Toxoplasma gondii infection in first-episode and inpatient individuals with schizophrenia

Open ArchivePublished:September 15, 2010DOI:https://doi.org/10.1016/j.ijid.2010.05.018

      Summary

      Background

      A high seroprevalence of Toxoplasma gondii infection has been detected in psychiatric patients, particularly in schizophrenia cases.

      Methods

      In the present study 98 patients suffering from schizophrenia (58 inpatients and 40 first-episode patients) and 96 control patients (50 healthy volunteers and 46 with a depressive disorder) were examined for the presence of both IgG and IgM antibodies against T. gondii by enzyme-linked immunosorbent assay (ELISA). We applied the Bradford Hill criteria to identify the weight of causal inference.

      Results

      The positivity rate of anti-T. gondii IgG antibodies among individuals with schizophrenia (57.1%) was significantly higher than in healthy controls (29.2%). There were no associations between immune status ratio (ISR) values and the risk of schizophrenia. The weight of evidence approach using the Bradford Hill criteria revealed a 92% probability of a causal association.

      Conclusion

      Our results show that exposure to T. gondii may lead to schizophrenia.

      Keywords

      1. Introduction

      Schizophrenia is a complex neuropsychiatric disorder and a challenging mental illness, with an uncertain etiology. This disorder is characterized by delusions, hallucinations, disturbances in thinking and communication, and withdrawal from social activity, and affects approximately 1% of the adult population in the USA and Europe.
      • Yolken R.H.
      • Dickerson F.B.
      • Fuller Torrey E.
      Toxoplasma and schizophrenia.
      Epidemiological data indicate that some infections including herpes simplex, rubella, polio, and varicella zoster virus may contribute to schizophrenia.
      • Amminger G.P.
      • McGorry P.D.
      • Berger G.E.
      • Wade D.
      • Yung A.R.
      • Phillips L.J.
      • et al.
      Antibodies to infectious agents in individuals at ultra-high risk for psychosis.
      • Flegr J.
      • Preiss M.
      • Klose J.
      • Havlcek J.
      • Vitkov M.
      • Kodym P.
      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?.
      • Carter C.J.
      Schizophrenia susceptibility genes directly implicated in the life cycles of pathogens: cytomegalovirus, influenza, herpes simplex, rubella, and Toxoplasma gondii.
      Recent studies have also corroborated the hypothesis that T. gondii, an important zoonotic and persistent infection in humans and livestock, is a prime candidate in this contribution, and it is now considered a possible cause of schizophrenia. Studies have shown that patients with schizophrenia have significantly increased levels of antibodies to T. gondii compared with healthy controls.
      • Yolken R.H.
      • Dickerson F.B.
      • Fuller Torrey E.
      Toxoplasma and schizophrenia.
      • Cetinkaya Z.
      • Yazar S.
      • Gecici O.
      • Namli M.N.
      Anti-Toxoplasma gondii antibodies in patients with schizophrenia—preliminary findings in a Turkish sample.
      T. gondii is an obligate intracellular protozoan parasite that is a member of the phylum Apicomplexa, order Coccidia,
      • Klevar S.
      Tissue cyst forming coccidian: Toxoplasma gondii and Neospora caninum as a cause of disease in farm animals.
      • Rorman E.
      • Zamir C.S.
      • Rilkis I.
      • Ben-David H.
      Congenital toxoplasmosis—prenatal aspects of Toxoplasma gondii infection.
      and felids are the most important definitive and reservoir hosts for this parasite.
      • Dubey J.P.
      The history of Toxoplasma gondii—the first 100 years.
      Humans become infected with T. gondii in three principal ways: congenitally, through the ingestion of raw or undercooked meat containing tissue cysts, and through the ingestion of water or poorly washed raw fruits and vegetables contaminated with sporulated oocysts from the feces of infected cats.
      • Dubey J.P.
      • Beattie C.P.
      Toxoplasmosis of animals and man.
      Research has revealed that individuals with schizophrenia have had a greater exposure to cats in childhood than adulthood.
      • Dubey J.P.
      The history of Toxoplasma gondii—the first 100 years.
      In most adults, T. gondii does not cause serious illness, and clinical manifestations are often not apparent.
      • Hunter C.A.
      • Reichmann G.
      Immunology of Toxoplasma infection.
      Blindness and mental retardation in children can result from a congenital infection and the disease can be severe in those with depressed immunity. Also, reactivation and replication of T. gondii in immunocompromised individuals can lead to neurological or psychiatric symptoms.
      Whereas epidemiological and neuropathological studies have shown that some cases of schizophrenia may be associated with toxoplasmosis,
      • Yolken R.H.
      • Dickerson F.B.
      • Fuller Torrey E.
      Toxoplasma and schizophrenia.
      the main aim of this study was to compare the prevalence of T. gondii infection between individuals with schizophrenia and depressive disorders (as a statistically confounding factor) and healthy controls, and to compare the immune status ratio (ISR) in individuals in the examined groups to investigate any association between T. gondii infection and schizophrenia based on psychiatric status.

