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TREPAR 2043 No. of Pages 11

Trends in Parasitology

Opinion

Toxoplasma gondii:

An Underestimated Threat?

Gregory Milne

1,2,

Joanne P. Webster,

1,2

and Martin Walker

1,2

Highlights

Accumulating evidence suggests that

latent infection with

Toxoplasma gondii

is associated with a variety of neuropsy-

chiatric and behavioural conditions.

Many of the conditions associated with

T. gondii

can be explained by emerging

understanding of the parasite’s neuro-

tropic activity.

The public health burden of latent infec-

tion may far outweigh that caused by

acute and congenital toxoplasmosis.

More powerful epidemiological studies,

in conjunction with further mechanistic in-

vestigations, are needed to strengthen

the evidence base for

T. gondii

as a

causal agent of a range of neuropsy-

chiatric and behavioural disorders.

Traditionally, the protozoan parasite

Toxoplasma gondii

has been thought of as

relevant to public health primarily within the context of congenital toxoplasmosis

or postnatally acquired disease in immunocompromised patients. However,

latent

T. gondii

infection has been increasingly associated with a wide variety

of neuropsychiatric disorders and, more recently, causal frameworks for these

epidemiological associations have been proposed. We present assimilated

evidence on the associations between

T. gondii

and various human neuropsychiatric

disorders and outline how these may be explained within a unifying causal frame-

work. We argue that the occult effects of latent

T. gondii

infection likely outweigh

the recognised overt morbidity caused by toxoplasmosis, substantially raising the

public health importance of this parasite.

Toxoplasmosis: Malaria’s Neglected Cousin

Evidence of exposure to the apicomplexan protozoan parasite

Toxoplasma gondii,

which is

capable of infecting all warm-blooded animals, is found in approximately 30% of the world’s

human population [1]. However, seroprevalence shows marked global variability: for example,

in highly endemic regions, such as parts of Africa, seroprevalence can reach almost 90% in

certain demographic groups, whereas in some European populations it can reach 60% [2].

Felines are the only known

definitive host

(see

Glossary)

for the parasite, shedding in their

faeces up to millions of

oocysts

per day, which sporulate and become infective in the environ-

ment [3]. While in domestic cats, oocyst shedding occurs for only 1–3 weeks after initial infection,

in wild feline species shedding may potentially continue intermittently for life [3]. Ingestion of these

sporulated oocysts, which contaminate crops, soil, and water sources [4,5], or consumption of

bradyzoites

from raw or undercooked meat comprise the two major horizontal routes of

transmission. Indeed, these parasite stages are responsible for a substantial burden of postnatally

acquired infections, causing both sporadic outbreaks of acute, symptomatic disease in immuno-

competent adults [6] and severe toxoplasmosis in immunocompromised individuals including

HIV/AIDS patients (following reactivation of bradyzoites into disseminating

tachyzoites)

[7].

In humans, congenital toxoplasmosis is acknowledged as a signiﬁcant public health problem

[8,9]. Vertical transmission of

T. gondii

from mother to foetus occurs most frequently following

a primary maternal infection during pregnancy (although other means of vertical transmission

are possible, including infection with an atypical genotype overriding acquired immunity from a

prior nonatypical exposure [10]). While the likelihood of maternofoetal transmission is highest in

the third trimester, the severity of congenital disease has the inverse relationship with gestational

age, such that

ﬁrst-trimester

infections generally result in the most severe clinical symptoms in

neonates, including spontaneous abortion or stillbirth [8]. While overall approximately 75% of

congenital cases are subclinical, congenital infection, amongst those surviving infants, can none-

theless result in various craniocerebral, ocular, and/or cognitive abnormalities in early or later life

(e.g.,

chorioretinitis, intracranial calcifications,

and learning difﬁculties [8]).

Latent

T. gondii

infection,

following postnatally acquired (acute) infection, has historically been

considered benign or even asymptomatic in immunocompetent individuals [1,11,12]. Yet a

Department of Pathobiology and

Population Sciences, Royal Veterinary

College, University of London,

Hertfordshire, UK

London Centre for Neglected Tropical

Disease Research, School of Public

Health, Imperial College London,

London, UK

*Correspondence:

[email protected]

(G. Milne).

