Toxoplasma gondii and personality

Running head: Toxoplasma Gondii Modifies Personality
Toxoplasma Gondii Modifies Personality
Cameron Aaron
Connecticut College
4/10/2020
Abstract

Toxoplasma Gondii Modifies Personality 1
Toxoplasma Gondii is a parasite that can only sexually reproduce in cats. (Dubey, J., & Jones, J.
2008) It is able to modify animal behavior and mood through a variety of means, such as
increasing the amount of Dopamine in the system through the release of the enzyme tyrosine
hydroxylase causing increased impulsivity and risk taking behaviors (Costa, R. M.,2017). It is
also able to modify genes through epigenetic means causing things like increased testosterone
causing its host to be more aggressive. Toxoplasma Gondii does all of these things in order to
make its main host, which is rats, more likely to be eaten by cats. The same things that are
modified in rats in order to help the parasite in its quest to sexually reproduce also causes
humans to have symptoms of psychosis and increased impulsivity. For example, people who are
infected are found to have 2.5 times the amount of car crashes likely due to the increase in risky
behavior (Flegr, J, 2002) and are more likely to develop disorders such as Schizophrenia and
Intermittent Rage Disorder (Emil F. Coccaro, MD , 2016). This is likely due to changes in
neurotransmitters and gene expression.
Keywords: Toxoplasmosis, Toxoplasma Gondii, DNA, Genes, Methylation
Discussion
Personality is a person's underlying way of interfacing with the world around them
(Carver, C. S., & Scheier, M.2017). Personality stays consistent across different moods and
situations but what really causes personality in psychological science are both biological and
social factors. While a person’s personality is stable for most of their life we do see certain traits
such as agreeableness, conscientiousness, and emotional resilience often change as we become
older. In this process of growing up and becoming more ready to take on the adult world around
us we become different, oftentimes better versions of ourselves. Though changes to personality

can be a good thing, sometimes when trauma is introduced in our most pivotal stages of
development we can develop personality disorders that include irrational thought processes. A
personality disorder is a psychological disorder in which the patient exhibits an unhealthy way of
thinking. Those with personality disorders often have trouble empathizing with others. This can
be worsened if the person already has underlying genetic vulnerability for personality disorders.
Personality disorders are distinguishable from mood disorders because they go beyond just a
chemical imbalance in the brain to unhealthy thoughts and irrational thinking that lie at the very
core of our being (Carver, C. S., & Scheier, M.2017). A mood disorder is a disorder in which
one's emotional state is distorted for a particular situation (Carver, C. S., & Scheier, M.2017).
Personality disorders are very hard to treat because unlike mood disorders they are
unnatural thoughts that also seem normal to the person who has them because it is part of their
personality and they are able to rationalize their thoughts. They often blame others when in
reality their actions are the ones that have caused harm. Because of this, it is very hard to treat
these types of disorders and it is very challenging for those afflicted to maintain meaningful
relationships.
While personality can change over time, it is usually a slow progression. Personality has
very important evolutionary underpinnings; for example, people who are more agreeable and
emotionally resilient are found to have more long-lasting relationships. Therefore, they are
thought to be more suitable mates. Also, due to the fact that personality is highly genetically
based, it is thought that these agreable traits persist as they are passed on to progeny in genes
through procreation.
It is also thought that certain combinations of personality traits now classified as
disorders may have at one point had a significant environmental advantage. For example,

ADHD, which is classified as a disorder in which people are very impulsive and unpredictable,
at one point may have helped us to more easily overcome livestock raids and robberies because
people could not predict their movements (Eisenberg and Campbell,2011). Additionally,
impulsivity may be a contributing factor to being novel and finding unique solutions to common
issues, such as what foods we can or cannot eat, it could have even helped drive us in finding
new lands to move our tribes in the hunter and gatherer days. The reason impulsivity is important
is that it helps drive us to do things that might not necessarily be safe or conventional, but might
reap a large reward(Eisenberg and Campbell,2011). In addition, we also have found DRD4 7R, a
gene that is commonly associated with ADHD and it’s novelty-seeking traits. In order to see how
this gene along with ADHD might have affected those in a hunter-gatherer society, African
tribes were studied. Those with ADHD and the DRD4 7R gene were found to be better nourished
and much better hunters. (Eisenberg and Campbell,2011)
Similarly, another neurological difference has been found in those with autism. Although
it can now be found to be debilitating and isolating in our hyper-social world, back when we
were still in a hunter and gatherer society these very same personality traits that cause us to seek
out a cure for this affliction actually helped us to survive. This is because those with autism
became super hunters that did not need to take so much time to socialize with others but rather
became very accomplished and fulfilled hunters because of their introversion ( Jared Edward
Reser, 2011.)
Both of these afflictions we know are genetic and deal with multiple genes and are not
rare diseases by any stretch of the imagination. We know that in both of these cases they are
strongly influenced by chemicals in the brain called neurotransmitters. Neurotransmitters are
chemicals found in the brain that regulate the human body’s actions and behaviors(Carver, C. S.,

