Regular Medical News Today readers may have cringed when reading our recent news story reporting on the whole genome sequencing of the Spirometra erinaceieuropaei tapeworm.
Why the cringe? Because the specimen sequenced was removed from the brain of a 50-year-old man, where the worm had happily taken up residence for the past 4 years.
Such shudder-worthy tales are standard fare within the study of helminths, the class of beasties to which the colloquially named tapeworms belong. However, a lot of these are “big fish” stories, so to speak. Although the internet is rife with tales of 108 ft monsters being pulled from the guts of unlucky souls, tapeworm infections in humans are uncommon in developed countries.
However, those who cannot resist the pull of such lurid details may be advised to read Nancy Butcher’s The Strange Case of the Walking Corpse: A Chronicle of Medical Mysteries, Curious Remedies, and Bizarre but True Healing Folklore, which mentions a Ms. Sally Mae Wallace of Great Grits, Mississippi, who, on September 5th, 1991, had 37 ft of continuous tapeworm removed by doctors.
“After about 20 ft of that thing had come out of my mouth, I just knew I had the record. I was really filled with joy,” she is reported as having said.
But what is a tapeworm? They are, in short, parasitic flatworms that make their homes within the insides of all main vertebrate creatures – fishes, mammals, birds and amphibians (tapeworm larvae are also parasites of invertebrates).
The name “tapeworm” represents the creature’s body, which is a flat ribbon – akin to tape – made up of many identical segments, capped by a “head.” This head – known as the scolex – is not a head as we know it, though, as the creature has no eyes, mouth or brain. Instead, it is simply a protrusion with suction cups and hooks that allow the worm to tightly grip the intestine of its host.
Possibly the most frightening aspect of the worm in magnified images, “the scolex looks like some sort of bizarre, antique tool you’d stumble upon in your grandfather’s shed,” blogged Discover magazine’s Rebbeca Kreston, “or a sex toy.”Not only does the helminth lack a mouth for eating, it has no gut or anus, so all of the nutrients necessary for its survival are absorbed straight from the host’s intestine, where they are processed in the parasite’s skin. Vitamin B12 is a favorite snack, with tapeworm hosts prone to weight loss, fatigue and anemia as a result of the host and parasite competing for B12.
So far, so gruesome. But more than just offering the kind of body-horror thrills that have inspired science fiction, such as the episode of The X-Files that featured a murderous helminth-human hybrid on the loose in the New Jersey sewers, tapeworms are fascinating from an evolutionary perspective.
Scientists are puzzled as to how the tapeworm diverged from the non-segmented flukeworm, resembling instead the only distantly related earthworms we find in our garden soil. Instead of evolving “up” like most organisms, the tapeworm ditched its guts and sensory organs as it evolved, reducing its anatomy to a very simple design.
“Tapeworms show the largest loss of developmental genes of any animal examined to date,” claimed geneticist Dr. Pete Olson, writing in the journal Evolve. Instead, their evolution has been concentrated in developing genes vital for resisting immune attacks from their hosts.
But this evolutionary stripping of the parasite’s bodily functions into the basic, modular entity the tapeworm is today has been to the creature’s advantage.
The tagliatelle-like succession of identical segments – or “proglottids” – that comprise the worm’s body are simply designed yet multi-functional.
Throughout its lifespan – which is reported as being potentially as long as the lifespan of its host – the tapeworm creates new proglottids, lengthening its body from the scolex down, while the older proglottids at the tail are shed as they age. As long as at least the scolex is intact, the worm can continue to produce these segments.
If proglottids are severed, then – like an earthworm – the tapeworm can grow a new body (perhaps disturbingly, some people infected with tapeworms have reported seeing shed proglottid ribbons still thrashing in their feces).
What is more, each proglottid segment features testes, ovaries and a uterus – the tapeworm is hermaphroditic. With each of its many segments carrying all the machinery a worm needs to make babies, an adult tapeworm living within a human is able to produce between 50,000 and a million eggs a day, depending on the species.