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https://doi.org/10.1016/j.pt.2020.08.005

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Trends in Parasitology

burgeoning number of epidemiological studies suggest that the parasite can be associated with a

number of long-term behavioural effects in hosts, including humans. In rodent

intermediate

hosts,

T. gondii

can cause a range of behavioural alterations, including altered activity levels,

decreased neophobic behaviour and a

‘fatal

feline attraction’ to cat urine, thereby increasing the

efﬁciency of transmission to the deﬁnitive host [13]. Similar by-product (nonadaptive, or residual

manipulative [14]) behavioural effects, from the subtle (e.g., changes in personality traits) to the

severe (e.g., increased risk of schizophrenia), have also been identiﬁed in

T. gondii-infected

humans

[15,16]. Here, we review the increasing number of human disorders that have been linked to

T. gondii

infection and argue that these occult effects raise the public health burden of this chronic

parasitic infection far beyond that recognised by the overt burden of acute disease.

Glossary

Bradyzoite:

the slowly dividing, sessile

parasite stage that is found within tissue

cysts in various body regions, including

skeletal and cardiac muscles, the eyes,

and central nervous system (with a

preference for neuronal and glial cells).

Bradyzoites are associated with latent

infection and cannot be cleared by any

current antiparasitic drugs.

Chorioretinitis:

inﬂammation of the

choroid layer of the eye, superior to the

retina and inferior to the sclera. Scarring,

particularly in the macular region of the

eye, and necrotic lesions are consistent

features.

Definitive host:

the organism(s) which

supports the sexually reproducing stage

of the parasite.

Effect size:

the magnitude of an

association between an exposure

(e.g., infection) and an outcome

(e.g., neuropsychiatric disease). Effect

sizes are frequently expressed as RRs

(or relative risks) or ORs, depending on

the study design.

Intermediate host:

the organism(s)

which harbour the asexual parasite stage

and which allow for transmission to the

deﬁnitive host. In the case of

T. gondii,

examples include rodents and birds

(domestic cat prey) and potentially

wildebeest and zebra (large wild cat

prey).

Intracranial calcifications:

deposits

of calcium within the brain, often

associated with infection with TORCH

agents (toxoplasmosis,

Cytomegalovirus,

and

Herpes simplex

virus). Infants with these lesions are

suspected to be at increased risk of

severe neuropsychiatric sequelae.

Latent

T. gondii

infection:

the phase

T. gondii

infection in which the parasite

remains in tissue cysts as bradyzoites.

Odds ratio (OR):

an effect size metric

which is most frequently calculated from

case–control studies to quantify the

magnitude of association between an

exposure and a disease.

Oocyst:

an environmentally resistant

parasite stage which contains the sexual

stage sporozoites that are produced in

intestinal cells of the felid deﬁnite hosts.

Depending on environmental conditions

(e.g., temperature and humidity),

oocysts can survive for many years.

Population attributable fraction

(PAF):

the burden of a particular

outcome (e.g., a neuropsychiatric

disorder) attributable to an exposure

(e.g., infection with

T. gondii).

How Does

Toxoplasma

Cause Neuropsychiatric Disease?

The

ﬁrst

studies describing potential associations between latent

T. gondii

infection and human

neuropsychiatric disorders, in this case schizophrenia, were published in the 1950s [16] (even

before the parasite's life cycle was completely understood [17]). After a lull in interest, the 21st

century has seen a proliferation of studies

–

followed by a

ﬂurry

of meta-analyses

–

on associa-

tions between

T. gondii

infection and a wide variety of cognitive and neuropsychiatric disorders,

including Alzheimer’s disease [18], bipolar disorder [19,20], epilepsy [21], and obsessive–

compulsive disorder (OCD) [19] (Figure

1).