& Scheier, M.2017). When neurotransmitters are balanced we tend to see neurotypical
behaviors; however when certain mutations known as single nucleotide polymorphisms are
found on specific genes we can see an imbalance in the neurotransmitters in the body causing
changes to mood and the increase or decrease of certain traits (Carver, C. S., & Scheier,
M.2017). For instance, low levels of dopamine are one of the contributing factors that cause the
impulsivity found in ADHD because they are constantly wanting to find something that will
boost the dopamine in there synapse such as fidgeting or daydreaming and in more extreme cases
things like risky sexual behaviors and gambling (Blum et al., 2008). Since we are able to show
that personality has a lot of biological underpinnings, I pose the question: what would happen if
we are able to manipulate gene expression to change a person's personality even as the result of
an organism opposed to a personal stress or trauma? What would the effects be? One creature
that is an expert in this area is a worm called Toxoplasma Gondii.
Toxoplasma Gondii is a single-celled parasite that travels to the brain of its host and
makes neurological behavioral changes. The parasite was discovered by Nicolle and Manceaux
in 1908 in a Gundi rodent. It can only sexually reproduce in the gut of felines. Upon
reproduction, the oocyst of Toxoplasma Gondii offspring are then excreted in the feces of cats.
Due to this unique breeding requirement, they are able to modify the behavior of the intermediate
host creature to be more likely to be consumed by cats (Wendy Marie Ingram et. al., 2013)
The worm can asexually reproduce in just about any creature it is able to successfully
infect. However, asexual reproduction only produces genetically alike offspring. This type of
reproduction is less desirable to the parasite because it does not allow it to diversify and therefore
evolve to become more adept at their environment. Failure to mutate and improve would cause
the Toxoplasma Gondii species to die out eventually and for that reason, it spends much of its

life cycle trying to do whatever it can to be as efficient as possible in finding a viable feline host
for sexual reproduction (Martorelli Di Genova et al., 2019).
The parasite furthers its quest for proliferation by altering the brains of its hosts to help
ensure that they will eventually find their homes in the guts of cats. For example, in the case of
rats, the parasite will inhibit the rat's natural instinct to run away by slowing down the reaction
and situational processing speed, causing the rat to be easy for the cat to consume. In addition,
the parasite even causes rats to love the smell of cat urine so that they will actually be attracted to
the cat.(Samojłowicz et al., 2019) This is done by lowering the methylation of the arginine
vasopressin promoter through epigenetic means, which affects the amygdala of rats, and numbs
their fear response. This dulls the fear that rats may potentially have when going into more
dangerous situations. Due to the increase in Tyrosine Hydroxylase caused by the worm, which
creates higher levels of dopamine which increase impulsivity increase in impulsivity and a less
fear in the rat, leading it to make poor decisions making it more likely to get into dangerous
situations and be eaten by cats. (Samojłowicz et al., 2019) When a rat is infected with the worm
we also see a high spike in the oxytocin (OT) and vasopressin (VP) pathways in rat’s brains
when exposed to cat urine meaning that they actually feel attachment and sexual arousal when
they smell cat urine. (Sapolsky, R. (2011)
It is interesting and quite environmentally smart that the worm takes the route of
working through the rat’s olfactory senses. Studies have shown that scents are one of the biggest
triggers of episodic memory. This is in part due to the way the brain processes smells by
bypassing the thalamus and going directly to the olfactory bulb, which is close to the
hippocampus and amygdala. As a result, the rat, due to the modifications done by the parasite to

