Saturday, July 14, 2007

Feline Parasite Alters Human Personality

Over the years, I've treated a few patients who were convinced that a sinister, unseen entity was trying to seize control of their minds. Such a delusional belief - usually regarded as clear evidence of psychosis - strikes us as obviously out of touch with reality. And yet straight from the front lines of scientific research comes evidence of a parasite that can exert its own version of mind-control . . .

The villain in this story is Toxoplasma gondii, a microscopic parasite that likes to take up temporary residence inside the brain and muscle tissue of warm-blooded animals. Once safely lodged inside the brain, Toxoplasma orchestrates a series of subtle neurological changes in the host. This process has numerous effects on the infected animal's behavior - effects that are especially pronounced in rodents. Afflicted mice and rats become fearless and disinhibited, and they exhibit particular boldness around housecats. Instead of avoiding cats at all costs (as any sane rodent would do), infected mice and rats will charge straight at a feline nemesis. It's a sure-fire strategy for winding up as cat chow, especially given the parasite's annoying tendency to induce slowed reaction times in its host.

If we didn't know any better, we might guess that Toxoplasma is trying to commandeer the rodent's body, to use it as a delivery vehicle to get itself inside a cat . . . and, in a very real sense, this is exactly what's going on. For it turns out that Toxoplasma can only reproduce effectively inside the feline small intestine. Its offspring can then hitch a ride back out into the world encased in cat feces, from which they make their way into the ground and then on into the body of any animal that happens to ingest them. Often the unsuspecting critter is something a cat can then prey on, after which the entire reproductive cycle repeats itself . . . Over the millenia, Toxoplasma has evolved a remarkable ability to make its way back inside cats to reproduce - primarily through altering the brain function of its temporary non-feline hosts.

Now, by this point in the story, some of you are no doubt wondering, "Hey, wait a minute; haven't I heard of this parasite somewhere before? Isn't it the one that causes toxoplasmosis, that disease pregnant women can catch from cats and pass on to their babies?"

Indeed it is. Fortunately, though, the best research suggests that Toxoplasma infection is no more likely among cat owners than it is among the general population, so the risk of contact with cats appears to be minimal. Medical experts do, however, urge some caution when cleaning out a litter box, as Toxoplasma is readily transmitted to humans through the accidental ingestion of cat feces. (As if such an unsavory fate weren't bad enough!)

Here's an unsettling fact to consider: according to the latest large-scale study, 16% of Americans are now infected with Toxoplasma. And most are completely oblivious to their infected status, since contracting the parasite usually brings about no obvious signs or symptoms. (Roughly 10%-20% of infected individuals will experience vague flu-like symptoms that can last for several days). In some countries, the rate of infection is even higher: it's nearly 50% in France and Germany.

But if people aren't typically contracting Toxoplasma from contact with cats, how are they getting it? The biggest culprit is the eating of undercooked meat. As we've seen, Toxoplasma can lodge itself in the muscle tissue (i.e., meat) of any warm-blooded animal, including pigs, cattle, sheep, and chickens. A non-trivial portion of our meat supply is infected. Luckily, heating tainted meat to at least 155 degrees Fahrenheit (67 degrees C) for a few minutes appears to be sufficient to prevent infection. Obviously, to be completely safe, you might want to consider going even hotter and longer . . .

Another common route to infection involves accidentally ingesting little bits of soil - e.g., by eating poorly washed vegetables or wiping one's mouth while gardening. (Kind of makes you think twice about all those mud pies we used to make back in kindergarten . . . )

Soon after entering the human body, the parasite begins taking up residence in muscle and brain tissue, forming very small cysts that - until recently - were thought to be completely harmless. But lately researchers have been wondering . . . hey, if Toxoplasma can hijack the brain of a rat and force the animal to do its bidding, what - if anything - can it do to a human host? Unfortunately, the answer is turning out to be: "quite a lot."

Even though research in this area is still in its infancy, the findings thus far have been unsettling, to say the least:

1) Just as Toxoplasma reduces reaction times in rodents - presumably, to make them easier for cats to catch - it also makes people react more slowly. In fact, infected individuals are at much higher risk of experiencing an automobile accident. (And remember, most people who have contracted Toxoplasma are completely oblivious to the fact.)

2) Women infected with Toxoplasma experience a suite of changes that might be considered generally positive. They tend to be very kindly, and to exhibit, on average, slightly higher IQ, conscientiousness, warmth, generosity, and guilt-proneness. Why in the world would the parasite engender these particular effects? Well, anything Toxoplasma can do to aid and abet its genetic kin will be subject to evolutionary selection pressure, since this will still effectively help pass along its genes. Thus, Toxoplasma appears to be trying to turn its female hosts into people who will be particularly nice to cats - aka, "cat ladies!"

3) Men, on the other hand, have a completely different set of reactions to Toxoplasma infection. They experience reduced IQ and initiative, and tend to become more stoic, laid-back, and slow-tempered. In other words, they become just the sort of guys who would be unlikely to interfere with their "cat lady" wives, to hinder them from tending to their brood of adoring felines!

Now, having outlined this array of creepy Toxoplasma effects, I need to give the following disclaimer: these effects on personality are often fairly subtle, and many infected individuals exhibit no measurable effects at all.

Nevertheless, the obvious question for each of us is: what if I'm infected, and if so, what can be done about it? I am duty-bound to urge you to consult with your physician on this one. Certainly, there are fairly straightforward blood tests that can determine whether or not you have Toxoplasma antibodies in your system (which would in turn indicate whether or not you've ever had an active infection, in which case it's likely that you would still have Toxoplasma cysts in muscle and nervous tissue).

There are a number of different antibiotics that seem to have at least some effect in combating the Toxoplasma cysts in the brain, although at this point it's premature to say that any single drug (or combination) is a guaranteed cure. Given the enormous potential public health implications of this issue - which is just now coming to widespread attention - I think it's safe to say that finding a reliable way to eradicate Toxoplasma will become a high clinical research priority in the years ahead.