ArticlePDF Available

Influence of latent Toxoplasma infection on human personality, physiology and morphology: Pros and cons of the Toxoplasma-human model in studying the manipulation hypothesis

Authors:

Abstract and Figures

The parasitic protozoan Toxoplasma gondii infects about one-third of the population of developed countries. The life-long presence of dormant stages of this parasite in the brain and muscular tissues of infected humans is usually considered asymptomatic from the clinical point of view. In the past 20 years, research performed mostly on military personnel, university students, pregnant women and blood donors has shown that this 'asymptomatic' disease has a large influence on various aspects of human life. Toxoplasma-infected subjects differ from uninfected controls in the personality profile estimated with two versions of Cattell's 16PF, Cloninger's TCI and Big Five questionnaires. Most of these differences increase with the length of time since the onset of infection, suggesting that Toxoplasma influences human personality rather than human personality influencing the probability of infection. Toxoplasmosis increases the reaction time of infected subjects, which can explain the increased probability of traffic accidents in infected subjects reported in three retrospective and one very large prospective case-control study. Latent toxoplasmosis is associated with immunosuppression, which might explain the increased probability of giving birth to a boy in Toxoplasma-infected women and also the extremely high prevalence of toxoplasmosis in mothers of children with Down syndrome. Toxoplasma-infected male students are about 3 cm taller than Toxoplasma-free subjects and their faces are rated by women as more masculine and dominant. These differences may be caused by an increased concentration of testosterone. Toxoplasma also appears to be involved in the initiation of more severe forms of schizophrenia. At least 40 studies confirmed an increased prevalence of toxoplasmosis among schizophrenic patients. Toxoplasma-infected schizophrenic patients differ from Toxoplasma-free schizophrenic patients by brain anatomy and by a higher intensity of the positive symptoms of the disease. Finally, five independent studies performed in blood donors, pregnant women and military personnel showed that RhD blood group positivity, especially in RhD heterozygotes, protects infected subjects against various effects of latent toxoplasmosis, such as the prolongation of reaction times, an increased risk of traffic accidents and excessive pregnancy weight gain. The modern human is not a natural host of Toxoplasma. Therefore, it can only be speculated which of the observed effects of latent toxoplasmosis are the result of the manipulation activity of the Toxoplasma aimed to increase the probability of its transmission from a natural intermediate to the definitive host by predation, and which are just side effects of chronic infection.
Content may be subject to copyright.
127
Toxoplasma as the model organism for studying the parasite
manipulation hypothesis
Toxoplasma gondii is a parasitic protozoan whose effects on human
behaviour, personality and other phenotypic traits have been
studied most thoroughly, often in the context of the manipulation
theory, the theory suggesting that many parasites change the
phenotype of their host to increase their chances of transmission to
a new host by, for example, predation. There are various reasons
why this particular protozoon has become a favoured model for
evolutionary parasitologists, biologists and also psychiatrists.
First of all, Toxoplasma is a very common parasite both in
developed and developing countries, and some forms of diseases
caused by Toxoplasma infection have very serious impacts on
human health; taken together, all forms of toxoplasmosis are a
serious socio-economic burdens throughout the world (Pappas et
al., 2009; Torgerson and Macpherson, 2011). It is also important
to note that the study of the influence of toxoplasmosis on the
behaviour of laboratory animals has a very long tradition; this
includes a series of about 20 studies that started in the laboratory
of William M. Hudtchison in the early 1980s, followed by studies
by Joanne P. Webster and Manuel Berdoy in the 1990s, which were
succeeded by several other teams (for reviews, see Skallová et al.,
2006; Webster, 2007; Webster and McConkey, 2010).
Toxoplasma is an excellent model for studying the manipulation
hypothesis as it is trophically transmitted from an intermediate to a
definitive host by predation. In contrast to behavioural patterns
induced by directly or, more commonly, sexually transmitted
parasites, the behavioural patterns induced by a trophically
transmitted parasite are relatively easy to recognize (Parker et al.,
2009). For example, in a sexually transmitted parasite, the parasite’s
and the host’s genes have similar interests: they both ‘try’ to program
the host to increase the probability of host reproduction. In contrast,
the interests of the host and its trophically transmitted parasite
radically differ. The intermediate host, e.g. the mouse, needs to
survive (and reproduce) for as long as possible while the parasite,
e.g. Toxoplasma, ‘wants’ the definitive host (here, a cat) to kill and
SUMMARY
The parasitic protozoan Toxoplasma gondii infects about one-third of the population of developed countries. The life-long
presence of dormant stages of this parasite in the brain and muscular tissues of infected humans is usually considered
asymptomatic from the clinical point of view. In the past 20years, research performed mostly on military personnel, university
students, pregnant women and blood donors has shown that this ʻasymptomaticʼ disease has a large influence on various
aspects of human life. Toxoplasma-infected subjects differ from uninfected controls in the personality profile estimated with two
versions of Cattellʼs 16PF, Cloningerʼs TCI and Big Five questionnaires. Most of these differences increase with the length of time
since the onset of infection, suggesting that Toxoplasma influences human personality rather than human personality influencing
the probability of infection. Toxoplasmosis increases the reaction time of infected subjects, which can explain the increased
probability of traffic accidents in infected subjects reported in three retrospective and one very large prospective case-control
study. Latent toxoplasmosis is associated with immunosuppression, which might explain the increased probability of giving birth
to a boy in Toxoplasma-infected women and also the extremely high prevalence of toxoplasmosis in mothers of children with
Down syndrome. Toxoplasma-infected male students are about 3cm taller than Toxoplasma-free subjects and their faces are
rated by women as more masculine and dominant. These differences may be caused by an increased concentration of
testosterone. Toxoplasma also appears to be involved in the initiation of more severe forms of schizophrenia. At least 40 studies
confirmed an increased prevalence of toxoplasmosis among schizophrenic patients. Toxoplasma-infected schizophrenic patients
differ from Toxoplasma-free schizophrenic patients by brain anatomy and by a higher intensity of the positive symptoms of the
disease. Finally, five independent studies performed in blood donors, pregnant women and military personnel showed that RhD
blood group positivity, especially in RhD heterozygotes, protects infected subjects against various effects of latent
toxoplasmosis, such as the prolongation of reaction times, an increased risk of traffic accidents and excessive pregnancy weight
gain. The modern human is not a natural host of Toxoplasma. Therefore, it can only be speculated which of the observed effects
of latent toxoplasmosis are the result of the manipulation activity of the Toxoplasma aimed to increase the probability of its
transmission from a natural intermediate to the definitive host by predation, and which are just side effects of chronic infection.
Key words: behaviour, parasite, polymorphism, Rhesus factor, personality questionnaire, toxoplasmosis.
Received 10 April 2012; Accepted 26 August 2012
The Journal of Experimental Biology 216, 127-133
© 2013. Published by The Company of Biologists Ltd
doi:10.1242/jeb.073635
REVIEW
Influence of latent Toxoplasma infection on human personality, physiology and
morphology: pros and cons of the Toxoplasma–human model in studying the
manipulation hypothesis
Jaroslav Flegr
Faculty of Science, Charles University, Department of Philosophy and History of Science, Prague, Czech Republic
flegr@cesnet.cz
THE JOURNAL OF EXPERIMENTAL BIOLOGY
128
eat the infected intermediate host. Some toxoplasmosis-associated
behavioural changes, e.g. the prolongation of reaction times in the
infected hosts (Hrdá et al., 2000), are rather simple and therefore
difficult to recognize from the side effects of the parasitic disease.
Other changes, however, are relatively complex and specific, e.g. the
fatal attraction phenomenon, i.e. the loss of the fear response to cat
odour (and not, for example, to dog odour) in infected rodents
(Berdoy et al., 2000). The existence of such complex behavioural
patterns suggests that the observed toxoplasmosis-associated changes
are the products of the parasite’s manipulative activity rather than
side effects of the acute toxoplasmosis (Poulin, 1995). This is further
supported by the fact that the intensity of some of the observed
behavioural changes increases with the length of time since the onset
of infection (Flegr et al., 1996; Havlíček et al., 2001). If the observed
behavioural patterns were merely the side effects of the acute form
of infection, their intensity would decrease over time from the
moment of the infection.
The probable role of Toxoplasma gondii in the origin and progress
of some important psychiatric diseases, including schizophrenia, is
another reason why this protozoan has become the most important
model for studying the influence of a parasite on human behaviour.
Schizophrenia afflicts about 0.5–1% of the population in all countries
worldwide and its health and socioeconomic impacts are
extraordinary (Saha et al., 2005). Since the 1950s it has been known
that the prevalence of toxoplasmosis in schizophrenic patients is
unusually high (Torrey et al., 2007). This systematic research was
initiated and developed by Edwin F. Torrey from the Stanley
Research Institute and Robert H. Yolken from the Johns Hopkins
University, who showed that the connection between schizophrenia
and toxoplasmosis is very strong and that the Toxoplasma infection
is most probably a very significant (but not exclusive) cause of
schizophrenia (Torrey and Yolken, 1995; Torrey and Yolken, 2005).