Whilst these studies and meta-analyses have

demonstrated consistent support for the link with schizophrenia [19,22,23], the strength of

evidence for other disorders is variable (Figure

2).

The association between

T. gondii

and neuropsychiatric conditions could partly be explained by

the inﬂuence of the parasite on the expression of several neurotransmitters. Dopamine dysregu-

lation, which has received the most research attention, results partly from the parasite’s ability to

synthesise tyrosine hydrolase (an enzyme involved in dopamine biosynthesis) [24].

T. gondii

also

alters the expression of a range of other neurotransmitters, including

γ-aminobutyric

acid (GABA),

glutamate, serotonin, and norepinephrine [25]. These effects might be mediated by the

encystment of bradyzoites in neural

–

and most often microglial or neuronal

–

cells, thereby

causing considerable alterations, both in host neurobiochemistry and in the expression of speciﬁc

receptors/transporters [24,25]. For example, it has been shown, in chronically infected mice,

that

T. gondii

decreases the expression of the glutamate transporter GLT-1, resulting in a twofold

increase in extracellular glutamate concentrations [26].

In humans, dysregulation of neurotransmitter expression is critically involved in many behavioural

and neuropsychiatric conditions, including bipolar disorder, depression, drug addiction, OCD,

schizophrenia, and suicide [25]. As examples, changes in dopamine-mediated neurotransmis-

sion have been proposed to be involved in the pathophysiology of both addictive and obsessive

behaviours [27,28], and serotonin is thought to play a central role in the aetiology of mood disor-

ders [29].

T. gondii

is auxotrophic for tryptophan

–

the precursor to serotonin

–

and low plasma

tryptophan concentrations have been associated with cases of severe depression [30,31].

A differing hypothesis is that changes in the endocrine system, particularly testosterone expres-

sion, may drive behavioural changes observed in

T. gondii-infected

hosts. The evidence to

support this comes from

in vivo

studies showing that infected male rodents have higher concen-

trations of testosterone and reduced innate aversion to the odour of cat urine compared with

uninfected controls (with castration prior to infection rescuing the aberrant behavioural

phenotype) [32,33]. Furthermore, castrated male mice given exogenous testosterone display a

reduced aversion to cat odour, which corresponds to changes in regulatory gene methylation

patterns in the extended medial amygdala [33]. Hence, it is plausible that both neurotransmitter

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(A)

No. published datasets

Disorder

Addiction disorder

Alzheimer’s disease

Bipolar disorder

Epilepsy

Major depression

Obsessive compulsive disorder

Parkinson’s disease

Rheumatoid arthritis

Schizophrenia

Suicide attempts

Traffic accidents

1960

1970

1980

1990

2000

2010

2020

(B)

No. published datasets

WHO Region

African region

Eastern Mediterranean region

European region

Region of the Americas

South-East Asian region

Western Pacific region

Risk ratio (RR):

an effect size metric

typically calculated from cohort studies

to quantify the risk of developing a

disease associated with a particular

exposure.

Secondary host:

the organism(s) which

harbour the asexual parasite stage. Unlike

intermediate hosts, secondary hosts do

not necessarily support transmission to

the deﬁnitive host. In the case of

T. gondii,

examples include non-prey animal

associates of the cat, for example, cow,

sheep, and human.

Tachyzoite:

the rapidly dividing

parasite stage associated with acute

infection. Tachyzoites disseminate

infection throughout the body prior to

the establishment of a latent infection

and may reactivate from bradyzoite

cysts in immunocompromised

individuals.

Toxoplasmic encephalitis:

inﬂammation of the brain which results

most often from

immunocompromisation and

subsequent reactivation of

T. gondii

bradyzoites in tissue cysts into

tachyzoites, which invade and replicate

in new cells.

1960

1970

1980

1990

2000

2010

2020

Trends in Parasitology

Year of publication

Figure 1. Publication History of Datasets from Meta-Analyses Associating

Toxoplasma gondii

with

Neuropsychiatric and Behavioural Disorders.

