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No. 34: Jul-Aug 1984

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Are parasites really the masters?

All animals harbor parasites; and some parasites even have their own parasites. The usual effect of a parasite upon its host is debilitation, often to the point of death. But parasites have to reproduce, and some settle for the modification of their hosts in ways that improve their chances. Parasites can change the size, color, and even the behavior of their host. The object is usually to encourage a specific predator to eat the host so the parasite can continue its life cycle. A classic example is the lan-cet fluke which infests ants and then sheep. The problem is that sheep don't normally eat ants, giving the flukes a chance to switch vehicles. So, the innovative flukes somehow force the ants to crawl to the tops of plants and lock themselves there with their jaws. The next hungry sheep that comes along has his meal seasoned with ants.

The bulk of the present article deals with thorny-headed worms, which are not as endearing as the lancet flukes. These parasites are merely bags of reproductive organs attached to a thorny probiscus, by which they attach themselves to the intestinal walls of vertebrates. Living in a sea of processed nutrients, the worms don't even have a digestive tract. Part of the life cycle of this parasite is spent in arthropods (insects, crustaceans).

As with the lancet fluke, the thorny-headed worm's big challenge is getting the arthropod eaten by a vertebrate. In most instances, it alters the behavior of the arthropod in a way that makes it more conspicuous to the predators. For example, infested pill bugs do not hide from birds, as they normally do, and are snapped up. Infested crustaceans move towards the light where ducks consume them. No one knows how a parasite floating in the body cavity of its host can control the host's behavior.

(Moore, Janice; "Parasites That Change the Behavior of Their Host," Scientific American, 250:108, May 1984.)

Comment. One cannot but wonder if human behavior is somehow controlled by parasites. Obviously we deny such dominance. Yet, some have speculated that our urge for space travel is only DNA's way of expanding its dominion.

Thorny-headed worm A thorny-headed worm that cycles between ducks and crustaceans. (Adapted from Scientific American).

From Science Frontiers #34, JUL-AUG 1984. 1984-2000 William R. Corliss