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14
Toxoplasmosis, Behavior, and Cognition
Jaroslav Flegr
Introduction
Toxoplasma gondii is a protozoan parasite that infects approximately one- third of
the human population worldwide (Tenter et al., 2000). During its lifecycle, T. gondii
must get from the body of an intermediate host, which can be any warm- blooded
animal, into the intestine of any felid species. Felidae, that is, cats, are denitive hosts
of T. gondii; only in their intestines can the parasite reproduce sexually and produce
resistant oocysts, which can then survive in the soil for years. T. gondii can be trans-
mitted between intermediate hosts by dierent routes, including a sexual route from
an infected male to an uninfected female (Hlavacova et al., 2021). However, the formation of sexual stages and oocytes in feline intestine is probably critical for the
stable survival of T. gondii in a given locality.
ere are several forms of toxoplasmosis found in humans (Pappas et al., 2009).
e most serious one is congenital toxoplasmosis, the result of a fetus being infected
by its mother during the acute phase of toxoplasmosis. Congenital infection in the
rst trimester of gestation can lead to a spontaneous abortion or severe developmental defects in the aected children. Postnatal infection with T. gondii results in
the second form of toxoplasmosis: acute toxoplasmosis. e symptoms of acute toxoplasmosis resemble u or other viral or bacterial diseases, but in addition, this form
is oen accompanied by various neuropsychiatric and behavioral conditions (Minto
& Roberts, 1959). In immunocompetent humans, a short phase of acute toxoplasmosis spontaneously resolves into a latent phase, which is why patients only rarely
realize that they were infected with T. gondii at all. e third form of the disease, la-
tent toxoplasmosis, is characterized by the presence of viable parasites in tissue cysts
(Figure 14.1) in various organs throughout human body. is form is not associated
with any specic clinical symptoms. Patients have in their serum anti- Toxoplasma
antibodies, whose concentration only slowly and irregularly decreases with time
elapsed since the end of the acute phase of the infection. Observed decrease of seroprevalence of toxoplasmosis in old age groups described in serological studies (Flegr,
2017c; Kolbeková et al., 2007), anecdotal clinical observations, and the results of a
handful of longitudinal studies (Kodym et al., 2007) all suggest that in many subjects
the concentration of anti- Toxoplasma antibodies ultimately decreases under the de-
tection threshold of serological tests. It is, however, mostly believed that the slowly
Jaroslav Flegr, Toxoplasmosis, Behavior, and Cognition In:
DOI: 10.1093/ oso/ 9780192870414.003.0015
Toxoplasmosis, Behavior, and Cognition 209
Figure 14.1 Tissue cyst of Toxoplasma in the brain of an infected mouse.
dividing parasites within the cysts remain viable until the end of the host’s life or
until they transform into fast replicating tachyzoites in individuals with highly compromised immunity. In such individuals, for instance, patients with human immunosuppressive virus, reactivated toxoplasmosis can lead to cerebral toxoplasmosis,
which is oen lethal (Lu & Remington, 1992; Smith et al., 2021).
It has been known since the 1980s that T. gondii aects the behavior of infected
rodents, including their cognitive functions (Hutchison et al., 1980a, 1980b). Since
the 1990s, these behavioral changes have been usually interpreted as the result of a
manipulative activity by T. gondii aimed at increasing the likelihood of transmission
from the infected intermediate host to the denitive host via predation (Webster,
1994; Webster et al., 1994). Still in the 1990s, there appeared the rst studies showing
that the behavioral changes associated with Toxoplasma infection can also be observed in humans (Flegr & Hrdý, 1994; Flegr et al., 1996).