its neurotransmitters, associates cat urine with fondness and wants to instinctively seek it out
(Wendy Marie Ingram,Leeanne M,et all,2013).
We also notice that host histones become differentially acetylated when the host gets
infected, contributing to changes in host cell gene expression (Angel, S. O. 2012,). If rats are
given hypermethylation in the medial amygdala to combat the lowering of the methylation
caused by the rat we see that the fear response returns, therefore, we can conclude that this is the
mechanism of action caused by the worm when lessening the fear response in rats (Angel, S. O.
2012,). It is important to understand that since rats can be used as animal models it is often easier
to experiment on and observe them. Similar experimentation on humans is sometimes impossible
due to much stricter regulations, however when humans are infected with Toxoplasma Gondii,
similar DNA methylation is seen, contributing to their lessened fear and hyper impulsivity (Hari
Dass & Vyas, 2014). There are several disorders associated with humans being infected with
Toxoplasmosis. Additionally, in correlational studies, there have been high correlations between
infected hosts and risks of suicide, Schizophrenia, and according to the evolutionary biologist
Jaroslav Flegr, people with Toxoplasmosis are 2.5 times more likely to get into a car accident
(Flegr, J, 2002). According to MRI data, one physical sign of this disease is visibly reduced
white matter in the brain (Harrison, W. T., & Hulette, C. 2017). This is also very interesting
because it also helps bolster the theory first suggested by correlational data that Toxoplasma
gondii contributes greatly to mental illness severity.
Once the cat eats the rat, the Toxoplasma Gondii oocysts attach to white blood cells
which the parasite causes to be mobile and are carried to the brain. In the brain, the oocysts
create cysts that contain the worm inside of them and produce an enzyme called Tyrosine
hydroxylase, which leads to a larger production of dopamine, the neurotransmitter associated

with reward (Costa, R. M.,2017) In a published scientific paper in, Nature Scientific Report,
scientists found potential links between Toxoplasma Gondii and a range of severe diseases, such
as some Schizophrenia, cancers, Parkinson’s, and epilepsy. The research was done by 32
researchers across 16 institutions from around the world (Huân M. Ngô, 2017). The study found
key genetic vulnerabilities in the structure of our neurological systems that allow for Toxoplasma
Gondii to potentially alter our brains in a way that would not only allow for these diseases to
come about but also become worsened by an infection of Toxoplasmosis (Huân M. Ngô, 2017.).
They found these vulnerabilities by finding the genes that Toxoplasmosis expresses in the human
brain through epigenetic means. Connections were found between those genes and other
neurological diseases, showing the potential for activation of genes related to diseases like
Schizophrenia and a host of other neurodegenerative and neuropsychiatric disorders (Huân M.
Ngô, 2017).
Toxoplasma Gondii has an effect on several diseases and behaviors. Interestingly,
Intermittent Explosive Disorder (IED), Risk-Taking Behavior, Schizophrenia, and certain
Personality Disorders are all affected negatively by the parasite. (Kavoussi, R. J. 2009,) Studies
have found that - men who had the parasite tend to be more introverted, neurotic, have lower
social intelligence, and to have more antisocial deviant behavior. Infected women, presented in
exactly the opposite way, they were more extraverted, trusting, self-aware, and followed the
rules more than uninfected women. (Lindová, J., Novotná, 2006) This can be partially explained
by the fact that higher basal testosterone levels increase the levels of aggressive behaviors in
men, while leading to substantially higher levels of self-directed personality traits in women
(Kazuhiro Tajima-Pozo, 2015).

