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Just What is the Brain-Eating “Amoeba” Naegleria fowleri?


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Cyst, trophozoite ("amoeba"), and flagellate forms of the protist Naegleria fowleri. Photos by CDC.

In the press this week were reports (see here and here and here) that the brain-eating amoeba Naegleria fowleri has killed three people this summer, as it does in a typical year. The only trouble is, Naegleria isn’t a true amoeba.

So why are they called amoebas if they are not? The organisms in question — which, like true amoebas are microbes called protists — do alternate between cysts, flagellate (swimming) forms, and amoeba-like (blobby, crawling) forms that are more properly called “trophozoites”. When times are good, these trophozoites crawl through the mud in search of bacteria to eat. When times are bad, they sprout tails and swim off like guided missiles in search of happier hunting grounds. Either of these forms can rarely, accidentally infect humans, typically in warm, shallow water in the southern U.S. in summertime. When times get *really* bad, they encyst. Click here for a CDC graphic of their life cycle (on the left). But the trophozoite forms only superficially resemble amoebas; their DNA tells us they are something much different indeed.

Naegleria, it turns out, is only a distant relative of the Amoebozoa, the true amoebae, which generally lack flagella. In fact, the true amoebae seem to be more closely related to fungi and animals than it they are to Heterolobosea, the phylum that includes Naegleria. Naegleria, in turn, seem to be much more closely related to Euglena — the flagellated (tailed) photosynthetic single-celled organisms from high school and college biology lab — and Trypanosoma, the causal organisms of sleeping sickness and Chagas disease. Take a look at this family tree of eukaryotes (nucleated organisms – everything except bacteria and archaea) for the groups Amoebozoa and Heterolobosea to see what I mean.

Heterolobosea are difficult to define in a sentence, since they have evolved into so many niches. What tells us they are related is their DNA. But generally speaking, the phylum that includes Naegleria is made up of organisms that alternate between amoeboid and swimming forms, and that, like the Euglenozoa, have “discoid” cristae, or folds, in their energy-producing mitochondria. In contrast, most animals, I believe, have sheet-like or laminar cristae. How exactly discoid is different from laminar I have not been able to discover. For extra randomness, the Heterolobosea also includes a group of slime molds that’s completely unrelated to the big showy ones most people (if they ever think of them) think of as slime molds.

The proposed kingdom to which the Heterolobosea belong — the Excavata — are even more of a mixed bag, although again one might say they are almost exclusively protists with either no or unusual mitochondria, two or more flagella for swimming, and a characteristic underside feeding groove supported by fibers called microtubules.

There are true amoebas that infect humans. Entamoeba histolytica comes to mind, the cause of the vastly more prevalent disease amoebic dysentary, which infects some 50 million worldwide. Naegleria, on the other hand, are not parasites of humans; they prefer bacteria and don’t seek out people. They only infect us when we swim into their habitat and happen to bump into them nose-first. Hungry and far from home, they crawl into our brains and start eating like crazy, killing the unfortunate host 95% of the time.

But as a result of its accidental nature, Naeglaria infection is quite rare in the United States — happening perhaps 2-3 times a year — especially compared to organisms that do seek us out in water. As a blog post at the L.A. Times points out:

According to the Centers for Disease Control and Prevention, the most commonly reported recreational water illness (RWI) is diarrhea, which can be caused by germs such as Cryptosporidium, Giardia, Shigella, norovirus and E. coli.  These can be introduced into the water through trace amounts of fecal matter that cling to people’s bodies. The agency reports:

Swimmers share the water — and the germs in it — with every person who enters the pool. On average, people have about 0.14 grams of feces on their bottoms which, when rinsed off, can contaminate recreational water. In addition, when someone is ill with diarrhea, their stool can contain millions of germs. This means that just one person with diarrhea can easily contaminate the water in a large pool or water park.

