Utente:Gianreali/prova3

An ectoparasite is an organism that derives benefit or fulfills a life cycle requirement through interaction with the outer, or cutaneous, surface of the host. Such organisms are generally members of the Arthropoda, a phylum of more than 1 million species. The term ectoparasite can include organisms that live on the host only long enough to obtain a blood meal as well as those that burrow into the superficial layers of the skin and remain there for weeks to months or even years if left untreated. Arthropods are invertebrates with a chitinous exoskeleton, an internal cavity containing a hemolymph-filled hemocoele and internal organs, a segmented body, and jointed appendages. There are five classes of arthropods that may affect the skin: Hexapoda, Arachnida, Diplopoda (millipedes), Chilopoda (centipedes), and Crustacea. Selected arthropods from Hexapoda and Arachnida are discussed in the following chapters. The class Hexapoda comprises insects, which are six-legged arthropods, and includes lice, bugs (e.g., bedbugs and kissing bugs), beetles, butterflies and moths, flies (including mosquitoes), fleas, ants, bees, and wasps. The class Arachnida comprises arachnids, which have eight legs, and include mites, spiders, and ticks. Stinging or vesicating arthropods, such as spiders, bees, ants, and caterpillars, inflict injury on the skin but do not obtain a blood meal and are therefore not considered true ectoparasites. Although adult flies do not parasitize the skin, their larval forms (maggots) can live off living or necrotic tissue, resulting in a condition known as myiasis. Although ectoparasitic infestations remain a more frequent health problem in nonindustrialized tropical nations of the world, a variety of vector-borne zoonoses are of increasing concern in the United States. [1] In particular, tick-borne bacterial diseases including Lyme disease, human monocytic ehrlichiosis, and human granulocytic ehrlichiosis cause increasing morbidity, occasional mortality, and considerable anxiety among both patients and physicians. Bartonella henselae and Bartonella quintana, the etiologic agents of bacillary angiomatosis-peliosis in patients with acquired immunodeficiency syndrome, have been linked to the age-old ectoparasites fleas and lice, respectively. [2] In tropical climates, few natives escape some type of ectoparasitic disease, and the consequences of such disease can be mutilating, incapacitating, debilitating, or fatal. Not only do ectoparasites cause enormous economic losses to humans by virtue of human parasitism, they also parasitize domestic animals used for food and contribute greatly to the malnutrition found in underdeveloped countries. Many bacterial, spirochetal, viral, rickettsial, helminthic, and protozoal diseases can be transmitted to humans by arthropod vectors. Table 282-1 lists some arthropod-borne diseases. ERADICATION, CONTROL, AND PREVENTION For most ectoparasites, eradication is not a practical approach because of the substantial reservoir of wild animals, which allows perpetuation of the species. Elimination of some species from domestic animals is reasonable in certain climates but is a virtual impossibility in other regions of the world. In many instances, improvements in sanitation and improved socioeconomic factors will do more to reduce parasitism for humans than will attempts to eliminate the arthropods. It is sometimes possible, however, to control important diseases in humans and livestock by controlling the arthropod vectors responsible for the transmission of those diseases. Such large-scale programs often require cooperation between governmental agencies at various levels for effective implementation. Methods used to control arthropod vectors include the use of insecticides, biologic interference with the arthropod's life cycle, and environmental manipulations that limit breeding and spread of the undesirable arthropod species. Unfortunately, the widespread use of insecticides may induce the development of resistance in the arthropods, thereby requiring the development of new and different toxic agents. Extensive use of insecticides may create anxiety and controversy because of the possible hazardous effects they may have on people, animals, and the environment. Diseases spread by arthropods can be limited not only by controlling the arthropod vector but also by preventing access of the arthropod to its host. When possible, fine screening should be used on windows and doors to prevent entrance of flying arthropods into dwellings. Homes built above the ground are less accessible to crawling arthropods. Regular grooming and treatment of infested animals can reduce animal reservoirs of arthropods. Protective clothing and insect repellents that are applied to the skin or clothes are also effective means of protecting humans from arthropods. The active ingredient in most of the insect repellents available in the United States is either N,N-diethyl- m-toluamide (DEET) or ethyl hexanediol. TABLE 282-1 -- Selected Diseases Transmitted by Arthropods Infectious Disease Vector Arbovirus diseases (including yellow fever, dengue fever, and encephalitis) Mosquitoes and ticks Babesiosis Hard ticks Boutonneuse fever (tick bite fever) (Rickettsia conorii) Rabbit flea Colorado tick fever Hard ticks Ehrlichiosis Hard ticks Endemic relapsing fever (Borellia duttonii) Soft ticks Epidemic relapsing fever (Borrelia recurrentis) Human body lice Epidemic typhus (Rickettsia prowazekii) Human body lice Filiariasis (Wuchereria bancrofti, Brugia malayi) Mosquitoes Leishmaniasis ( Leishmania spp.) Phlebotomid flies Loiasis (Loa loa) Tabanid flies Lyme disease (Borrelia burgodorferi) Hard ticks Malaria ( Plasmodium spp.) Mosquitoes Murine typhus (Rickettsia mooseri) Rat fleas, lice Onchocerciasis (Onchocerca volvulus) Black flies Plague (Yersinia pestis) Rat fleas Q fever (Coxiella burnetii) Hard ticks, fleas Rickettsialpox (Rickettsia akari) Mouse mites Rocky Mountain spotted fever (Rickettsia rickettsii) Hard ticks Scrub typhus (Rickettsia tsutsugamushi) Mites (chiggers) 2972 REFERENCES 1. Walker DH, Barbour AG, Oliver JH, et al. Emerging bacterial zoonotic and vector-borne diseases: Ecological and epidemiological factors. JAMA 1996;275:463-469. 2. Koehler JE, Sanchez MA, Garrido CS, et al. Molecular epidemiology of Bartonella infections in patients with bacillary angiomatosis-peliosis. N Engl J Med 1997;337:1876-1883