There have been new studies conducted by the University of Chicago that suggest there is
a link between Toxoplasmosis and Intermittent Explosive Disorder (IED) (Emil F. Coccaro, MD
, 2016) which is a disorder characterized by overly intense rage for the stimuli. IED is found in
the section of the DSM-5, "Disruptive, Impulse-Control, and Conduct Disorders,". In a study
conducted on a sample of 385 test subjects that had Intermittent Explosive Disorder, they found
that while only 9% of subjects without IED had Toxoplasma Gondii, 22% of people diagnosed
with IED had Toxoplasmosis. (Emil F. Coccaro, MD , 2016) This study is a follow up on the
research done by Emil Coccaro, a professor of psychiatry at the University of Chicago who has
been studying IED since the early 90s and has been looking for some type of biological
connection to why people with IED experience such sudden and unexplainable rage for
seemingly no reason. (Emil F. Coccaro, MD , 2016) To be involved in her study, subjects had to
display aggression that was far beyond the normal response to stimuli. (Emil F. Coccaro, MD ,
2016) This response was assessed using structured interviews conducted by research
psychiatrists using DSM 5 Personality Interview Questions and the DSM Axis 1 Disorder
Interview Questions. Those with IED can be acting perfectly rational and fine until something
happens and they blow up in a fit of rage.
Dr. Coccaro wrote that IED appears to be a disorder that has a high genetic predisposition
and usually appears to manifest itself in the early teens (Emil F. Coccaro, MD , 2016).
Toxoplasma Gondii has been shown to increase levels of Testosterone leading to heightened
levels of aggression. We have noticed that heightened levels of testosterone is also a contributing
factor to higher levels of aggression in humans. (Book, A. S., Starzyk, K. B., & Quinsey, V. L.
(2001, November 21)) This heightened level of aggression caused by Toxoplasma Gondii can

only have the effect of worsening the manifestations of the disorder in those who suffer from
IED. (Emil F. Coccaro, MD , 2016)
Heightened levels of testosterone are also found in diseases such as conduct disorders
(Bukstein, O. G.2012, 2012). If it further increases testosterone in these patients, the severity of
their disorder would also increase (Bukstein, O. G.2012, 2012).
There is also currently research being conducted to look at the correlation between a
woman with borderline personality disorder and Toxoplasma gondii. In studies, it has been found
that women with borderline personality disorder are more likely to have an infection of
toxoplasmosis (Ociskova, M. 2019). This would explain some of the symptoms of affective
dysregulation, aggression, suicidality, or anxiousness that are included as symptoms of both
borderline personality disorder and an infection of Toxoplasmosis. Borderline personality
disorder is a personality disorder characterized by a pattern of disordered thinking, strong
emotional reactions caused in part by highly neurotic symptoms, and a distorted sense of self.
Those with borderline personality disorder tend to have a very hard time maintaining healthy and
close relationships due to these disordered thoughts and inappropriate interactions (NIMH,
Borderline Personality Disorder). Unlike mood disorders, such as depression, that may lead to
momentary shifts in mood altered thought processes, a personality disorder like borderline
personality disorder is caused by unhealthy thinking at the core of a person’s personality because
this is deeply embedded into who they are and the way they interface and perceive the world
around them (Carver, C. S., & Scheier, M.2017). A person with this disorder can rationalize
their thinking and behavior because it seems natural even when it is not. This makes treatment
very difficult because it is very hard to change a person’s personality especially when they are
unable to recognize that their behaviors are unhealthy. It is only through very intensive therapy

such as Dialectical Behavioral Therapy that they are able to get better and start to be able to have
close relationships with others in a healthy way (Carver, C. S., & Scheier, M.2017).
. Schizophrenia is a neurological disorder characterized by both positive and negative
symptoms (Picchioni & Murray, 2007). Negative symptoms such as motivation, expression of
emotion, and trouble with attention are reduced or suppressed in those with Schizophrenia, while
positive symptoms that are not present in neurotypical people, such as hallucinations, delusions,
and disordered speech are present (Picchioni & Murray, 2007). Schizophrenia is very strongly
connected to genetics and can often be activated by certain environmental factors, such as things
like doing drugs or being in stressful environments (Picchioni & Murray, 2007). Patient
diagnosis is usually in the late teens and early adult years and is usually diagnosed after a bout of
psychosis which includes symptoms of hallucination, talking incoherently and being delusional
(Picchioni & Murray, 2007). Schizophrenia is very strongly connected to genetics and oftentimes
can be activated by certain environmental factors, such as things like doing drugs or being in
stressful environments. In this disease, there is a diathesis. This means that factors such as
environmental stressors, genetic factors, and a person’s inherent resilience to stress determine
whether or not they will actually end up developing the disease. Children with parents who have
schizophrenia are ten percent more likely to develop schizophrenia.
Schizophrenia is very stigmatized. It is portrayed in the media as having people with
many personalities and is often fictitiously associated with psychopaths. As a result,
schizophrenics are often the victims of crimes and people with it do not always feel that they can
seek help or tell people about their mental illness. Although Schizophrenia's cause is not fully
known, it is believed that it has to do with the dysregulation of dopamine in the human brain and
because of this, the antipsychotic medication that is used to block dopamine receptors seems to