I think I could have gone my whole life without knowing each and every bottom in the pool contains 0.14 grams of feces. It certainly makes a hard sell for the bidet. This fact alone should be *much* more frightening than the chance that a little, lost, hungry protist might somehow find its way into your brain, no matter how food-crazed it might be once it gets there. And if you’re still worried, there’s always nose clips.

Jennifer Frazer About the Author: Jennifer Frazer is a AAAS Science Journalism Award-winning science writer. She has degrees in biology, plant pathology/mycology, and science writing, and has spent many happy hours studying life in situ.
Nature Blog Network
Follow on Twitter @JenniferFrazer.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. kdimoff 7:49 pm 08/17/2011

    ew. and i grew up in a rural area, swimming in swimming holes. i’m lucky i survived ;)

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  2. 2. Jennifer Frazer in reply to Jennifer Frazer 10:17 am 08/18/2011

    I think you had more reason to feel lucky you survived the snakes, spiders, ticks and mosquitoes than this little guy. : ) And possibly also the deep fryer-based Southern cooking, if the rural area in question was such. Fried food = way more deadly than Naeglaria.

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  3. 3. Lethe 11:27 am 08/18/2011

    It’s the first time I find that Amoeba can kill human beings.

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  4. 4. okestra 11:53 am 08/18/2011

    Actually, I think I’m lucky even to be alive at this moment. There are countless possibilities which can lead to death. I’m feeling so lucky that I stop concerning about that feeling. haha ;)

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  5. 5. Dogs Can Detect Lung Cancer | Con Games 4:21 pm 08/18/2011

    [...] Brain-eating amoeba don't sound quite as frightening after reading this. If you haven't heard of this brain-eating amoeba recently, well, that's nice. We, however, have learned that the extremely rare amoeba (found in warm pond-like waters) killed three people this week. It "burrows up into the skull and destroys brain tissue," relayed the AP. Countering the frenzy is Scientific American, which notes that the brain-eating amoeba isn't actually an amoeba and it doesn't seek out people: "They only infect us when we swim into their habitat and happen to bump into them nose-first." Still, when that happens it's lethal. [Scientific American] [...]

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  6. 6. Unbeliever 3:49 am 08/19/2011

    Everyone is much more likely to be killed by a Muslim terrorist than by an amoeba or a naegleria.
    Of course, Scientific American readers probably fear Global Warming™ or Nordic shooters more so than the most obvious dangers. Include black youth populated “flash mobs” among the current dangers…

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  7. 7. Unbeliever 3:55 am 08/19/2011

    I’ll wait in vain for the Sciam publicized study explaining the phenomenon of black flash mobs.

    Actually, i stopped waiting already.

    Of course Sciam will never touch the subject. But the minute a couple of Amish youths go on a crime spree, we’ll all be treated to an issue devoted to the dysfunctional Amish lifestyle…

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  8. 8. Unbeliever 4:08 am 08/19/2011

    Somebody should do a demographic on the populations these infected individuals were among.
    I suspect that dirty bottoms are prevalent among certain types of people.
    I know my bottom is spiffy. You could eat a salad off of my sphincter and be assured of its freshness and sanitation.
    I bet the bedbug population correlates with that of Naegleria.

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  9. 9. Unbeliever 4:13 am 08/19/2011

    Goodbye public pools. You’ve gone the way of diving boards and lawn darts.
    Unclean idiots ruin it for everybody.
    Eugenics should be around the corner.

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  10. 10. American Muse 5:31 pm 08/22/2011

    Methinks “Unbeliever” is a sphincter-hole?

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  11. 11. zimmi 5:48 pm 08/22/2011

    people who do not wash their hands or always only rinse them are a much greater risk to those around them. Food is the greatest Trojan horse for pathogens to enter, trashing our beneficial bacteria and handicapping our entire system. Wash up everyone, & speak up to those who blatently disregard hand washing.

