Does Toxoplasma gondii Related to Schizophrenic Symptoms? A Narrative Systematic Review

Background: Accumulating evidence associated infectious agents with schizophrenia. Majority of these studies analyzed Toxoplasma. gondii association with schizophrenia. Aims and Objectives: the present study aimed to perform systematic search on studies conducted on investigating association between T. gondii and schizophrenia using IgG antibodies against T. gondii. Secondary objective was to discuss possible mechanisms by which T. gondii linked to schizophrenia. Methods: Systematic search performed using Google Scholar, PubMed, Web of Science and University of Manchester databases.


INTRODUCTION
Behavioral and psychiatric disorders significant affects public health since they frequently occur and affect 10-16% of community world-wide [1,2]. They are not only associated with increased level of individual's distress, premature mortality, and disability, but led to substantial increase of cost related to lost productivity and utilization of health services. These diseases constitute 12% of global disease burden [3].

Definition and burden
Schizophrenia is serious neuropsychiatric disorders of unknown origin [4], disease affects approximately fifty million individuals worldwide. Disease burden on economy, either directly or indirectly. Direct impact of the disease constitutes 1.5-3% of total national healthcare budget worldwide, which includes hospitalization, medications, procedures, diagnosis and prolonged care services [5], whereas indirect burden includes increased premature mortality rate as result of suicide and unemployment rate and reduced productivity in work place. Disease's cost indirect burden exceeds those of direct costs by 20% [6,7].

Symptoms and clinical manifestation
Schizophrenia onset time differ between males and females. In females, disease onset usually begins in late adolescence to middle adulthood, while in men disease onset usually develops earlier [8]. Disease characterized by two types of symptoms: negative and positive. Positive symptoms manifested as hallucination, delusions and unusual motor behaviors with different severity grade, whereas negative symptoms presented as reduction in emotion expression, induction of goal-directed behavior and hostile thinking style to the people. In addition, alogia and anhedonia present in some patients [9,10]. Cognitive symptoms occur and grouped as third category. These symptoms include disorganized thought, speech, and potentially attention, eventually debilitating person's capability to communicate [11].

Risk factors
Unfortunately, until now no causative agent discovered in relation to schizophrenia, however, there are some risk factors that play role in causing disease. These include: abnormalities in neurons development, genetic susceptibility, and environmental associated factors [12]. Pathogens as cytomegalovirus, Herpes simplex virus, influenza virus, and Toxoplasma. gondii, contribute in causation of some behavioral and psychiatric disorders. Many studies focused on schizophrenia association with T. gondii infection [13].

Definition and background
T. gondii is commonest protozoan parasite that belongs to Apicomplexa phylum, a phylum that contains 5000 species. Only minority of species (e.g. Plasmodium spp. and Cryptosporidium spp.) responsible for causing disease in humans [14]. Parasite is categorized into three major genotypes, which vary in their virulence characteristics and distribution (epidemiology). For instance, type I strain is highly virulent in mice and commonly found in patients with ocular toxoplasmosis [15]. Both type II and III strains are not virulent in mice, but frequently involved in human infections in Europe and North America, although type III is less frequent than type II [16]. Type I and II involved in congenital diseases and HIV patients [17,18].

