Our tragedy opens somewhere in the North Country, as a lowly marsh slug prowls the leaf litter. Slurp, munch, slurp. Munch, slurp, wriggle. The slug is quick, for a gastropod at least, but its movements have not gone unnoticed.

From stage right enters our story's leading actors, the larvae of a brainworm. As the curtain falls on Act One, the brainworms have surreptitiously drilled their way into the slug's foot. The slug slurps on, unaware, and a hush falls upon the forest.

Act Two. There's a chill in the air this morning, but that won't stop ol' sluggy. Spring has finally come to the woods, and fresh herbs sprout here and there on the south-facing slopes. The slug climbs up a particularly choice-looking sprig. "Mmm," mumbles the slug as its radula saws through the herb. "Life is good."

But the slug isn't the only creature eating herbs for breakfast. White-tailed deer have been pushing their way north due to a pattern of warmer winters, and they too have found the herb patch. One deer lowers its head to graze, nipping sprouts with its front teeth. The fresh growth is a welcome change to the bitter acorns and twigs it has had to gnaw on all winter, and the white-tailed deer gobbles greens with abandon. So much abandon, in fact, that it never notices the slug. "Mmm," mumbles the deer as it swallows the slug and the brainworms the slug harbors. "Life is good."

Act Three opens with the stage bathed in darkness. We are in the fourth stomach of the white-tailed deer. Somehow sensing their cue, the brainworms burst forth from the slug like any proper gun of Chekhov and make a break for the abdominal wall. The symphony's pace picks up — we're all strings and woodwinds now — and the worms pry through the transparent membranes that hold mammal guts together. At last, they single out a nerve, and they climb — oh, how they climb — straight up the spinal cord until they reach the deer's brain.

[Intermission: With some time to kill, you crack open the program to learn more about the actors cast as the leads. Parelaphostrongylus tenuis, you read, is a parasitic nematode. In college, people started calling it "brainworm" because it sounded cool, but it's more accurately described as a meningeal worm. In its final stage of growth, P. tenuis looks like a slimy thread and can grow to be nearly 2 inches long. You're impressed, but note the nematodes did not attend Juilliard. The lights in the theater begin to flicker.]

When the curtains draw back on Act Four, two adult brainworms have just made love. We know this because the female smokes a cigarette, then ambles over to a blood vessel at the base of the deer's cranium and starts dropping eggs into it like one of those old pneumatic mail tubes. "Mmm," says the female nematode after a drag. "Life is good."

The audience starts to fidget and wonder where all of this is going.

At least the eggs know — they enter the bloodstream and head for the lungs, by way of the heart. The eggs hatch in the fine tissue of the lungs. Slowly, surely, the larvae grapple through the branching air sacs until they reach the trachea and its rows and rows of finger-y cilia. The cilia's job is to remove debris from the lungs with a wave-like motion, and they attend to the worms accordingly. But the brainworms want to be ejected. They ride the mucociliary escalator straight up to the top of the deer's windpipe. But barely have the worms emerged before they give the audience a dramatic glance, turn, and plunge again into the darkness of the esophagus. The deer swallows, and the worms brace themselves for the horrors of the digestive track.

A burst of white light signals the start of Act Five. The larvae fall through the sky straddling deer pellets like that bomb-dropping scene in Dr. Strangelove.

Clearly, this is the perfect place for the play to end. A soft rain washes the larvae off the deer feces and into the leaf litter. Another slug (or snail) comes along and gets infected. Another deer eats the gastropod and the circle of life continues as it has for millions of years. The nematodes keep reproducing, and neither the deer nor the gastropods are any worse for wear.

But the play does not end here, because as I warned you, this is a tragedy.

Act Six. A moose clomps onto the stage. It's a male, a bull, and he's seven feet tall at the shoulder and 1,500 pounds if he's an ounce. The moose is so big, he has to get down on his knees to eat spring's first sprigs — and this behemoth can eat a lot of roughage before he gets full.

"Mmm," says the moose as he swallows a particularly slimy mouthful of herbs. "Life is good."

But inside the moose, life is not good. A new generation of P. tenuis slithers like sidewinder snakes up the moose's spine. The nematodes penetrate the meningeal layers of tissue encasing the brain and start squirming around like brainworms do. But before long, they notice that something is wrong.

The sweet sound of the symphony has devolved into raucous, rhythm-less percussion. Outside, the moose loses its hearing. The lights surge on and off. The moose goes blind. The stage starts to pitch and roll and spin in circles. The moose loses control of its hindquarters and stumbles about as if drunk. Sandbags fall out of the rafters and crash through the floor. The brain and central nervous system sustain irreparable damage. And one by one, the drums explode until there's nothing but dark, dead silence.

The host is dead.

The curtain falls with our brainworms strewn about the rubble. Writhe, moan, writhe. Moan, writhe, wince.

There is no exit. And there will be no next generation.

***

Epilogue: Nobody knows for sure why the meningeal worm should have such a disastrous effect on moose and not white-tailed deer. One theory is that deer have co-evolved with the parasite for so long that they are able to develop a resistance after the initial infection. Whatever the case, P. tenuis is one of the main suspects in the dramatic decline of moose populations in Minnesota — along with ticks, liver flukes, and climate change. You can learn more about the story here, here, and here.

Many thanks to Professor Emeritus Murray Lankester, who started his career investigating the meningeal worm and has continued investigating the little buggers even now, in his retirement. Thanks, Murray, for helping me see through the eyes of the meningeal worm — not that it has any.