The Laptev Virus

Now that she thought about it, she remembered that Emile had said something else. He had looked up her story about the ants, the acacia tree and the giraffes. It turns out that recent research indicated that the ants were not protecting the tree after all. They were mere bystanders, stepping in, yes, but not effectively keeping the grazers away. When scientists had done an experiment in which they isolated the trees, putting fences around them so that no large animals could eat the shoots, the trees had not thrived and the ants had all disappeared. She had meant to follow up on that story, but had become distracted with something else.

“Earth to Sarah,” said John. He had stopped cutting the tomatoes and was waving a hand at her.

“Oh, sorry. Tell me, how do you think that Toxo affects humans?”

“Well, I guess let’s first start with mice. What I was reading is that they become more bold and daring. They are not afraid of cats and in general they are more likely to take risks. It’s the kind of behavior that was puzzling me before we knew what was going on. ”

“Not afraid of cats?” Sarah echoed. “That does sound crazy. That could get them killed.”

John nodded.

“And you’re sure it’s because of the Toxoplasmosis infection? It’s one thing to not react because they are paralyzed with fear as you expected, but it’s quite another to not be afraid of cats.”

John tested a piece of pasta, then stirred the pot some more. “We saw this in our lab, but I’ve since verified that labs in other parts of the world have seen the same behavior.”

Having finished with the carrots, Sarah put down the peeler, crossed her arms and leaned back in her chair. “But, it doesn’t make sense to me. If the mice take more risks, as a result of having a Toxoplasmosis infection, as you’ve seen, then fine, it must be that Toxoplasmosis is somehow contributing to having this happen. But what I don’t see is how can that behavioral change be advantageous to the organism causing the change? If the host dies because it is taking more risks, then the parasitic organism won’t live on either because it will no longer have a home. So it doesn’t make evolutionary sense, does it?”

John shook his head, having tested the pasta again. Then he donned heavy oven mitts and carefully poured the pot of boiling water and spaghetti into a large colander placed in the sink. “I’m not sure either,” he said, speaking through a cloud of vapor, “but I did read that it has another host…”

Sarah grunted and smiled, shaking her head. “Dear me! I’m forgetting all my parasitology. My professor would have killed me. Of course, I get it. And it does make sense, now that I think about it. Do you remember ever hearing about the life cycle of Dicrocoeleum dendriticum?”

John returned the strained pasta to the pan and added a pat of butter, then began stirring the pasta with a fork, burying the butter so that it would melt. “Can’t say that I have,” he said, looking over at her with a mischievous smile, “but I bet I’m about to.”

Sarah grinned. “Oh, it’s a cool story. You’ll enjoy it, I promise. Hand me those plates and I’ll get us set up here.”

John reached for the carrots, rinsed them and then placed a couple on each plate. Then he served the pasta and the sauce, and Sarah began her narrative.

“So, Dicrocoeleum is a protozoan, just like Toxoplasmosis. Scientists had studied it for years, but they had never been able to map out its complete lifecycle. They knew that there were three animal hosts that it required, and that it would spend only part of its lifecycle in each host.”

John nodded as he ate, and Sarah reached for the parmesan cheese. Then she began swirling forkfuls of pasta onto her fork, and continued telling her story between bites. “So, this parasite, Dicro, would mate in one host and lay eggs, but the eggs would have to be transferred to a second host, a totally different animal, in order to develop. In this second animal host the little larvae would grow until they got to a particular stage, but after that, they couldn’t develop any further until they got moved into a third animal host. Scientists had worked out two of the animal hosts, sheep and snails, but it took them years to come up with the third one. Any guesses as to what it was?”

John scratched his chin and frowned in an exaggerated manner, as if deep in thought. “I don’t know, maybe a wolf or a fox?”

Sarah shook her head, taking advantage of the pause to eat more.

“Maybe a mountain lion—or some big predator that eats sheep? Humans?”

“You’re cold,” said Sarah. “Think smaller.”

“Okay,” said John, drawing out the last syllable of the word and rolling his eyes toward the ceiling. “If it’s not a predator of the sheep, then maybe it’s something that eats the snails? A bird or something?”

Sarah shook her head again, clearly enjoying the game. “Nope, nothing that eats the snails or the sheep.”