The effect of latent toxoplasmosis on the risk of schizophrenia is
stronger than that of any schizophrenia-associated gene variant
identified in genome-wide analyses (The International Schizophrenia
Consortium, 2009). A prospective study performed on personnel of
the American Army revealed that specific anti-Toxoplasma
antibodies show up in the serum of subjects before they contract
schizophrenia (Niebuhr et al., 2007). It was shown later that
toxoplasmosis increases the concentration of dopamine in the brain
of infected hosts, including humans (Flegr et al., 2003), and its
genome even contains unique genes for enzymes (tyrosine
hydroxylases) that play an important role in the synthesis of
dopamine (Gaskell et al., 2009). The increased concentration of
dopamine in certain regions of the brain is believed to play a key role
in the origin and progress of schizophrenia and the inhibition of
dopamine receptors is the basis of the function of all modern drugs
used in the treatment of schizophrenia (Tandon et al., 2010). Other
studies have shown that the symptom profiles of Toxoplasma-
infected and Toxoplasma-free schizophrenia patients differ and the
positive symptoms of the disease (hallucinations, delusions) are more
severe in Toxoplasma-infected patients (Wang et al., 2006). Modern
imaging techniques revealed that the morphology of the brain of
schizophrenics differs from that of controls by having a lower density
of grey matter (GM) in certain parts of the brain (Shenton et al.,
2001). A magnetic resonance imaging (MRI) study published in 2011
showed that these differences (reduction of GM volume bilaterally
in the caudate, median cingulate, thalamus and occipital cortex, and
in the left cerebellar hemisphere) are observed only in Toxoplasma-
infected patients while Toxoplasma-free patients (as well as
Toxoplasma-infected controls) have the same brain morphology as
Toxoplasma-free controls (Horacek et al., 2012). This observation
suggests that toxoplasmosis can induce morphological changes in the
brain of genetically predisposed subjects, which, possibly together
with a toxoplasmosis-associated imbalance in the levels of dopamine
and other neurotransmitters, e.g. serotonin (Henriquez et al., 2009)
or nitric oxide (NO) (Kaňková et al., 2010a), can result in
schizophrenia.
Changes in the personality profile of humans with latent
toxoplasmosis
The personality profile of Toxoplasma-infected subjects was
studied using three standard psychological questionnaires, i.e.
Cattell’s 16PF, Cloninger’s TCI, NEO-PI-R (Big Five), and one
special psychological questionnaire, Toxo94, that searched for
specific changes expected to occur in subjects infected by the
parasite transmitted from prey to predator (Flegr, 2007). Several
studies have shown that infected men exhibited lower scores on
Cattell’s factor G – superego strength (they have tendency to
disregard rules) and higher scores on Cattell’s factor L – protension
(they are more suspicious and jealous). In women, the shift in these
two factors is opposite to that of men; they mainly show a positive
shift in Cattell’s factor A – affectothymia (they are more warm-
hearted, outgoing and easy-going than the more reserved, detached
and critical Toxoplasma-free women). With a new version of
Cattell’s questionnaire (v. 5), the infected men showed increased,
rather than decreased, scores on superego strength [(Flegr, 2010b)
for an explanation of discordant results between studies, see chapter
4]. Cloninger’s TCI showed that infected subjects, both men and
women, have decreased scores on factor NS – novelty seeking, i.e.
a lower tendency to search for new stimuli (Flegr et al., 2003;
Skallová et al., 2005). Ethopharmacological studies have shown
that lower novelty seeking scores are characteristic for individuals
with an increased concentration of dopamine in the brain tissue,
which is in an agreement with the increased synthesis of dopamine
in tissue cysts of Toxoplasma found in the brain of infected hosts
and with results of ethopharmacological studies performed with
Toxoplasma-infected mice (Hodková et al., 2007; Skallová et al.,
2005). Some studies also suggest that infected subjects have higher
scores on Cloninger’s ST – self-transcendence (Novotná et al.,
2005; Skallová et al., 2005). The NEO-PI-R questionnaire showed
more extraversion in infected subjects, both men and women, and
less conscientiousness in comparison with Toxoplasma-free
subjects (Lindová et al., 2012).
On the basis of predictions of the manipulation theory and
introspection of the Toxoplasma-infected author, a special
questionnaire called Toxo94 was constructed (Flegr, 2010b). This
questionnaire consisting of only 10 questions was distributed to
several groups of subjects tested for toxoplasmosis, such as two
large groups of university students and a group of women screened
for toxoplasmosis during pregnancy (Flegr, 2010b). The results
showed that infected men more often reported that diplomacy is not
their strong point, that their instinctive (reflex) behaviour under
imminent danger is rather slow and passive, that they believe that
some people have the power to impose their will on others with
hypnosis or through other means and that when they are attacked,
physically or otherwise, or when they should fight for something
important, they stop fighting at a certain moment because their own
subconsciousness betrays them and they loss the will to fight back.
The infected women more often report that diplomacy is not their
strong point, that their instinctive (reflex) behaviour under
imminent danger is rather slow and passive, that they believe that
some people have the power to impose their will on others with
hypnosis or otherwise and that they have a weak instinct for self-
The Journal of Experimental Biology 216 (1)
THE JOURNAL OF EXPERIMENTAL BIOLOGY
129Toxoplasmosis and human behaviour
preservation: in situations where somebody else might be afraid,
for example being alone in a forest at night or in an empty house,
they remain calm.
Influence of latent toxoplasmosis on human behaviour
Toxoplasma-infected subjects have prolonged reaction times, as
measured by a test of simple reaction times (Havlíček et al., 2001).
The psychomotor performance gets worse with the level of
development of the infection (estimated on the basis of a decrease
in the concentration of specific anti-Toxoplasma antibodies). The
performance of the subjects in the 3min simple reaction time test
suggests that toxoplasmosis impairs long-term concentration ability
rather than maximum performance. The largest performance
decrease in the test occurred in RhD negative subjects while the
performance of RhD-positive heterozygotes was not influenced by
the infection (Flegr et al., 2010; Novotná et al., 2008). The impaired
psychomotor performance of infected subjects can explain the
higher risk of traffic accidents and work accidents observed in four
retrospective studies (Alvarado-Esquivel et al., 2012; Flegr et al.,
2002; Kocazeybek et al., 2009; Yereli et al., 2006) and one
prospective study (Flegr et al., 2009). The risk of traffic accident
is again increased in RhD-negative drivers (Flegr et al., 2009). A
double-blind observational study showed that Toxoplasma-infected
men scored lower in clothes tidiness than uninfected men, whereas
infected women scored higher (but not significantly so) than
uninfected women (Lindová et al., 2006). Similarly, infected men
scored lower and infected women scored higher in sociability.
These outcomes match the results of the personality questionnaires.
The infected rural male students scored higher in suspiciousness
while infected rural female students scored lower in suspiciousness
than their non-infected peers (Lindová et al., 2006), which again
agrees with the results obtained with Cattell’s 16PF questionnaire.
However, the very opposite was true for students of urban origin –
infected male students showed lower and infected female students
higher suspiciousness than their Toxoplasma-free peers. Using the
method of experimental games, it was shown that both infected men
and infected women were less altruistic than Toxoplasma-free
subjects in the Dictator game while in the Trust game, the infected
men were less altruistic and infected women were more altruistic
than Toxoplasma-free men or women (Lindová et al., 2010).
Influence of Toxoplasma on the human phenotype
In addition to its influence on personality profile and behaviour,
Toxoplasma is known to affect other phenotypic traits in humans.
For example, infected male university students (age 19–22years)
have increased concentrations of testosterone (Flegr et al., 2008a;
Flegr et al., 2008b) and, from photographs, their faces are rated as
more masculine and dominant by females (Hodková et al., 2007).
In contrast, infected female students have decreased levels of
testosterone – which corresponds to decreased levels of testosterone
in infected male and female mice (Kaňková et al., 2011). Infected
male students are 3cm taller than non-infected male students and
both male and female students have a lower index finger to ring
finger ratio (Flegr et al., 2005b), which is considered as an
indication of being exposed to higher concentrations of testosterone
during pregnancy (Manning, 2002). The increased concentration of
testosterone was also recently reported in Toxoplasma-infected
men, women (Shirbazou et al., 2011) and rats (Vyas, 2013). It
should be noted, however, that recent studies performed on two
independent populations did not find increased levels of
testosterone in infected male soldiers and immunology clinic
patients (see Table1). An alternative explanation for the observed
increase in the level of testosterone in males (and expected increase
in the level of oestrogen) was suggested by James (James, 2010).
He proposed that high testosterone and high oestrogen individuals
are more susceptible to any infection, including the Toxoplasma
infection. This model can explain the increased concentration of
testosterone in men; however, it cannot explain the increased
concentration of testosterone in laboratory-infected rodents.
