Also see Humanopathogenic worms
More icky stuff about parasites. Every part of the Earth is teeming with life, and your body is no exception. When you think about it, humans are basically big walking hotels full of tiny little squirming things. If you don't like that idea, well, this ain't the node for you.
Class Cestoda
- Subclass Cestodaria (only a few species)
- Subclass Eucestoda (many species, including the most common and widespread tapeworms)
The basics
Tapeworms are big ol' worms that live in your gut. They're flatworms from class Cestoda and different ones can infect just about any vertebrate species. There are around 4,000 known species altogether, which if I were a creationist I'd have to discuss with God, because there's nothing that justifies that. Anyway, tapeworms have extremely simple bodies comprising a scolex, the head, which attaches to the intestinal wall of the host; a neck made of undifferentiated cells that gives rise to new segments; and a strobilla, which is long and flat and makes up most of the body length of a tapeworm. The strobilla's shape is of course the source of the word "tapeworm"; it's composed of a string of one or more proglottids (the aforementioned segments) which contain the tapeworm's reproductive equipment, along with the hairlike projections that absorb food, and the specialized cells that excrete wastes. Each proglottid contains both male and female gametes, and self fertilization is common. Fully mature proglottids fall off and are released in the host's feces, or else they just crawl on out of the host's ass.
The scolex is not really a head in any real sense; it's simply an organ to attach to the intestinal lining of the host. Different tapeworm species have various combinations of suckers and hooks to accomplish this purpose. The morphology of the scolex is one of the most useful ways to distinguish between different species, but this is of little use in diagnosing infection since it remains inside the host. Tapeworms are not smart; there's not much of a nervous system to speak of, so there's nothing resembling a brain in the scolex. The parasitic bastards have no digestive tracts of their own; they simply absorb predigested food from their surroundings.
The most important tapeworms are in order Cyclophyllidea, whose members infect humans and livestock. Among these are the beef tapeworm, Taenia saginata and the pork tapeworm, Taenia solium.
The life and times of a tapeworm
So mama tapeworm is hanging around in someone's intestine. Maybe mine. Maybe yours. Most likely, though, it's my damn cat's. When a proglottid matures fully, it falls off of mama and makes its way out, either to be deposited in the feces or to spontaneously crawl out onto my carpeting as what looks like a slightly large, slightly wiggly grain of rice. Upon seeing this, I run screaming from the room and come back armed with enough paper towel to mummify my own pharaoh, gingerly pick up the proglottid, and flush it.
Should this happen in the wild, though, the proglottid chills until the eggs it contains are consumed by the right host. This is the intermediate host - you see, almost all tapeworms require two hosts to complete their life cycle. Within the intermediate host, the eggs hatch and turn into little hooked larvae (called oncospheres) which enter the critter's circulatory system and encyst in some part of the intermediate host's tissue, becoming cysticerci.
So the intermediate host eventually gets eaten by something. If our little tapeworm pals are lucky, that something is the definitive host, which is the organism inhabited by the tapeworm's adult forms. Each larva then turns into its final form - the scolex of a new adult tapeworm, ready to make its own way in the world. It attaches to the definitive host's intestinal lining, and starts growing little proglottids of its own.
Yes, the cat has been treated. On three separate occasions. Eventually we wised up and decided that tapeworms might be the ickiest things lurking outside, but probably not the most dangerous. She lives indoors now, worm-free.
Tapeworms we know and love
It's known as the beef tapeworm because cows are the intermediate host for T. saginata while humans are normally the definitive host. They can grow up to twelve meters in length. I am not making this up. It's fairly common in areas of the world with inadequate sanitation. Cows graze on fields contaminated with human feces, and their tissue becomes infected with cysticerci. When people eat them - voilà, now their intestines are filled with worms. Building adequate sanitation systems, inspecting meat, and thoroughly cooking (or freezing) it prevents infection. They are members of family Taeniidae, from order Cyclophyllidea, and like all cyclophyllids they have four suckers on their scolex.