Over the following three decades, dozens of studies have demonstrated that
T. gondii infection is in humans associated with specic changes in personality, behavior, and performance in various cognitive tests (Flegr, 2013). It has also been
shown that dierences between infected and non- infected individuals oen increase
with time elapsed since the acute phase of the infection, suggesting that the behavioral traits associated with toxoplasmosis are most likely the eect rather than the
cause of T. gondii infection (Flegr et al., 1996, 2000). Moreover, the positive correla-
tion between the intensity of behavioral eects of latent toxoplasmosis with duration
of the infection (or the negative correlation between the intensity of these eects and
the concentration of anti- Toxoplasma immunoglobulin (Ig)- G antibodies) suggests
Infectious Disease and Neurocognition
that what is probably responsible for the observed changes is the cumulative eect of
a lifelong latent infection rather than diminishing aereects of the acute phase of
Toxoplasma infection.
Personality changes
e rst evidence of behavioral eects of latent toxoplasmosis in human hosts
was published in the early 1990s (Flegr & Hrdý, 1994). A comparison of personality proles of Toxoplasma- infected and Toxoplasma- free students, soldiers, blood
donors, pregnant women, and later also of the general internet population showed
that infected subjects scored dierently on many personality factors measured with
Cattell’s Sixteen Personality Factor (16PF) questionnaire, Cloninger’s Temperament
and Character Inventory (TCI), and the Neuroticism– Extraversion– Openness
Personality Inventory Revised (NEO- PI- R) questionnaire (Flegr, 2013). Some effects were identical in men and women: for instance, the lower levels of novelty
seeking measured with the TCI (Flegr et al., 2003; Skallová et al., 2005) and lower
conscientiousness measured with the NEO- PI- R (Akgul, 2020; Lindová et al.,
2012) observed in Toxoplasma- infected individuals. Other factors, however, shied
in opposite directions in men and women: Cattell’s superego strength, aectothymia,
and protension. e opposite direction of shi in these factors could be explained
by the stress- coping hypothesis as part of specic sex- typical stress- coping strategies (Lindová et al., 2006). ere are robust indications to the eect that people
with latent toxoplasmosis can have impaired health and be thus exposed to chronic
stress (see “Manipulation or side eects of the disease?”). Moreover, it is known that
women and men react to long- term stress by shiing some behaviors in the opposite
direction. e sex- specic patterns of these reactions correspond to those observed
in Toxoplasma- infected men and women (Lindová et al., 2006, 2010).
It has been reported that toxoplasmosis is associated with higher aggression in
women but not in men and with higher levels of impulsive sensation- seeking in
younger men (Cook et al., 2015). Higher aggression and impulsivity of Toxoplasma-
infected subjects were also observed in 358 adult subjects with and without psychiatric disorders (Coccaro et al., 2016) and in 950 individuals with no clinical disorders
(Peng et al., 2018).
A cross- sectional study published in 2022 showed that 477 Toxoplasma- infected
subjects diered in their political beliefs and values from 1838 Toxoplasma- free
subjects (Kopecký et al., 2022). is study was based on the results of an electronic
version of the 34- item Political Beliefs and Values Inventory (PI34), which sought
to identify dierences in four independent areas: tribalism, economic equity, cultural liberalism, and anti- authoritarianism. Infected and non- infected subjects
signicantly diered in three of these four areas. Separate analyses of women and
men showed that infected women scored higher on tribalism and lower on cultural
Toxoplasmosis, Behavior, and Cognition 211
liberalism than the Toxoplasma- free control group, while infected men scored
higher on economic equity.
Dierences between infected and non- infected subjects were also demonstrated
with a projective test (Flegr, 2017a), where the subjects were asked to specify which
of ten dierent animal species they would prefer to be if they were born as an animal in their next life. In Toxoplasma- free men, the most popular animal was a
lion and in Toxoplasma- free women a dog. In Toxoplasma- infected men, the most
popular animal was a dog and in infected women a cat. It should be noted that the
most prominent negative dierence between popularity in Toxoplasma- infected
and Toxoplasma- free men— that is, the largest Toxoplasma- associated decrease in
popularity— was observed for the lion (p = 0.0001), while the largest increase in popularity was found for the cat (p = 0.0001) and the squirrel (p = 0.0001). In women, the
smallest increase in popularity was found for the dog (statistically non- signicant),
parrot (statistically non- signicant), and lion (statistically non- signicant), while
the most signicant increase was measured for the cat (p = 0.0001) and mouse
(p = 0.0001). ese results could suggest that infected men and women have lower
ambitions than their Toxoplasma- free peers.
