban areas, along with the incidence ofLyme disease, due to the proliferation of deer in these habitats.

Rhipicephalus sanguineus completes its development on domestic and feral dogs, and it has adapted to urban environments in many parts of the world. This species originated in Africa, but has been introduced into the Americas, Europe, Asia, and Australia, where it is well-established. This tick requires relatively high temperatures to complete development. In temperate countries itis associated with dogs indoors; in warm climates it occurs outdoors in suburban areas and is a vector of RMSF in the USA and a vector of boutonneuse fever in the Mediterranean region ofEurope.


Allergic disease is a common disorder affecting about 40% of the world population. The allergen proteins that induce allergic reactions may be inhaled, ingested, and absorbed through the skin, or mucous membranes. Typical allergic reactions include swelling, itchy and watery eyes and nose, difficulty breathing, headaches, skin rash, and itching. Many species of arthropods are the sources of allergens that sensitize and cause allergic reactions in humans. These allergens are proteins and the physiological response to exposure is the same as it is for other allergen sources, such as plant pollen, molds, and some foods. Arthropods in the urban environment that induce allergic reactions in humans include flies, fleas, beetles, and moths in stored food, and stinging insects such as bees, wasps, and ants. However, the prevalence of cockroaches and dust mites in the living space and their potent allergens make these two very important sources of allergic reactions.

Cockroaches common in and around human dwellings are an important source of allergenic proteins. Sensitivity to cockroaches is worldwide and ranges from 23 to 60% of the population; it is evident as respiratory asthma and dermatitis. In some inner-city neighborhoods in the USA 37% of children may be allergic to cockroach allergens. The cockroaches known to be the sources of allergen include species of Blattella, Blatta, and Periplaneta. However, Blattellagermanica and P. americana are the prevalent indoor pests, and contribute the most to health problems. The allergens from these insects are found in the fecal material, oral secretions, exoskeleton fragments, and dead bodies. Particles bearing cockroach allergen are mainly carried on particles less than 10 |j.m diameter; these particles settle quickly and reduce exposure.

Dustmites are in stored food products and inhabit the living and working space worldwide, and they are the source of allergens. Sensitivity to mite allergens is well-known; in the USA and Europe 20-35% of allergic individuals are sensitive to dust mites. Most homes and work environments inhabited by dust mites contain several species, including Dermatophagoides fari-nae, D. pteronyssinus, D. microceras, and Euroglyphus maynei. Dust mite populations require a source of protein-rich food and environmental conditions of 10-30 ° C and atleast 50% relative humidity (RH). The 0.5-1.0 g of skin scales humans shed every day provide sufficient food, and carpets and textile materials on beds, furniture, and clothing provide harborage and breeding sites for these mites. Allergens ofDermatophagoides species are produced in the posterior midgut and hindgutas digestive enzymes, excreted fecal pellets (10-40 /xm diameter), and in cast skins ofmites. These allergenic particles are relatively large and rapidly fall in undisturbed air. However, excrement pellets become dry and fragment, and small particles may become airborne.

The mites associated with stored foods and fungi include Acaris siro, Glycyphagus domesticus, Lepidoglyphus destructor, and Tyrophagus putrescentive. They feed on mold and fungi that grow on household foods, but are also found on textiles and on wall and ceiling surfaces. Mostspecies require 70-98% humidity for development. Exposure to stored-food mite allergens can be by ingestion or by inhalation, and sensitization to these mites has been reported in many developed countries. The confused flour beetle, Tribolium confusum, is probably the most common contaminant of flour, cereal, prepared flour mixes, dried fruits and nuts, and various spices. In these materials, there may be fragments ofall the life stages ofthis beetle, and for some infested material, there may be live adults and larvae. Persons who are sensitive to insect allergens may have an allergic response when ingesting contaminated flour products.