This reproductive system also gives the tapeworm the opportunity of either self-fertilization or of cross-fertilizing with other worms in the same host.
In the species of tapeworm most common in human infections, Taenia solium – encountered in humans in areas of Latin America, China and South-East Asia, where pigs are common – the tapeworm eggs are shed in the feces of the infected person.
These eggs are then swallowed by pigs rummaging on the ground for food, establishing a curious life cycle, where the larvae hatch inside the pig and burrow into its blood vessels, forming cysts.
The worms return to a human host when the pig is slaughtered and eaten (if the meat is undercooked), where they will mature and reproduce, beginning the cycle again.
While an adult tapeworm establishing itself in a human intestine is not a pleasant thought, the medical implications of these kinds of infections are relatively benign.
Where serious complications occur is if the eggs enter a human host rather than a pig – for example, if someone shedding tapeworm eggs touches food that other people then eat.
In these cases, the T. solium larvae will burrow throughout the host’s body – as they do in pigs – getting caught in ventricles and forming cysts in the brain. As the cysts grow they can block the flow of cerebrospinal fluid, causing water to build in the brain at high pressure that may result in stupor, coma or death.
Although the larval cyst will eventually die if it is unable to progress to the tapeworm’s adult stage, the inflammation attacks unleashed by the host body onto the cyst may also cause significant problems, potentially causing the surrounding brain tissue to become dangerously inflamed, which can result in seizures or coma, depending on the affected area.
Praziquantel, a drug developed in the 1980s, can easily kill tapeworm larvae nesting in the brain, but it is also prone to triggering an immune reaction that causes the brain to swell – effectively causing the same condition the drug is attempting to treat.
There are estimated to be 11-29 million cases of this infection of the brain – known as neurocysticercosis – in Latin America alone. However, neurocysticercosis is commonly misdiagnosed because the symptoms are easily confused with other brain disorders, and because the developing nations where the disease is a concern lack the medical support to make accurate diagnoses using computed tomography (CT) scanners and blood tests.
Although T. solium is a problem in regions of the globe with poor sanitation, another tapeworm species – Diphyllobothrium latum, or fish tapeworm – has been reported among some Jewish and Scandinavian communities in developed countries due to the popularity of Jewish gefilte fish, Swedish fish balls or other delicacies involving freshwater fish.
The fish tapeworm is the largest parasite known to infect humans, growing up to 82 ft in length at a rate of 22 cm a day.
If you want to see what one of these little horrors looks like, below is a clip of a surgeon chancing across one during a colonoscopy:
A fairly nasty case of fish tapeworm infection – involving hundreds of tapeworms in one host – was reported recently in a Chinese man thought to have eaten contaminated sushi, though cultural changes in eating behaviors – as well as improved refrigeration and transport systems – has seen a decline in this infection across many cultures.
Experts say that only 53 tapeworm species have been reported in humans from a family of more than 9,000 formally identified species.
Until now, the genome for these parasites has remained undeciphered. But the case of the unfortunate man who lived for 4 years with a tapeworm in his brain may have a silver lining.
For decades, scientists have been fascinated by the parallels between tapeworm infection and cancer. Both cancer development and tapeworm survival are influenced chiefly by how the host’s immune system responds.
What is more, these two seemingly very different conditions have been shown to share similar drug targets. As Dr. Olson writes:
“A major impetus for sequencing tapeworm genomes is to find specific targets for chemotherapy. By identifying genes that code for proteins known to be inhibited by pre-existing drugs, researchers can inhibit the growth of tapeworms. In research so far, a surprising discovery has been that many of the targets discovered were the same as those used to control the growth of tumors. Thus not only is the spread of the disease similar to that of cancer in some cases, but potentially the treatment is, too.”
Some rumored benefits of tapeworm infections – such as the belief by some people that they are an effective, alternative slimming aid – are mostly superstition. But how great would it be if this much-loathed intestinal scourge helped us beat cancer?