Published datasets (N = 202) collected across 39 countries and cited

in 15 systematic reviews, assessing the relationship between

T. gondii

infection and neuropsychiatric disorders, grouped

by: (A) speciﬁc neuropsychiatric and behavioural disorders (arrows indicate years in which systematic reviews were

published); (B) World Health Organization (WHO) global region in which the study was conducted (for all studied disorders

combined).

and endocrine dysregulation have roles to play in the behavioural changes associated with

T. gondii

infection.

A notable exception to the neuromodulation-based (or endocrine-based) mechanistic explanation

is the association of

T. gondii

infection with epilepsy. Development of epilepsy is likely primarily

determined by the pattern and extent of cyst presence and/or rupture in the brain and the subse-

quent formation of scar tissue [34]. In agreement with a nonspeciﬁc infectious aetiology of some

cases of epilepsy, a case–control study conducted in sub-Saharan Africa found the prevalence

of active convulsive epilepsy to be higher among individuals seropositive for

T. gondii,

although

also among those seropositive for a range of other parasitic infections, including

Onchocerca

volvulus

and

Toxocara canis

(with numerous other studies reporting associations between epilepsy

and the aetiological agent of neurocysticercosis,

Taenia solium

[35]) [36]. Interestingly, the

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Traffic accidents

Suicide attempts

Schizophrenia

Rheumatoid arthritis

Parkinson’s disease

Obsessive-compulsive disorder

Major depression

Epilepsy

Cryptogenic epilepsy

Convulsive epilepsy

Bipolar disorder

Alzheimer’s disease

Addiction disorder

Odds ratio (95% CI)

Trends in Parasitology

Figure 2.

Toxoplasma gondii

and Associated Human Behavioural and Neuropsychiatric Disorders.

The

association of

T. gondii

IgG seroprevalence in humans with various disorders compared with IgG seroprevalence in

healthy control subjects. Each odds ratio (OR) shows the strength of association stated in a published systematic review

and meta-analysis [18–21,23,46,47,71–77]. For some disorders more than one meta-analysis was published, and hence

multiple ORs are presented in these cases. Note that the ORs for cryptogenic and convulsive epilepsy are calculated using

subgroup analysis of one of the epilepsy metanalyses [21]. Studies were found using PubMed and Google Scholar with

terms

“Toxoplasma

gondii

OR toxoplasmosis” AND [name of disorder]. Abbreviation: CI, conﬁdence interval.

combined effects of certain coinfecting agents was more than additive [36],

ﬁndings

which are in

agreement with the increasingly recognised notion that many human neuropsychiatric disorders

may have a multi-infectious agent causation (Box

[37,38].

Another potential key and interrelated mechanistic factor in the propensity of

T. gondii

to cause

behavioural alterations is the host's immune response. Following infection, immune cells in the

small intestine, including innate lymphoid cells, are stimulated to produce a range of cytokines

and transcription factors. Notable amongst these defences are cytokines, including interferon-

gamma (IFN-γ), interleukin-12 (IL-12), and tumour necrosis factor-alpha (TNF-α), and

chemokines, including CCL2 and CXCL2, which are produced by activated immune cells such

as macrophages and dendritic cells, prompting further immune cell activation and anti-T.

gondii

gene expression (for a review see [39]). While this immune response mediates the effective control

of acute infection, it also contributes to the maintenance of parasite latency and, potentially, to the

development or aggravation of neurological sequelae. Studies using rodent models have

observed excessive levels of proinﬂammatory cytokines, including TNF-α and IL-6, in the

serum of mothers of offspring who later developed psychotic-like symptoms [40]. Interestingly,

TNF-α and IL-6 have both been shown to promote bradyzoite formation in murine and cell culture

models [41], and in humans, perinatal exposure to TNF-α and IL-8 has been linked to develop-

ment of schizophrenia in offspring [42].