Behavior
More recently, ethological experiments have shown that at least some of the
toxoplasmosis- associated personality traits could aect not only the results of personality questionnaires but also the actual behavior of men and women in doubleblind behavioral experiments. For example, Toxoplasma- infected men scored lower
on clothes tidiness than uninfected men did, while infected women scored higher but
not statistically signicantly so on clothes tidiness than uninfected women (Lindová
et al., 2006). Similarly, infected men scored lower and infected women higher on sociability than their uninfected peers. Infected rural men also scored higher on suspiciousness, while infected rural female students scored lower on suspiciousness than
their non- infected peers. ese outcomes match the patterns observed in previous
questionnaire studies.
Toxoplasmosis status also aects strategies used in experimental games. Both infected women and men were less altruistic than Toxoplasma- free individuals in the
Dictator game, while in the Trust game, infected men were less altruistic and infected women more altruistic than Toxoplasma- free men and women, respectively
(Lindová et al., 2010). e increased prevalence of cysts in the brains of victims of
risky behavior that lead to their death suggests a possible association between risky
behavior and latent toxoplasmosis (Samojlowicz et al., 2019). On the other hand, a
study that used experimental games found no dierences between 39 Toxoplasmainfected and 40 Toxoplasma- free women in their attitude to risk and their level of
risk aversion (Lanchava et al., 2015).
Infectious Disease and Neurocognition
A case– control study performed on 347 female and 208 male university students
showed that toxoplasmosis aects the performance in endurance trials, in particular in weight- holding and grip tests (Flegr et al., 2018b). e direction of the effect of toxoplasmosis depended on the rhesus (Rh) blood group of the individual,
a phenomenon observed in many other studies (see below). e results showed
that Toxoplasma- infected Rh- negative subjects performed worse and Toxoplasma-
infected Rh- positive subjects better in the endurance tests than corresponding
Toxoplasma- free subjects.
Toxoplasmosis also seems to aect entrepreneurial behavior. A study performed
on 1495 students showed that Toxoplasma- infected individuals are 1.4 times more
likely to major in business and 1.7 times more likely to emphasize “management and
entrepreneurship” over other business- related characteristics than Toxoplasma- free
individuals (Johnson et al., 2018). Toxoplasma- infected participants of an entrepre-
neurship event were 1.8 times more likely to have started their own business than
other participants (Johnson et al., 2018). Another study performed on 3672 subjects
showed that Toxoplasma- infected men reported having signicantly fewer subordinates than their Toxoplasma- free peers, and Toxoplasma- infected individuals were
less satised with their economic situations than Toxoplasma- free individuals. ese
two eects were much stronger in subjects over 30 years of age (Flegr, 2017a).
Latent toxoplasmosis also aects the sexual behavior of human hosts. A large
cross- sectional internet study performed on a population of 5087 Toxoplasma- free
and 741 Toxoplasma- infected subjects showed that infected men express a preference
for sexual masochism: they are more sexually aroused by their own submission, fear,
and pain than Toxoplasma- free men are (Flegr & Kuba, 2016). e infected subjects
were more attracted to masochism- related sexual practices but less frequently reported actual performance of such activities than Toxoplasma- free subjects did.
Nevertheless, later re- analyses of the same data showed that the increase in attractiveness of sexual masochism was merely relative. Infected subjects reported lower
attraction to all non- conventional sexual practices, but the decrease in attraction to
sexual masochism was the lowest (Flegr, 2017b).
