Blog | Tuesday, June 12, 2012

'Flesh-eating bacteria'--science or hype?


The media are once again abuzz with cases of "flesh-eating bacteria." Stories about this condition (technically "necrotizing fasciitis") started making the rounds about 10 or 15 years ago, and they make great headlines (and, in all fairness, the incidence of the disease started to rise with the increase in people with damaged immune systems such as HIV patients, chemotherapy patients, and organ transplant patients).

Since the original stories, reporting has improved--we've evolved from "flesh-eating virus" to "flesh-eating bacteria." It's not much, but I'll take it. To really understand this nasty disease, we need to step back and learn a little science.

First, some perspective. There are an estimated 3.5 per 100,000 cases of necrotizing fasciitis and related diseases in the U.S. yearly (about 10,500 cases), and perhaps 2,500 deaths. In contrast, "invasive pneumococcal disease," caused by Streptococcus pneumoniae, causes about 4,400 deaths per year, out of about 40,000 cases. Necrotizing fasciitis is quite deadly, but relatively rare.

Necrotizing fasciitis (we'll call it NF for now) is a bacterial disease. Pathogenic bacteria can come from the environment, or even from ourselves. We are hosts for many species, many of which aren't even known yet. Some of these are harmless, some are beneficial, and some can cause disease given the proper conditions.

Bacteria can harm us in a number of ways: they can directly invade and damage tissue, they can produce toxins, and they can cause our immune systems to react so strongly that our bodies are destroyed in the fight. (The specifics of all this are quite complex, too much to get into here.)

One of the most common pathogenic bacteria is group A streptococcus or GAS. This is the most common cause of "strep throat," these days a relatively benign disease. But strep throat can give us hints of the potential of GAS.

Scarlet fever, a rash often associated with strep throat, is caused by a toxin produced by the bacterium. It's not serious, but GAS can cause another, less-benign rash, the one seen in toxic shock syndrome (toxic shock syndrome and NF are not complications of strep throat).

NF can be caused by a variety of bacteria, but often the cause is GAS. We'll use GAS alone for simplicity. GAS can enter the skin through injuries or other skin disruptions such as chicken pox (which is vaccine-preventable) or IV drug use.

With a bit of bad luck, the bacteria will get comfy and start producing some pretty useful (for the bug) toxins. Some of them prevent our own white cells from eating the bacteria. This allows the bacteria to continue to destroy tissue. Other toxins cause high fevers and shock.

With shock, blood pressure falls, which can cause further tissue damage as infected areas receive less blood and oxygen. Low oxygen environments are also fertile ground for other dangerous bacteria that require oxygen-poor environments. White cells, already being inhibited by toxins, are further inhibited by low oxygen levels.

All this leads to a synergy and positive feedback loop, increasing tissue damage. The infection can be unimpressive at first, but can rapidly lead to massive tissue destruction as the feedback loop progresses.

Antibiotics alone aren't effective. Since the blood flow is so poor to the affected tissue, antibiotics can't really reach them, so surgical removal of affected tissue is the only real cure. Due to the shock, most patients will require specialized care, often in an ICU.

Necrotizing fasciitis makes for good headlines (especially if the victim is a previously-photogenic young woman), but it's relatively rare. As a physician, I treat all skin infections seriously, and so should you, but rarely is there cause for panic.

(NB: Don't Google Fournier's Gangrene, another type of NF).

Peter A. Lipson, ACP Member, is a practicing internist and teaching physician in Southeast Michigan. After graduating from Rush Medical College in Chicago, he completed his internal medicine residency at Northwestern Memorial Hospital. This post first appeared at his blog, White Coat Underground. The blog, which has been around in various forms since 2007, offers "musings on the intersection of science, medicine, and culture." His writing focuses on the difference between science-based medicine and "everything else," but also speaks to the day-to-day practice of medicine, fatherhood, and whatever else migrates from his head to his keyboard.