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  12. 12. hanasi 12:45 am 08/23/2011

    The problem of swimming pools should not be difficult to solve. In some public pools it is required to walk through a trough containing an anti-fungal solution before entering the deck area. That requirement simply needs to be extended appropriately. Implementation is left as an exercise for the reader.

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  13. 13. blackbird79 2:58 pm 08/23/2011

    I wouldn’t doubt that Unbeliever’s sphincter is spotless — since it appears to be blocked up by his head. He and other right-wing political anti-rationalists will, of course, have missed that ‘New Scientist’ ran an ‘Upfront’ article on his so-called “flash mob” phenomenon IMMEDIATELY in the first possible weekly issue following the U.K. incidents (New Scientist, August 13-19, 2011). ‘Scientific American’, being a monthly, has had NO chance as yet to pipe in; but then, facts and evidence, nor making any attempt to achieve a meaningful education in the sciences themselves, hold little interest for racist, crypto-Fascist dogmatists of this ilk.

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  14. 14. Jennifer Frazer in reply to Jennifer Frazer 6:02 pm 08/23/2011

    While I appreciate the comments of everyone who has chimed in, we have wandered pretty far off-topic here. Let’s keep the comments on this post relevant to the post itself. As a reminder of this blog’s comments policy contained on my “About” page, comments containing obvious hate speech will be deleted, and some of the above comments stray dangerously close. Consider this a warning.

    And one other note: As I mentioned in the post, Naegleria is only a problem in freshwater. People who pick up Naegleria were usually swimming in natural ponds, creeks, river. Remember that their home is silt and mud. Chlorine is effective in killing Naegleria; thus, chlorinated swimming pools do not contain it.

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  15. 15. brumon 10:24 am 08/25/2011

    Tank you, Jennifer, for this revealing article. I’ve always felt an instinctive hesitation before entering a crowded public pool. Now I know my inner command deck was right. Only please have mercy on poor old Latin: “flagella” (not “flagellae”) is plural of “flagellum”.

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  16. 16. Jennifer Frazer in reply to Jennifer Frazer 2:23 pm 08/26/2011

    Fixed! Thanks for the reminder.

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  17. 17. Wayne Williamson 7:19 pm 08/30/2011

    excellent article and thanks for the link to the tree of life project…

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  18. 18. Jennifer Frazer in reply to Jennifer Frazer 11:25 am 09/3/2011

    You are welcome!

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  19. 19. ronzzz 11:23 pm 04/9/2012

    Jennifer, Is there enough chlorine in most tap water to kill Naegleria ? I use warm tap water in a sinus rinse every day and have for years. While 2 people died from doing this, both were in the same place in Louisiana, so I think this was probably a very rare problem with their water supply. What are your thoughts on this ?

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  20. 20. Micronerd 12:15 pm 04/28/2012

    Jennifer, I regret I did not find your blog sooner than this. I appreciate your work on Naegleria, nice job. I started dabbling in Naegleria in 1964, before we knew about N. fowleri. By the way, except for a matter of minutes, it was almost N.butti. Butts and Fowler published the first descriptions in the same journal at nearly the same time. N. butti seems more appropriate with your description of swimming pools. The chlorine or bromine concentration in water must be kept at sufficient levels to protect. An oil platform worker died after nasal ablution with untreated water from a storage tank out in the sun. Infections have occurred form sources as diverse as a chlorinated swimming pool (special case) to a rain puddle. Naegleria is a grazer and may be found on the surface of mud, however it seems to prefer reasonably clean water. Thanks.

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  21. 21. Jennifer Frazer in reply to Jennifer Frazer 3:17 am 04/29/2012

    You are welcome! Thanks so much for your comment.

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  22. 22. Primary amebic meningoencephalitis: brain-eating parasite in southwestern Indiana « It's Interesting 2:03 pm 09/8/2012

    [...] a single-celled living organism that lives in warm, fresh water, according to the CDC. (It’s not actually an amoeba, despite the colloquial term for it.) It can travel up your nose while swimming in a lake or [...]

    Link to this

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