Two studies showed that latent toxoplasmosis was associated with sexual promiscuity (Alvarado- Esquivel et al., 2006, 2021). e authors of the latter study compared the seroprevalence of toxoplasmosis in 315 sexually promiscuous subjects
(9.2 percent) and 3618 not sexually promiscuous subjects (4.6 percent). ey found a
highly signicant association between these two traits (odds ratio: 2.09, p = 0.0003).
eoretically, it seems possible that sexual transmission could be responsible for
this association, but it has been shown that the sexual transmission of toxoplasmosis
probably takes place only in the direction from infected men to uninfected women
(Hlavacova et al., 2021). In the abovementioned study, an association between toxoplasmosis and promiscuity was observed in men (odds ratio: 2.90, p < 0.0001)
but not in women (odds ratio: 1.34, p = 0.17). erefore, toxoplasmosis seems to
be the cause— rather than the eect— of higher promiscuity of Toxoplasma- infected
subjects.
Toxoplasmosis, Behavior, and Cognition 213
is result lends supports to a hypothesis, based on animal studies, about a
coactivation of hypothalamic circuits related to danger and sex in Toxoplasma-
infected individuals (Dass & Vyas, 2014; Flegr & Markos, 2014). It is known that
Toxoplasma- free rodents try to escape from places where they smell a cat, while
Toxoplasma- infected rodents are in fact attracted to such places (Berdoy et al., 2000;
Vyas et al., 2007a). Such behavioral change might increase the chance of transmission of Toxoplasma from an infected intermediate host to the denitive host, a feline
predator. A similar eect of toxoplasmosis, called the “fatal attraction” phenomenon,
was later observed in chimpanzees (Poirotte et al., 2016) and even in humans (Flegr
et al., 2011, 2018a). It should be noted, though, that the results of the two relevant
human studies signicantly diered. In the rst study (Flegr et al., 2011), the infected men rated the pleasantness of smell of diluted cat urine— but not of dog, horse,
tiger, and brown hyena urine— higher and women lower than the corresponding
non- infected controls. e second study (Flegr et al., 2018a) found the opposite pattern: infected men rated the pleasantness of cat urine smell lower and women higher
than the corresponding non- infected controls. is discrepancy could be explained
by an older observation that showed that the relationship between the attractiveness
of cat urine smell for Toxoplasma- infected rodents has a reverse U- shape (Vyas et al.,
2007b). It is high for medium urine concentrations and low for very low and very
high urine concentrations. erefore, the direction of the Toxoplasma- induced shi
in the attractiveness of cat urine smell could depend on the concentration of stimuli
and the experimental setup.
Speed of information processing
e rst study on the eect of latent toxoplasmosis on neurocognitive functions,
namely on reaction times, was published in 2001 (Havlicek et al., 2001). It showed that
60 Toxoplasma- infected blood donors had longer reaction times than 56 Toxoplasma-
free blood donors. e study also showed that in the Toxoplasma- infected subset, re-
action times positively correlated with time since infection, suggesting that reaction
times were over time worsening as more time elapsed since the acute phase of toxoplasmosis. ese data were later re- analyzed with three new sets of data: two sets
of blood donors N = 439 and one set of soldiers N = 315 (Novotná et al., 2008). All
datasets conrmed the existence of signicant eects of latent toxoplasmosis on reaction times, and all showed the eect of concentration of anti- Toxoplasma IgG
antibodies (i.e., of time since the acute phase of the infection) on reaction times. In
all four datasets, researchers have also detected an interaction between toxoplasmosis and the Rh blood group: Toxoplasma seropositivity was associated with much
longer reaction times only in Rh- negative subjects. In the blood donors, serological genotyping enabled identication Rh- positive subjects who were probably D/ D
homozygotes and those who were most likely D/ d heterozygotes. Analyses had shown
that Toxoplasma- seropositive D/ D homozygotes had longer reaction times, while
Infectious Disease and Neurocognition
Toxoplasma- seropositive D/ d heterozygotes had shorter reaction times than corresponding Toxoplasma- seronegative controls. e existence of an interaction between
toxoplasmosis, Rh factor, and reaction times was further conrmed in another study
performed on 436 university students (Flegr et al., 2008c).
