You’ll Either Love This One Or Hate It
All of you have Carl Zimmer to blame for this one. Go and read everything he’s ever written ever. I’m in the process of doing just that. And I thought I already wrote this article months ago, and I was just going to do an update on it. However, I skimmed the archives and I can’t find it anywhere, which means I have to write the whole thing from scratch. *sigh*

Today I’m going to discuss parasites, and no, Brent, I don’t mean corporate CEOs (although…). Oxford considers a parasite “an organism which lives in or on another organism and benefits at the other’s expense”. That’s a pretty broad spectrum to look at, really. Anything could be considered a parasite with such loose standards, so lets tighten up what we’ll be talking about here.

So, let’s instead focus on my personal favorite parasite, a cute little protozoan called Toxoplasma gondii. Since the species is the only know form of its genus, we’ll drop the “gondii” to make life simpler. Toxoplasma can be found in all mammals and birds, and it’s quite possibly the single most successful protozoan parasite on the planet. And what makes it so successful? Well, it’s extremely durable, easily contaminable, and it spends its life cycle in two of the most successful and spread out mammals on the planet: rats and cats.

Cats eat the contaminated rats. Toxoplasma can only enter the sexual portion of its life within the muscle and brain tissue of the Felidae family, so rats make the perfect host body for the parasites. The immature parasites inside the rat grow and reproduce inside the cat, and the baby parasites are sent out into the world through the cat’s waste system. Once in the dirt, rats and other mammals pick up the newborn parasites, where the cycle begins anew.

Of course, not just rats and mice pick up the baby Toxoplasma. Virtually any mammal or bird that comes in contact with the cat’s feces can become contaminated, as ingesting the matter or the soil around it causes infection. The cysts formed by Toxoplasma are extraordinarily durable, and they can survive for up too a year without latching onto a host body. Not only that, but anything else that eats the an infected mouse or rat can catch it right there as well. All of this helps to contribute to the parasites overwhelming success rate. It’s estimated that in humans 3 BILLION people are infected with Toxoplasma, 50 million in the USA alone. In case you’re wondering, that’s 1/6 of the country’s population. Any one who’s spent a great deal of time around cats has a good chance of getting infected, which means that yours truly probably has it (I’m making a point to find out next time I have blood work done. I’m curious, is all).

All this though would make Toxoplasma a mere passing interest, but it gets much better. You see, what makes Toxoplasma so interesting to me is what it does to its host body. Lots of parasites have staggeringly brilliant abilities to survive and prosper. Whether it’s Plasmodium(which causes malaria)’s ability to completely bypass the body’s immune system and survive inside of a red blood cell, or Trichinella’s ability to rewrite the genetic structure of it’s host’s cells, many parasites do rather fantastic things.

Toxoplasma, however, is a much more subtle parasite, which has contributed greatly to it’s success. An infected rat looks and acts like a regular, healthy rat. This is to the parasite’s advantage: If the infection caused the rat to look diseased or abnormal, a cat wouldn’t want to eat it.

Despite the fact that cats have been eating rats and mice for ages, statistically speaking, there have to be a lot more mice than cats for the mice to still be around (The average for a stable environ is one predator animal to 250 prey animals). Since cats and mice have been predator and prey for so long, natural selection has brought about a very important instinct within mice that is a big contributing factor to a mouse’s survival. As soon as a mouse smells cat urine it flees away. This is a fairly basic but effective adaptation: Mice that run from the smell of a cat are far less likely to be eaten than mice that don’t.

The only thing Toxoplasma does to change the mouse is to turn off this adaptation. A Toxoplasma Positive mouse won’t run away from cat urine. In fact, some studies suggest that an infected subject may linger at the scent for an abnormal amount of time. In effect, Toxoplasma causes the mouse to be more likely to get itself killed. Crazy, no? The parasite causes suicidal tendencies in the mouse, to advance it’s own life cycle. And it does more than that, too. Recent studies have found that the parasite actually regulates its host’s immune system to prevent the body from attacking it before it takes root.

Did I mention that half the human race is infected with this stuff?

That’s where things get interesting. Humans have no natural aversion to the smell of a cat, obviously, so it doesn’t cause us to leap into a lion pen or anything. But due to its prominence (and also because parasitology is only recently getting the mainstream scientific attention that bacteriums and viruses have enjoyed for a century) studies are being done to examine the effects of Toxoplasma in humans. Toxoplasmosis (the disease caused by the protozoan) is latent in most people with the condition. Again, the fact that half the planet has this condition and odds are you’ve never heard of it before now should suggest that this is generally not a dangerous parasite. Cysts can form on muscle and brain tissue, but they aren’t particularly dangerous except in the case of pregnancy, where the infant may possibly become infected. Acute toxoplasmosis occurs rarely upon initial infection, generally only in immunodeficient people. Symptoms are generally flu-like, but brain and eye damage can occur.

But does Toxoplasma have an effect as a behavior modifier on humans? Studies aren’t completely conclusive yet. Statistically speaking, people with Toxoplasma are at least twice as likely to suffer a car accident than an uninfected person. Some studies have shown that infected women are more out going, and infected men more socially paranoid, though these studies are currently in question.

However, the most interesting and cryptic possibly comes from Toxoplasma’s connection with schizophrenia. Research into a possible correlation between the two has been going on since the fifties, but only recently has much attention been paid to it. Infection with toxoplasmosis has been associated with the damage of astrocytes, a class of neuron. So has schizophrenia. In the early 2000s, E. Fuller. Torrey of the Stanley Medical Research Institute began compiling all the reports of a correlation between the two, and se out to find some results of his own. He found that women with high levels of Toxoplasma antibodies in their blood (indicating a fairly large infection) were much more likely to have children that would develop schizophrenia than women who were clean. And people with schizophrenia are twice as likely to have a Toxoplasma infection than people without the condition. Interestingly, Torrey discovered that many drugs that are used to treat schizophrenia were effective at causing Toxoplasma cells to stop growing.

No one’s sure yet how Toxoplasma may cause the condition, or if, perhaps, having schizophrenia increases the likelihood of catching toxoplasmosis. Perhaps it’s some strange combination of the two. But research continues, and no matter what the results, it seems clear that Toxoplasma gondii is one of the most successful parasites on the planet.