Naegleria fowleri

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 ( /nəˈɡlɪəriə/; also known as "the brain-eating amoeba") 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 swimming pools in an amoeboid or temporary flagellate stage. There is no evidence of this organism living in ocean water. It belongs to a group called the Percolozoa or Heterolobosea. Although not a true amoeba, the organism is often referred to as an amoeba for convenience.

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 mortality rate is estimated at 98%.^[3]

History of discovery

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) in order 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 that the first documented case of PAM possibly occurred in Ireland in 1909.^[6]

Lifecycle

Life Cycle Stages: Cyst, Flagellate, Trophozoite.

Naegleria fowleri exists in nature in three forms: a cyst, a trophozoite (ameboid) and a flagellate.

Cyst stage

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]

Trophozoite stage

The reproductive stage of the protozoan organism, which grows fastest at around 42°C and proliferates by binary fission. The trophozoites are characterized by a nucleus and a surrounding halo. The trophozoites 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, trophozoites phagocytize red blood cells and white blood cells and destroy tissue.^[7]

Flagellate stage

A biflagellate form occurring 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]

Infection

Life cycle of N. fowleri and other free-living Amoebae. Click to enlarge and view caption.

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 amoebae 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 that 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 there is no evidence that this treatment affects 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]

Timely diagnosis remains 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 U.S. from 1937 through 2007, including six in 2007 (three in Florida, two in Texas, and one in Arizona).^[11] The illness killed one in 2008 in California, one in 2009 in Florida,^[14] and in 2010 there were 3 reported cases; one in Arkansas,  one in Minnesota,  and one in Texas,   with a fourth case of an unidentified amoeba in South Carolina.   In 2011, there were individual deaths in Virginia,^[15] Louisiana,^[16] Florida,^[17] and Kansas.^[18]

Symptoms

Onset symptoms of infection start 1 to 14 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 3 to 7 days, with death occurring from 7 to 14 days after exposure.^[19]

Detection

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 CSF sediment added to it. Plates are then incubated at 37 degrees Celsius and checked daily for clearing of the agar in thin tracks, which indicate that the trophozoites have fed on the bacteria.^[20] Detection in water is performed by centrifuging a water sample with Escherichia 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.^[21] 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 flagellar state. Pathogenicity can be further confirmed by exposure to high temperature (42°C): Naegleria fowleri is able to grow at this temperature, but the non-pathogenic Naegleria gruberi is not.

Current research

Diagnostics

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.^[22] Another group has multiplexed three real-time PCR reactions as a diagnostic for N. fowleri, as well as Acanthamoeba spp. and Balamuthia mandrillaris.^[23] This could prove to be an efficient diagnostic test.

Pathogenicity factors

As there is no effective treatment for PAM, the development of a therapeutic is an area of great research interest. Currently, much work is being done to determine what 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 Nfa1 protein. When Nfa1 is expressed in non-pathogenic N. gruberi and the amoebas are co-cultured with target tissue cells, it was observed that the protein was located on the food cup which is responsible for ingestion of cells during feeding.^[24] 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.^[25] This method could, potentially be a technique applicable for knockdown of expression of pathogenicity factors in N. fowleri trophozoites.

Incidents and outbreaks

Czechoslovakia

Histopathology of amoebic meningoencephalitis

• Between years 1962–1965, 16 young people died of acute meningoencephalitides in Ústí nad Labem as a consequence of bathing in an indoor swimming pool.^[26]

New Zealand

• Between 1968 and 1978, eight fatal cases of primary amoebic meningoencephalitis occurred after the victims had been swimming in geothermal water at locations between Taupo and Matamata, in the Waikato Region.^[27]

United States

According to the Centers for Disease Control and Prevention, the amoeba 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.^[28]