The infected students differ from non-infected students in
various morphological traits; however, at least some of the
observed differences could be caused by differences between the
populations of students coming from towns and from villages
(where the prevalence of latent toxoplasmosis is much higher than
Table1. Concentration of steroid hormones in Toxoplasma-infected and Toxoplasma-free subjects
Men Women
Toxoplasma free Toxoplasma infected Toxoplasma free Toxoplasma infected
NMean s.d. NMean s.d. tPNMean S.D. NMean s.d. tP
Immunology clinic patients
Testosterone 26 15.16 5.99 12 14.40 4.11 –0.029 0.796 128 1.21 0.61 65 1.37 0.79 0.064 0.186
Cortisol 26 200.90 82.60 13 204.90 74.15 0.040 0.717 129 226.10 125.40 65 226.80 125.40 –0.021 0.661
Estradiol 26 0.0986 0.0417 13 0.1013 0.0309 0.066 0.556 129 0.2049 0.2537 65 0.2985 0.3811 0.066 0.169
Military personel
Testosterone 50 15.87 4.55 50 15.19 3.04 –0.032 0.526 46 1.49 0.53 47 1.52 657.40 –0.029 0.682
Cortisol 50 542.50 122.30 50 466.90 108.30 –0.269 0.000 46 712.80 365.80 47 741.90 342.70 0.039 0.578
Estradiol 50 0.1146 0.0271 50 0.1123 30.70 –0.091 0.178 46 0.2883 0.3457 47 0.3009 0.3416 0.028 0.687
Students 2003–2006
Testosterone 1 68 0.393 0.234 22 0.496 0.251 0.161 0.026 143 0.272 0.274 29 0.252 0.400 –0.282 0.007
Testosterone 2 68 0.349 0.242 21 0.428 0.219 0.152 0.035 133 0.272 0.248 28 0.230 0.379 –0.142 0.003
Students 2007–2010
Testosterone 1 64 0.776 0.845 9 0.840 0.707 0.116 0.148 148 0.361 0.505 22 0.738 1.165 –0.186 0.000
Testosterone 2 63 0.561 0.645 8 0.489 0.222 0.060 0.461 155 0.186 0.136 24 0.176 0.115 –0.241 0.000
Cortisol 1 67 4.820 3.538 9 5.003 4.306 0.013 0.868 148 5.456 3.322 25 4.177 3.983 –0.137 0.007
Cortisol 2 67 3.049 1.630 9 2.040 1.236 –0.197 0.013 149 2.929 1.586 24 3.118 2.434 0.030 0.559
The concentration (nmoll–1) of total hormones was determined in serum of immunology patients and military personal, and that of free hormones was
determined in saliva of university students. Two samples were collected, the first at 09:00h (before ethological and psychological testing of students) and the
second at 11:30h. Statistical significance of the effect of toxoplasmosis on hormone concentration was estimated with a partial Kendall test, with age of
subjects as covariate. Significant results are shown in bold. (J.F., Š. Kaňková, M. Bičíková and J. Klose, unpublished.)
THE JOURNAL OF EXPERIMENTAL BIOLOGY
130
in Prague) (Kodym et al., 2000). Infected pregnant women have an
increased probability of giving birth to a boy; the shift in the sex
ratio is especially high in women with relatively recent latent
infection. The women with high levels of anti-Toxoplasma IgG
antibodies (but with low levels of IgM antibodies) gave birth to 250
boys per 100 girls while the women with low levels of anti-
Toxoplasma IgG antibodies gave birth to more girls than boys
(Kaňková et al., 2007b). The same effects have been confirmed in
mice infected with Toxoplasma in the laboratory (Kaňková et al.,
2007a). Pregnant women with toxoplasmosis have increased
weight gain: in the subpopulation of RhD-negative Toxoplasma-
infected women, the weight gain was nearly twice as high in the
16th week of pregnancy as in other pregnant women (Kaňková et
al., 2010b). The rate of early fetal development is lower and the
length of pregnancy is about 1.5days longer in Toxoplasma-
infected than in non-infected mothers (Flegr et al., 2005a; Kaňková
et al., 2010b). The children of Toxoplasma-infected mothers have
lower rates of motor development in the first 18months of life
(Kaňková et al., 2012). Most differences in the reproduction-
associated traits between infected and non-infected women can be
explained as being a result of immunosuppression and the resulting
(expected) decrease in the stringency of embryo quality control
(Neuhäuser and Krackow, 2007), which has been observed in both
humans (Flegr and Stříž, 2011) and mice (Kaňková et al., 2010a)
with latent toxoplasmosis. A large proportion of embryos with
various developmental defects, as well as a large percentage of
more immunogenic male embryos, are aborted in the early weeks
of pregnancy. In immunosuppressed Toxoplasma-infected women,
a fraction of such embryos are saved. This phenomenon can explain
not only the decreased rates of prenatal and postnatal development
of children of infected mothers but also the increased sex ratio in
their offspring. The lower stringency of embryo quality control can
also explain the observation published in the early 1960s of a
dramatically higher prevalence of toxoplasmosis in mothers of
children with Down syndrome, 84% versus 32% in controls
(Hostomská et al., 1957).
An endocrine hypothesis for the increased sex ratio of recently
infected women and decreased sex ratio of women infected for a long
time – namely, originally (before the infection) higher oestrogen and
testosterone levels in Toxoplasma infection-sensitive subjects and a
reduced concentration of these hormones as consequence of long-
term infection – has also been suggested (James, 2008; James, 2010).
The increased sex ratio of recently infected women can also be
explained by Catalano’s stress hypothesis, i.e. selective abortion of
male embryos of chronically stressed women (Catalano et al., 2012).
It should be noted that the immunological and the endocrine or stress
hypotheses are compatible as the increased level of steroids is known
to impair the function of the immune system.
An analogous effect to the fatal attraction phenomenon (Berdoy
et al., 2000; Kannan et al., 2010; Webster and McConkey, 2010)
was observed in Toxoplasma-infected humans. Infected men rated
the smell of cat urine as relatively more pleasant while infected
women rated it as relatively less pleasant compared with non-
infected controls (Flegr et al., 2011). Using urine from four other
animal species (tiger, dog, horse, brown hyena), a similar but
weaker effect was observed for hyena urine. Like the cat, the hyena
is a member of the Feliformia suborder; however, it is not known
whether any representatives of this superfamily other than cats
(family Felinidae) can be definitive hosts of Toxoplasma. The fatal
attraction phenomenon was not observed with tiger urine. This is
rather surprising because large cats are definitive hosts of
Toxoplasma, and monkeys and apes are a regular component of
their prey. It may be that the difference in the effects of the smell
of cat and tiger urine on human behaviour is due to the fact that the
important pheromone felinine is present in the urine of small cats
(Felinae subfamily) but absent in the urine of large cats
(Pantherinae subfamily) (Hendriks et al., 1995). It is, however,
possible that chance strongly influences which of the urine samples
is active in the fatal attraction test. In our study (Flegr et al., 2011),
samples of five individuals of each species were used in smell-
rating experiments. However, the relative attractiveness of
particular samples can still depend on the sample concentration and
the time elapsed from sample collection. It has been observed that
the effect of toxoplasmosis on olfactory preference follows an
inverted-Ufunction – the effect on mice is not observed when using
either a high or a very low amount of cat urine (Vyas et al., 2007).
Therefore, the results of odour studies partly depend on the dilution
of the samples tested. In this context, interesting side results were
obtained in one of our evolutionary psychology studies run in
parallel with the fatal attraction study. We found that the smell of
urine of men and of women in the fertile phase of the menstrual
cycle was relatively more pleasant for Toxoplasma-infected male
raters (Fig.1). No significant effect of toxoplasmosis was observed
with urine of women in infertile phases of the menstrual cycle. It
is possible that the smell of strange male urine might signal a
potential danger, which is not avoided to the same extent by
infected men – as has been suggested in a similar context by the
stress-coping hypothesis (Lindová et al., 2010).
Advantages and disadvantages of the Toxoplasma–human
model for studying the manipulation hypothesis
The greatest advantage of the Toxoplasma-human model for
studying the manipulation hypothesis is the convenience of
obtaining empirical data. Practically any clinical, ethological,
anthropological or psychological study could be supplemented with
testing the experimental subject for the presence of anamnestic anti-
Toxoplasma antibodies and with the comparison of the data from
Toxoplasma-infected and Toxoplasma-free subjects. Moreover, all
pregnant women are being screened for toxoplasmosis in some
countries. Here, we could just ask the women tested to provide
informed consent for the use of their clinical data or to complete a
special, e.g. psychological, questionnaire.
The human is a long-living animal, especially in contrast with
laboratory rodents. This is another very important advantage in
manipulation hypothesis studies (but see Webster et al., 2013). Acute
toxoplasmosis is usually only a mild disease in humans, a short event
in a long human life. The life-long latent toxoplasmosis is mostly
considered asymptomatic from the clinical point of view. Therefore,
there is little risk of mistaking manifestations of Toxoplasma’s
manipulative activity for side effects of the parasitic disease suffered.
The possible side effects of acute infection can be identified by
searching for a positive or negative correlation between the time
elapsed from the infection (which can be derived from the patient
medical records or estimated from the concentration of anamnestic
antibodies) and the intensity of the observed Toxoplasma-associated
phenotypic changes. Of course, the existence of such a positive
correlation cannot distinguish whether the observed changes are
manifestations of the manipulative activity or only symptoms of the
chronic disease. In the case of human parasites, we cannot run a
predation study, i.e. we cannot tell whether the manipulation activity
objectively increases the efficiency of parasite transmission from
intermediate to definitive host by comparing the prevalence of the
parasite in intermediate hosts captured and eaten by the definitive
host with that in a population of the intermediate host living in the
The Journal of Experimental Biology 216 (1)
THE JOURNAL OF EXPERIMENTAL BIOLOGY
131Toxoplasmosis and human behaviour
same area. Theoretically, it would be possible to compare the
intensity of behavioural manifestations of toxoplasmosis in high-
prevalence areas (with high rates of superinfections, i.e. new
infections in hosts previously infected with Toxoplasma) with the
intensity in low-prevalence areas (where the rates of superinfections
are low). The virulence of non-manipulative parasites increases in
the high-prevalence areas because in the competition between
different genetic lines of the parasite in the body of a superinfected
host, the winners are the lines with the highest rates of reproduction
and therefore usually those with the highest virulence (Ewald, 1994).
