Clues about how the parasites affect the animal come from several observations. First, the density of cysts in the amygdala is almost double that in other brain structures involved in odor perception. Parts of the amygdala have been linked to anxiety and the sensation of fear. Second, the genome of T. gondii contains two genes related to mammalian genes involved in the regulation of dopamine, the molecule associated with reward and pleasure signals in the brain, including in ours. So perhaps the creepy protozoa makes suicidal activities, such as hanging around places frequented by cats, feel more pleasurable for the infected rodent?
What elevates this vignette about evolution and life in the wild to epic proportions for humanity is that about a tenth of the U.S. population is infected by T. gondii (in some countries, such as France, the infection rate is seven to eight times higher, possibly because of the widespread consumption of uncooked and undercooked meat). Human toxoplasmosis is usually considered to be symptom-free (what doctors refer to as asymptomatic). Exceptions are patients with a weakened immune system and the unborn (hence the need for pregnant women to avoid cleaning cat-litter boxes).
Science has known for a long time that schizophrenic patients are two to three times more likely to carry antibodies to T. gondii than are controls who are not schizophrenic. Furthermore, antipsychotic drugs that block the action of dopamine, such as haloperidol, commonly used in the treatment of schizophrenia, are also effective in combating toxoplasmosis in both rats and people. And some infected adults go on to develop psychotic symptoms similar to schizophrenia. Little is known about the mode or site of action of this pathogen in the human brain. The exact link between T. gondii and psychiatric diseases is tantalizing but remains murky.
Recent claims go so far as to argue for a role of T. gondii in shaping distinct cultural habits, depending on the rate of infection in the population. A prospective study tracking the road safety in Czech recruits during their 18 months of compulsory military draft found a rate of accidents six times higher in affected drivers. Are the young men with toxoplasmosis infection simply slowed down? Or do they drive more aggressively?
In my November 2009 column, I described the discovery by cognitive neuroscientists that the feeling of freely willing an action (called authorship or agency) is a subjective, conscious sensation no different, in principle, from the conscious awareness of seeing the azure blue sky or feeling the sharp pain of a toothache. When I engage in a dangerous pursuit, such as taking the end of the rope on a steep section of a granite wall in Yosemite Valley while climbing, I feel as if “I freely decided” to do so, whatever this might mean in a metaphysical sense. Yet my action is most likely caused by an inexhaustible multiplicity of factors not accessible to my conscious introspection, including, yes, possibly some tiny single-celled parasites lodging in my brain and making me act out their silent commands. The wonder of it all.