Humans have known about the pork tapeworm since ancient times, mainly because of its size, but it wasn't until scientists of the mid-19th century turned their minds to it that anyone knew how this two- to ten-meter-long parasite gets into a human intestine in the first place. Early research methods were logical but often unethical.
Taenia solium Today
Over the millennia T. solium has been extraordinarily successful. No one knows how many people today have a pork tapeworm living in their intestines, but one estimate suggests a number between 11 and 29 million in Latin America alone. The same authors (Coyle et al, "Neurocysticercosis: Neglected But Not Forgotten") estimate that almost 15 million people in Latin America have neurocysticercosis, a disease where the larvae of T. solium live in the brain. Both the worm and neurocysticercosis are common in Africa, India, and parts of East Asia too.
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A drawing of a pork tapeworm showing
the head (at lower left and center right)
with its suckers and a rosette of hooks,
and the long string of segments.
Delorieux drew this picture in 1831 for
Johann Gottfried Bremser.
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One acquires a pork tapeworm by eating the larval form, or cysticercus, in undercooked pork. Pigs are infected by swallowing eggs produced by the adult worm and passed in human feces. Those eggs cause neurocysticercosis as well, when humans accidentally swallow them. Today, as in the past, the pork tapeworm is common where sanitation is poor.
In 2010, the World Health Organization added cysticercosis – larvae of T. solium in any body tissue – to the list of major neglected tropical diseases. It was another step in the long battle that began in earnest when Friedrich Küchenmeister, a German doctor and parasitologist, demonstrated that a particular large intestinal tapeworm of humans and the tiny cysticerci of pigs are different stages of the same creature.
Friedrich Küchenmeister
Taenia solium was a common European parasite in Küchenmeister's day. In his book On Animal and Vegetable Parasites of the Human Body, he described examining “freshly expelled Taeniae,” and wrote that his “wife found cysticeri in the water used in washing sausages.” He also gives a detailed and colorful account of the scientists and theories of the day.
According to Küchenmeister, a few of his contemporaries almost figured out how the worm's life cycle worked and “might have saved... science from a most unpleasant debate.” They concluded, however, that the small tissue parasites of pigs were degenerate tapeworms that had strayed into the wrong host: “It appears to have been a peculiar fate,” Küchenmeister wrote, “which prevented the earlier solution of this zoological problem; naturalists either could not correctly comprehend the true direction of progress, or...they ignored the labors of their predecessors.”
Discovering the Life Cycle of T. solium
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Humans get T. solium by eating the larval cysticerci in undercooked pork.
Pigs get it by eating human feces that contain worm eggs. In cysticercosis,
humans eat the eggs and harbor larvae in the tissues, like the pig.
Life cycle diagram courtesy of United States CDC.
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Küchenmeister wrote: “the thought... suggested itself to my mind, of administering various cystic worms [cysticerci] to different animals.” In 1851 he tried this with dogs and cats, and when the animals developed tapeworm infections, he concluded “the cystic worms constitute a necessary step in the development of the Taeniae.”
He figured out neurocysticercosis as well. In 1853, Küchenmeister fed the eggs of T. coenurus (a tapeworm of dogs) to a sheep to prove this caused a disease called vertigo. His experiment went slightly awry when the farmer, “Herr Kärmsen... only sent [him] the head for examination; the rest of the body was kept to be eaten by the people on the farm.” Nonetheless, he found “fifteen young vesicles of Coenurus, partly on the surface of the brain, which was reddened by inflammation, partly in the substance of the brain, and even in the ventricles.” Küchenmeister described similar cases in humans – neurocysticercosis.
Küchenmeister's Unwitting Volunteer
Küchenmeister and his peers continued to perform various experiments of this sort. He believed that raw pork was the source of the human tapeworm: “We find this Taenia very abundant wherever the breeding of pigs flourishes..." he wrote, "and especially amongst those engaged in trades which bring them in contact with raw pork... (as butchers, cooks, eating-house keepers and etc.) or in people who obtain portions of ready cooked or smoked sausages and hams from the butchers' shops.”
Getting definitive proof was difficult however: He couldn't experiment on humans as he did on animals. Or could he? Küchenmeister devised a plan, and then set his most famous - and most controversial – experiment in motion: without the man's knowledge or consent, Küchenmeister fed cysticerci found in pork to a convict who was sentenced to hang, and then dissected the body after the hanging to see if the man had intestinal tapeworms.
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Friedrich Küchenmeister details his theories
and experiments in his book On Animal and
Vegetable Parasites of the Human Body, a
Manual of Their Natural History, Diagnosis,
and Treatment.
Book cover: Friedrich Küchenmeister.
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“[I made] the following experiments upon a murderer condemned to death," he wrote. 72, 60, 36, 24 and 12 hours before execution 12, 18, 15, 12, and 18 specimens of Cysticercus cellulosae were administered to the criminal partly in rice or vermicelli soup cooled to a blood heat, and partly in blood puddings from which the fat was removed and replaced by cysticercae.
"On dissection, forty-eight hours after execution, I found ten young Taeniae... The little Taeniae were 4 – 6 millimeters in length, had exserted [sic] their hooks and proboscis and attached themselves therewith to the intestine...
“Subsequent experiments of the same kind will certainly give us the means of tracing the progress of this Taenia as it increases in age, according to the various times of the administration of Cysticercus cellulosae.”
Despite criticism, Küchenmeister repeated his experiment a few years later, again using a criminal facing the death sentence. This time he administered the larvae months ahead and recovered young T. solium tapeworms at a later stage of development. His experiments established that the worms were indeed the adult stage of the parasite found in pigs.
Today, no ethics committee would approve such an experiment, but research into the worm is ongoing, with many scientists focusing on ways to eradicate this serious and neglected tropical disease.
Further reading on Friedrich Küchenmeister and Taenia solium
Coyle, C.M., Mahanty, S. et al. (2012). "Neurocysticercosis: Neglected But Not Forgotten." PLoS Neglected Tropical Diseases 6(5): e1500. doi:10.1371/journal.pntd.0001500. Accessed April 3, 2017.
Küchenmeister, Friedrich. On Animal and Vegetable Parasites of the Human Body, a Manual of Their Natural History, Diagnosis, and Treatment. Translated from the 2d German ed., by Edwin Lankester. (1857). Sydenham Society. Accessed April 3, 2017.
World Health Organization. Taeniasis/Cysticercosis. Fact Sheet N°376. (2013). Updated March 2017.
