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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/01/24 16:14:49」(JST)
| Naegleria fowleri | |
|---|---|
| Different stages of Naegleria fowleri | |
| Scientific classification | |
| Domain: | Eukaryota |
| Phylum: | Percolozoa |
| Class: | Heterolobosea |
| Order: | Schizopyrenida |
| Family: | Vahlkampfiidae |
| Genus: | Naegleria |
| Species: | N. fowleri |
| Binomial name | |
| Naegleria fowleri Carter (1970) |
|
Naegleria fowleri (pron.: /nəˈɡlɪəriə/) is a free-living excavate form of protist typically found in warm bodies of fresh water, such as ponds, lakes, rivers, and hot springs. It is also found in soil, near warm-water discharges of industrial plants, and unchlorinated or poorly chlorinated swimming pools in an amoeboid or temporary flagellate stage. There is no evidence of this organism living in salt water. It is an amoeba belonging to the groups Percolozoa or Heterolobosea.
N. fowleri can invade and attack the human nervous system. Although this occurs rarely,[1] such an infection nearly always results in the death of the victim.[2] The case fatality rate is estimated at 98%.[3]
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Physicians M. Fowler and R. F. Carter first described human disease caused by amebo-flagellates in Australia in 1965.[4] Their work on amebo-flagellates has provided an example of how a protozoan can effectively live both freely in the environment, and in a human host. Since 1965, more than 144 cases have been confirmed in a variety of countries. In 1966, Fowler termed the infection resulting from N. fowleri primary amoebic meningoencephalitis (PAM) to distinguish this central nervous system (CNS) invasion from other secondary invasions caused by other true amoebas such as Entamoeba histolytica.[5] A retrospective study determined the first documented case of PAM possibly occurred in Britain in 1909.[6]
Naegleria fowleri occurs in three forms: a cyst, a trophozoite (ameboid) and a flagellate.
Trophozoites encyst due to unfavorable conditions. Factors that induce cyst formation can include food deprivation, crowding, desiccation, accumulation of waste products, and cold temperatures.[7] N. fowleri has been found to encyst at temperatures below 10°C.[8]
This reproductive stage of the protozoan organism, which transforms near 25°C and grows fastest at around 42°C, proliferates by binary fission. The trophozoites are characterized by a nucleus and a surrounding halo. They travel by pseudopodia, temporary round processes which fill with granular cytoplasm. The pseudopodia form at different points along the cell, thus allowing the trophozoite to change directions. In their free-living state, trophozoites feed on bacteria. In tissues, they phagocytize red blood cells and white blood cells and destroy tissue.[7]
This biflagellate form occurs when trophozites are exposed to a change in ionic concentration, such as placement in distilled water. The transformation of trophozoites to flagellate form occurs within a few minutes.[7]
In humans, N. fowleri can invade the central nervous system via the nose (specifically through the olfactory mucosa and cribriform plate of the nasal tissues). The penetration initially results in significant necrosis of and hemorrhaging in the olfactory bulbs. From there, the amoeba climbs along nerve fibers through the floor of the cranium via the cribriform plate and into the brain. The organism begins to consume cells of the brain piecemeal by means of a unique sucking apparatus extended from its cell surface.[9] It then becomes pathogenic, causing primary amoebic meningoencephalitis (PAM or PAME). PAM is a syndrome affecting the central nervous system.[10] PAM usually occurs in healthy children or young adults with no prior history of immune compromise who have recently been exposed to bodies of fresh water.[11]
Amphotericin B is effective against N. fowleri in vitro, but the prognosis remains bleak for those who contract PAM, and survival remains less than 1%.[11] On the basis of the in vitro evidence alone, the CDC currently recommends treatment with amphotericin B for primary amoebic meningoencephalitis, but no evidence supports this treatment affecting outcome.[11] Treatment combining miconazole, sulfadiazine, and tetracycline has shown limited success only when administered early in the course of an infection.[12]
While miltefosine had therapeutic effects during an in vivo study in mice, chlorpromazine (Thorazine) showed to be the most effective substance - the authors concluded: "Chlorpromazine had the best therapeutic activity against N. fowleri in vitro and in vivo. Therefore, it may be a more useful therapeutic agent for the treatment of PAME than amphotericin B."[13]
Untimely diagnoses remain a very significant impediment to the successful treatment of infection, as most cases have only been discovered post mortem. Infection killed 121 people in the United States from 1937 through 2007, including six in 2007 (three in Florida, two in Texas, and one in Arizona).[11]
Onset symptoms of infection start about five days (range is from one to seven days) after exposure. The initial symptoms include, but are not limited to, changes in taste and smell, headache, fever, nausea, vomiting, and stiff neck. Secondary symptoms include confusion, hallucinations, lack of attention, ataxia, and seizures. After the start of symptoms, the disease progresses rapidly over three to seven days, with death occurring from seven to 14 days after exposure.[14]
N. fowleri can be grown in several kinds of liquid axenic media or on non-nutrient agar plates coated with bacteria. Escherichia coli can be used to overlay the non-nutrient agar plate and a drop of cerebrospinal fluid sediment is added to it. Plates are then incubated at 37°C and checked daily for clearing of the agar in thin tracks, which indicate the trophozoites have fed on the bacteria.[15] Detection in water is performed by centrifuging a water sample with E. coli added, then applying the pellet to a non-nutrient agar plate. After several days, the plate is microscopically inspected and Naegleria cysts are identified by their morphology. Final confirmation of the species' identity can be performed by various molecular or biochemical methods.[16] Confirmation of Naegleria presence can be done by a so-called flagellation test, where the organism is exposed to a hypotonic environment (distilled water). Naegleria, in contrast to other amoebae, differentiates within two hours into the flagellate state. Pathogenicity can be further confirmed by exposure to high temperature (42°C): Naegleria fowleri is able to grow at this temperature, but the nonpathogenic Naegleria gruberi is not.