Again humans are the definitive hosts. It's difficult to distinguish the proglottids and eggs of T. solium from those of the beef tapeworm, but treatment is the same. The scolex of the pork tapeworm, however, can be distinguished by its two rows of hooks (in addition to the suckers). Rarely, a patient infected with pork tapeworms can get eggs into their stomach, usually because they vomit, allowing the contents of the small intestine to back up into the stomach. If this happens, the patient becomes the intermediate host and the eggs hatch and form larvae in the body. This can lead to a severe illness known as cysticercosis, which can result in cysts all over the body. If they form in the central nervous system, epilepsy can result. This may be the commonest cause of epilepsy worldwide.
Another variety from family Taeniidae. Tapeworms of genus Echinococcus frequently choose humans (as well as many types of livestock and wildlife) as intermediate hosts, causing hydatid disease (or echinococcosis). The definitive host is usually a carnivore that predates or scavenges on the intermediate host. Hydatid disease causes cysts or tumor-like growths (depending on the particular species) that can be fatal and require surgery to treat. In advanced disease, the cysts can grow to the size of soccer balls. No shit. You don't want that to happen.
A cyclophyllid from family Dilepididae. The intermediate host is the flea, and the definitive host can be anything afflicted with fleas. Dogs and cats are commonly infected, but it can infect small children as well. It rarely causes severe symptoms; infection is treated with the same drugs as most tapeworm infections.
The broadfish tapeworm, from order Pseudophyllidea, has two intermediate hosts. First is a copepod, a small shrimp-like crustacean. Next is a fish that feeds on the copepod. They can pass up the food chain into larger fish before finding a definitive host, which may be a human, bear, dog, seal, or other fish-eater. In some cases, they can cause sparganosis, in which humans (rather than fish) serve as the second intermediate host by drinking water contaminated with copepods. They can migrate to just about anywhere in the body and turn into 15 centimeter long cysts.
Another cyclophyllid. Humans and mice (primarily mice) can serve as definitive hosts, while beetles infecting grain are the intermediate hosts. However, the insect intermediate host is not required: if eggs are consumed by humans or mice, they will develop into cysticerci in the lining of the intestines, and then grow into adulthood in the same host. They are thus particularly adapted to high population densities if sanitation is poor. However, infection generally causes only minor symptoms. This is the most common tapeworm in the United States.
So what's it like to have tapeworms?
Aside from diseases caused when humans are the intermediate hosts, tapeworm infestation is often asymptomatic and isn't terribly dangerous. Symptoms - when they occur - include abdominal pain, nausea, malaise, and in more severe cases, weight loss, malnutrition, and problems related to inflammation of the intestines. In very severe cases, bowel obstruction can result, and masses of proglottids can cause appendicitis. The broadfish tapeworm can cause deficiency of vitamin B12, which results in pernicious anemia. With most varieties of tapeworm, though, infection is often not discovered until the patient discovers proglottids falling out of their asshole.
Tapeworm infections are usually diagnosed from examination of stool samples. Treatment for almost all types of tapeworms is with niclosamide.
So how might I go about getting myself a case of tapeworms?
What the hell is wrong with you? Why in the world would you want to catch tapeworms? You don't want your intestines crawling with gigantic worms. Are you some kind of freak? Thank God you have me around to stop you from doing stupid things like this.
But just hypothetically, most infections are caused by consumption of undercooked meat. In the United States, few varieties of tapeworm are common; adequate sanitation means that the eggs released by infected individuals don't have a chance to perpetuate their species. So head for the developing world, preferably somewhere without adequate inspection of the food supply. Eat some undercooked meat. And you too can thus have the friend that's with you wherever you go.
The tapeworm diet
There's an old urban legend that says that during the early part of the twentieth century, people would embark upon the tapeworm diet - deliberately consuming tapeworm cysticerci in order to deliberately develop malabsorption and lose weight. Snopes says it's possible but undetermined - there seem to have been advertisements for such products, but there's no telling what the pills actually contained. Check out the link if you want to read about the old rumors surrounding Maria Callas. Yes, she had tapeworms. But it was not on purpose - she just liked steak tartare. Don't follow her example.
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