It is therefore likely that a number of mechanistic pathways, including endocrine and neurotrans-

mitter dysregulation and the proinﬂammatory immune response to infection (including, potentially,

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Box 1. A Multiagent Model of Neuropsychiatric Disease

Accumulating evidence suggests that the abnormal behaviour of

T. gondii-infected

intermediate/secondary

hosts

may

result from coinfection with other neurotropic pathogen(s). For example, serological testing of schizophrenic patients

and matched controls showed that schizophrenic patients more often had IgG antibodies to various coinfecting agents,

including

T. gondii

but also

Chlamydia trachomatis,

human herpesvirus-6, and measles virus [37,38]. In agreement with

these

ﬁndings,

our recent research has shown that UK foxes housed in sanctuaries with aberrant behaviours indicative

of neuropsychiatric impairment have a higher prevalence of

T. gondii/vulpine Circovirus

(FoxCV) coinfections relative to wild

foxes [64]. Thus,

T. gondii

infection and its interaction with other coinfecting neurotropic pathogens may be a factor

contributing to the aetiology of human neuropsychiatric disorders like schizophrenia.

An extension to this model could include additional interactions with host and pathogen genetics [22]. In terms of host

genetics, dysregulated expression or changes in the 'disrupted in schizophrenia ' (DISC1) protein have been associated

with a predisposition to schizophrenia [22]. Moreover, this protein has been implicated in the immune response against

T. gondii,

with individuals with altered DISC1 having higher titres of

T. gondii

antibodies [65]. Furthermore, the HLA-D al-

leles (e.g., HLA-DQ3, HLA-DQA1/B1) have been shown to play a key role in determining the likelihood and outcome of

both

T. gondii

congenital disease and disease in immunocompromised patients [66]. These

ﬁndings

support a tripartite

model of clinical disease outcome, which could be further extended given the addition of strain-speciﬁc differences in par-

asite pathogenicity.

Parasite genotype may partly determine patterns of neuropsychiatric disease [63]. For example, mothers infected with

genotype I parasites have almost two times the odds of birthing a child who develops psychosis in later life, compared with

seronegative mothers [67]. This pattern is not seen for other genotypes, perhaps suggesting differing parasite tropisms

between lineages or differences in strain virulence factors, which could exacerbate the development of psychoses. Fur-

ther, so-called atypical genotypes (those not captured by the traditional type I–III classiﬁcation) circulating in the

Americas have been associated with a greater burden and severity of ocular disease [63].

T. gondii

epidemiology is therefore

multifactorial, with different factors interacting and combining to alter the likelihood and outcome of clinical

–

and plausibly

neuropsychiatric

–

disease. These factors likely include: (i) host genetics (and epigenetics); (ii) parasite genetics (types I–III,

atypical); (iii) population disease endemicity (seroprevalence); and (iv) coinfecting neurotropic agents.

interactions between pathways [24]), act in parallel to explain the diversity of neuropsychiatric

disorders associated with

T. gondii

infection (Figure

2).

Indeed, emerging evidence from rodent

models suggests that

T. gondii

may even transgenerationally modulate host behaviour, including

anxious and depressive symptoms, via paternally inherited epigenetic changes [43].

Tip of the Iceberg: An Underestimated Burden of Disease?

Notwithstanding the general scarcity of data on the causal nature of

T. gondii

infection and human

neuropsychiatric disorders (with the exception of some cases of schizophrenia [13,22]), it is

possible to approximate the

population attributable fraction

(PAF) from the

effect sizes

(odds

ratios, ORs)

estimated from meta-analyses (Box

2).

This approach has been used to

estimate that 21.4% (13.7–30.6%) of schizophrenia cases are associated with

T. gondii

infection

[44]. Assuming a global incidence of schizophrenia of 15.2 per 100 000 [45], this means that

between 150 000 and 335 000 cases per year may be attributable to

T. gondii.

While this PAF

is based on an OR of 2.71 from an older systematic review [46,47] (higher than a more recent

review [19];

Figure 2),

these numbers nonetheless highlight the potentially signiﬁcant global

burden of

T. gondii-associated

schizophrenia. Indeed, this is particularly the case considering

that the psychopathology of

T. gondii-related

schizophrenia has been reported to be more severe

and of longer duration than schizophrenia unrelated to

T. gondii

[48].

