For Weil, an infectious-disease specialist who spends a lot of his time flying to far-flung places like Indonesia to study tropical diseases like elephantiasis, a bacterial infection that causes gross swelling of the legs and genitals, it's a refreshing change to be able to look at something closer to home.

"It's like Alien," he says excitedly. "The astronaut's exploring a new planet, and he sees this egg thing. He touches it, and it explodes all over him. They take him to the clinic, and he seems fine, but this thing has gotten into him, and it starts to grow. And then the mature alien bursts out of his chest! That's science fiction, but this is real."

It's also considerably less dramatic. Like the titular alien, Paragonimus kellicotti begins as an egg. Unlike the alien egg, however, it's a tiny, translucent yellow blob that's been expelled from its animal host via phlegm (which the doctors, with medical delicacy, call "sputum") or shit ("stool"). The egg lands in the water supply where it's ingested by an unsuspecting snail. There it hatches and waits for the snail to become food for the next intermediate host, the crawfish. Once it reaches the crawfish, the parasite makes a little home for itself in the heart where it grows into a very tiny worm called a fluke. It'll reach maturity in its final host, an unsuspecting dog or cat or possum...or a drunken human.

Dr. Michael Lane: "If you've seen [paragonimiasis] once, it sticks with you."
Kelly Hogan
Dr. Michael Lane: "If you've seen [paragonimiasis] once, it sticks with you."

If a Paragonimus kellicotti hits the right combination of snail and crawfish and if that crawfish manages to get ingested by a human or another mammal without getting cooked first, which would kill both the crawfish and any other living creatures it harbors, it has won the parasitic lottery. Paragonimus kellicotti like the inside of warm-blooded creatures. Particularly their lungs.

As soon as the crawfish hits the first stretch of the small intestine, an onslaught of digestive enzymes warns the parasite that it's time to move. It burrows through the intestinal wall and the diaphragm, the muscle that separates the digestive system from the heart and lungs. Since the fluke is now approximately a centimeter long, some human hosts can feel the burrowing. Some get diarrhea. In at least one patient, doctors mistook the pain for a gall-bladder attack. Once the gall bladder was removed, it turned out to be perfectly normal.

In the lungs, the fluke settles in for the long haul and goes about the business of living and reproducing. (They're hermaphrodites, but they're more successful if they travel in pairs.) Its life expectancy is between five and ten years. "We don't know a lot about the biology," Lane admits.

What the doctors do know is that the parasite causes the lungs to fill up with fluid, similar to the effects of pneumonia. If it goes untreated, the fluid can spread throughout the chest cavity and surround the heart and, eventually, the pressure will stop it from beating. Lane hasn't seen this happen in any of his patients, though one did die when the fluid became infected. "It was a bad set of circumstances," he says, "two bad things coming together."

Sometimes the flukes go astray. Lane and Weil have seen them as far afield as the hand and, in one case, the face, where the lost fluke formed a small but visible — and moving — nodule under the skin. In two cases, flukes wandered into the brain, specifically the part that affects vision. One patient suffered from partial blindness. Another had bad headaches and saw "floaters," blind spots in his vision.

Because paragonimiasis looks a lot like several other, more common, medical problems, a lot of money has been spent on unnecessary treatments: rounds of steroids, repeated draining of the lungs, the aforementioned gall-bladder removal. One patient had been misdiagnosed with lymphoma and was about to start chemotherapy.

Weil's lab is working on developing a test for paragonimiasis similar to the CDC version, except that it will detect antibodies or DNA from Paragonimus kellicotti instead of westermani. But the doctors think it would be preferable to keep people from getting infected in the first place, or at the very least let their colleagues around the state know what to look for.

"We don't want to have patients having unnecessary surgery," says Lane. "It's expensive, and they don't do well. It's not the best medicine."

To that end, last summer the Missouri Department of Health and Senior Services issued a physicians' advisory and started hanging posters in stores that sell equipment for float trips that read "Do NOT eat raw crayfish." (In the .gif version saved on Weil's office computer, he's added the phrase "you idiot.")

[Editor's Note: A correction ran concerning this section; please see end of article.]

Lane and Weil haven't seen any cases this year (though they did hear of one at the St. Louis VA Medical Center in the spring), but it's still early. "In prior years," Weil says, "people who were infected over Memorial Day or the Fourth of July start showing symptoms in August or September."

The whole phenomenon still puzzles him. "You've got to be nuts to eat a raw crawfish." He walks over to the fish tank in his lab that holds a single live specimen. "Look! They've got pinchers!" He shakes his head. "If you want to eat something, eat a tadpole. It's soft, and it's not infectious."

Correction published 7/15/11: In the original version of this story, we we laid out an incorrect chain of events that led to doctors' diagnosis of Adam Brewer's parasitic infection. The nurse's son who was diagnosed with lymphoma was a different patient, not Brewer.
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