help with with the positive symptoms of schizophrenia, however, they seem to sometimes make
the negative symptoms worse.
We are slowly finding that more than one neurotransmission system contributes to the
symptomatology of schizophrenia (McCutcheon, R.,2020). One enzyme we see that is increased
is the neurochemical tyrosine hydroxylase, which increases dopamine and is thought to
contribute to the abnormalities in the dopaminergic transmission in schizophrenia. Tyrosine
hydroxylase is a chemical that is also modified by the parasite Toxoplasma Gondii Costa, R.
M.,2017. Toxoplasma Gondii increases Tyrosine Hydroxylase as a means of controlling their
prey. Tyrosine Hydroxylase causes a change in dopamine receptors Costa, R. M.,2017. This
change increases dopamine, causing rodents to become sexually attracted to their urine and also
suppressed their fear response. When humans are infected by the parasite, Toxoplasma Gondii,
the change in the dopamine receptors caused by the increase in tyrosine hydroxylase is
oftentimes enough to cause initial dysregulation of the dopamine system, causing a person with
underlying vulnerability for schizophrenia to get the disorder. Increased dopamine has also been
associated with psychosis Costa, R. M.,2017.
In a controlled study, it was found that individuals with schizophrenia are three times
more likely to have antibodies in their blood to the Toxoplasma Protozoan. Dalimi, (Dalimi, A.,
& Abdoli, A. 2012) This shows that those who have schizophrenia are more likely to have
Toxoplasmosis than the population that do not have the disorder (Dalimi, A., & Abdoli, A. 2012)
.
Conclusion
Toxoplasma Gondii is an incredible and highly interesting parasite. The way that it is
able to manipulate host cell genetics in order to benefit itself and affect multiple types of brains

in different ways is absolutely incredible. The study of Toxoplasma Gondii’s ability to activate
genes epigenetically (Sullivan, W. J. 2010) could bring great advancements in the study of
Neuroscience and Genetics. I am very excited to see what kind of future advancements will come
from learning more about how they are able to manipulate behavior on a cellular level. Perhaps
even one day, we will be lucky enough to synthesize the process and be able to modify animal
behavior in an easier way or be able to turn off genes in our own body that are responsible for
diseases such as cancer. It has always been fascinating how amazing the creatures on this planet
have evolved in a way that allows them to alter biological systems. It is my hope that like
CRISPER was discovered from bacteria we will be able to learn more from other creatures, such
as Toxoplasma Gondii, and utilize their techniques to learn more about our genome and cure
illness. I believe that this is very interesting in showing just how much of our behavior has
underlying mechanisms of neurobiology that drive us. This worm is able to so precisely modify a
rats neurobiology that it is able to take away the fear of its natural predator. Every living creature
instinctually avoids predators in order to prolong their lives as a means of procreation. It is
deeply ingrained in all animals to stay away from predators. Amazingly, Toxoplasma Gondii has
the ability to change the brains, not only diminishing its fear of predators , but causing them to be
attracted to cat urine and their predators through the dopamine-producing enzyme tyrosine
hydroxylase. In humans we are able to see that those very same modifications made to rats brains
for the case of increasing the survival of these parasites increase the chances of the development
and proliferation of mental illness, for example, the fact that it modifies dopamine in both rats
and humans is oftentimes enough to cause the onset of psychosis in genetically vulnerable
individuals due to the nature of schizophrenia being caused by dysregulation in the dopamine
system. In addition, because of the way that toxoplasma gondii causes genes to be modified due

to epigenetic means it is able to cause people who otherwise did not have genetic vulnerability to
gain mental illnesses such as psychosis and compulsive behaviors. By knowing and studying the
way that these parasites work, I believe we will be able to learn more about the biological
mechanisms of personality based behaviors.
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