My friend Dan mentioned “Shivers,” an old Cronenburg horror
flick on sexually-transmitted, libido-stimulating parasites in his journal
today… Which made me think about parasites in general and their wild and wacky
life cycles.
Okay, so most parasites are pretty straightforward. Animal
ingests eggs, eggs hatch into parasites, parasites lay eggs, animals defecate,
rinse, wash, repeat. Some are a bit more complex: blood-sucking insect with
stage A parasite bites animal, injects parasite into blood, parasite morphs
into Stage B, has sex, replicates, etc, etc, is picked up by blood-sucking
insect during biting, morphs back into stage A in insect, etc.
Some, however, do exactly what Cronenburg’s film suggests -
they have a humongous impact on the behavior of their host. One makes its
rodent hosts stupid and visually impaired, so they’re more likely to be eaten by
a more desirable host - a cat. But my favorite (and I suspect the darling of
the parasitic world in many respects) is the lancet fluke.
The lancet fluke, Dicrocoelium dendriticum isn’t really a
human problem, because we don’t graze. It’s a serious problem for sheep,
however. Without getting into the great chicken-egg, debate, I’ll start the
life cycle with the egg of the parasite, which is defecated onto the grass by
infected sheep. The eggs are like extra protein for certain terrestrial snails,
who gobble them up. Inside the snail, the eggs hatch, and the parasite goes
through an intermediate larval stage called a miracidium, and then becomes a
more stable, ball-shaped form called a cercariae. The cercariae, to the snail,
are like grains of sand to an oyster - very, very irritating. So the snail does
what we would all do, if we were snails, which is to bundle it up in mucus and
secrete it in the waste all snails leave behind - yes, that shiny, sticky trail
on your garden path may well contain little balls of potential ovine infection.
But this is where it starts to get really cool. Turns out,
these little parasite slime balls are the equivalent of candy to ants (my
textbook calls them “gastronomic prizes”). There may even be ants who stalk
snails hoping they’ll excrete cercariae, though I haven’t heard of any. At any
rate, the ants grab, fight over, and consume these little balls as fast as
possible. Then the plot thickens. The cercariae turn mobile and move into the
“brain” of the ant (ants don’t really have brains - they have subesophageal
ganglions, but whatever. In a head that small, who needs a brain?).
Somehow, mysteriously, the presence of parasites in ant
brains makes the ants a little nuts, perhaps a little Don Quixote-style in
their philosophy. They decide that the best thing in the world is to form a
traveling acrobatic company. Now, having a limited repertoire (no clown school
for ants), they only have one option, apparently. The infected ants crawl up
blades of grass and dangle from the tips by their mandibles. A typical
conversation might look like this:
non-parasitized ant, looking up: “Barry! What the hell are
you doing up there?”
Barry, who has a parasite: “mmphmmmphh mhhpmm” (translation:
Oh, nothing, just hanging out.”)
N.P.A.: “Why on earth would you want to do that? You’re an
ant! We like to work!”
Barry: “mmphmmtmymph mmphhpermmph” (I don’t know, it seemed
like a good idea at the time)
This behavior is not just weird, it’s temporally controlled.
Ants only do this at dusk or early morning, the favorite time for many grazers
to knosh on a bit of bedtime or breakfast greenery. So hanging ants get nibbled
up along with the grass, and BLAMMO! The parasite has a new home in the sheep
of its dreams. It turns into the adult fluke, crawls up the bile duct, lays
eggs, and the whole thing starts again.
Now, this is just bizarre. I mean, why would the lancet
fluke have any reason to do this? It could be like lots of other flukes and
just happily go from egg to snail to larva on grass to sheep. Or it could be
super-simple and just go from egg to adult to egg. But no…. at some point the
lancet fluke must’ve been given a book on “how to be an evil overlord” or
something along those lines, because it came up with the most pointless,
complex, fascinatingly silly life cycle ever. “Muahahaha,” it said, rubbing its
little membranous hand-like bits together, “I shall eenvade zee brainz of antz!
And zen! I shall make zem crazee! And zey shall commeet zueecide een zee jaws
of sheep! Muahahahaha!”
Makes you wonder what more successful and more subtle bugs
do, doesn’t it? I mean, sneezing when sick may be a good way of ejecting extra
mucus, but it’s also a great way for spreading colds - so is it to the virus’s
best interest to make us sneeze more? Are we just pawns of our plagues?
Awesome post, but don't know who wrote it, if you know, let me know... you know?
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