Martijn ter Haar, a person from the Netherlands I’ve never met, clearly knows what sort of movies I like to watch. The ones with big parasitic worms crawling around inside a sealed box of fish at the supermarket. Warning: Safe for work, not safe for lunch.
This bad boy is Anisakis, a worm that would much rather be off in the sea than in your gut. Its eggs float through alll the world’s oceans until they’re scarfed up by a crustacean, inside of which they mature into a new larval stage. Their next host is the fish that eats the crustacean–they escape from its gut and drill into its muscle. Finally, the fish is eaten by a dolphin or a seal, whereupon the worm becomes an adult and lives harmlessly and happily, churning out eggs that the predators kindly squirt out with their poop. In other words, the good life.
But if some thoughtless fisherman trawls up an Anisakis-infected fish, and you end up eating that fish raw–as sushi or herring–or without cooking it thoroughly, the parasite is doomed. Inside a human host it will die–often without the host even knowing what happened. But sometimes things get a little nasty for the host, too. You may feel a tingle in your throat and cough the worms up. Or you may feel like your appendix just popped. What’s happening is that the poor confused Anisakis is drilling its way out of the gut and wandering around the abdominal cavity. Your immune system may go wild over this visitor and launch a frenzied allergic reaction. Even when there’s no worm crawling around in the fish, you can get an allergic reaction–a few loose proteins left behind are enough. If Anisakis makes you sick, a doctor may have to go in and fish the critter out (as seen in this even more disgusting video from Dr. Peter Kelsey of Harvard).
Out in the ocean, by contrast, Anisakis doesn’t do much harm to its hosts, and they don’t harm it. It’s a relationship fine-tuned by millions of years of coevolution. And when Anisakis populations drop, it’s a sign that the entire food web is not well–thanks to overfishing or pollution. Obviously nobody wants to get to be close friends with Anisakis (note: don’t eat sushi out of the back of a truck). But how about a little sympathy for the ocean-wandering worm trapped in the supermarket?
[Update: A commenter asks, "What is the actual rate of 'things getting a little nasty' for humans"? Severe cases of Anisakiosis are rare, as detailed here. But in Madrid, where people eat a lot of raw fish, 12% of people test positive to exposure--either to the worm or to its proteins.]