Prevalence
T. gondii infection is widely distributed and approximately one third of world's population infected with Toxoplasma [19]. Disease incidence differs depending on weather, hygienic and nutritional habits and geographical area [11]. Antibodies prevalence to T. gondii in London was 22%. These different stages vary in their resistance to environmental conditions. Tachyzoite is most sensitive form to extreme environmental condition as temperature, high salt and proteolytic enzymes (e.g. pepsin found in GIT of both secondary and primary host). Because of this high sensitivity, they rarely survive outside host and hence, they are of less concern epidemiologically except for vertical transmission (mother to baby) [ Primary stage (tachyzoites) replicates rapidly by frequent endogeny in all body cells. Once immune system starts to suppress infection, final generation of tachyzoites generates second stage of asexual reproduction, which results in tissue cysts generation. In tissue cysts, cystozoites or bradyzoites replicate slowly by endodyogeny [25,24]. Tissue cysts increased tropism to muscular and neural tissues.
They are mainly found in eyes, CNS and skeletal and cardiac muscles; however, they may locate in kidney, lung and liver, but less common [24]. Tissue cysts remain infectious and persist for lifespan of intermediate hosts [23]. 2) Cats can be infected by eating prey animal whose tissues contain T. gondii cysts. Once cat infected by tissue cysts, bradyzoites undergo asexual dissemination which consist of many stages of endopolygeny in feline's small intestine. Final stages of this dissemination create sexual life cycle replication phase. Oocysts and gamogony also formed in small intestine epithelium. Unsporalated oocysts discharged into intestinal lumen and excreted into environment via cat feces. Sporogony happens in environment and result in infectious oocysts formation which possess 2 sporocysts and four sporozoites in each [25,24]. Not only bradyzoites cause infection to cats, but also tachyzoites and oocysts. Successful infection rate is not as high as with bradyzoite cysts [24]. While in humans, T. gondii infection acquired via four routes: 3) by ingesting meat from cattle, pigs, sheep or poultry whose tissues contain bradyzoites cysts. 4) by ingesting oocysts after handling cat feces or materials contaminated with cat feces. 5) from mother to baby via placenta if mother infected with T. gondii during pregnancy (by tachyzoites stage) [26]. 6) by organ transplantation where T. gondii can be in form of tachyzoites or bradyzoites. T. gondii transmission in tachyzoites form occurs during blood transfusion. Infection chances via this route are rare and occur mainly if blood donor has acute infection [23].

Prevalence of different infection routes in human
Both ingesting sporulated oocysts after handling cat feces and ingesting lightly-cooked meat from cattle and pigs are common infection routes in human. gondii [31]. Most common method used higher titers of anti-T. gondii antibodies in schizophrenic patients versus controls [34]. Although some investigations revealed association, others did not find. It is still debatable whether T. gondii causes schizophrenia or behavioral changes caused by disease favored T. gondii acquisition [35].
Primary objective of the present study was to evaluate association between schizophrenia and T. gondii by performing systematic search on studies conducted in last ten years (2010-2020). Second objective was to discuss possible mechanisms by which T. gondii linked to schizophrenia.

Databases
Studies that investigated association of T. gondii with schizophrenia were retrieved using PubMed, Google Scholar, Web of Science, and University of Manchester Library databases. Key words used were Toxoplasm* and Schizo*. Title and abstract of articles were examined and selection made based on inclusion/ exclusion criteria. Additionally, reference lists of reviews and retrieved articles were screened and relevant articles were cross-referenced. Only 24 studies were included in the study.

Procedure
The total 24 research articles that met inclusion criteria were subjected to the review study. Firstly, it was identified 122 articles after duplicates removed. Of these 32 were reviews. Title and abstract of remaining 90 articles screened and 49 were assessed for eligibility. In total, articles number that met all bellowmentioned criteria were 24. Flow chart (PRISMA. http://www.prisma-statement.org/) presenting information through different stages of systematic review was shown in Fig. 2.

Inclusion/ Exclusion criteria
Inclusion criteria were; case-control study, primary research, published in English, published in last ten years (2010 to 2020), cases in studies definitely diagnosed by schizophrenia according to DSM-IV or ICD-10 criteria, specifically examining association of T. gondii with schizophrenia, included measurement of anti-T. gondii IgG antibodies titer rather than IgM antibodies as IgM antibodies affected by presence of high IgG level, which leads to falsenegative results. Rheumatoid factors existence influences IgM titers, resulting in false-positive results [36], Presence of statistical comparison between control and schizophrenia.

Exclusion criteria
Exclusion criteria were; review articles, articles that are not completed (e.g. letter to editor), maternal or neonatal studies, animal studies (but these referred to in discussion sections), cohort studies, studies investigating effects of T. gondii seropositivity on schizophrenic patients without controls, studies measured risk ratio of schizophrenia development.

