Yahoo News reports, "Half of the world's human population is infected with Toxoplasma, parasites in the body—and the brain". Toxoplasma gondii is a common parasite found in the guts of cats; it sheds eggs that are picked up by rats and other animals that are eaten by cats. Toxoplasma forms cysts in the bodies of the intermediate rat hosts, including in the brain. Oxford scientists discovered that the minds of the infected rats have been subtly altered. In a series of experiments, they demonstrated that healthy rats will prudently avoid areas that have been doused with cat urine. In fact, when scientists test anti-anxiety drugs on rats, they use a whiff of cat urine to induce neurochemical panic. However, it turns out that Toxoplasma-ridden rats show no such reaction.In fact, when scientists test anti-anxiety drugs on rats, they use a whiff of cat urine to induce neurochemical panic. However, it turns out that Toxoplasma-ridden rats show no such reaction. In fact, some of the infected rats actually seek out the cat urine-marked areas again and again. The parasite alters the mind (and thus the behavior) of the rat for its own benefit.
Dr. E. Fuller Torrey (Associate Director for Laboratory Research at the Stanley Medical Research Institute) noticed links between Toxoplasma and schizophrenia in human beings, approximately three billion of whom are infected with T. gondii: Toxoplasma infection is associated with damage to astrocytes, glial cells which surround and support neurons. Schizophrenia is also associated with damage to astrocytes. Pregnant women with high levels of antibodies to Toxoplasma are more likely to give birth to children who will develop schizophrenia. Human cells raised in petri dishes, and infected with Toxoplasma, will respond to drugs like haloperidol; the growth of the parasite stops. Haloperidol is an antipsychotic, used to treat schizophrenia. Slashdot has a good discussion thread on this. In particular this conveys the seriousness. It goes on to say that the implications of this research are enough to send a frisson of fear down the spine of anyone, devoted parasitologist or otherwise. Toxoplasma infection is common amongst humans. It has been estimated that 30% of the global human population may be infected, with prevalence in specific countries ranging from 22% in the UK to 84% in France. Can the parasite affect human behaviour in the way in which it affects that of rats? The answer appears to be "yes". One manner in which this happens is via direct damage to the host's brain and central nervous system. Babies born to mothers infected with Toxoplasma early in fetal development can suffer from widespread disease, including mental retardation . Infection later in development can lead to a persistant infection with no apparent symptoms, with the parasite forming cysts in the brain. With any luck the immune system can keep the parasite under control; depression of the immune system, however, can result in its reactivation, with consequent neurological or psychiatric effects. The next one looks more levelheaded. It says, "Before we get too carried away, note that the numbers indicate the number of people with the antibodies to toxoplasma gondii, not the number of people with active infection. Antibodies just mean you have been infected at some point in your life - and the mental status changes seem to be primarily in those infected as infants or born to infected mothers". This connects well with the known etiology of toxoplasmosis, and is why the MD tells your pregnant wife/girlfriend/mom to stay away from cats. Still, it is really interesting how many diseases have been found recently to be of infectious etiology - ulcers (no, it's not the pizza), many forms of heart disease, and now possibly some forms of schizophrenia. Makes prevention at least plausible...