In contrast, the virulence of manipulative parasites decreases in the
high-prevalence areas, because in the competition within the body of
an infected host, the winners are the lines of non-manipulators that,
instead of wasting resources in the manipulation activity, invest the
maximum resources in reproduction (and leave the manipulation to
their competitors). Such studies, however, should be performed in a
long-term stable area and it is clear that the human is not a suitable
model. Even studies conducted in some suitable model animal in a
stable environment would not differentiate between the direct and
indirect manipulation activity. It is highly probable that some of the
observed effects, for example the shift of the sex ratio in infected
humans and mice (Kaňková et al., 2007a; Kaňková et al., 2007b),
are only side effects of the manipulative activity of Toxoplasma,
primarily aimed at suppressing the activity of the immune system of
the infected host and therefore increasing the survival of the parasite
in the host organism.
For obvious reasons, a laboratory infection experiment using a
Toxoplasma–human model to study the manipulation hypothesis is
not feasible. This is an important obstacle to the study of the causal
relationship between Toxoplasma infection and various
toxoplasmosis-associated traits. For example, the lower NEO PI-R
conscientiousness in infected subjects (Lindová et al., 2012) could
be an effect of the infection or it may be that there is a higher
probability of infection in persons with lower conscientiousness who
may have a lower tendency to adhere to hygienic standards. In some
cases, the causality direction is quite obvious. It is more probable that
toxoplasmosis causes impairment of reaction times than the persons
with longer reaction times having a higher probability of infection.
Sometimes, a longitudinal study can help; however, a large number
of subjects, preferably several thousand, would be needed for such a
study when the incidence of the parasitosis is relatively low. Before
the presence of genes for dopamine-synthesizing enzymes in the
genome of Toxoplasma was revealed (Gaskell et al., 2009) and
before an increased dopamine synthesis rate was found in
Toxoplasma tissue cysts (Prandovszky et al., 2011), it was not
possible to decide whether the positive association between
toxoplasmosis and schizophrenia was more probably caused by the
effect of Toxoplasma on the brain of predisposed individuals or by
a higher risk of Toxoplasma infection in schizophrenics. The results
of a prospective longitudinal study performed on US army personnel,
however, showed that Toxoplasma infection often precedes the first
episode of schizophrenia (Niebuhr et al., 2008).
An important hint concerning the causality can be provided by
measurement of the correlation between the duration of Toxoplasma
infection and the amount of observed phenotypic change. The
existence of a positive correlation suggests that the difference
observed between the infected and non-infected subjects is probably
the effect of latent infection. A negative correlation suggests that the
difference is a fading-out effect of past acute infection and the
absence of any correlation is likely to indicate that subjects with a
particular phenotype have an increased risk of infection. The
published results of similar studies suggest that many statistical
associations between latent toxoplasmosis and phenotypic traits are
caused by the effect of toxoplasmosis on the host phenotype;
however, some associations are probably caused by the effect of a
particular trait on the risk of infection and certain associations by
parallel effects of some third, known or unknown, factor on the host
phenotype and on the risk of Toxoplasma infection.
The last but yet very serious disadvantage of the human model is
connected with extreme genetic polymorphism in the human
population as well as with extreme heterogeneity of the
environmental factors that affect individuals in the study population.
Such genetic and non-genetic heterogeneity has a strong negative
influence on the observed effect size of any factor studied, including
latent toxoplasmosis. In statistics, the effect size is often estimated
as the proportion of the total variability of a dependent variable (e.g.
Non-infected Infected Non-infected Infected
Women Men
Men
Fertile women
Non-fertile women
Cats
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
Odour pleasantness score
Fig.1. Differences between Toxoplasma-infected
and Toxoplasma-free men and women in the
odour pleasantness scores attributed to urine
samples of men, fertile and non-fertile women,
and cats. The signs denote the mean raw scores
attributed by a particular population to the
samples tested (at least five samples from
different individuals of the same species were
used); the vertical bars denote 0.95 confidence
intervals. Higher scores were attributed to urine
samples with a more pleasant smell (in
comparison with other urine samples). The
statistical significance of the toxoplasmosis–sex
interaction for men, women in the fertile phase of
the menstrual cycle, women in the non-fertile
phase of the menstrual cycle and cats was 0.021,
0.021, 0.067 and 0.006, respectively. The urine
odour was rated by 36 Toxoplasma-free women,
9 Toxoplasma-infected women, 26 Toxoplasma-
free men and 7 Toxoplasma-infected men on a 7-
point scale. (J.F., P. Lenochová, Z. Hodný and M.
Vondrová, unpublished.)
THE JOURNAL OF EXPERIMENTAL BIOLOGY
132
of a personality factor) that can be explained by the independent
variable studied (e.g. toxoplasmosis). While in studies performed on
inbred laboratory animals or on F1 hybrids we can often see factors
explaining a high percentage of the total variability of a particular
behavioural variable, in ecological and evolutionary studies
performed on outbreeding organisms, we mostly see factors
explaining 2–7% of variability (Moller and Jennions, 2002);
therefore, to find significant effects, we may need to use an order of
magnitude larger samples than in inbred animal studies.
A large within-sample and between-samples variability of
human populations is also the cause of the fact that various studies
performed on different populations often provide different, even
opposite, results. In laboratory experiments on inbred animals, we
study genetically identical animals that have been exposed to very
similar environmental factors during their lives. Therefore, they
will probably react identically to the same factor, for example to
Toxoplasma infection. In humans, the situation is very different for
various reasons. For example, toxoplasmosis influences the human
body and mind through several independent pathways. Infected
men have a higher concentration of testosterone (Flegr et al., 2008a)
and, therefore, are likely to be more competitive, but at the same
time they have impaired reaction times (Havlíček et al., 2001).
Therefore, if in one study a self-administered simple reaction time
test is distributed to groups of 20 draftees during regular military
testing while in another study the same test is individually
administered by an attractive female PhD student to male university
students, it can be expected that in the first study, the negative
influence of toxoplasmosis on reaction times will prevail while in
the second, the higher competitiveness of the infected students will
prevail in the final effect (Flegr et al., 2008c). In both studies, we
would find a significant effect of toxoplasmosis on performance in
the test; however, in the first study the effect would be negative but
in the second it would be positive.
Most physiological processes are regulated on various levels, from
the molecular to the psychological. If, for example, Toxoplasma
causes an increase in the dopamine concentration in certain regions
of the brain, the dopamine-synthesizing cells in other areas of the
brain may degenerate. Therefore, at a certain stage of infection, we
can detect, paradoxically, a decreased, rather than an increased, level
of dopamine in the brain of infected individuals. If toxoplasmosis
induces a decrease in superego strength, it could increase the
tendency of certain individuals to lie while filling out a questionnaire
and therefore we could detect seemingly increased rather than
decreased superego strength in these subjects in questionnaire
studies. When a subject recognizes some personality change that
he/she does not like, for example a toxoplasmosis-associated increase
of extraversion, he/she may try, consciously or unconsciously, to
mask this change while completing the questionnaire and he/she can
even overcompensate for the real personality change by moving from
extraversion to introversion.
Some biological theories suggest that a large part of the genetic
polymorphism is sustained in a natural outbreeding population as
a result of epistatic interactions and frequency-dependent selection
(Flegr, 2010a; Mayr, 1963; Templeton, 2008). The particular alleles
cannot be fixed or eliminated from the population because they
increase a trait positively in the context of one genotype and
negatively in the context of another, or because they are
advantageous when rare and disadvantageous when common. The
population cannot get rid of various mutations by selection and
remains polymorphic, and its members react to the same stimuli in
different, often even opposite, ways. This affects the results of our
ethological and psychological studies. Toxoplasmosis quite often
influences the same personality factor in different populations (men
and women, rural and urban populations, or RhD-positive and
RhD-negative subjects); however, the direction of the effect of a
factor may vary between populations. When we study the effect of
a factor on a heterogeneous population, we often find a significant
increase of variance in certain dependent variables, rather than a
significant difference between the population means of particular
variables (see Poulin, 2013). For example, comparison of Cattell’s
16PF personality profiles of young women screened for
toxoplasmosis during pregnancy showed that infected females
scored higher in intelligence and lower in guilt proneness than
Toxoplasma-free females. At the same time, they differed in the
variance of four other personality factors, namely surgency,
protension, shrewdness and self-sentiment integration (Flegr and
Havlíček, 1999). In technical articles, tests for equality of variance
are commonly used only to check preconditions of the statistical
tests. Our experience with real data and the present knowledge of
the genetic architecture of phenotypic characters, however, suggest
that many genetic and environmental factors influence the variance
rather than the mean values of particular characters in polymorphic
populations. Therefore, the results of the tests for equality of
variance should be published as full-bodied results of such studies.
There are several objective reasons why Toxoplasma gondii is
now used as the most important model for studying manipulative
activity in humans, which are summarized in the first paragraphs
of the present article. However, the most important are subjective
reasons and also chance. A large number of parasitic organisms
probably exist in helminths, protozoa, fungi, bacteria, archea and
viruses that may influence the phenotype of their human host even
more than the Toxoplasma. These organisms are, however, still
waiting for research teams to engage in a systematic study of their
influence on the human host.
Acknowledgements
I would like to thank the organizers and all participants of The Journal of
Experimental Biology 2012 symposium ʻNeural parasitology: how parasites
manipulate host behaviourʼ for creating a friendly environment for stimulating
discussions.