Another recent study estimated, assuming equivalence of ORs and risk ratios (RRs) (Box

2),

that

T. gondii

may account for approximately 17% of trafﬁc accidents (6–29%) and 10% of

suicide attempts (3–19%) [23]. The World Health Organization (WHO) estimates that there are

20–50 million non-fatal trafﬁc-related injuries per year [49]. Taking the central value of 35 million

accidents,

T. gondii

may therefore be associated with between 2.1 and 10.2 million non-fatal

trafﬁc accidents per year. A similar illustrative calculation is possible for suicides and non-fatal

suicide attempts (NFSA). For every suicide fatality there are roughly 20 attempts [50] and

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therefore, given average annual global suicide rates of 9.94 per 100 000 [50], between

0.46 million and 2.91 million NFSAs per year may be attributable to infection with

T. gondii.

How do these numbers compare with what is known about the overt burden of disease? It is

estimated that between 179 300 and 206 300 cases of congenital toxoplasmosis (CT) occur

annually [51], which result in an estimated 5900 cases of foetal loss and neonatal death,

24 700 cases of chorioretinitis in the

ﬁrst

year of life, and 9300 cases of hydrocephalus

and other central nervous system (CNS) abnormalities [52]. Disease in immunocompromised

individuals also poses a considerable burden. For example, a recent meta-analysis estimated

that there are 13.1 million HIV patients coinfected with

T. gondii

[7]. While this

ﬁgure

is substantial,

current data nonetheless suggest that the now routine use of combination antiretroviral therapy

(cART) in individuals with HIV diagnoses has signiﬁcantly mitigated the risk of

T. gondii-associated

sequelae [including

toxoplasmic encephalitis (TE)]

[53]. Furthermore, acute outbreaks of ac-

quired toxoplasmosis, the largest of which resulted in 1400 human infections, occur locally and

sporadically [6]. The case numbers of the overt manifestations of

T. gondii

infection are therefore

likely dwarfed by those of neuropsychiatric disorders, non-fatal trafﬁc accidents, suicide, and

NFSAs associated with latent

T. gondii

infection.

While a sizeable (yet inestimable) burden of sequelae may still be present amongst

immunocompromised/HIV-positive subpopulations either not seeking healthcare nor complying

Box 2. How Well Can the Odds Ratio Approximate the Risk Ratio?

Epidemiological studies are designed to determine whether there exists an association between an exposure (e.g., infection

with

T. gondii)

and an outcome (a particular neuropsychiatric disease). They can be broadly split into observational or

experimental study designs. Observational designs include case–control studies and cohort studies.

Case–control studies are particularly useful when an outcome is rare in a population. Case participants (with the disease)

and controls (without the disease) are included, and the exposure status of each participant is ascertained retrospectively.

The proportion of cases with the exposure of intertest is then compared with the corresponding proportion of controls,

usually expressed as an OR. A large majority of studies that have identiﬁed associations between neuropsychiatric

disorders and exposure to

T. gondii

have used case–control designs.

Cohort studies are an alternative to case–controls studies, with the key advantage that they measure exposure before the

outcome of interest, a more powerful indicator of causality. Cohort studies involve tracking groups of participants (cohorts),

either retrospectively or prospectively, with and without the exposure of interest and measuring the frequency (incidence)

of the disease in each group. The relative frequency of disease occurrence in exposed and nonexposed cohorts is typically

expressed as a

risk ratio

(RR).

The PAF can be calculated to determine what proportion of cases in a population can be attributed to the exposure of

interest and is given by:

PAF

RR−1

Þ þ

�½I

where

is the proportion of the population exposed (e.g., to

T. gondii).

To calculate the PAF using the results from a

case–control study, one can either assume equivalence of the OR and RR, or adjust the OR using an estimate of the ab-

solute risk of the outcome in the unexposed population,

[68],

1−R

Þ þ

�½II

If a disease is uncommon in the population, the denominator in Equation

tends to unity and gives equivalence of the RR

and the OR (the so-called rare disease assumption [69]). For diseases like schizophrenia, with a worldwide incidence of

15.2 per 100 000 [45], the OR will provide a good approximation of the RR. For more common outcomes, like addiction

disorder, with an incidence of approximately 2000 per 100 000 [19,70], an OR >1 will provide a biased overestimate of the

RR and can be adjusted using Equation

[68] (Figure

I).