e main eect of toxoplasmosis on simple reaction times was later observed in
data collected from 4200 adults examined in the third National Health and Nutrition
Examination Survey (NHANES III) from the United States Centers for Disease
Control and Prevention (Pearce et al., 2014). is study also showed some signicant eects of interaction between toxoplasmosis and socioeconomic status on the
results of other performance tests, namely the symbol– digit substitution test and serial number– digit learning test.
Another study performed on NHANES III data used three dierent cognition
tests (simple reaction time, symbol– digit substitution, and serial digit learning)
and serological data about eight dierent pathogens, including Toxoplasma (Gale
et al., 2016). e results showed a signicant association of toxoplasmosis with the
performance of 5662 young and middle- aged subjects in all three tests. It should be
noted, though, that the eects of some viruses (herpes 1, cytomegalovirus, hepatitis
A virus) were even stronger than those of Toxoplasma.
It has been known for a long time that toxoplasmosis is oen positively associated with schizophrenia and that schizophrenia patients have a prolonged startle
response latency. For this reason, researchers investigated the startle reaction to
acoustic stimuli in Toxoplasma- infected and Toxoplasma- free schizophrenia patients
and controls using prepulse inhibition (Massa et al., 2017; Pearce et al., 2013) and
prepulse facilitation (Příplatová et al., 2014) techniques. e rst study showed that
both Toxoplasma- infected and Toxoplasma- free patients with schizophrenia had sig-
nicantly a longer startle latency than corresponding controls. e longest startle
latency was observed in Toxoplasma- infected schizophrenia patients. e authors
concluded that both schizophrenia and toxoplasmosis slow down neural processing.
In a follow- up study (Massa et al., 2017), the authors analyzed data from 365 outpatients treated at an inner- city hospital, mostly with some psychiatric diagnoses. In
this study, Toxoplasma- infected subjects had an acoustic startle response signicantly
higher than Toxoplasma- free subjects, and both groups had a similar startle latency.
Another study measured prepulse facilitation (Příplatová et al., 2014). e authors
detected a signicant eect of toxoplasmosis and toxoplasmosis– sex and prepulse–
toxoplasmosis interactions only in the third part of the prepulse inhibition test, suggesting that toxoplasmosis might aect the ability of long- term concentration rather
than the maximum performance of subjects in tests. A similar observation has been
discussed in earlier papers that measured the performance of Toxoplasma- infected
subjects in simple reaction- time tests. Again, the eect of prepulse increased with
the decrease of anti- Toxoplasma IgG antibodies.
e observed eects of toxoplasmosis on reaction times were usually relatively
weak, explaining just 2– 8 percent of total variability. Still, this could have a signicant impact on the life of modern humans. A case– control study published in 2002
Toxoplasmosis, Behavior, and Cognition 215
showed that Toxoplasma- infected subjects had a 2.65 times higher risk of trac ac-
cidents than Toxoplasma- non- infected controls (Flegr et al., 2002). In contrast to
the situation with simple reaction times, the eect was weaker in 99 subjects with
low concentrations of anti- Toxoplasma antibodies (odds ratio: 1.86, 95 percent
condence interval: 1.14– 3.03), stronger in 34 subjects with moderate concentrations of anti- Toxoplasma antibodies (odds ratio: 4.78, 95 percent condence
interval: 2.39– 9.59), and much stronger in six subjects with very high concentrations of anti- Toxoplasma antibodies (odds ratio: 16.03, 95 percent condence in-
terval: 1.89– 135.66). is eect of antibody concentrations possibly suggests that
during, or immediately aer, the acute phase of toxoplasmosis, subjects are at the
highest risk of involvement in a trac accident. Two meta- analytic studies published in 2018 and 2019 found 12 studies, 11 of which suggested the eect of toxoplasmosis on the risk of being involved in a trac accident (Gohardehi et al., 2018;
Sutterland et al., 2019). e authors of the second (larger) study calculated not only
the aggregate odds ratio (1.69, 95 percent condence interval: 1.20– 2.38) but also
the population- attributable fraction. ey showed that if toxoplasmosis were fully
eradicated, the number of causalities due to trac accidents would decrease by approximately 17 percent.
