When the song ended and the applause died away, Melinda turned to Trent. “As I said, what about this bug intrigues you?”
Surprised that Melinda might really be interested in his work, Trent responded, “First, I find it interesting that you could have a parasite in your brain for the rest of your life, with no clinical manifestations unless your immune system somehow becomes compromised. Second, this organism’s reproductive cycle and its effects on its hosts are bizarre. Its primary host is the cat; house cats or jaguars, size doesn’t matter. So, imagine you’re this parasite, Melinda, and I’ll walk you through—”
“Now you’re calling me a parasite,” she said sharply, but with affection. “It’s no wonder we were never lovers.”
“Very funny,” Trent said. “Okay, so you’re this parasite and you need to be in a cat to survive and carry out your genetic destiny, but you can’t just hop from one cat to another because you can’t travel in a sneeze like a cold virus, and cats don’t normally eat other cats.”
“That doesn’t sound like a big problem, Pavlov,” Melinda said. “One cat just has to lick another cat with its sandpaper tongue or nip it with a sharp tooth in a friendly fight and presto, there’s a new victim.”
Trent shook his head. “It’s not that easy for you to guarantee the continuance of your species, my parasitic beauty. Your kind is not present in saliva or blood in sufficient quantities to infect another cat. Your kind is buried deep in a cat’s tissues, and without a transfer to successive new cats by way of infected meat you would die when the cat you called home died. In time, your species would become extinct,” he said. “Toxoplasma has solved the survival problem in a very ingenuous way. It reproduces sexually only in the intestinal tract of a cat, where it splits into male and female cells that recombine, in the process exchanging DNA with any related strains that may be present. This DNA sharing allows the parasite to evolve to meet changing environmental conditions.”
“Okay, I’ll play your little game,” she said. “How do I get from one cat to another?”
“You recall that I said gondii is mainly in the intestines of the cat. That means there is only one way out, and that is with the feces, the cat poop. That’s a good step, but not sufficient to spread the parasite because cats have good hygiene habits and don’t eat poop.”
“Pavlov, that’s disgusting,” Melinda said. “I don’t like your game anymore, especially while I eat my pasta.”
“That’s too bad, because the interesting part is yet to come,” he said. “The part about how T. gondii changes the sexual nature of human beings and may be shaping the behavior of whole cultures, even as we dine here tonight.”
“All right,” she said. “I’ll try to forget why biology always made me nauseous and distance myself enough from the disgusting details to listen to your story. After all, you did invite me out tonight for some important reason, I suppose.”
“Good, I’m glad I have your rapt attention.” Trent smiled at her. “When T. gondii exits a cat, it’s in a noninfectious form that matures in two or three days to a highly infectious, spore-like form that can persist in soil or water for months or maybe years. Now here comes the bizarre part. Rats forage for food and sometimes they ingest T. gondii in the process. When male rats are infected, they lose their fear of cats and are, surprisingly, attracted to cats, with the result that you’d expect: dinner for the cat and a new home for T. gondii.”
“That’s crazy!” she exclaimed. “What would drive a rat to seek out a cat? That’s suicidal behavior!”
“The most powerful of all animal forces, my dear Melinda: sex.”
~~~~~~~~~~~~~~~~~
“Wait a minute,” she said, putting her fork down. “You mean a rat tries to have sex with a cat?”
“In a manner of speaking, yes,” he said. “When an infected male rat smells a cat, he feels a strong attraction and even gets an erection, exhibiting all the behavior he would display if he smelled a female rat eager for mating.”
~~~~~~~~~~~~~~~~~
Toxoplasma gondii organism (left, half-moon figure) moving over cells in culture and infecting one cell. The image to the right is of a parasite that has entered a cell and divided into two daughter cells. Video provided by Doctor Gary Ward, Professor of Microbiology and Molecular Genetics at the University of Vermont