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Figure I. The Odds Ratio (OR) as an Approximation of the Risk Ratio (RR).

Simulated cohorts were constructed

to illustrate the relationship between the incidence of an outcome, the OR and the RR. Each line indicates the change in OR

with increasing incidence and a

ﬁxed

RR. Data on the incidence of outcomes and their OR associations with

Toxoplasma

gondii

were extracted from the literature: addiction disorders (using incidence data for all substance-use disorders) [19,70],

schizophrenia [45,47], suicide attempts [23,50], and trafﬁc accidents (using incidence data for all transport-related injuries)

[70]. Adapted from [68].

with cART or TE prophylaxis [54], it remains plausible that, without accounting for the burden of

T. gondii-associated

neuropsychiatric and behavioural conditions, we may be seeing only the tip

of the iceberg of

T. gondii’s

effects on human populations.

Implications and Applications for Public Health

While accurate estimates of the burden of latent

T. gondii

infection are required, so too are parallel

efforts towards improving public health interventions. Current interventions against

T. gondii

primarily focus on prevention of congenital transmission and treatment of acute disease.

Prevention is largely limited to health advice on avoiding exposure during pregnancy and, in

some countries, antenatal screening programmes that offer treatment with spiramycin to

women who seroconvert during pregnancy. Treatment of acute disease is with pyrimethamine

and sulfadiazine, although treatment failures are signiﬁcant [55].

Whether current treatment and prevention practices have any tangible effect on neuropsychiatric

sequelae will crucially depend on how the age of infection inﬂuences the likelihood of developing

neuropsychiatric disease. For example, if congenital acquisition of infection was particularly likely

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to cause neuropsychiatric sequelae later in life, then antenatal screening and treatment programs

may be an effective intervention strategy.

By contrast, if the development of neuropsychiatric sequelae was mostly associated with

(horizontally) acquired infections, treatment of acute cases may be ineffectual since the vast

majority of such cases are unidentiﬁed because of mild and vague symptomology. While different

strategies [11] could be employed to reduce the burden of horizontally acquired infections, a

tailored response

–

one that efﬁciently distributes control resources

–

would require setting-

speciﬁc information on the predominant route of infection (bradyzoite vs oocyst). Such informa-

tion could be estimated using recent developments in sporozoite-speciﬁc serology [56].

Future Directions

Ultimately, to achieve better control of

T. gondii

and reduce its public health burden, it will be

important to further basic science research (including behavioural studies on non-human

primates with more similar neurological structures to humans) to better understand the mecha-

nisms driving neuropsychiatric associations, the role of host and parasite genetics on sequelae

(Box

1),

and to conduct more powerful epidemiological research to strengthen the evidence

base for these associations (Box

2).

As the majority of epidemiological studies to date have been either cross-sectional or case–

control, they cannot determine the temporality of an association (Box

2).

Cohort studies, by

contrast, can provide supportive evidence of causation by identifying the presence of exposure

(infection) prior to the onset of the outcome (neuropsychiatric disease). Ideally, this is achieved

by following individuals for many years measuring exposure and outcome through time. However,

in the case of schizophrenia, as the average age of onset is 23 years [16], such a long time to

disease onset realistically precludes the use of prospective cohorts. This limitation could be

ameliorated through the improvement of assays which provide estimates of the timing of infection

by measuring

T. gondii-speciﬁc

antibodies, for example, by improving the temporal resolution

obtained from IgG avidity tests. Nevertheless, in the absence of such advances, retrospective

cohorts, which are less resource-intensive and costly designs, could potentially be constructed

to identify temporality of the exposure–outcome relationship.

