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Pathogenicity of Naegleria
A few Naegleria species have been shown to be pathogenic in humans and animals most notably N. fowleri. This amoeba is a facultative pathogen capable of living many generations without infecting a host. Trophozoites are thought to enter the nose during swimming in warm water and thereafter the brain by locomotion and destruction of neurons. N. fowleri causes Primary Amoebic Meningoencephalitis (PAM) in man Naegleria fowleri posesses secreted proteases (Ferrante & Bates, 1988), phospholipases (Fulford et al, 1985; Barbour & Marciano-Cabral, 2001), and pore-forming peptides (Herbst et al, 2002), all of which have been implicated in the pathogenic process. Although man can act as a host, this occurs so infrequently that we are very unlikely to be the primary host. This host (if indeed there is one) has not yet been clearly identified, but many any other mammals have been reported to be infected or infectable. These include; mice, cotton rats, squirrels, muskrats (John & Hope, 1990), guinea pigs (Culbertson et al, 1972), and sheep (Young et al, 1980). Many wild animals (but not all) have significant titres of Naegleria reactive antibodies suggesting that they come into contact with this group (Kollars & Wilhelm, 1996). It is possible that the main host are fish as these are frequently found to be parasitized by Naegleria and other amoebae (especially the gills). Invertebrates too cannot be ruled out as snails, amphibians and reptiles are also associated with Naegleria (Franke & Mackiewicz, 1982).
As in other amoebae there is a strong correlation between temperature tolerance and pathogenicity of Naegleria species (Griffin, 1972). Naturally, any amoeba that lives within a human host must be able to survive the normal 37oC and elevation above this that occurs in disease associated fevers. The majority of human PAM cases are caused by Naegleria fowleri which can grow at temperatures as high as 45oC. N. fowleri and other temperature tolerant Naegleria sp. multiply in bodies of water both natural and man-made that are warm. Most PAM cases where Naegleria is involved have been caused by swimming or other intimate contact with these waters and the introduction of the amoebae is assumed to be via the nose. However there have been at least three cases have been documented where no such association has been made (Lawande et al, 1979; Sugita et al, 1999; Shenoy et al, 2002).
Treatment of Primary Amoebic Meningoencephalitis (PAM)
Of the 300 or so cases of this disease world-wide, only seven or so have been survived (Jain et al, 2002). The drug of choice has been amphotericin B (Anderson & Jamieson, 1972;Seidel et al, 1982;Jain et al, 2002 ) it is usually given intravenously at 1mg/kg/day (see Shenoy et al, 2002). In the cases where patients have survived early diagnosis has been crucial. Symptoms are generally like bacterial meningitis but with no bacteria in the cerebrospinal fluid, the presence of amoebae can however be detected by observation of the CSF under a microscope. The incubation period is usually between 3 and 8 days and the patient usually dies 7-10 days after infection (Butt, 1966; Anderson & Jamieson, 1972; Barnett et al, 1996).
Molecular Biology of Naegleria
The nucleus of Naegleria is similar in overall structure to a number of protists in that the nucleolus is very prominent (Figure 1). Although the ploidy (how many copies of the genome exist in the nucleus) of Naegleria is not known it is suspected that it is polyploid (Clarke et al, 1990). There are about 23 chromosomes with total genome of about 104 Mb (Clarke et al, 1990). The most remarkable thing about the molecular biology of Naegleria is that the rRNA genes are carried on a 14 Kb plasmid present as a multi-copy (4,000) episome! (Clark & Cross, 1987). These circular plasmids are a feature of the Vahlkampfiid amoeba (Clark & Cross, 1988) and are easily isolated from the amoeba (click here for method).
Classification of Naegleria species
Many different strains of Naegleria have been isolated and most described as belonging to the N. gruberi groups, however molecular analysis indicates that these are not monophylectic and the genus has now been reclassified into some 24 species (this number will soon expand). In order to simplify the classification, only the current situation will be discussed based on molecular data (usually 5.8S rDNA or SSUrDNA gene data). See (De Jonckheere 2002) for details.
Amoeba to Flagellate transformation.
The flagellate stage is presumed to be a mechanism whereby Naegleria can colonies new regions rapidly by swimming rather than by crawling. One of the stimuli for this to occur is by dilution of the culture media which may imitate rainfall? Certainly, Naegleria flagellates are to be found in puddles after heavy rain.