Entomophobiaand delusional parasitosis Most people do not like having insects and spiders in their living space, and some may be fearful of their presence. Fear is a natural extension of human experience, and a reasonable and appropriate response to situations that involve potential danger. It has some value in protecting the individual from potentially harmful situations. However, irrational anxiety in situations of limited danger or health threat is a phobia. For some individuals, the presence of insects or spiders in their immediate surroundings produces an unreasonable level of fear, and this is considered entomophobia. The general symptoms of a phobia are characterized as sudden and intense feeling of anxiety, shortness of breath and increased heart rate, shaking, and sweaty palms. An important component of any phobia is avoidance and people who are extremely fearful of insects and spiders avoid them. Another componentis the generalization ofthe fear to include other organisms, such as spiders and spider webs, or to all insects that make a buzzing sound.

The emotional condition in which individuals believe that live organisms are present on or in their skin, or periodically biting them, is called delusional parasitosis. In the late 1800s, delusional parasitosis was described by Georges Thibierge in the French literature as acarophobia. This condition was called presenile Dermatozoenwahn by Ekbom in the 1930s. He was a Swedish physician who described several cases, and for whom the condition is named. Ekbom's syndrome has been variously called dermatophobia, parasitophobia, and, more recently, monosymptomatic hypochondriacal psychosis. About 25% of the reported cases exhibited folie l deux involving a family member or close associate (see below); thus this conviction of cutaneous infestation may be regarded as a contagious mental state.

It is defined as a false belief (delusions) held in spite of no evidence that there are external or internal organisms biting or stinging the skin. The apparent cause of the skin irritations is tiny, almost invisible insects or mites. This emotional state may develop quicklyandpersistformonths or even years. It is believed that delusions of infestation are more common with advancing age, and gender (primarily females), but often patients less than 50 years of age are males. Victims are able to provide a detailed description of the supposed parasite. Individuals typically characterize the supposed parasites as black or white bugs; the bugs crawl on the skin for shortperiods. The supposed parasites sometimes tunnel in the skin, or jump on and off the person during various times of the day or in specific locations, which are usually indoors. The origin of the bugs can be almost any household material, including furniture and paper. The bugs may infest any portion of the body, including hair, arms, legs, and genital region; commonly the infestation will be centered in areas that are within reach of their hands. The bugs bite or sting, and often cause intense localized pain on the skin. Sometimes skin irritations develop in response to the supposed bite or sting, and the typical response to the pain or itch is intense scratching. The infestation can be so severe that the person leaves the house seeking relief, but the bugs usually reappear in the new location after a few hours or days. Others living in the household, including family members, may be convinced of the presence of these biting animals and share in the delusion (folie a deux).

An itch on the skin is a sensation which is sometimes described as a mild form of pain. The sensation of itching is apparently a result of chemical or physical stimulation of receptors on the skin. However, itching may not be accompanied by a stimulus to the skin. The causes of itches are many and range from medical conditions, such as diabetes, to mild irritants, such as laundry detergents, fabric sizing and conditioners, and dry skin in winter. Persons suffering from the sensation of itching of the skin often have the idea that they have mites, fleas, or some other microscopic animal. The supposed mites may be called cable mites or paper mites, and they are assumed to be associated with the wires that supply electricity to office or manufacturing equipment, or with paper that accumulates in offices or storerooms. Fleas are often suspected because these insects are usually associated with bites and intense itching.

Cable mites or paper mites are often reported by groups of people performing routine and repetitive tasks, such as secretarial and bookkeeping personnel, or assembly-line workers. Cable or paper mite dermatitis is a delusional parasitosis in the sense that the victims may believe they are being exposed to the mites, although these mites do not exist. There are no such animals as paper or cable mites, and fleas are not generally present in office or manufacturing environments.

Delusions of cleptoparasitosis is an extension of the concept of delusional parasitosis, but the patients imagine arthropod infestations in their house or living area. This condition is not accompanied by the sensations of insects or mites on the body; instead the apparentinfestation may be imagined in household items. Wood-infesting insects are sometimes implicated as the cause of the problem.

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