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!