A very interesting study I just came across:
The neurotropic parasite Toxoplasma gondii increases dopamine metabolism.
Prandovszky E, Gaskell E, Martin H, Dubey JP, Webster JP, McConkey GA.
During the chronic stage of infection, infected rodents, which are a key intermediate host for T. gondii, exhibit a distinct repertoire of specific behavioral changes, including a loss of aversion to cat odors. Infected rodents are, conversely, attracted to these odors, and this may be responsible for increased predation and for an increase in successful transmission of the parasite to the feline host; as cats are the only animal that can shed the environmentally-resistant stage of the parasite known as oocysts. This behavior change in infected rodents during the chronic stage of infection appears highly specific to feline odor, as a similar change is not evoked by other predators and has no effect on conditioned fear and anxiety.
...
In this study, significant levels of dopamine was detected by immunohistochemistry in T. gondii tissue cysts in the brain, as well as, increased dopamine release from dopaminergic cells infected with T. gondii. Based on these novel findings, this is the first study to suggest that a parasite can directly alter dopamine signalling to mediate host behavior changes.
...
Tissue cysts have been detected throughout the brain, although higher percentages of cysts were reported in the amygdala and nucleus accumbens. These limbic brain regions are well known to contain dopamine that plays important functions in the control of movements (basal ganglia), reward to stimuli, pleasure, dependency (nucleus accumbens and hippocampus), motivation and cognition, and species and stimuli specific fear (amygdala).
So in rats, this parasite alters the brain to turn fear of cats (specifically) into an attraction, toward its own ends. This is unlikely to have as specific a correlate in humans, but it seems that T. gondii infection has been shown to be a higher risk factor for schizophrenia than both genetic and environmental factors.
This is fairly novel research, so it's yet unknown if other neurotransmitters are affected, and how this increased dopamine propensity plays out in an active brain. It's possible that it manifests as dysregulation, with entirely negative consequences.
However, another very interesting possibility (not mentioned in this study) is that the neurochemical impact could be beneficial. Enhanced levels of limbic dopamine can have many beneficial effects related to reward and cognition. Raises some interesting possibilities for technological brain manipulation that may be feasible before the nanobots....
"It is estimated that one quarter of the population (over 12 years of age) in the United States is positive for T. gondii infection (Center for Disease Control, USA, 2008). Prevalence in some areas can be as high as 95% in older populations."
See this excellent post by Live Forever with pictures detailing T gondii life cycle.