• In October 2002, two Peoria, Arizona 5-year-olds died after being exposed to contaminated water supplied by Rose Valley Water.^[29]
• In August 2005, two Oklahoma boys, ages 7 and 9, were killed by N. fowleri after swimming in the hot stagnant water of lakes in the Tulsa area.^[30]
• In 2007, six cases were reported in the U.S., all fatal:^[11]
  • In July, the amoeba caused the deaths of three boys in lakes around Orlando, Florida. Possible causes of the infections include higher temperature and droughts in that area of Florida.^[31]
  • N. fowleri can be found in all non-chlorinated bodies of fresh water in Texas except colder water, typically spring fed. In late summer, the amoeba caused the death of a 12-year-old boy and a 22-year-old young man in Lake LBJ in Texas.^[32][33]
  • In September, a 14-year-old boy, Aaron K. Evans, was killed by the amoeba after likely having caught it while swimming in Lake Havasu in Arizona. The doctors suspected meningitis before the boy died, but did not know the etiology until the CDC confirmed it as N. fowleri.^[34][35]
• In August 2008, a 9-year-old boy was killed after having been exposed to the amoeba while swimming several times in Lake Elsinore in California. The boy was the first ever confirmed case in Riverside County.^[14]
• On September 23, 2009, a 22-year-old man hospitalized in Florida died from a confirmed case of N. fowleri after having contracted it at the Orlando Watersports Complex.^[36]
• On August 17, 2009, 10-year-old Philip Thomas Gompf died from a confirmed case after having contracted the parasite while tubing on Lake Arietta in Polk County, Florida.^[37]
• There were four cases in 2010:
  • In July 2010, 10-year-old Liza Hollingsworth of South Carolina died from amoebic meningitis, though the exact amoeba was not specifically identified.^[38]
  • In August 2010, Minnesota state epidemiologists reported that a 7-year-old girl died of meningitis caused by the amoeba, after swimming in several different bodies of water in the state. This was the first ever reported case in Minnesota and the first ever documented in a northern state.^[39]
  • In August 2010, 7-year-old Kyle Lewis died after contracting the amoeba from swimming in warm water near Glen Rose, Texas.^[40] Texas authorities say this is the tenth case since 2000.^[41]
  • On August 15, 2010, a 7-year-old boy died of a confirmed case of N. fowleri contracted from swimming in an Arkansas lake/pond.^[42]
• In June, 2011, a young Louisiana man died after flushing his sinuses with infected tap water.^[43]
• In August 2011, three cases are known to have occurred:
  • 16-year-old Courtney Nash became ill and died after swimming with her family in the St. Johns River in Brevard County, Florida.^[44]
  • The Virginia Department of Health confirmed that a case of N. fowleri infection had occurred in the state for the first time since 1969.^[45] Local media reports cited the death of a 9-year-old boy, Christian Strickland of Henrico County, who died August 5 of meningitis contracted from a N. fowleri infection.^[46] He had been at a fishing day camp the week before his illness.^[47][48]
  • A Kansas resident died after contraction of the amoeba after swimming in a lake near Winfield, Kansas.^[49]

Pakistan

• In 2010, a 39-year-old male patient died from amoebic meningitis, though the exact amoeba was not specifically identified.
• On October 15, 2010, a 54-year-old male Ghulam Mustafa Khalid was admitted to a Karachi hospital, suffering from typical signs of meningitis.^[50] It was quickly diagnosed as PAM and attempts to administer Amphotericin B, initially via IV then by intrathecal injection, had no effect. Cranial decompression of the patient likewise yielded no results, and the patient died on October 22, 2010.^[51] He was in perfect health, had no history of swimming, however, he had a habit of deep nasal cleansing with tap water while doing wudu, which could possibly be the reason of this infection. Laboratory tests confirmed the presence of N. fowleri.

Public health prevention strategies

Currently there are no widespread efforts for prevention because of the low prevalence of N. fowleri infections. However, because of the fatality of the ensuing meningoencephalitis, there are efforts in research and development of both diagnostics and treatment (see above). Additionally, a case can be made for increased awareness of N. fowleri and its infection for more accurate reporting.

References in the media

N. fowleri was featured in the second season of House MD in the two-part episode Euphoria (Part 1) and Euphoria (Part 2). The amoeba was determined to be living in the water supply that the deceased patient used to irrigate his marijuana garden.

Briefly referenced by Mulder in the first season X-Files episode "Darkness Falls"

See also

• List of parasites (human)

References

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External links

• Naegleria Infection fact sheet from the website of the Centers for Disease Control and Prevention (The link no longer resolves and appears to have been replaced by FAQ page linked below.)
• Naegleria FAQs from the website of the Centers for Disease Control and Prevention
• Tree of Life: Naegleria