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.
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
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?