Toxoplasma gondii may be the most prevalent human parasite. As many as 50% of humans worldwide, and up to 80% in urban areas, have been infected with it at some time in their lives. An estimated 60 million people in the US have active cases at any given time. It’s a single celled parasite whose favored host is cats. However it can infect and live in a host of other creatures including rats and humans. Most infected people, and most infected rats, show no particular signs of illness when infected. They continue on with their daily life and work completely unaware they’ve been parasitized. But they may not be as unaffected as they seem.

There are some interesting studies showing striking behavior differences between rats that have been infected with Toxoplasma and those that haven’t. Normal rats are very reactive to the smell of cat urine – an unsurprising survival instinct. If they encounter cat urine in their environment they have an extreme fear reaction, and they will avoid that spot thereafter. Rats infected with Toxoplasma don’t do this. They have no fear reaction to the smell of cat pee; they don’t avoid the areas where they smell it. In fact some of the studied rats preferentially returned to the sites where they had smelled the urine. It’s hard to see how this could benefit the rat, but easy to see how it could benefit Toxoplasma, which could return to its preferred host to complete its life cycle if the rat gets eaten.

Now consider that statistic from the first paragraph again. Up to one half of humans worldwide are, or have been, infected with Toxoplasma. Can something that affects the behavior of one mammalian host so drastically have no effect on the other?

The answer has always been no for some people. A small minority of people have strong psychological effects from toxoplasmosis, including delusions, hallucinations, and paranoia. The

majority of the infected however, show no such drastic symptoms. Most people with toxoplasmosis have no idea they’re infected, but that doesn’t mean that they’re unaffected. At Charles University in Prague, parasitologist Jeroslav Flegr administered psychological tests to people infected with Toxoplasma, and compared them to a control group. He found alterations in the psychology of the infected individuals that seemed to be gender-based. Infected men appeared more jealous and suspicious. Infected women appeared more warm-hearted and outgoing. Both sexes seemed to be more self-reproachful than the control group.

Those results are fairly subtle. E. Fuller Torrey of the Stanley Medical Research Institute has found evidence of some that may be more drastic. Toxoplasma is associated with damage to the brain’s astrocytes – glial cells that function as an interface between neuronal and non-neuronal tissues. Astrocyte damage has also been associated with schizophrenia. Now add in that pregnant women with high levels of Toxoplasma antibodies are more likely to have children who later develop schizophrenia, and you have something to give most people pause. Torrey’s study also found that some of the drugs used to treat schizophrenia have an inhibiting effect on Toxoplasma growth.

What exactly the connection is between Toxoplasma and schizophrenia has not been determined, though it seems clear there is one. What alterations Toxoplasma makes to human psychology in general is even more unclear. What is abundantly clear is that whatever those alterations are, they affect a huge number of people. Can we afford to be as ignorant of them as we currently are?