Only one study analyzed Rh- positive and Rh- negative subjects (military drivers)
separately. is prospective cohort study found a signicant eect of toxoplasmosis
in Rh- negative subpopulation: Rh- negative subjects with high concentrations of
anti- Toxoplasma antibodies were at a six times higher risk of a trac accident than
Toxoplasma- free or Rh- positive subjects (Flegr et al., 2009). An ecological study
performed on aggregate data from 87 countries showed that the incidence of trac
accidents correlated with seroprevalence of toxoplasmosis if the prevalence of Rhnegative individuals in particular countries was controlled for (Flegr & Dama, 2014).
A study published in 2012 showed that in specic socioeconomic groups, latent toxoplasmosis might also be associated with workplace accidents (Alvarado- Esquivel
et al., 2012).
A recent meta- analytic study identied nine studies that investigated the eects
of toxoplasmosis on processing speed. Aggregate data suggest that the eect of latent toxoplasmosis is relatively low but highly signicant, with standardized mean
dierence equal to 0.12 (p = 0.001). It must be, however, borne in mind that all
these studies analyzed mixed populations of Rh- positive and Rh- negative subjects.
e aggregate eect of toxoplasmosis on processing speed in the Rh- negative
subpopulation is probably stronger.
A widely cited New Zealand study reported no eect of toxoplasmosis on processing speed and performance in various performance tests and no association
between toxoplasmosis and personality, schizophrenia, major depression, suicidal
attempts, or criminality (Sugden et al., 2016). On the other hand, the research design (a cohort study) and the number of participants (837, of whom 238 were
Toxoplasma infected) almost precluded the detection of any association of toxoplasmosis with rare disorders or events. For example, the authors found a relatively
Infectious Disease and Neurocognition
strong association of toxoplasmosis with suicide attempts (odds ratio: 2.63, 95 percent condence interval: 0.97– 4.44), but due to the low number of individuals with a
history of suicide attempts (24), even this eect was non- signicant. e probability
of demonstrating signicant eects of toxoplasmosis on performance or personality
was higher. Still, the majority of published eects explain just 2– 8 percent of variability, which is why the likelihood of getting signicant results for any particular
(randomly selected) test was not high. e authors performed no aggregate tests.
When one checks the results presented in the paper, one nds that the observed
shis of studied parameters took place in the anticipated direction in 17 out of 22
comparisons. A binomial test shows that the probability of obtaining such results
by chance is only 0.006. One can thus conclude that this study, too, in eect suggests
that toxoplasmosis could have a signicant eect on human behavior and mental
functioning.
It should be mentioned that in action- control tests, Toxoplasma- infected
subjects score better than their Toxoplasma- free peers. Compared to Toxoplasma-
free individuals, young men with latent toxoplasmosis achieved superior behavioral performance in challenging situations demanding cognitive control (Stock
et al., 2014, 2017). Toxoplasma- infected subjects also showed a larger improve-
ment of attention/ vigilance in the course of a test expressed by the Hit Reaction
Time Block Change parameter in Conners’ Continuous Performance Test II, although this overall acceleration of reaction times may be related to attention deterioration of Toxoplasma- infected individuals at the beginning of the test (Holub
et al., 2008).
