Invasion of the body snatchers!

Parasites that control the minds of their hosts -- whether those hosts be fish, 'zombie' caterpillars, 'brainwashed' grasshoppers or even humans -- make for great headlines, resonanting all kinds of uncomfortable thoughts on the edge of consciousness. [1] In general, 'parasite' is a term of abuse in our culture, and scientists, notes Carl Zimmer [2], took a long time to get beyond this. Konrad Lorenz, for example, saw their only virtue as a warning to humans. "A retrogression of specific human characeristics and capacities conjures up the terrifying specter of the less than human, even of the inhuman". [3] But, argues Zimmer,

parasites are complex, highly adaptive creatures at the heart of the story of life. If there hadn't been such high walls dividing scientists who study life --the zoologists, the immunologists, the mathematical biologists, the ecologists -- parasites might have been recognised sooner as not disgusting, or at least as not merely disgusting.

Parasites find themselves a vast number of ecological niches, he notes; they even find out a way to parcel out the human eye: one species of worm in the retina, one in the chamber, one in the white of the eye, one in the orbit. Zimmer also suggests that an assertion made back in 1845 by Johann Steenstrup, a pioneer in the study of flukes, is still relevant:

I believe that I have given only the first rough outline of a province of a great terra incognita which lies unexplored before us and the exploration of which promises a return such as we can at present scarcely appreciate.[4]

Footnotes:

1. Humans fear parasitic infestations, of course, and often with good reason. At least as disturbingly, we apply the term to other people and groups of people, sometimes with horrendous political consequences. And there is sometimes for some people a lurking sense that we ourselves as a species are parasitic (the borderland between parasite and predator not being well defined).

2. Parasite Rex (2000). The book is sensationally subtitled Inside the Bizarre World of Nature's Most Dangerous Creatures.

3. According to Kenan Malik's account (Man, Beast and Zombie. 2000), Lorenz saw civilisation as a degradation of the natural state of Man, the great hunter of the Upper Paleolithic. Lorenz was, notes Malik, highly sympathetic to Nazi ideology.

4. Just one example of a recent step to better understand and manage a remarkable protozoan is the work by Hugo D. Luján et al reported here. Unlike most of its fellow eukaryotes, giardia has no mitochondria. Even stranger, each giardia cell has two nuclei.

Image: hookworm