|
::''Microfilaria may also refer to an informal "collective group" genus name, proposed by Cobbold in 1882. While a convenient category for newly discovered microfilariae which can not be assigned to a known species because the adults are unknown,〔Purnomo; Bangs, M.J. (1995) "''Microfilaria sundaicus'' sp.n., a ''Chabfilaria''-like parasite (Filarioidea: Onchocerciae) from the blood of the horseshoe bat (''Rhinolophus affinis'') in Flores, Indonesia". ''Journal of the Helminthological Society of Washington'' 62(1):32-34.〕 it is seldom used today.'' The microfilaria (plural microfilariae, sometimes abbreviated mf) is an early stage in the life cycle of certain parasitic nematodes in the family Onchocercidae.〔Anderson, R.C. (2000) Nematode Parasites of Vertebrates: Their Development and Transmission. Second Edition. CABI Publishing: Wallingford, England, pp. 472-475.〕 In these species, the adults live in a tissue or the circulatory system of vertebrates (the "definitive hosts"). They release microfilariae into the bloodstream of the vertebrate host. The microfilariae are taken up by blood-feeding arthropod vectors (the "intermediate hosts"). In the intermediate host the microfilariae develop into infective larvae that can be transmitted to a new vertebrate host. The presence of microfilariae in the host bloodstream is called "microfilaraemia". The success of filariasis eradication programs is typically gauged by the reduction in numbers of circulating microfilariae in infested individuals within a geographic area.〔Farid, H.A., Z.S. Morsy, H. Helmy, R.M.R. Ramzy, M. El Setouhy, and G.J. Weil (2007) "(A critical appraisal of molecular xenomonitoring as a tool for assessing progress toward elimination of lymphatic filariasis. )" American Journal of Tropical Medicine and Hygiene'' 77(4):593-600.〕 ==Escaping the circulatory system== All parasites need a mechanism for spreading to new individual hosts. Parasites in the lower gastrointestinal tract usually shed eggs in the host feces. Tissue-dwelling parasites, such as ''Trichinella spiralis'' (cause of trichinosis), rely on new hosts eating the tissues of their current host. For members of the family Onchocercidae whose adults live in the "closed" vertebrate circulatory system, transmission to a new host is achieved by the microfilaria stage, with the help of blood-feeding arthropod vectors. This system is seen in the life cycle of ''Elaeophora schneideri''.〔Hibler, C.P. and C.J. Metzger (1974) (Morphology of the larval stages of ''Elaeophora schneideri'' in the intermediate and definitive hosts with some observations on their pathogenesis in abnormal definitive hosts ). ''Journal of Wildlife Diseases'' 10(4):361-369.〕 The adults of ''E. schneideri'' typically reside in the carotid artery of its definitive host, the Mule deer. The female may be up to 12 cm (almost 5 inches) long, and releases microfilariae which measure 207 by 13 µm (or 0.008 by 0.00051 inches) into the bloodstream of the host. The blood flow carries the microfilariae away from the female in the carotid artery, and directly into the branching arteries of the head and face. Because of their size, the microfilariae pass easily through successively smaller vessles, becoming physically lodged in the small capillaries near the skin surface of the face and head. Attracted by the carbon dioxide exhaled by the mule deer,() the blood-feeding female horse fly often lands on the head or face to feed. The horse fly uses its scissor-like mouthparts to cut the surface of the skin, creating a pool of blood which it takes in through its sucking mouthparts. The microfilariae, which were just under the surface of the skin, are small enough to be ingested whole by the horse fly. Once inside the horse fly, the microfilariae bore through the stomach wall, and mature into infective larvae about two weeks later. These larvae migrate to the head and mouthparts of the horse fly, and enter the bloodstream of another vertebrate host when the horse fly feeds again. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「microfilaria」の詳細全文を読む スポンサード リンク
|