Dear Straight Dope:
A friend of mine says she keeps getting infected by tapeworms and can't get rid of them since she is constantly being reinfected. I asked her how she keeps getting reinfected, and she claims the eggs are in the air and we all breathe them. What's the straight dope?
SDStaff Doug replies:
Laurel, your friend may be inhaling something funny, but it isn’t tapeworm eggs. There are two ironclad reasons she’s wrong about this, and an obvious deductive corollary.
First, tapeworms that attack humans can belong to one of several different species, but all of them enter the human body as cysts – not eggs – in undercooked meat. You could ingest millions of tapeworm eggs and they’d have absolutely no effect, because humans are the second host in this critter’s life cycle. The eggs only hatch into larvae when ingested by a first host, such as fish, pigs, or cows.
Even if the eggs could hatch inside a human, tapeworm larvae form cysts in muscle tissue, and don’t enter the digestive tract. There’s no way to get an adult tapeworm in your gut other than by eating muscle tissue containing larval cysts.
Second, the eggs are packed inside the small parts of the tapeworm’s body that break off as the tapeworm grows and matures, and these exit the host’s body in the feces, from which they’re eventually washed into the soil (where they can be ingested by pigs, cows or other grazing animals) or into the water (to be eaten by fish). At no point can these eggs ever become airborne.
The obvious corollary is that if we all breathed tapeworm eggs and if we could possibly be infected that way, then everyone, everywhere would have tapeworms, and we don’t — which means at least one of those premises must be incorrect, or in this case, both.
If your friend is genuinely infested with tapeworms, the only way this could be happening is if she eats a lot of undercooked meat from less-than-ideal sources. Generally, a restaurant selling things like steak tartare or sushi will obtain the meat from sources certified as safe for human consumption.
Given how wrong she is about the life cycle of tapeworms, maybe she doesn’t even have tapeworms. She could be pulling your leg or delusional, or she might be one of those people who are simply wrong about everything. As Mark Twain said: “The trouble with the world is not that people know too little, but that they know so many things that ain’t so.”
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