Funding
This study was supported by the Grand Agency of the Czech Republic [grant no.
P303/11/1398] and Charles University of Prague [grant no. UNCE 204004].
References
Alvarado-Esquivel, C., Torres-Castorena, A., Liesenfeld, O., Estrada-Martínez, S.
and Urbina-Álvarez, J. D. (2012). High seroprevalence of Toxoplasma gondii
infection in a subset of Mexican patients with work accidents and low socioeconomic
status. Parasit. Vectors 5, 13.
Berdoy, M., Webster, J. P. and Macdonald, D. W. (2000). Fatal attraction in rats
infected with Toxoplasma gondii. Proc. Biol. Sci. 267, 1591-1594.
Catalano, R. A., Saxton, K. B., Bruckner, T. A., Pearl, M., Anderson, E., Goldman-
Mellor, S., Margerison-Zilko, C., Subbaraman, M., Currier, R. J. and Kharrazi, M.
(2012). Hormonal evidence supports the theory of selection in utero. Am. J. Hum.
Biol. 24, 526-532.
Ewald, P. W. (1994). Evolution of Infectious Disease. Oxford: Oxford University Press.
Flegr, J. (2007). Effects of Toxoplasma on human behavior. Schizophr. Bull. 33, 757-
760.
Flegr, J. (2010a). Elastic, not plastic species: frozen plasticity theory and the origin of
adaptive evolution in sexually reproducing organisms. Biol. Direct 5, 2.
Flegr, J. (2010b). Influence of latent toxoplasmosis on the phenotype of intermediate
hosts. Folia Parasitol. (Praha) 57, 81-87.
Flegr, J. and Havlíček, J. (1999). Changes in the personality profile of young women
with latent toxoplasmosis. Folia Parasitol. (Praha) 46, 22-28.
Flegr, J. and Stříž, I. (2011). Potential immunomodulatory effects of latent
toxoplasmosis in humans. BMC Infect. Dis. 11, 274.
Flegr, J., Zitková, S., Kodym, P. and Frynta, D. (1996). Induction of changes in
human behaviour by the parasitic protozoan Toxoplasma gondii. Parasitology 113,
49-54.
Flegr, J., Havlícek, J., Kodym, P., Malý, M. and Šmahel, Z. (2002). Increased risk of
traffic accidents in subjects with latent toxoplasmosis: a retrospective case-control
study. BMC Infect. Dis. 2, 11.
The Journal of Experimental Biology 216 (1)
THE JOURNAL OF EXPERIMENTAL BIOLOGY
133Toxoplasmosis and human behaviour
Flegr, J., Preiss, M., Klose, J., Havlícek, J., Vitáková, M. and Kodym, P. (2003).
Decreased level of psychobiological factor novelty seeking and lower intelligence in
men latently infected with the protozoan parasite Toxoplasma gondii. Dopamine, a
missing link between schizophrenia and toxoplasmosis? Biol. Psychol. 63, 253-268.
Flegr, J., Hrdá, Š. and Kodym, P. (2005a). Influence of latent ʻasymptomaticʼ
toxoplasmosis on body weight of pregnant women. Folia Parasitol. (Praha) 52, 199-
204.
Flegr, J., Hrusková, M., Hodný, Z., Novotná, M. and Hanusová, J. (2005b). Body
height, body mass index, waist-hip ratio, fluctuating asymmetry and second to fourth
digit ratio in subjects with latent toxoplasmosis. Parasitology 130, 621-628.
Flegr, J., Lindová, J. and Kodym, P. (2008a). Sex-dependent toxoplasmosis-
associated differences in testosterone concentration in humans. Parasitology 135,
427-431.
Flegr, J., Lindová, J., Pivoñková, V. and Havlícek, J. (2008b). Latent toxoplasmosis
and salivary testosterone concentration – important confounding factors in second to
fourth digit ratio studies. Am. J. Phys. Anthropol. 137, 479-484.
Flegr, J., Novotná, M., Lindová, J. and Havlícek, J. (2008c). Neurophysiological
effect of the Rh factor. Protective role of the RhD molecule against Toxoplasma-
induced impairment of reaction times in women. Neuroendocrinol. Lett. 29, 475-481.
Flegr, J., Klose, J., Novotná, M., Berenreitterová, M. and Havlícek, J. (2009).
Increased incidence of traffic accidents in Toxoplasma-infected military drivers and
protective effect RhD molecule revealed by a large-scale prospective cohort study.
BMC Infect. Dis. 9, 72.
Flegr, J., Novotná, M., Fialová, A., Kolbeková, P. and Gasová, Z. (2010). The
influence of RhD phenotype on toxoplasmosis- and age-associated changes in
personality profile of blood donors. Folia Parasitol. (Praha) 57, 143-150.
Flegr, J., Lenochová, P., Hodný, Z. and Vondrová, M. (2011). Fatal attraction
phenomenon in humans: cat odour attractiveness increased for toxoplasma-infected
men while decreased for infected women. PLoS Negl. Trop. Dis. 5, e1389.
Gaskell, E. A., Smith, J. E., Pinney, J. W., Westhead, D. R. and McConkey, G. A.
(2009). A unique dual activity amino acid hydroxylase in Toxoplasma gondii. PLoS
ONE 4, e4801.
Havlíček, J., Gasová, Z. G., Smith, A. P., Zvára, K. and Flegr, J. (2001). Decrease
of psychomotor performance in subjects with latent ʻasymptomaticʼ toxoplasmosis.
Parasitology 122, 515-520.
Hendriks, W. H., Moughan, P. J., Tarttelin, M. F. and Woolhouse, A. D. (1995).
Felinine: a urinary amino acid of Felidae. Comp. Biochem. Physiol. 112B, 581-588.
Henriquez, S. A., Brett, R., Alexander, J., Pratt, J. and Roberts, C. W. (2009).
Neuropsychiatric disease and Toxoplasma gondii infection. Neuroimmunomodulation
16, 122-133.
Hodková, H., Kodym, P. and Flegr, J. (2007). Poorer results of mice with latent
toxoplasmosis in learning tests: impaired learning processes or the novelty
discrimination mechanism? Parasitology 134, 1329-1337.
Horacek, J., Flegr, J., Tintera, J., Verebova, K., Spaniel, F., Novak, T., Brunovsky,
M., Bubenikova-Valesova, V., Holub, D., Palenicek, T. et al. (2012). Latent
toxoplasmosis reduces gray matter density in schizophrenia but not in controls:
voxel-based-morphometry (VBM) study. World J. Biol. Psychiatry 13, 501-509.
Hostomská, L., Jírovec, O., Horáčková, M. and Hrubcová, M. (1957). The role of
toxoplasmosis in the mother in the development of mongolism in the child (in
Czech). Českosl. Pediatr. 12, 713-723.
Hrdá, Š., Votýpka, J., Kodym, P. and Flegr, J. (2000). Transient nature of
Toxoplasma gondii-induced behavioral changes in mice. J. Parasitol. 86, 657-663.
James, W. H. (2008). Further support for the hypothesis that parental hormone levels
around the time of conception are associated with human sex ratios at birth. J.
Biosoc. Sci. 40, 855-861.
James, W. H. (2010). Potential solutions to problems posed by the offspring sex ratios
of people with parasitic and viral infections. Folia Parasitol. (Praha) 57, 114-120.
Kaňková, Š., Kodym, P., Frynta, D., Vavrinová, R., Kuběna, A. and Flegr, J.
(2007a). Influence of latent toxoplasmosis on the secondary sex ratio in mice.
Parasitology 134, 1709-1717.
Kaňková, Š., Šulc, J., Nouzová, K., Fajfrlík, K., Frynta, D. and Flegr, J. (2007b).
Women infected with parasite Toxoplasma have more sons. Naturwissenschaften
94, 122-127.
Kaňková, Š., Holáň, V., Zajícová, A., Kodym, P. and Flegr, J. (2010a). Modulation
of immunity in mice with latent toxoplasmosis – the experimental support for the
immunosuppression hypothesis of Toxoplasma-induced changes in reproduction of
mice and humans. Parasitol. Res. 107, 1421-1427.
Kaňková, Š., Šulc, J. and Flegr, J. (2010b). Increased pregnancy weight gain in
women with latent toxoplasmosis and RhD-positivity protection against this effect.
Parasitology 137, 1773-1779.
Kaňková, Š., Kodym, P. and Flegr, J. (2011). Direct evidence of Toxoplasma-
induced changes in serum testosterone in mice. Exp. Parasitol. 128, 181-183.
Kaňková, Š., Šulc, J., Křivohlavá, R., Kuběna, A. and Flegr, J. (2012). Slower
postnatal motor development in infants of mothers with latent toxoplasmosis during
the first 18 months of life. Early Hum. Dev. 88, 879-884.
Kannan, G., Moldovan, K., Xiao, J. C., Yolken, R. H., Jones-Brando, L. and
Pletnikov, M. V. (2010). Toxoplasma gondii strain-dependent effects on mouse
behaviour. Folia Parasitol. (Praha) 57, 151-155.
Kocazeybek, B., Oner, Y. A., Turksoy, R., Babur, C., Cakan, H., Sahip, N., Unal,
A., Ozaslan, A., Kilic, S., Saribas, S. et al. (2009). Higher prevalence of
toxoplasmosis in victims of traffic accidents suggest increased risk of traffic accident
in Toxoplasma-infected inhabitants of Istanbul and its suburbs. Forensic Sci. Int.
