Wondrous tapeworms

Posted: Sunday, June 01, 2008

My hiking friends love to laugh at me when I stop on the trail to inspect every bear scat I see.

I can easily see what fruits the bear has been eating, usually by recognizing the seeds. Sometimes I find deer bones. In spring, most bear scat is composed of fragments of green vegetation, which requires a microscope to identify with certainty. But I can learn quite a lot by peering into bear scats as we hike along.

Sometimes there is an extra bonus. Years ago, my crew and I went up an old logging road in a river valley on Chichagof Island, collecting bear scats for analysis. Among the fascinating items we found was a scat dribbled out over some distance as the bear ran along the road. This scat contained a prodigious number of tapeworm segments, altogether about 18 yards of worm. Remarkably, the head end of the worm may still have been in the bear, producing more segments. Each segment holds lots of tapeworm eggs. I hate to think how many eggs that tapeworm was sending out into the world.

Tapeworms are commonly viewed as disgusting, revolting, and generally icky. Attached to the gut of a mammal such as a bear or human, they take up nutrients and effectively rob the host of some of its nutrition. We tend to loathe all such parasites.

Nevertheless, parasites offer us some entrancing lessons in adaptation. For example, ordinary tapeworms - which are probably the kind we found in that bear scat - have a very complex life cycle involving a series of host animals. The eggs dropped in mammalian feces hatch in water. The larvae are eaten by small crustaceans and there transform into a second larval type. When the crustacean is eaten by a fish, the second larva migrates from the fish's gut to its muscles and transforms again.

If the first fish is eaten by another, this part of the cycle may be repeated. Then, when the fish is eaten by a bear (or a human), the last larval form infects the mammal, become sexually mature, and produces lots of eggs. And so the cycle can begin again.

What a complicated way to make a living! Why in the world would any organism have such a complex life cycle? At every stage, it seem as if something could easily go awry, breaking the cycle and signaling the demise of the parasite. And that must sometimes happen. But there must be a pay-off for the parasite in terms of numbers of offspring or dispersal of those offspring to more hosts.

Not all parasites are so complicated: Some have only one host, some have only one larval stage, some seldom or never reproduce sexually. Some multiply copies of themselves (usually asexually but sometimes sexually) during the larval stages, thus amplifying the total number within that host (probably to the detriment of the host).

The amazing things that parasites do would fill a fat book of thousands of pages. Here I just want to call attention to the problems of moving from one host to another, and some of the ways that parasites arrange this. One fundamental rule is probably that the higher the risks of failing to be transmitted to the next host, the more the parasite multiplies itself in the previous host. The idea is simply that sheer numbers may compensate for the difficulties of transmission (just as animals with high juvenile mortality, such as herring, tend to make lots of them, whereas animals with low juvenile mortality, such as bears, tend to make fewer).

But some parasites have gone beyond such games of probability and found ways of manipulating host behavior to increase the probability of transmission to the next host.

For example, a kind of spiny-headed worm infects freshwater amphipods (shrimp-like crustaceans). Normal, uninfected amphipod hosts prefer to hide in dark places, under rocks or debris. But infected individuals behave differently. They tend to stay in light areas, often swimming near the surface of the water. And there they are more likely to be eaten by a duck, which is the next host.

Another example is a fluke whose life cycle involves a terrestrial snail, an insect, and finally a vertebrate. The snails eat parasite eggs that they find as they cruise around. The eggs hatch in the snail and the larval parasites multiply asexually there. Infected snails produce more slime than usual, expelling balls of slime full of larvae, and the slime balls stick to vegetation.

Some insect, often an ant, eats the slime ball and gets infected.

An infected ant behaves normally during the daytime. But at night, instead of going to bed, it climbs up a blade of grass, becomes inactive, and stays perched there. In the morning, a grazing sheep, rabbit, or other herbivore munches the grass-and the infected ant-and becomes infected in turn. The parasites enter the liver and gall bladder, mature, and produce eggs. The eggs are passed with the feces, and some of them are encountered by snails, which eat them. And so the cycle continues.

A recent Natural History Magazine cites another example. A certain kind of ant in Panama is normally black. But some individuals of this species have round red abdomens, which they hold up high when they walk. Those red abdomens are packed full of the eggs of a parasitic roundworm. The round red abdomens may mimic berries and be attractive to berry-eating birds, which are the next host for this worm. If the bird eats the ant and excretes some of the worms, and worker ants feed bird scat to their larvae, the young ants become infected, completing the cycle.

Pretty tricky! Thanks to the friend that told me of the article about the Panamanian ants and reminded me of more things to think about when I poke into bear scats.

• Mary F. Willson is a retired professor of ecology and a Trail Mix board member.

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