RESULTS
Twenty-four studies included in this report summarized in Table 1

DISCUSSION
This systematic review was carried to find studies that examined association between schizophrenia and T. gondii using IgG antibody titers, aiming to investigate whether there is an association between T. gondii and schizophrenia. Further, it aimed to discuss the nature of this association (whether T.gondii causes schizophrenia or behaviuor changes caused by schizophrenia increased the acquisition of T. gondii). As indicated in studies summarized in Table 1 that in most studies, there is an association between schizophrenia and T. gondii, with two thirds of studies indicated that schizophrenic patients were more likely to have anti-T. gondii IgG antibodies versus controls with statistically significant, P value ranging from P=0.00006 to <0.05. This finding of reviewed studies consistent with other meta-analysis examining association between schizophrenia and T. gondii, that were out of current research scope, despite having some of studies from these meta-analyses in this review. Torrey et al [59] reported in their meta-analysis that schizophrenic patients had higher seroprevalence of T. gondii seropositivity than controls with odd ratio 2.73, and 2.54 for first episodes. The meta-analysis covered 38 studies reported odds ratios of 2.71 (95% confidence interval (CI) 1.93-3.78), acknowledging relation between T. gondii and schizophrenia disorder [60]. Sutterland et al [35] carried meta-analysis on 42 studies and found significant association between T. gondii and schizophrenia with odd ratio 1.81, P<0.00001. Taken together, current study and past meta-analyses point clearly to presence of association. Indeed, association between schizophrenia and T. gondii observed in many other study types, not only epidemiological studies. Studies summarized in two main categories; first category is association could be explained by T. gondii being risk factor for schizophrenic symptoms development, or alternatively schizophrenia might favor T. gondii acquisition via behavioral changes caused by schizophrenia.
Behavioral changes that T. gondii causes in infected individual observed clinically in schizophrenic patients, leading to hypothesis that T. gondii might proceed to schizophrenia. However, these findings comes from descriptive studies, not causative ones. There is a need of the causative research dealing with DOPA release with the T. gondii infection [61][62][63][64].
Schizophrenia is multifactorial disease, and needs combination of both genetic and environmental factors explanations. T. gondii is important risk factor for its development in individuals with both genetic and environmental susceptibility [65], and risk exceeded risk of any other human gene with OR< 1.40, as reported by genome-wide linkage study [66]. Improvement in both sensitivity and specificity of results needed to solve these differences. This achieved by conducting studies that uses IgG antibodies and PCR. Only few studies used both methods to investigate association.
Exposure to cats and eating lightly-cooked meat effects investigated only in few studies and significant effect on association was found. Research could be conducted on psychiatric outpatients with healthy controls in same area of residence or carrying out cohort studies using PCR and IgG antibodies to ascertain association.
Another question for future research is determining infection time as this would identify relation between schizophrenia and T. gondii. Niebuhr et al [67] found association before schizophrenia onset, it was difficult to ascertain this association due to nature of military personnel's lifestyles that made them more exposed to T. gondii [49]. It would be interested to examine combination of antimicrobial effects of antipsychotic drugs with antiparasitic treatment. This might provide possible cure for disease.
Finally, prenatal infection with T. gondii results in severe consequences on fetus. Hence, determining exact mechanism by which T. gondii cause these consequences is point that needs further research.

CONCLUSION
Many epidemiological studies associated T. gondii with schizophrenia. However, other epidemiological studies did not find this association. So, it was difficult to confirm aetiopathogenesis association between T. gondii and schizophrenia. However, given that schizophrenia is multifactorial disease and not all schizophrenic patients infected with T. gondii, T. gondii identified as an important risk factor for schizophrenia development. Therefore, upcoming research should ascertain association between T. gondii and schizophrenia, to open up new perspective in treatment and prevention of this disease. It is vital to continue research on this area.

LIMITATIONS
There are two major limitations for the present study. First, review did not include studies that published in languages other than English, which could lessened methodology power. However, exclusion of these studies aimed to construct a replicable systematic search, which is regarded as gold standard. Second limitation was the limiting date of included studies to years 2010 to 2020. This could also affect methodology power. Nevertheless, this aimed to clearly indicate association between T. gondii and schizophrenia, as serological methods sensitivity improved during last ten years.