Histopathology of primary amebic meningoencephalitis due to Naegleria fowleri. Direct fluorescent antibody stain.
|Classification and external resources|
Naegleriasis, also known as primary amoebic meningoencephalitis (PAM), amebic encephalitis, and naegleria infection, is an infection of the brain by the free-living protist Naegleria fowleri, also known as the "brain-eating amoeba".
N. fowleri is 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 in poorly chlorinated, or unchlorinated swimming pools, in an amoeboid or temporary flagellate stage. There is no evidence of it living in salt water.
Signs and symptoms
Onset symptoms of infection can start from one to seven days after exposure. Initial symptoms include changes in taste and smell, headache, fever, nausea, vomiting, and a 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 usually from seven to fourteen days later, although it can take longer. In 2013, a man in Taiwan died twenty five days after being infected by Naegleria fowleri.
N. fowleri invades the central nervous system via the nose, specifically through the olfactory mucosa and cribriform plate of the nasal tissues. This usually occurs as the result of the introduction of water into the nasal cavity with water that has been contaminated with N. fowleri, during activities like swimming, bathing, or nasal irrigation.
The amoeba follows the olfactory nerve fibers through the cribriform plate of the ethmoid bone into the skull. There, it migrates to the olfactory bulbs and subsequently other regions of the brain, where it feeds on the nerve tissue, resulting in significant necrosis and bleeding.
The organism then begins to consume cells of the brain, piecemeal, by means of an amoebostome, a unique actin-rich, sucking apparatus extended from its cell surface. It then becomes pathogenic, causing primary amoebic meningoencephalitis (PAM or PAME). PAM is a disease affecting the central nervous system. 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.
Naegleria fowleri propagates in warm, stagnant bodies of freshwater (typically during the summer months), and enters the central nervous system after insufflation of infected water by attaching itself to the olfactory nerve. It then migrates through the cribriform plate and into the olfactory bulbs of the forebrain, where it multiplies itself greatly by feeding on nerve tissue.
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. 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. 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.
Michael Beach, a recreational waterborne illness specialist for the Centers for Disease Control and Prevention, stated in remarks to the Associated Press that the wearing of nose-clips to prevent insufflation of contaminated water would be an effective protection against contracting PAM, noting that "You'd have to have water going way up in your nose to begin with".
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%. On the basis of the in vitro evidence alone, the Centers for Disease Control and Prevention (CDC) currently recommends treatment with amphotericin B for primary amoebic meningoencephalitis, but no evidence supports this treatment affecting outcome. Treatment combining miconazole, sulfadiazine, and tetracycline has shown limited success only when administered early in the course of an infection. An Iranian infant of five months was successfully treated with Amphotericin B and Rifampicin.
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."
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.
The disease is rare and highly lethal: there had been fewer than 200 confirmed cases in recorded medical history as of 2004, 300 cases as of 2008. Drug treatment research at Aga Khan University in Pakistan has shown that in-vitro drug susceptibility tests with some FDA approved drugs used for non-infectious diseases have proved to kill Naegleria fowleri with an amoebicidal rate greater than 95%. The same source has also proposed a device for drug delivery via the transcranial route to brain.
This form of nervous system infection by amoeba was first documented in Australia in 1965. In 1966, four cases were reported in the USA. By 1968 the causative organism, previously thought to be a species of Acanthamoeba or Hartmannella, was identified as Naegleria. This same year, occurrence of 16 cases over a period of two years (1963–1965) was reported in Ústí nad Labem, Czechoslovakia. In 1970, the species of amoeba was named N. fowleri.
The number of cases of infection is likely to increase as its range through climate change is increasing. Also, the numbers of reported cases are expected to show an increase, simply because of better informed diagnoses being made both in living patients and also in autopsy findings.
|Costa Rica||1 in 2014 |
|Czechoslovakia||16 from 1962-1965 |
|India||3 from 2001 to 2009 (two infants and one adult) |
|Iran||1 infant in 2012 |
|New Zealand||8 from 1968 to 1979 in geothermal water |
|Pakistan||69 from 2012 to 2014 likely from ablution |
|Taiwan||1 in 2013 from a thermal spring |
|United Kingdom||1 in 1979 in the restored Roman baths in the English city of Bath |
|United States||133 between 1962 and 2015  with most cases in the Southeast U.S.|
|Venezuela||1 in 1998 and 2 in 2006 |
Physicians M. Fowler and R. F. Carter first described human disease caused by amebo-flagellates in Australia in 1965. 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 different 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 made by other amoebae such as Entamoeba histolytica. A retrospective study determined the first documented case of PAM possibly occurred in Britain in 1909.
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 fluorescent-labeled hybridization probes targeting the MpC15 sequence – which is unique to N. fowleri. Another group has multiplexed three real-time PCR reactions as a diagnostic for N. fowleri, as well as Acanthamoeba spp. and Balamuthia mandrillaris. This could prove to be an efficient diagnostic test.
As no effective treatment for PAM has been found, research to develop a therapeutic is searching for what factors, specific to N. fowleri, make it pathogenic; virulence factors may 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 non-pathogenic Naegleria gruberi and the amoebae are co-cultivated with target tissue cells, the protein is found to be located on the food cup which is responsible for ingestion of cells during feeding. Following up that research, Nfa1 gene expression knockdown experiments were performed using RNA interference. In these experiments, double-stranded RNA targeting the Nfa1 sequence was introduced and subsequently expression levels of the gene product dramatically decreased. This method could potentially be a technique applicable for knockdown of expression of pathogenicity factors in N. fowleri trophozoites.
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