More parasite fun!

Two more parasite stories to share today.

First, another mind-control parasite. The host organisms we looked at last time were all invertebrates; snails, ants, crickets, moths, and the like. But here's a parasite that invades the brains of mice and rats. The protozoan Toxoplasma gondii invades a mouse host, and can only further its life cycle if that host is eaten by a cat. Mice normally have a fear response to the scent of cat urine, but Toxoplasma camps out in the mouse's amygdala and causes the host to like the scent. Here's an example of a finely tuned adaptation; rather than just disrupting the host's neural mechanisms for danger avoidance willy-nilly, the parasite interferes with a single mechanism most likely to result in a useful death.

Now, for a change of pace, how about a story wherein the host get the better of a parasite? Even better, how about global eradication of a painful human parasite? The New England Journal of Medicine reports that the days of the Guinea worm are numbered, thanks to two decades of work by the global Dracunculiasis (Guinea worm disease) Eradication Program. The report includes this graph demonstrating the annual number of cases of Guinea worm infection:



Guinea worm larvae are carried by microscopic water fleas (copepods). After a human drinks water containing infected copepods, the Guinea worm spends about a year maturing. Female worms then emerge from the skin and will expel the next generation of larvae upon making contact with water.

The incredible thing about the eradication effort is that it hasn't required any drugs or vaccines, just education about the parasite's life cycle. Drinking water is filtered to remove the larvae-bearing copepods. Water sources are monitored. Infected individuals are given assistance or incentive to submit to quarantine while the worms are emerging.

The program seems on track to eradicate the Guinea worm completely by its goal of 2009. This marks the first global eradication of a disease since smallpox, and the first ever eradication of a parasite without use of drugs or vaccines. Score one for science and health education!

Polio: A Virus' Struggle

One of the best (okay, most amusing) science sites around is The Science Creative Quarterly (RSS). They've just done a second (third?) repost of one of the classic pieces of science narratives, "Polio: A Virus' Struggle" by James Weldon.

Brain-Controlling Parasites

I've found a bunch of awesome videos on YouTube demonstrating a fascinating aspect of nature: parasites that take over part of the host's brain, resulting in a kind of behavior that will further the parasite's life cycle.

First, a parasite, having outgrown its cricket host, convinces the cricket to drown itself. Thus, the parasite has a chance to swim away (sorry, no sound on this one):



It's amazing, the kind of stuff that evolution makes possible. If you're having trouble thinking of how something like this could evolve, think of it this way. (This is just a hypothetical, of course, and an over-simplified one at that, so forgive me. :-P) That parasitic worm's ancestor also needed to grow in a cricket and seek out water. Some crickets were bound to drown just by accident, so that was just fine: few of the parasites would make it from cricket to water, but enough would to continue the species. Now, imagine a random mutation that made some of that worm's offspring have the slightest effect on their hosts, so those hosts tended to be slightly more attracted to water. This small effect couldn't guarantee that the cricket would drown, but over a large population, a slightly greater proportion of these mutated worms would reach the water compared to the non-mutated ones. Thus, the mutants pass on their genes, and the water-seeking allele increases in frequency. Now, just repeat the process. More water-directing mutations stack upon the first, gradually increasing the effect until the parasite practically has total control over the water-seeking portion of the cricket's little brain.

Next (and my personal freak-out favorite), this poor snail has worms. I bet he's all the rage at the local discotheque, but I don't know that it would be worth it to have pulsating worms in your freaking brain:



And last but not least, mind-controlling fungi attack insects. The time-lapse growth of the fruiting body (~1:07 to 1:45) is particularly cool, as is the fungus-covered moth:



Speaking of fungi, I've got a post up at my blog Synapostasy talking about a recent study of melanin-containing fungi and radiation. Feel free to check it out! (There's also some stuff in that post about fish and limb development.)