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.
I looked further into the relationship of T. Gondii infection and schizophrenia as I wondered if antipsychotic treatments were merely masking the symptoms of infection (by affecting dopamine levels) versus treating the disease and further wondered to what extent schizophrenic patients may be misdiagnosed and given a treatment that does not address the underlying cause of the disease. However, it appears some antipsychotics actually inhibit the growth of T. Ghondii in cell cultures which means the antipsychotic treatment can address the underlying cause rather than mask symptoms - found in article by Torrey & Yolken on CDC site: http://wwwnc.cdc.gov/eid/article/9/11/03-0143_article.htm
ReplyDeleteI remember being fascinated at the effect of T. gondii on human personality and behavior when the fascinating bestseller Survval of the Sickest by Sharon Moalem was requred reading in an undergrad Evolutionary Medicine class. According to Moalem, Jaroslav Felgr of CHarles University in Prague has found that human women and men respond differently to infection. Infected women are more likely to spend more money on slothes, are rated as more attractive, warm-hearted, and eastygoing but less trustworthy. Infected men, however, are more likely to be loners, disobey rules, and be less well groomed.
ReplyDeleteMore recent work by Flegr (2011: http://www.ncbi.nlm.nih.gov/pubmed/22087345) has shown that infected men find domesticated cat urine odor more attractive than uninfected men, whereas infected women find domestecated cat urine odor less attractive than uninfected women. Since the findings were not true for urine odor from large wild cats, we can deduce that the mechanism for host modification that T. gondii uses evolved relatively recently- at least since the domestication of cats.
Google books search link to relevant Survival of the Sickest passage
http://books.google.com/books?id=-S-tYNGP_k8C&pg=PA108&lpg=PA108&dq=T.+gondii+survival+of+the+sickest&source=bl&ots=xIi4bgbxdg&sig=dFKQja4vl_fXuzFx5OfFXW0fMa4&hl=en&sa=X&ei=rjRmUNSnJo-i8gT4jYHoBg&ved=0CCIQ6AEwAQ#v=onepage&q=T.%20gondii%20survival%20of%20the%20sickest&f=false