“Can the common brain parasite, Toxoplasma gondii, influence human culture?” by Kevin D.

Lafferty

Presented by Sara Brinda

What is Toxoplasma

gondii?•Protozoan parasite capable of infecting all endothermic vertebrates

•Felines are primary hosts: only sexually reproductive in feline gut epithelium, disperses through oocytes in feces

•Remains alive in soil for up to a year, consumed by cats and other hosts

•If they are consumed by intermediate (secondary) hosts, parasites are under selection to increase the chance that primary (definitive) hosts eat intermediate hosts (allowing them to re-enter their primary hosts)

•T. gondii alter the behavior of intermediate hosts to increase predation risk

Webster, 2001

• “Could aspects of human culture result from a parasite selected to predispose its host to predation by cats?”

How does T. gondii affect behavior of rodents?

• Infected rodents are: more active, first to enter traps, less fearful of cats and associated smells

• Mechanism in brain = elevates dopamine, which is known to alter novelty-seeking and neuroticism

Does T. gondii affect people?

• YES, it modifies the behavior even of “dead end” hosts

• People can ingest infected soil, or infected meat

• Mild flu-like symptoms initially, then the parasite becomes dormant in the brain (or other tissues)

• Mechanism in brain = unknown; speculated to also be elevated dopamine levels caused by altered cytokine levels (brain’s local immune response against T. gondii)

• WHY? “T. gondii has nothing to lose evolutionarily speaking, in trying manipulative strategies in other host species.”

• Long-term personality changes

Webster, 2001

Goals of study• To determine whether aggregate

personalities of cultures are affected by T. gondii

• Whether prevalence of T. gondii infection could explain variation in cultural dimensions among human populations

Methods• Collected data on infection prevalence in

different countries– Women of childbearing age– Correct for countries with older mothers

• Collected data on relevant population-level personality: neuroticism (affected by guilt-proneness), uncertainty avoidance, and culturally “masculine” sex roles

• Performed General Linear Model to test for correlation between aggregate neuroticism, uncertainty avoidance, and masculinity

Results

Factors determining infection prevalence

• Climate• Dwelling

among cats• Working with

soil/poor hygeine

• Cuisine including rare or undercooked meats

What are the societal implications?

• Extremely widespread parasite explains some aspects of individual personality

• Also may explain a small part of two (sex roles, uncertainty avoidance) of the four principal cultural dimensions (others = individualism, class distinction)

• Could have medical or economic implications

What if we did not recognize evolution?

• We would not understand anything about this parasite!– The selection pressures that led to its

ability to manipulate behavior – Its non-adaptive behavior in human hosts

(kind of a correlated character, a “side-effect of prior selection in intermediate hosts…it is unlikely that any parasite would have the recognition and modulatory mechanisms able to restrict expression to within only those hosts likely to be predated.”)

References

• FOCAL PAPER: Lafferty, Kevin D.. "Can the common brain parasite,

Toxoplasma gondii, influence human culture?." Proceedings of the Royal Society B: Biological Sciences 273(2006): 2749-2755.

• ADDITIONAL SOURCE:Webster, Joanne P.. "Rats, cats, people and parasites:

the impact of latent toxoplasmosis on behaviour." Microbes and Infection 3(2001): 1037-1045.

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