biology zoology blog benno meyer gut feeling

Have you also had that “Gut Feeling”?

Maybe it was a tapeworm

Tapeworms are not the most pleasant guests to entertain in your intestine, but zoologically they are some of the weirdest and most interesting animals I can think of. They live in the gut, but they themselves have none; they reproduce sexually, but don’t need a partner (they practice self-fertilization), which should preclude genetic variety; they are evolutionary far more ancient than their vertebrate hosts, so how did they get there; they are not terribly common, but each one of them can produce millions of eggs; they are usually not dangerous, but one species (the dog tapeworm Echinococcus) can be deadly in humans, if by mistake larvae get into a human body causing hydatidosis or a cyst in the lungs, which my first wife almost died from as a child. And talking about life cycles: actually the larvae of most tapeworms awaits a nightmarish search to find their suitable final hosts (see below).

If you happen to host a tapeworm (and I was a host once after having lived with tribal people and eaten their food in Papua Niugini), how do you know there’s one inside you? Some people with a tapeworm (and me too) noted that they felt tight, a feeling that could stem from the host body’s attempts in the form of heightened gut peristalsis to dislodge the worm. Others have reported that their tummy “grumbled” a lot more than usual and that there was a hollow sort of feeling and a slight discomfort after a meal. Most people, however, did not even realize that they harboured in their gut a cestode (the scientific term for a tapeworm) until they noticed some flat, whitish, fingernail-like pieces in their faeces. These pieces are shed tapeworm segments, full of eggs. If it happens to be the fish tapeworm (Diphyllobothrium latum) you entertain in your gut, then the animal may be your constant companion for many years, eventually reaching a length of 12 m with daily segment shedding rates of at least 5 -8 of such “glottids”, each containing up to 80,000 eggs on average.

With hooks and suckers on its head (called the “scolex” in tapeworms), the worm holds tight and will resist all attempts by the host’s gut movements to get rid of it Some of its tiny eggs may reach a river, pond, or lake before they dry out. These lucky eggs hatch into microscopically small, free-swimming first larvae, which immediately start searching for a host (the intermediate host number 1): a small waterflea-like creature known as a copepod and being part of zooplankton will do just fine. Inside the copepod the larva assumes a somewhat more elongated shape, hoping all the time that its copepod might fall victim to a perch, pike or other freshwater fish (the intermediate host number 2). Once inside the fish, the larva changes its shape to an even more wormlike creature again (known as plerocercoid) and seeks out the fish’s musculature as the ideal waiting place to be eaten.

Eventually a bear, fox or fisherman may come along and catch the infected, but otherwise totally healthy looking fish. Ingesting the fish raw, or eating it a little undercooked or improperly smoked gives the tapeworm larva its chance of a lifetime: after months, even years of uncertainty and anxiety, what relief to finally end up in the right host! As a small token of thanks for being accepted, the one successful tapeworm usually tries its best to keep the place to itself by preventing another tapeworm from settling in. Let that be a consolation (but read this recent BBC news item as a dampener of premature solace).

© Dr V.B. Meyer-Rochow and, 2020.
Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.

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