187, 103-108.
Kodym, P., Malý, M., Švandová, E., Lekatková, H., Badoutová, M., Vlková, J.,
Beneš, C. and Zástěra, M. (2000). Toxoplasma in the Czech Republic 1923-1999:
first case to widespread outbreak. Int. J. Parasitol. 30, 11-18.
Lindová, J., Novotná, M., Havlícek, J., Jozífková, E., Skallová, A., Kolbeková, P.,
Hodný, Z., Kodym, P. and Flegr, J. (2006). Gender differences in behavioural
changes induced by latent toxoplasmosis. Int. J. Parasitol. 36, 1485-1492.
Lindová, J., Kuběna, A. A., Sturcová, H., Krivohlavá, R., Novotná, M., Rubesová,
A., Havlícek, J., Kodym, P. and Flegr, J. (2010). Pattern of money allocation in
experimental games supports the stress hypothesis of gender differences in
Toxoplasma gondii-induced behavioural changes. Folia Parasitol. (Praha) 57, 136-
142.
Lindová, J., Příplatová, L. and Flegr, J. (2012). Higher extraversion and lower
conscientiousness in humans infected with Toxoplasma. Eur. J. Pers. 26, 285-291.
Manning, J. T. (2002). Digit Ratio: A Pointer to Fertility, Behavior, and Health.
Piscataway, NJ: Rutgers University Press.
Mayr, E. (1963). Animal Species and Evolution. Cambridge, MA: Harvard University
Press.
Moller, A. P. and Jennions, M. D. (2002). How much variance can be explained by
ecologists and evolutionary biologists? Oecologia 132, 492-500.
Neuhäuser, M. and Krackow, S. (2007). Adaptive-filtering of trisomy 21: risk of Down
syndrome depends on family size and age of previous child. Naturwissenschaften
94, 117-121.
Niebuhr, D. W., Cowan, D. N., Millikan, A. M., Yolken, R., Li, Y. and Weber, N.
(2007). Risk of schizophrenia and antibodies to Toxoplasma gondii among U.S.
military personnel. Schizophr. Bull. 33, 243-244.
Niebuhr, D. W., Millikan, A. M., Cowan, D. N., Yolken, R., Li, Y. Z. and Weber, N.
S. (2008). Selected infectious agents and risk of schizophrenia among U.S. military
personnel. Am. J. Psychiatry 165, 99-106.
Novotná, M., Hanušová, J., Klose, J., Preiss, M., Havlíček, J., Roubalová, K. and
Flegr, J. (2005). Probable neuroimmunological link between Toxoplasma and
cytomegalovirus infections and personality changes in the human host. BMC Infect.
Dis. 6, 54.
Novotná, M., Havlícek, J., Smith, A. P., Kolbeková, P., Skallová, A., Klose, J.,
Gasová, Z., Písacka, M., Sechovská, M. and Flegr, J. (2008). Toxoplasma and
reaction time: role of toxoplasmosis in the origin, preservation and geographical
distribution of Rh blood group polymorphism. Parasitology 135, 1253-1261.
Pappas, G., Roussos, N. and Falagas, M. E. (2009). Toxoplasmosis snapshots:
global status of Toxoplasma gondii seroprevalence and implications for pregnancy
and congenital toxoplasmosis. Int. J. Parasitol. 39, 1385-1394.
Parker, G. A., Ball, M. A., Chubb, J. C., Hammerschmidt, K. and Milinski, M.
(2009). When should a trophically transmitted parasite manipulate its host? Evolution
63, 448-458.
Poulin, R. (1995). ʻAdaptiveʼ changes in the behaviour of parasitized animals: a critical
review. Int. J. Parasitol. 25, 1371-1383.
Poulin, R. (2013). Parasite manipulation of host personality and behavioural
syndromes. J. Exp. Biol. 216, 18-26.
Prandovszky, E., Gaskell, E., Martin, H., Dubey, J. P., Webster, J. P. and
McConkey, G. A. (2011). The neurotropic parasite Toxoplasma gondii increases
dopamine metabolism. PLoS ONE 6, e23866.
Saha, S., Chant, D., Welham, J. and McGrath, J. (2005). A systematic review of the
prevalence of schizophrenia. PLoS Med. 2, e141.
Shenton, M. E., Dickey, C. C., Frumin, M. and McCarley, R. W. (2001). A review of
MRI findings in schizophrenia. Schizophr. Res. 49, 1-52.
Shirbazou, S., Abasian, L. and Meymand, F. T. (2011). Effects of Toxoplasma gondii
infection on plasma testosterone and cortisol level and stress index on patients
referred to Sina hospital, Tehran. Jundishapur J. Microbiol. 4, 167-173.
Skallová, A., Novotná, M., Kolbeková, P., Gasová, Z., Veselý, V., Sechovská, M.
and Flegr, J. (2005). Decreased level of novelty seeking in blood donors infected
with Toxoplasma. Neuroendocrinol. Lett. 26, 480-486.
Skallová, A., Kodym, P., Frynta, D. and Flegr, J. (2006). The role of dopamine in
Toxoplasma-induced behavioural alterations in mice: an ethological and
ethopharmacological study. Parasitology 133, 525-535.
Tandon, R., Nasrallah, H. A. and Keshavan, M. S. (2010). Schizophrenia, ʻjust the
factsʼ 5. Treatment and prevention. Past, present, and future. Schizophr. Res. 122,
1-23.
Templeton, A. R. (2008). The reality and importance of founder speciation in
evolution. Bioessays 30, 470-479.
The International Schizophrenia Consortium (2009). Common polygenic variation
contributes to risk of schizophrenia and bipolar disorder. Nature 460, 748-752.
Torgerson, P. R. and Macpherson, C. N. L. (2011). The socioeconomic burden of
parasitic zoonoses: global trends. Vet. Parasitol. 182, 79-95.
Torrey, E. F. and Yolken, R. H. (1995). Could schizophrenia be a viral zoonosis
transmitted from house cats? Schizophr. Bull. 21, 167-171.
Torrey, E. F. and Yolken, R. H. (2005). Toxoplasmosis and schizophrenia: is the cat
in the hat? Schizophr. Bull. 31, 241.
Torrey, E. F., Bartko, J. J., Lun, Z. R. and Yolken, R. H. (2007). Antibodies to
Toxoplasma gondii in patients with schizophrenia: a meta-analysis. Schizophr. Bull.
33, 729-736.
Vyas, A. (2013). Parasite-augmented mate choice and reduction in innate fear in rats
infected by Toxoplasma gondii.J. Exp. Biol. 216, 120-126.
Vyas, A., Kim, S. K. and Sapolsky, R. M. (2007). The effects of toxoplasma infection
on rodent behavior are dependent on dose of the stimulus. Neuroscience 148, 342-
348.
Wang, H. L., Wang, G. H., Li, Q. Y., Shu, C., Jiang, M. S. and Guo, Y. (2006).
Prevalence of Toxoplasma infection in first-episode schizophrenia and comparison
between Toxoplasma-seropositive and Toxoplasma-seronegative schizophrenia.
Acta Psychiatr. Scand. 114, 40-48.
Webster, J. P. (2007). The effect of Toxoplasma gondii on animal behavior: playing
cat and mouse. Schizophr. Bull. 33, 752-756.
Webster, J. P. and McConkey, G. A. (2010). Toxoplasma gondii-altered host
behaviour: clues as to mechanism of action. Folia Parasitol. (Praha) 57, 95-104.
Webster, J. P., Kaushik, M., Bristow, G. C. and McConkey, G. A. (2013).
Toxoplasma gondii infection, from predation to schizophrenia: can animal behaviour
help us understand human behaviour? J. Exp. Biol. 216, 99-112.
Yereli, K., Balcioğlu, I. C. and Ozbilgin, A. (2006). Is Toxoplasma gondii a potential
risk for traffic accidents in Turkey? Forensic Sci. Int. 163, 34-37.
THE JOURNAL OF EXPERIMENTAL BIOLOGY
... In heavy acute infection, the organism can be free in the blood and peritoneal exudates. It may inhabit the host cell's nucleus but usually lives in the cytoplasm (2). ...
Article
Full-text available
Toxoplasma gondii is a pervasive parasite that causes a variety of diseases in both humans and animals; it infects the neural cells of the central nervous system. There have been previous reports linking this infection with autism. Autism is a combination of complex neurodevelopmental disorders leading to alteration in social communication. This study was done to detect the frequency of seropositive Toxoplasmosis in autistic children attending psychiatric units in Azady Teaching Hospital in the Kirkuk governorate of Iraq. This case-control study was done on 40 autistic children (30 male, 10 female) with an average age of 5.4±6 years and 40 healthy children (8 male, 32 female) with an age range of 6.1±2 years, from April 2018 to January 2020. Sample collection was done by obtaining four ml of blood from all study group children for the detection of both IgM and IgG Toxoplasma gondii antibodies using the ELISA technique. Chi-square and paired T-tests were used for data analysis. The IgM serology for both autistic and control children was negative, while there was just one positive result of T. gondii IgG in autistic children and two positive results in normal children, which made the result insignificant (X2=0.45, p-value>0.05). Regarding gender, there was no significant difference in detecting positive results between boys and girls of the study groups (X2=0.45, p value > 0.05). The study also demonstrated that autistic parents were statistically older than healthy children of the same age. We couldn't uncover any connection between ASD and toxoplasmosis. The age of parents & family history has a significant impact on the development of ASD.
