The old wives' tale is true -- stepping on a rusty nail has the potential to cause tetanus. But so can a perfectly clean nail, a sewing needle or a scratch from an animal.
Tetanus is caused by bacteria known as Clostridium tetani, which is commonly found in soil, dust and animal feces. Because of its presence in soil and manure, gardeners and others who work in agriculture are particularly at risk for exposure to this bacteria; indeed, some farmers may even have it on their skin. But city-dwellers aren't completely safe -- a dusty sidewalk or street may harbor just as many bacteria.
In the soil or on the skin, C. tetani isn't dangerous, because it can only reproduce in an oxygen-deprived setting. A puncture wound, such as one that might occur from stepping on a nail, can provide that breeding ground. Within the wound, C. tetani releases a neurotoxin known as tetanospasmin, which may be the second most powerful toxin after botulinum [source: Krasner]. It takes only a small amount of tetanospasmin, which causes tetanus's signature muscle contractions and spasms by affecting the nerves, to do the trick.
Rust is not in and of itself a C. tetani carrier; rather, the thinking goes that if the nail has been outside long enough to get rusty, then it's probably been exposed to soils containing the bacteria. The crevices of the rust give the soil a place to hide, and the deep puncture wound gives the C. tetani a place to do its work. Any injury related to puncture is reason for concern, though, no matter how clean the piercing object seemed to be. That includes gunshot wounds and knife stabbings.
But while deep wounds best provide that environment, don't shrug off surface injuries. Every injury, from sewing needle and gardening tool mishaps to animal bites and scratches, carries with it the potential for tetanus. People who perform their own tattooing or piercing are at risk, as are intravenous drug users.
If you do step on a rusty nail or suffer a similar injury, bear in mind that the spores of C. tetani are resistant to antiseptics that are used to clean wounds. You'll have to head to the hospital for an antitoxin known as tetanus immune globulin. The antitoxin must be administered soon after the injury as it can only attack circulating toxins, not the tetanospasmin which may have already attached itself to a nerve ending. Those who don't seek treatment face the risk of their entire body freezing up; the rigorous treatment regimen includes sedatives, muscle relaxers, days spent in a nonstimulating environment (which gives the nervous system time to recuperate) and possibly even surgery.
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