Blogger's note: I am away for the next several weeks. In the meantime, I'm bringing you some classic Artful Amoeba posts. This one was originally posted on January 18, 2010. Unlike rinderpest, the subject of the last post, Guinea worm still awaits eradication. A major factor holding this up: the Civil War in Sudan.
Though I could find little about the biology of rinderpest for the last post, guinea worm is a case of the opposite: Way Too Much Information. Guinea worm inspires horror not so much by its life history (many infectious organisms find ways to wander about your body at will), but by its size, Homo sapiens-escape method, and terrifying treatment.
So how does one go about acquiring a guinea worm? I’m glad you asked. It all starts with a copepod. During its life, an aspiring guinea worm must pass through both humans and a freshwater copepod. Remember the bioluminsecent bomb firing marine copepods I covered here?
Well, this isn’t one of them. It’s another marine copepod species, but the best I could do right now by way of illustration. Marine copepods, in turn, have freshwater cousins, and these cousins are hosts for the young aquatic guinea worm larvae. After a few weeks in the copepods, the larvae are ready. Drink this water without filtering out the copepods and congratulations! You’ve just acquired your own pet guinea worm, and will become host to one of the most gruesome human parasites on Earth.
For when the copepods hit the stomach, the acid dissolves the copepods but not the guinea worm larvae. Instead, the females migrate to the lining of the small intestine, burrow through, get knocked up by tiny males who then die and dissolve, and then grow into two-foot long spaghetti strands that spend a year sightseeing your body. I don’t know about you, but the only entities I want roving my body are blood, immune cells, and the occasional miniaturized submarine.
Strangely enough, you usually don’t notice all this until the worm is full term, about a year after you drank their larvae. When the blessed moment arrives, the worm migrates to a patch of skin most commonly located on feet or legs, but which can also include “the head, torso, upper extremities, buttocks, and genitalia” (eep!) and release chemicals that cause a searingly painful blister to form, which then pops. Mrs. Worm emerges — but just her tip. The pain is so intense victims are driven mad by desire to plunge the extremity into cool water. When they do, the worm immediately secretes a cloudy liquid containing scores of her copepod-seeking young, thus beginning the cycle anew.
The treatment, known since ancient times, is hardly better. You take a matchstick, twig, or pencil, wrap the end of the worm around it, and then slowly pull her out a few centimeters a day, like (brace yourself) this:
Pull any faster and she breaks, defeating your efforts. It can take weeks or months to pull the whole thing out. In the meantime, your open sore can become infected by bacteria, and the pain is so bad you find it hard to move, work, or care for others. This is not a living organism that it is easy to feel sorry for anihilating.
Like rinderpest, guinea worm is an ancient scourge whose prevention has been long understood but which thrived on ignorance and poverty. All one has to do to prevent guinea worm is drink clean water, but clean water is a luxury for millions. The nuclear option is dosing local water bodies with copepod-icide. They (and anything else that happens to depend on copepods for food) can’t be happy about that. The alternative is behavior change — persuading people to filter their water through cloth (carefully checked for stray holes!) to strain out the fairly large copepods. That’s fine for adults, but often the victims are small children who don’t know any better when they get thirsty.
Dracunculus medinensis, as this pest is most formally known, is, believe it or not, a nematode, or round worm. Roundworms are distinguished from flatworms because they have a round (duh) body and true digestive tract: a tube that opens at the mouth and exits at the you-know-where. Nematodes crawl invisibly throughout your environment every day, in soil and fresh- and saltwater. They are among the most diverse groups on Earth, and probably heaviest by biomass, on earth. They’re everywhere. I’ll never forget teaching introductory microscopy lab during my first year of grad school and seeing a very surprised nematode crawling around a dish with a thinly sliced apple we were observing. So believe me, you have almost certainly consumed many of these little guys in your day. As with most nematodes, it looked like this.
Obviously, pregnant guinea worm females are the ultra-super-uber-heavyweights of the nematode world, and, at least in my experience, atypcial. Many nematodes are harmless free-living soil-dwellers, like the Caenorhabditis elegans that has contributed so much to our knowledge of basic development and gene function. But there are also scores of nasty parasites of both plants and animals: root-knot nematodes, hookworms, pinworms, whipworms, heartworms, and Trichinella spiralis, the reason you should not eat undercooked pork. To see where the nematodes fit into the rest of the animals, click here.
In December Nigeria announced it was the latest country to be free of Dracunculus medinensis, leaving only four in Africa that are still beset. Jimmy Carter’s on the case, so you know it won’t be long. To see a slide show from Time that vividly illustrates the worm’s toll, click here, and to read the latest news about the eradication, see here and here.
And of final note, dracunculuiasis, the disease’s formal name, means “afflicted with little dragons.” Quite so. I am glad I will never experience that firsthand, I hope that soon no one else will either.