According to the Centers for Disease Control and Prevention, the protist killed 33 people between 1998 and 2007. In the 10 years from 2001 to 2010, 32 infections were reported in the U.S. Of those cases, 30 people were infected by contaminated recreational water and two people were infected by water from a geothermal (naturally hot) drinking water supply.[25]
Current research is focused on development of real time PCR diagnostic methods. One method being developed involves monitoring the amplification process in real time with hybridization of fluorescent-labeled probes targeting the MpC15 sequence – which is unique to N. fowleri.[32] Another group has multiplexed three real-time PCR reactions as a diagnostic for N. fowleri, as well as Acanthamoeba spp. and Balamuthia mandrillaris.[33] This could prove to be an efficient diagnostic test.
As no effective treatment for PAM has been found, the development of a therapeutic is an area of great research interest. Currently, much work is being done to determine what factor specific to N. fowleri makes it pathogenic and if these virulence factors can be targeted by drugs. One potential factor in motility of the "amoeba" is the protein coded by Nfa1. When the Nfa1 gene is expressed in nonpathogenic N. gruberi and the amoebae are cocultivated with target tissue cells, the protein was found to be located on the food cup which is responsible for ingestion of cells during feeding.[34] Following up that research, Nfa1 gene expression knockdown experiments were performed using RNA interference. In this experiment, double-stranded RNA (dsRNA) targeting the Nfa1 sequence was introduced and subsequently expression levels of the gene product dramatically decreased.[35] This method could potentially be a technique applicable for knockdown of expression of pathogenicity factors in N. fowleri trophozoites.
There is no licensed vaccine in the U.S. to protect against N. fowleri infection. However, because of the fatality of the ensuing meningoencephalitis, there are some efforts in vaccine research. Current research shows intranasal administration of Cry1Ac protoxin alone or in combination with amoebic lysates increases protection against Naegleria fowleri meningoencephalitis in mice. Studies are ongoing investigating whether the STAT6-induced Th2 immune response is essential for the resistance to N. fowleri infection, conferred by immunization with amoebic lysates plus Cry1Ac. Protected STAT6+/+-immunized mice elicited a Th-2 type inclined immune response that produced predominantly humoral immunity; unprotected STAT6-/- mice exhibited a polarized Th1-type cellular response. These findings suggest that the STAT6-signalling pathway is critical for defence against N. fowleri infection. Immunization with Nfa1 protein on experimental murine PAM because of N. fowleri, BALB/c mice were intra-peritoneally or intranasally immunized with a recombinant Nfa1 protein. The mean survival time of mice immunized intra-peritoneally with rNfa1 protein was prolonged compared with controls (25.0 and 15.5 days, respectively).
N. fowleri was featured in the second season of House M.D. in the two-part episode "Euphoria (Part 1)" and "Euphoria (Part 2)". The amoeba was determined to be living in the water supply the deceased patient used to irrigate his marijuana garden.
The X-Files Season 1. Episode 20 "Darkness Falls" makes mention of "N. Fowleri".
In August 2009, N. fowleri was featured in the Animal Planet television series called Monsters Inside Me on the sixth episode entitled "Hijackers". The episode documented the story of how 11-year-old Will Sellars got infected with it by wakeboarding on a lake in Orlando, Florida, in July 2007.[36]
On November 30, 2012, Monsters Inside Me (Season 3 Episode 9) featured Kyle Lewis's story.
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| リンク元 | 「原発性アメーバ性髄膜脳炎」「ネグレリア属」 |