A recent report from Stanford University describes the
ability of the single-celled protozoan parasite Toxoplasma
gondii to manipulate rat behavior for the parasite's benefit. Cat odors induce innate defensive
behaviors in rats, a seemingly adaptive response to the evolutionary pressures
of predation. Amazingly, rats infected
with Toxoplasma approach the cat odors they would
otherwise avoid. In order for Toxoplasma to reproduce sexually it requires
the cat intestine, is shed in cat feces, and must make its way from the ground
to another cat host (typically through a rat that was infected by consuming
food/water contaminated with cat feces).
The report indicates that Toxoplasma
infection alters neural activity in limbic brain areas necessary for innate
defensive behavior in response to cat odor. In addition, the researchers found that Toxoplasma increases activity in nearby
limbic regions of sexual attraction when the rat is exposed to cat urine. The end result: rats infected with Toxoplasma not only ignore basic fear
survival instincts but they also develop a type of sexual attraction to the
normally aversive cat odor. These
results raise important questions for humans given that one-third of
humans test positive for exposure to Toxoplasma and the Toxoplasma genome
includes a gene that can induce a host’s brain to create dopamine, the
neurotransmitter most closely linked with feelings of pleasure or reward. Interestingly,
people who are Toxo-infected have three to four times the likelihood
of being killed in car accidents involving reckless speeding. In addition, elevated levels of
dopamine are a hallmark characteristic of schizophrenia, and some studies show
that people with schizophrenia had a higher rate of exposure to Toxoplasma
as a fetus or in early childhood.
Similarly, medications currently used to treat schizophrenia, which
generally work by reducing dopamine activity, are as effective at reducing Toxoplasma-related
behavior changes in rats as normal antibiotic treatments for the infection.