Intelligence
e rst data showing a possible eect of latent toxoplasmosis on intelligence were
published in 1996. In this study, 190 Toxoplasma- infected men diagnosed with acute
toxoplasmosis in various Prague hospitals completed Cattell’s 16PF questionnaire
6 months to 13 years aer being diagnosed with acute toxoplasmosis (Flegr et al.,
1996). Cattell’s factor B (intelligence) had signicantly decreased with time since the
diagnosis even when the eect of age was controlled for. e same study repeated
with 230 women found no correlation with intelligence, and most cross- sectional
studies that used the same 12- item intelligence test likewise found no eect of latent
toxoplasmosis on factor B (intelligence). An exception was a study performed on
191 women tested for toxoplasmosis during pregnancy, where Toxoplasma- positive
mothers achieved higher intelligence scores (Cattell’s B) than Toxoplasma- negative
ones (Flegr & Havlíček, 1999). Another exception was a recent study performed on a
large cohort of internet users (Flegr et al., 2023). In this study, Toxoplasma- infected
men and women achieved higher Cattell’s intelligence scores than Toxoplasma- free
controls. is study will also be discussed later in this chapter under the heading
“Manipulation or side eects of the disease?”
Toxoplasmosis, Behavior, and Cognition 217
Dierent psychometric instruments were used in studies performed in the rst
decade of the twenty- rst century. In a study performed on 857 military conscripts
(male) (Flegr et al., 2003), the authors showed that Toxoplasma- positive subjects
scored signicantly worse in the OTIS verbal intelligence test (Otis, 1954). But in
that study, intelligence negatively correlated with anti- Toxoplasma antibodies titer
and thus positively with the duration of infection. is correlation suggests that the
observed lower performance of Toxoplasma- positive subjects in intelligence tests
was most likely a transient eect of recent acute toxoplasmosis rather than a cumulative eect of latent toxoplasmosis. Another group of 502 male soldiers was tested
with two intelligence tests: the Wiener Matrizen- Test (nonverbal test of general intelligence) and the OTIS verbal intelligence test (Flegr et al., 2013). In this study,
the RhD- positive Toxoplasma- infected subjects scored lower and RhD- negative
Toxoplasma- infected subjects higher on intelligence than their Toxoplasma-
free peers.
No association between toxoplasmosis and intelligence core measured with
the Stanford– Binet Intelligence Scale was observed in 109 patients with nonschizophrenia neurodevelopmental disorders (58, i.e., 53.2 percent Toxoplasma pos-
itive) (Shehata et al., 2016).
Memory
Several studies examined a possible relationship between memory functions and
toxoplasmosis. Most focused on investigating a possible association between toxoplasmosis and dementia. ey were therefore performed on clinical populations: patients with schizophrenia (Boronow et al., 2002), bipolar disorder (Dickerson et al.,
2014; Hamdani et al., 2015), Alzheimer’s disease, Parkinson’s disease, or other psychiatric disorders. In general, these studies found only a very weak association between latent toxoplasmosis and dementia. For example, a meta- analytical study
found no association between latent toxoplasmosis and Parkinson’s disease (N = 8,
odds ratio: 1.14; 95 percent condence interval: 0.78– 1.68) and only a trend toward
such association between latent toxoplasmosis and Alzheimer’s disease (N = 8,
odds ratio: 1.38; 95 percent condence interval: 0.99– 1.92) (Bayani et al., 2019).
At the same time, though, some studies performed on clinical populations found
worse performance in memory (and other) performance tests in the Toxoplasmaseropositive subpopulation of patients infected with HIV (Bharti et al., 2016; Ene
et al., 2016), in subjects infected with Helicobacter pylori (Gale et al., 2015), in patients with schizophrenia (Boronow et al., 2002; Veleva et al., 2022), and patients
with bipolar disorder (Hamdani et al., 2015).
A signicant complication of the studies performed on clinical populations is the
diculty of controlling for the patients’ biological age, which can oen dier from
their chronological age. Moreover, various disorders, including dementia, might
have numerous mutually independent causes. If the form related to toxoplasmosis