Randomised controlled trials (RCTs) can test the efﬁcacy of

T. gondii

treatment on the alleviation

of neurological symptoms, an approach that has been taken before [57,58]. At least

ﬁve

RCTs

have been performed in

T. gondii-positive

schizophrenic patients to assess the impact of adjunc-

tive antiparasitic drugs on schizophrenic symptoms [57,58], although none have documented

any impact on the severity of schizophrenic symptoms. This

ﬁnding

could be due to the choice

of adjunctive treatment: azithromycin, trimethoprim, artemisinin, artemether, and valproate have

no demonstrated

in vivo

efﬁcacy against bradyzoites [57]. It is also possible that treatment did

not affect existing symptoms because behavioural changes resulting from cyst formation are

irreversible, even in instances when cysts degrade [59].

Recently, spiramycin combined with metronidazole (a blood–brain barrier efﬂux pump inhibitor)

has been shown to signiﬁcantly reduce bradyzoite cysts in latently infected mice [60] (several

other candidates, including miltefosine and guanabenz, have also shown promise; for a review

of antibradyzoite drug targets see [61]). This combination may therefore overcome drug brain

penetration issues, allowing antiparasitics to reach therapeutic concentrations. Nonetheless, if

behavioural abnormalities are indeed irreversible [59], future efforts should instead focus on pre-

ventative measures such as cat, human, and livestock vaccines. While an effective vaccine for

preventing

T. gondii-induced

sheep and goat abortions exists, the development of cat vaccines

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Trends in Parasitology

is in its infancy, and no human vaccine exists [though promising targets include, amongst others,

rhoptry proteins and surface antigen (SAG) proteins] [62].

Ensuring representation of different geographic regions in which epidemiological studies are

conducted

–

particularly where high-quality data remain scare, such as in South America and

sub-Saharan Africa (Figure

1B)

–

will also be important for several reasons. Firstly, parasite

genotype appears to be geographically heterogeneous and may possibly be predicted to drive

differences in the likelihood and severity of various clinical disease outcomes (Box

[63].

Secondly, host genetics and gene–environment interactions (including epigenetics [43]) may also

play a role in determining disease burden (Box

1).

Finally, criteria for diagnosis of different neuropsy-

chiatric disorders are likely highly geographically variable, making outcome ascertainment differ by

global region. Therefore, to verify the robustness of these associations, it is imperative that new

epidemiological studies be performed in currently under-represented populations.

Outstanding Questions

What are the mechanisms underlying

epidemiological associations between

latent

T. gondii

infection and speciﬁc

neuropsychiatric disorders?

To what extent does parasite genotype

inﬂuence the onset, maintenance, and

type of neuropsychiatric disease?

How does age of infection inﬂuence the

likelihood of developing neuropsychiatric

disease?

Do interactions between

T. gondii

and

other neurotropic agents affect the

likelihood of developing neuropsychiatric

disease?

How effective are antiparasitic drugs at

reducing the symptoms of psychiatric

illness?

Concluding Remarks

The burgeoning number of associations of

T. gondii

with various neuropsychiatric disorders

–

alongside compelling causal explanations supporting such links

–

suggest that the impact

of this pervasive parasite on global populations has been greatly underestimated. More re-

search is needed to strengthen the epidemiological evidence base for these associations,

but crucially also to improve understanding of the causative mechanisms (see Outstanding

Questions). Ultimately, better prevention, treatment, and transmission control will be required

to reduce the public health burden of

T. gondii.

There currently exists no effective treatment

for latent

T. gondii

infection; acute disease treatments, and prophylaxis to prevent vertical

transmission, are imperfect and there are no human or cat vaccines. Together with its com-

plex epidemiology,

T. gondii

is a substantial and incompletely understood global public

health challenge.

Acknowledgments

We are grateful for funding from the Biotechnology and Biological Sciences Research Council (BBSRC) [London Interdisciplinary

Doctoral Training Programme grant to G.M. (grant number BB/M009513/1)] and from the London International Development

Centre (LIDC), (pump-priming grant to M.W. and J.P.W.).

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