Behavioral changes
It has been found that the parasite has the ability to change the behaviour of its host: infected rats and mice are less fearful of cats—in fact, some of the infected rats seek out cat-urine-marked areas. This effect is advantageous to the parasite, which will be able to proliferate as a cat could eat the infected rat and then reproduce.[23] The mechanism for this change is not completely understood, but there is evidence that toxoplasmosis infection raises dopamine levels and concentrates in the amygdala in infected mice.[24]
The findings of behavioural alteration in rats and mice have led some scientists to speculate that Toxoplasma may have similar effects in humans, even in the latent phase that had previously been considered asymptomatic. Toxoplasma is one of a number of parasites that may alter their host's behaviour as a part of their life cycle.[25] The behaviors observed, if caused by the parasite, are likely due to infection and low-grade encephalitis, which is marked by the presence of cysts in the human brain, which may produce or induce production of a neurotransmitter, possibly dopamine,[26] therefore acting similarly to dopamine reuptake inhibitor type antidepressants and stimulants.
Correlations have been found between latent Toxoplasma infections and various characteristics:[27]
- Decreased novelty-seeking behaviour[28]
- Slower reactions[citation needed]
- Lower rule-consciousness and greater jealousy (in men)[28]
- Promiscuity and greater conscientiousness (in women)[28]
Studies have found that toxoplasmosis is associated with an increased car accident rate in people with Rh-negative blood. The chance of an accident relative to uninfected people is increased roughly 2.5 times.[26][29][30]
This may be due to the slowed reaction times that are associated with infection.[29] "If our data are true then about a million people a year die just because they are infected with Toxoplasma," the researcher Jaroslav Flegr told The Guardian.[31] The data shows that the risk decreases with time after infection, but is not due to age.[26] Ruth Gilbert, medical coordinator of the European Multicentre Study on Congenital Toxoplasmosis, told BBC News Online these findings could be due to chance, or due to social and cultural factors associated with Toxoplasma infection.[32] However there is also evidence of a delayed effect which increases reaction times.[33]
Other studies suggest that the parasite may influence personality. There are claims of Toxoplasma causing antisocial attitudes in men and promiscuity[34] (or even "signs of higher intelligence"[31] ) in women, and greater susceptibility to schizophrenia and bipolar disorder in all infected persons.[34] A 2004 study found that Toxoplasma "probably induce[s] a decrease of novelty seeking." [35]
According to Sydney University of Technology infectious disease researcher Nicky Boulter in an article that appeared in the January/February 2007 edition of Australasian Science magazine, Toxoplasma infections lead to changes depending on the sex of the infected person. [36][37]
The study suggests that male carriers have shorter attention spans, a greater likelihood of breaking rules and taking risks, and are more independent, anti-social, suspicious, jealous and morose. It also suggests that these men are deemed less attractive to women. Women carriers are suggested to be more outgoing, friendly, more promiscuous, and are considered more attractive to men compared with non-infected controls. The results are shown to be true when tested on mice, though it is still inconclusive. A few scientists have suggested that, if these effects are genuine, prevalence of toxoplasmosis could be a major determinant of cultural differences.[27][37][38]
[edit] Toxoplasma's role in schizophrenia
The possibility that toxoplasmosis is one cause of schizophrenia has been studied by scientists since at least 1953.[39] These studies had attracted little attention from U.S. researchers until they were publicized through the work of prominent psychiatrist and advocate E. Fuller Torrey. In 2003, Torrey published a review of this literature, reporting that almost all the studies had found that schizophrenics have elevated rates of Toxoplasma infection.[39] A 2006 paper has even suggested that prevalence of toxoplasmosis has large-scale effects on national culture.[40] These types of studies are suggestive but cannot confirm a causal relationship (because of the possibility, for example, that schizophrenia increases the likelihood of Toxoplasma infection rather than the other way around).[39]
- Acute Toxoplasma infection sometimes leads to psychotic symptoms not unlike schizophrenia.
- Several studies have found significantly higher levels of Toxoplasma antibodies in schizophrenia patients compared to the general population.[41]
- Toxoplasma infection causes damage to astrocytes in the brain, and such damage is also seen in schizophrenia[citation needed].
