Fish Fertilize Trees
By Maya C. Lemaire
Parasites & Mind Control
When you hear the words 'mind control', you may be picturing Charles Xavier or Kilgrave or any other fictional superhero/villain, but instead picture this:Toxoplasma gondii.
Toxoplasma gondii are parasites with felids as their natural hosts, however, they are carried in the brain by a third of the human population. Despite their sheer numbers, they are relatively harmless to those not immunosuppressed - at most only causing a mild flu in humans.
Toxoplasmosis occurs when the parasite invades a cell and starts reproducing for the first few days before building cysts and hiding within them. They use our own immune systems to prevent overpopulation (as they need their host to survive) and they do this by releasing a molecule that causes the creation of more inflammatory T cells – essentially destroying themselves by eliminating all toxoplasma that have not yet been hidden in cysts.
The mind control of rats
For several years, many researchers have been studying the effects of this parasite on rats/mice and how it changes their behaviour in regard to cats. Researchers built an enclosure and placed nest boxes with different odours in different areas - including a nest box with cat urine. They then released healthy and infected rats and unsurprisingly, the healthy rats avoided the cat urine scented nest. The odour of a cat normally triggers an intense anxiety in healthy rats and leads them to avoid those areas, but the scent appeared to have no effect whatsoever on the infected rats. There was no difference between the mating or feeding habits of both rat groups, but not only did the infected rats show signs of a decreased aversion towards the cat urine, some even expressed a mild attraction to it – thus decreasing their chances of survival. This way, toxoplasma increases its chances of entering their natural host and continuing its life cycle.
How do they do it?
Studies indicate that T. gondii may manipulate their hosts by causing a large increase in the production of dopamine (a hormone and neurotransmitter that helps control the brain’s reward and pleasure centres and regulates emotional responses such as fear) as they encode the enzyme for producing dopamine in its genome. However, research has shown that even after the parasites have been removed, the rodents’ lack of fear towards felines remain - indicating a permanent structural change in the brain, not the temporary effects of dopamine.
What about humans?
There have been links to strange behaviour in humans where the infected are more likely to be involved in traffic accidents (due to slower reaction times and increased impulsivity) and other studies have even shown correlation between suicide and homicide rates and rates of T. gondii infection.
A number of studies have also shown that there may be some association with T. gondii and neurological disorders (such as schizophrenia, bipolar disorder and Parkinson’s disease) as some preliminary evidence suggests that T. gondii may induce subtle behavioural and personality changes in those infected by it by altering dopamine levels. However, that is not to say that this parasite causes these disorders, as the observational studies only looked at correlation between antibodies to T. gondii (to determine if they are infected or not) and schizophrenia - so no evidence of causation was collected.
Nonetheless, many studies have shown that this parasite does indeed influence behaviour to a certain extent and even though toxoplasmosis itself is innocuous, their influence may be far more impactful than we originally thought.
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 Barford, E. (2019). Parasite makes mice lose fear of cats permanently. [online] Nature. Available at: https://www.nature.com/news/parasite-makes-mice-lose-fear-of-cats-permanently-1.13777 [Accessed 10 Jun. 2019].
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 Neuroscientifically Challenged. (2019). What's really the deal with toxoplasma gondii and human behavior?. [online] Available at: https://neuroscientificallychallenged.com/blog/toxoplasma-gondii-human-behavior [Accessed 10 Jun. 2019].
Year 12 Student at Bangkok Patana School