... Consequently, the expression of traits that are merely side effects of the pathological processes in the infected organism also increases. However, if a trait is the result of parasitic manipulation, the frequency of superinfections should have the opposite effect on the virulence of the parasites and thus on the expression of the trait (Flegr 2013). In such cases, parasites would benefit more from investing resources in reproduction rather than manipulation, effectively shifting the costs of manipulation to competitors sharing the same host. ...
Article
In this article, I recount the journey of discovering the effects of latent toxoplasmosis on human psychology, behaviour, morphology, and health as I observed it from the closest perspective over the past 30+ years, during which our laboratory has been intensely focused on this research. I trace how we moved from the initial observations of differences between infected and uninfected individuals in certain personality traits to the systematic study of similar differences in behaviour, both in the laboratory and in everyday life, as well as in physiological and even morphological traits. This eventually led us to investigate the causal relationships behind these observed associations and their molecular basis. I describe some of the unexpected discoveries our research revealed - whether it was the impact of toxoplasmosis on the human sexual index, the prenatal and postnatal development, the sexual preferences and behaviour, the modulatory effect of blood Rh factor on toxoplasmosis, or the discovery of sexual transmission of toxoplasmosis. In exploring whether the toxoplasmosis-associated effects were merely side effects of an ongoing latent infection, we gradually uncovered that seemingly asymptomatic toxoplasmosis has profound (and certainly not positive) effects on the mental and physical health of infected individuals. The article also includes three separate boxes that discuss some key methodological challenges we encountered along the way, such as how to distinguish the effect of infection from mere statistical association, or how to differentiate parasitic manipulation from a simple side effect.
... Analogous side effects, which can be nonadaptive or residual manipulative (Flegr et al., 2011), encompass a spectrum of behavioural consequences in T. gondii-infected humans, ranging from subtle shifts in personality traits to more severe outcomes, such as an elevated susceptibility to schizophrenia (Flegr 2013a(Flegr , 2013b, and association with a higher risk to develop other several neuropsychiatric disorders (Milne et al., 2020a) This association has been investigated also in other animal species during natural infection, especially in wildlife. Recent studies have brought attention to an intriguing phenomenon among wild red foxes (Vulpes vulpes), known as Dopey Fox Syndrome (DFS), characterized by unusual behavioural traits, in which T. gondii seems to hold a pivotal role. ...
... Furthermore, during the chronic stage of infection, the immune response within the central nervous system may potentiate neuronal plasticity and other cognitive patterns in infected hosts (7). Alternatively, it is suggested that T. gondii manipulates host behavior to increase the transmission rates by infected brain cells (8). Latent toxoplasmosis is also associated with the development of schizophrenia, anxiety disorder, aggressivity and impulsivity, suicidal attempts and depression (9). ...
Article
Full-text available
Introduction Depressive syndrome (DS) is a common complication during pregnancy and the postpartum period, and is triggered by multiple organic/genetic and environmental factors. Clinical and biochemical follow-up is essential for the early diagnosis and prognosis of DS. The protozoan Toxoplasma gondii causes infectious damage to the fetus during parasite primary-infection. However, in long-term infections, pregnant women develop immune protection to protect the fetus, although they remain susceptible to pathological or inflammatory effects induced by T. gondii. This study aimed to investigate plasma inflammatory biomarkers in pregnant women seropositive and seronegative for T. gondii, with diagnoses of minor and moderate/severe DS. Methods Pregnant women (n=45; age=18–39 years) were recruited during prenatal care at health centers in Ouro Preto, Minas Gerais, Brazil. Participants were asked to complete a socio-demographic questionnaire to be submitted to well-standardized DS scale calculators (Beck Depression Inventory Questionnaire, Edinburgh Postnatal Depression Scale, and Major Depressive Episode Module). Additionally, 4 mL of blood was collected for plasma neuroserpin, CCL2, IL-17A, and IL-33 analysis. Results Pregnant volunteers with chronic T. gondii contact were all IgG+ (44%; n=21) and exhibited increased plasma IL-33, IL-17A, and neuroserpin levels, but not CCL2, compared to uninfected pregnant women. Using Beck’s depression inventory, we observed an increase in plasma IL-17A and IL-33 in women with T. gondii infeCction diagnosed with mild DS, whereas neuroserpin was associated with minor and moderate/severe DS. Discussion Our data suggest a close relationship between DS in pregnant women with chronic T. gondii infection and neurological conditions, which may be partially mediated by plasma neuroserpin, IL-33, and IL-17A levels.
... According to certain studies, toxoplasmosis has negative correlation to incidence of COVID-19. Indeed, this intracellular parasite exhibits broad antiviral effects by secreting into host cells a 'Dense Granule Protein-7' which has immune stimulatory effects and inhibits replication of virus (Fekadu et al. 2010, Flegr 2013, Weeratunga et al. 2017. Additionally, apicoplast proteins exhibit remarkably immune-mediated properties (Can et al. 2020). ...
... According to certain studies, toxoplasmosis has negative correlation to incidence of COVID-19. Indeed, this intracellular parasite exhibits broad antiviral effects by secreting into host cells a 'Dense Granule Protein-7' which has immune stimulatory effects and inhibits replication of virus (Fekadu et al. 2010, Flegr 2013, Weeratunga et al. 2017. Additionally, apicoplast proteins exhibit remarkably immune-mediated properties (Can et al. 2020). ...
Article
Full-text available
Susceptibility to COVID-19, the most devastating global pandemic, appears to vary widely across different population groups. Exposure to toxoplasmosis has been proposed as a theory to explain the diversity of these populations. The aim of the present study was to investigate the possible association between latent toxoplasmosis and COVID-19 and its probable correlation with markers of oxidative stress, C-reactive protein (CRP) and ferritin. In a case-control study, blood samples were collected from 91 confirmed (48 non-pneumonic; NP, and 43 pneumonic; P) COVID-19 patients and 45 healthy controls. All participants were tested for IgG anti-Toxoplasma gondii antibodies and oxidative stress markers (nitric oxide [NO], superoxide dismutase [SOD] and reduced glutathione [GSH]), and CRP and serum ferritin levels were determined. In COVID-19 patients, IgG anti-T. gondii antibodies were found in 54% compared to 7% in the control group, with the difference being statistically significant (P ˂ 0.001). However, no significant correlation was found between the severity of COVID-19 and latent T. gondii infection. Latent toxoplasmosis had a strong influence on the risk of COVID-19. NO and SOD levels were significantly increased in COVID-19 patients, while GSH levels decreased significantly in them compared to control subjects (P ˂ 0.001 for both values). CRP and ferritin levels were also significantly elevated in P COVID-19 patients infected with toxoplasmosis. This is the first study to look at the importance of oxidative stress indicators in co-infection between COVID-19 and T. gondii. The high prevalence of latent toxoplasmosis in COVID-19 suggests that T. gondii infection can be considered a strong indicator of the high risk of COVID-19.
... According to certain studies, toxoplasmosis has negative correlation to incidence of COVID-19. Indeed, this intracellular parasite exhibits broad antiviral effects by secreting into host cells a 'Dense Granule Protein-7' which has immune stimulatory effects and inhibits replication of virus (Fekadu et al. 2010, Flegr 2013, Weeratunga et al. 2017. Additionally, apicoplast proteins exhibit remarkably immune-mediated properties (Can et al. 2020). ...
Article
Susceptibility to COVID-19, the most devastating global pandemic, appears to vary widely across different population groups. Exposure to toxoplasmosis has been proposed as a theory to explain the diversity of these populations. The aim of the present study was to investigate the possible association between latent toxoplasmosis and COVID-19 and its probable correlation with markers of oxidative stress, C-reactive protein (CRP) and ferritin. In a case-control study, blood samples were collected from 91 confirmed (48 non-pneumonic; NP, and 43 pneumonic; P) COVID-19 patients and 45 healthy controls. All participants were tested for IgG an-ti-Toxoplasma gondii antibodies and oxidative stress markers (nitric oxide [NO], superoxide dismutase [SOD] and reduced glutathione [GSH]), and CRP and serum ferritin levels were determined. In COVID-19 patients, IgG anti-T. gondii antibodies were found in 54% compared to 7% in the control group, with the difference being statistically significant (P ˂ 0.001). However, no significant correlation was found between the severity of COVID-19 and latent T. gondii infection. Latent toxoplasmosis had a strong influence on the risk of COVID-19. NO and SOD levels were significantly increased in COVID-19 patients, while GSH levels decreased significantly in them compared to control subjects (P ˂ 0.001 for both values). CRP and ferritin levels were also significantly elevated in P COVID-19 patients infected with toxoplasmosis. This is the first study to look at the importance of oxidative stress indicators in co-infection between COVID-19 and T. gondii. The high prevalence of latent toxoplasmosis in COVID-19 suggests that T. gondii infection can be considered a strong indicator of the high risk of COVID-19.
... whereas the age group of 36 to 40 years old had the lowest infection rate (9.09%). The results of the statistical analysis revealed a significant variation (P<0.05) on the overall prevalence of Toxoplasma among women groups as shown in figure (1). The relation between infection with Toxoplasma and number of abortion has been studied , the results were showed The ratio of one miscarriage in infection women was 72.7% (40/55) higher than two and three miscarriages 23.6% (13/55 ) and 3.6% ( 2/55 ) respectively figure (2). ...
