Friday, August 19, 2011

When a Microbe "Eats" a Human

There they go again. My guess is that the science & health "editors" at the major television media outlets felt a frisson of excitement when they heard of the deaths of some teenage kids exposed to pond or lakewater from an extremely rare amoeba known as Nagleria fowleri. Why? That's lot's of eyes of worried parents zooming in to their website and passing it along to other worried parents. It's good for the news business. ABC News's piece is here; CBS's story, with a link to a piece giving tips on staying safe, is here; MSNBC's take is here. Of the majors, only CNN appears to have taken a pass at the time I write this; at Fair & Balanced, the story is buried in the "Children's Health" tab here.

That the media Bigs love a good scare story, particularly with respect to some spooky infection, isn't saying anything new (and is discussed thoroughly in Marc Siegel's great book False Alarm: The Truth About the Epidemic of Fear). Suffice it to say that, depending on how you slice the numbers, thousands of American children die every year and that the three deaths so far due to Nagleria hardly indicates that we need to take all of our children out of the lake. Indeed, about a thousand kids die annually due to drowning, but this substantially larger problem isn't grabbing headlines and isn't even being mentioned as a comparison in the Nagleria stories to give some sense of proportion. Yes, lakes can be dangerous places: but mostly because teenagers drink alcohol and do stupid things on boats, not because a microscopic beast lurks underwater.

Which is actually what the Rubin blog is preoccupied with at the moment: the description of Nagleria. "Microscopic beast" is something of a contradiction in terms, right? Nagleria is smaller than a speck of dust and almost pretty to look at under a microscope. Beasts, by contrast, are big. They look scary! They have big, giant...teeth. And with those teeth, they eat. No surprise then, that a sensationalistic news item indulges in a little sensationalist imagery, as every one of the news stories above refer to Nagleria as a brain-eating amoeba.

But it's nonsense for the most part. Humans are, for Nagleria, what we call an accidental host: it makes its living by hanging out in the water feeding on tiny little bacteria. Yes, it does consume brain cells once it finds itself inside a human head, but to call it "brain-eating" just amps up the raise-the-hair-on-the-back-of-your-neck factor. Why not just call it "lethal", as it is almost universally so?

While we're on the subject, "flesh-eating bacteria" is--are you at all surprised?--likewise a misnomer. There is no particular species of flesh-eating bacteria, as it could be any number of bacteria. The most common bug to cause the condition of necrotizing fasciitis (the phenomenon that is caused by so-called flesh-eating bacteria) is from the family streptococcus, which lives harmlessly in the nasal passages, mouth and gut of humans. The problem isn't the bacteria per se; the real problem is when the bacteria manage to get deep into the soft tissues of the body (usually the legs, sometimes the arms, less commonly the trunk or face). In the upper layers toward the skin, bacteria have lots of physical impediments in their way to cause infection, and by the time they've lumbered along to a new patch of tissue, the immune system usually kicks in and clears the infection. We call that cellulitis.

In rare cases, though, these bacteria can dive deep and get down to an area called the fascia. Once there, there are no physical impediments, and the bacteria can move rapidly and make people incredibly sick very quickly, and typically the only "cure" is to filet the person's limb, take out the dead tissue, and hope that they survive. Often the affected limb needs to be amputated, and there's a high mortality rate. But there's nothing special about the bacteria themselves, although you wouldn't know that from seeing the news stories put out by the august organizations noted above.

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