The Persistent Parasites

The thing the curious doctor was busy disentangling and studying last week looked like an endless skein of white rubber band. Actually, he explained happily, it was 100 ft. of rare tapeworm which he found in the intestine of a whale captured off Catalina Island. Although his specialization is the dwarf mouse tapeworm, a common human parasite, Dr. Donald Heyneman, 32, of the University of California at Los Angeles, finds all tapeworms fascinating. He hates to pass up a chance to find a new species, for the surface of tape-wormology has hardly been scratched.

United Family. Tapeworms have no personality, and folk tales about long-established worms that grow to be intimate friends of their human hosts are not based on fact. But tapeworms do have a talent of sorts: their strange and ingenious way of perpetuating their species. Some tapeworms are almost microscopic; others may be more than 100 ft. long. They are all following the same basic way of life. The adult consists of a "head" (scolex) that clings with hooks and suckers to the host's intestine. Below the head is a short neck that grows continually and differentiates into segments that become filled with eggs and sperm as they mature. The younger segments are chiefly male, and their sperm fertilizes eggs released by older segments farther downstream. A single worm can do the whole job and sometimes produces several billion eggs in its lifetime.

Parasitologists do not agree on whether a tapeworm is a single individual or a family tree of ancestors and descendants. The head has the only brains (a trace of nerve tissue), but the segments are practically independent.

The great crisis of a tapeworm's life comes in the egg stage. When it is first launched on the world, usually in fecal matter, an egg cannot survive unless it happens to be swallowed by an animal that can serve as intermediate host. Most eggs perish, but the survivors that find a home spring into a burst of frantic activity. Out of their capsules come hook-armed embryos that claw their way through the intestines of the intermediate host and form cysts in its tissues.

From Patting Dogs. When humans eat infected meat that is not thoroughly cooked, the cysts are digested, releasing a tapeworm head that attaches itself to the wall of the intestine. The "beef tapeworm of man" (Taenia saginata) may grow to be 40 ft. long and half an inch wide. This is the species whose heads are occasionally sold as active ingredients of "reducing pills." They may be slightly effective by causing general debility (it is not true that tapeworms increase appetite), but a society of tapeworms coiled in the abdomen cannot improve the figure.

More dangerous to man than the beef tapeworm is Echinococcus gramdosus. Sometimes the microscopic eggs adhere to the fur of dogs and find their way into the mouths of dog patters. Once established in human tissue, a single egg can multiply asexually into millions of tapeworm heads enclosed in a cyst as big as a grapefruit.

From Uncooked Fish. Another tapeworm partial to humans, Diphyllobothrium latum, is acquired by eating uncooked fish. It is common in Scandinavia, where raw fish livers are considered a delicacy. In some parts of Finland, 80% of the people are infected.

Dr. Heyneman studies all the variations of tapeworm life, but most of his professional time goes to Hymenolepsis nana, the dwarf mouse tapeworm that infects between 1% and 2% of the population of U.S. Southern states. Its intermediate host is the flour beetle, which may be ground with grain and eaten by humans. It can also be carried to man by mouse droppings that get into the food. A person infected by H. nana soon develops immunity, ejects the established worms and does not acquire new ones for several years at least. Dr. Heyneman hopes to discover the mechanism of this immunity. The knowledge should be useful against more dangerous kinds of tapeworm.

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