Hey smart people, Joe here.
What do poke, sushi and sashimi, and all these dishes have in common?
They’re made with raw fish.
And they could all give you parasites.
The ocean is a parasite playground.
That warning about raw or undercooked fish at the bottom of every menu?
It’s there for a reason.
At least 15,000 different species of wormy parasites use fish as a host.
Raw fish dishes like sashimi are, well… raw.
They’re never cooked--which means any parasites or parasite eggs in the fish can end up in
In case you’re wondering, let me make this next part extra clear for ya: Parasites are
so common in fish, if you’ve eaten raw fish more than a few times, you’ve almost definitely
eaten a parasite egg.
The real question is… did it hatch?
Ok, so Anisakis is one of the parasites you could get from eating sashimi.
One investigation found that 10% of salmon sushi sampled from Seattle restaurants contained
dead Anisakis worms.
Now, if that raw fish is treated according to regulations--previously frozen for at least
7 days at -20 degrees C -- then the parasites and the eggs should all be dead.
But that means you did still definitely eat them.
If the fish isn’t frozen correctly or it’s eaten fresh, viable worms and eggs can make
it into your gut.
Like all parasites, these worms live in a carefully evolved life cycle: The eggs are
released into the ocean through marine mammal poop, which are then eaten by crustaceans,
which are then eaten by fish.
When people eat this fish, we interrupt this natural cycle, and the nematode worm larvae
can take up residence inside our throat, stomach, or intestinal lining instead.
On rare occasions people report feeling a tingling after eating sushi--that’s not
the wasabi people, it’s worms!
Well, it’s probably wasabi.
But it COULD be worms!
Of course parasites don’t have to be worms, they can come in a ton of forms from microbes
to ticks to plants to fungi.
Humans have recognized the weirdness of parasites for a long time, from the ancient Greeks and
Egyptians to the Chinese.
The first written records show up on medical papyruses from ancient Egypt, and we’ve
found parasite eggs in actual 3000 year old mummies.
They’re even described in the Bible.
The 'fiery serpents' mentioned in the Old Testament are thought to be an early description
of a painful condition caused by Guinea worms.
But back then “doctors” didn’t understand how people actually got parasites.
In the 17th century, folks thought parasites just spontaneously generated in the human
body because they never saw the worms go in, they only ever saw the worms come out–if
you know what I’m sayin.
Of course now we know parasites don’t have to go in as whole worms.
You only need to eat their eggs.
That’s how the life cycle of another ocean parasite goes: Euuuu’ve gotta be kidding
me, I can’t say that name.
THAT thing resides in salt marshes off the California coast.
It gets passed around by three different hosts: Birds poop out the parasite eggs, and the
eggs get eaten by horn snails.
Then the parasite castrates the snail and reproduces inside it.
Then, the parasite swims out of the snail, and into the gills of the unsuspecting California
It digs through the gills and into the fish’s head, where it weaves a little parasite carpet
over the little fish’s brain.
Killifish that are infected with parasites jerk around and swim near the surface of the
water, which make birds THIRTY times more likely to snatch that killifish up for a snack
than one that isn’t infected with parasites.
The parasites are promoting their own survival… by getting the fish eaten.
Finally, after the fish get munched, the parasites burst out into the birds’ guts and the cycle
starts all over again.
Now, that is the circle of life.
Ok, so fish are one thing.
But parasites are everywhere, land, sea, even air--they make up as many as half of all animal
Scientists think more than 200 different forms of parasitism have evolved independently from
It’s difficult to even estimate the number of parasites on Earth because they can come
in so many forms.
From the beginning of multicellular life on Earth, parasites have been locked in quiet
evolutionary arms races with almost everything living in their environment.
Hosts and parasites always trying to stay one step ahead of each other.
And here’s the thing: Even if you’ve never gotten sick from eating worm-infected sushi,
parasites have impacted your life.
Human DNA is full of battle scars from our species’ past run-ins with parasites.
One of the most famous and nasty parasites out there is Plasmodium vivax, carried by
mosquitoes, it’s responsible for millions of cases of malaria a year.
Today, a whopping 99% of people living in sub-Saharan Africa have a specific type of
a protein receptor on their red blood cells that doesn’t allow the parasite to gain
They evolved a natural malaria parasite defense!
We think this change in the genome only took 8000 years to become widespread in that region,
meaning it must have been much easier to survive in the environment with that variant of the
gene than without it.
Parasites cause us harm, so the simple and obvious answer is that getting rid of them
is a good thing, but biology is rarely simple and obvious.
Preserving parasites could be more important than we realize.
Let’s return to the California salt marshes and see what happens if we disrupt the parasite
Snails that aren’t infected reproduce super quickly.
This could lead to fewer plants, fewer fish, less for birds to eat… by messing with a
parasite, we’ve broken the ecosystem.
This is important because parasites are actually at risk--climate change could cause the extinction
of one third of them by 2070.
Parasites are often gross, but they are critical to keeping balance in ecosystems--and we aren’t
even close to knowing how diverse the parasite world really is.
The conservation of parasites doesn’t inspire a warm and fuzzy feeling.
But it’s just as important to the environment that we save not only the whales… but also
the worms inside their guts.