Article
Toxoplasmosis as a zoonotic disease has a worldwide distribution and can infect a wide range of animal hosts, as well as at least one third of the world's human population. The disease is usually mild or asymptomatic in immunocompetent individuals, but dormant tissue cysts survive especially in the brain for the host lifespan, known as latent toxoplasmosis (LT). Recent studies suggest that LT can have certain neurological, immunological psychological and behavioural effects on human including schizophrenia, bipolar disorder, Alzheimer's disease, depression, suicide anxiety and sleeping disorders. LT effects are controversial, and their exact mechanisms of action is not yet fully understood. This review aims to provide an overview of the potential effects, their basic mechanisms including alteration of neurotransmitter levels, immune activation in the central nervous system and induction of oxidative stress. Additionally, beneficial effects of LT, and an explanation of the effects within the framework of manipulation hypothesis, and finally, the challenges and limitations of the current research are discussed.
Article
A BSTRACT Introduction The novel coronavirus causes the deadly disease, severe acute respiratory syndrome (SARS). Therefore, it is necessary to identify prognostic factors for patients hospitalized with SARS-coronavirus-2 infection to identify patients with increased mortality risk. Several studies have shown that polymorphisms within the ABO gene are associated with different traits, including risk factors for coronavirus disease 2019 (COVID-19) mortality. Methods We conducted a prospective, single-center study at the Pulmonologist Department of Rouiba Hospital, Algiers. The medical records of 498 COVID-19 patients enrolled in this cohort study were hospitalized between March 2020 and March 2022. The patient reported the blood group (grouping card) or was carried out on site. The patients were divided into 10 groups: A+, A−, B+, B−, O+, O−, AB+, AB−, (Rhesus+ABO/Rh blood groups), and Rhesus-groups, according to the distribution of ABO blood group. The different ABO blood group of the 498 patients was analyzed and compared according to their prognosis, death, or transfer to an intensive care unit. Results Four hundred and ninety-eight patients were evaluated, 213 (42.8%) were male, and the mean age was 55.2 years (standard deviation: 15.3). A total of 473 (94.98%) patients were discharged, and 25 (5.02%) died during hospitalization. The number of patients with ARh negative was significantly higher in the dead patients 30% among 10 patients ( P = 0.0002) versus 5.6% for A+, compared to improved individuals and more computed tomography scan lesion extension 40% ( P = 0.021) between 50% and 75% vs. 13.2% ± 34.0% ( P = 0.034) for the A(+) group, and hypernatremia 141.2 ± 2.7 ( P = 0.056) versus 137.4 ± 4.36 for A(+) group. The distribution of ABO blood groups was statistically different between the two groups. The dead patients had the blood groups A(−), (30%), A+ (5.6%), B− (0%), B+ (1.5%), AB− (0%), AB+ (0%), O− (20%), and O+ (4.3%), respectively. Conclusion The protective nature of all blood groups (AB, B, and O+) was observed in patients presenting with COVID-19 symptoms of varying severities. Blood grouping A and especially A− and O− are marked by a high risk of COVID-19 infection, serious form, and mortality and therefore deserve particular attention in their management.
Article
Intracellular parasite Toxoplasma gondii infects up to a third of the world population. The agent is neurotrophic, with a specific affinity to glia. It is possible that latent toxoplasma infection plays a role in the pathogenesis of schizophrenia. The alteration of cognitive functions is considered as an element of vulnerability for the development of some schizophrenia subtypes in the current state of knowledge. In theory, the changes of neurotransmission, the effect of cytokines, autoimmune reactions and disorders of brain development are considered to be responsible pathogenetic mechanisms. The hypothesis of the link between Toxoplasma infection and schizophrenia is supported by these findings: 1) statistically significant increase of antibodies to Toxoplasma in antipsychotic-naive, first-episode patients, 2) the PANSS total score related to the levels of IgG antibodies to TG and the levels of IgM antibodies depending on PANSS negative symptoms in First-Episode Schizophrenia, 3) prenatal exposure to toxoplasmosis and intrauterine infections with Toxoplasma as a risk factor for schizophrenia, 4) increased levels of cognitive impairment in persons with schizophrenia who have serologic evidence of Toxoplasma infection in comparison to Toxoplasma seronegative patients, 5) cognitive and behavioural changes in infected non-schizophrenic subjects 6) specific behavioural changes in infected animals, 7) adults with schizophrenia or bipolar disorder had more exposure than controls to the definitive host of Toxoplasma in childhood - cats. Neurocognitive assessment, functional brain imaging methods and molecular genetic techniques improve our possibilities to test the involvement of Toxoplasma in the etiology of schizophrenia.
Article
Introduction and objective: Toxoplasma gondii is an intra cellular protozoan parasite which infects 30 to 60% of the world population. A wide range of toxoplasmosis conditions has been studied, but there are still unknown damages which must be explored. In the present research, the effect of this parasite on testosterone and cortisol changes in the infected men and women was examined. Materials and methods: A total of 180 patients (73 females and 107 males), were examined for lgG anti-Toxoplasma antibody, cortisol and testosterone in their plasma. In addition, the patients also filled in personality questionnaire DASS21 (depression, anxiety, and stress). Results: Results showed that 24(%13/33) females and 39(%20) males were positive with lgG anti-Toxoplasma antibody, respectively. A statistically significant correlation between Toxoplasma infection and testosterone and cortisol increase in women and men were observed. Stress and anxiety index also increased in men and women whereas depression index increased only in men. Conclusion: The results show a direct relation between raise Toxoplasma infection and cortisol, and testosterone increase in bout men and women and high DASS21 test score in Toxoplasma infected group. Significance and impact of the study: Our results may indicate that following Toxoplasma infection, the probability of stressful condition is high.
Article
Objective: To compare two methods of calculating total antipsychotic drug dosage with a representative patient sample in Hong Kong. Methods: Antipsychotic prescription data from 8,620 inpatients and outpatients in the New Territories West Cluster hospitals of Hong Kong were collected. The total antipsychotic dose was calculated using the chlorpromazine equivalent method and the percentage of maximum recommended method. Results: There was 97.2% concordance between these two methods, with a Spearman's rank correlation of 0.92. High doses of antipsychotic medications were prescribed for 3.2% and 2.8% of the sample based on the chlorpromazine equivalent and percentage of maximum recommended methods, respectively. Conclusions: Total antipsychotic dose expressed as a percentage of the maximum recommended dose is easy to calculate and avoids the ambiguity associated with the chlorpromazine equivalent method, besides it helps to identify patients receiving high doses of antipsychotics, thus facilitating monitoring and adherence to current guidelines.
Article
Typically, female rats demonstrate clear mate choice. Mate preference is driven by the evolutionary need to choose males with heritable parasite resistance and to prevent the transmission of contagious diseases during mating. Thus, females detect and avoid parasitized males. Over evolutionary time scales, parasite-free males plausibly evolve to advertise their status. This arrangement between males and females is obviously detrimental to parasites, especially for sexually transmitted parasites. Yet Toxoplasma gondii, a sexually transmitted parasite, gets around this obstacle by manipulating mate choice of uninfected females. Males infected with this parasite become more attractive to uninfected females. The ability of T. gondii to not only advantageously alter the behavior and physiology of its host but also secondarily alter the behavior of uninfected females presents a striking example of the 'extended phenotype' of parasites. Toxoplasma gondii also abolishes the innate fear response of rats to cat odor; this likely increases parasite transmission through the trophic route. It is plausible that these two manipulations are not two distinct phenotypes, but are rather part of a single pattern built around testosterone-mediated interplay between mate choice, parasitism and predation.
Article
We examine the role of the protozoan Toxoplasma gondii as a manipulatory parasite and question what role study of infections in its natural intermediate rodent hosts and other secondary hosts, including humans, may elucidate in terms of the epidemiology, evolution and clinical applications of infection. In particular, we focus on the potential association between T. gondii and schizophrenia. We introduce the novel term 'T. gondii-rat manipulation-schizophrenia model' and propose how future behavioural research on this model should be performed from a biological, clinical and ethically appropriate perspective.
Article
The past decades have seen mounting evidence that parasites alter their host's behaviour in ways that benefit transmission, based on differences in the expression of behavioural traits between infected and control individuals, or on significant correlations between trait expression and infection levels. The multidimensional nature of host manipulation has only recently been recognised: parasites do not target single host traits, but instead suites of interrelated traits. Here, I use recent research on animal personality (behavioural differences among individuals consistent across time and situations) and behavioural syndromes (correlations at the population level among distinct behavioural traits, or between the same trait expressed in different contexts) to provide a framework from which simple testable patterns of host behavioural changes can be predicted. Following infection, a manipulative parasite could (i) change the temporal consistency of its host's behavioural responses, (ii) change the slope of a host reaction norm, i.e. the way host behavioural traits are expressed as a function of an environmental gradient, or (iii) decouple two or more host behavioural traits and/or change the way in which they correlate with each other. Two case studies involving trematode parasites and their freshwater hosts are used to provide empirical illustrations of the above scenarios. These clearly illustrate the full richness of behavioural alterations induced by parasites, and how these effects would go unnoticed using the classical trait-by-trait comparisons of mean values between parasitised and non-parasitised individuals. However, the power of animal personality and behavioural syndromes to inform research on host manipulation by parasites will only be fully realised when underlying mechanisms are elucidated and linked to their phenotypic impacts.