      2. Materials and methods

      This cross-sectional study was undertaken in four populations: inpatients and first-episode individuals with schizophrenia, individuals with depressive disorders, and healthy volunteers. Clinical diagnoses were confirmed by means of the Structured Clinical Interview for the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV).
      • First M.
      • Spitzer R.
      • Gibbon M.
      • Williams J.
      Structured clinical interview for DSM-IV axis 1 disorders (SCID-1), clinician version.
      Of the schizophrenia patients, 40 were first-episode individuals and 58 were inpatients; they were not immunodeficient and did not have any other abnormalities or neurological disease. The control groups consisted of 46 patients with depressive disorders and 48 healthy volunteers.
      The median age of the patients and controls was 33 years (range 18–58 years). After receiving permission from the patient and their family (at the neuropsychiatry hospital), blood samples were taken under sterile conditions by means of venipuncture. Samples were uniformly handled, centrifuged at 1000 × g, and the sera were stored at −20 °C until serological examination.
      Commercially available enzyme-linked immunosorbent assay (ELISA) kits (Trinity Biotech, USA) were used according to the manufacturer's instructions, and a microtiter plate reader (405–690 nm; Dynatech, Iceland) was used to determine the IgG and IgM antibodies in serum samples. The ISR of each sample was determined by dividing the optical density of the sample by the cut-off value (cut-off = mean optical density of the calibrators × correction factor). As per the manufacturer's recommendation, sera with an ISR ≤ 0.90 were regarded as negative, those with an ISR between 0.90 and 1.10 were regarded as suspicious, and those with an ISR ≥ 1.10 were considered positive. International standard unit values were calculated by the formula presented below.
      ex=derivedIU/ml;e=Neperiannumber;X=(Y+b)/a;a=0.864,b=1.566,Y=ISR


      International standard units were evaluated statistically for all groups to compare the intensity of seropositivity and seronegativity in specimens from schizophrenic patients and controls.
      Statistical analyses were carried out using SPSS software version 16 (SPSS Inc., Chicago, IL, USA). The Chi-square test was performed to compare IgG levels in all groups, as well as Fisher's exact test for IgM levels. Odds ratios (OR) with 95% confidence intervals (95% CI) were also determined to estimate the strength of association. International standard units were compared by one-way analysis of variance (one-way ANOVA). We also applied the Bradford Hill criteria to identify the weight of causal inference.
      • Swaen G.
      • van Amelsvoort L.
      A weight of evidence approach to causal inference.

      3. Results

      Individuals with schizophrenia had significantly increased levels of serum IgG antibodies to T. gondii (57.1%) as compared to controls (29.2%) (p < 0.05). The OR for this association was 2.99 with 95% CI = 1.65−5.41.
      Of the 192 individuals examined, anti-T. gondii IgG antibodies were present in 60% (24 of 40) of those with first-episode schizophrenia, 55.2% (32 of 58) of inpatient cases with schizophrenia, 32.6% (15 of 46) of those with a depressive disorder, and in 29.2% (14 of 48) of healthy volunteers (Table 1).
      Table 1Analysis of anti-Toxoplasma gondii IgG and IgM antibodies in schizophrenia patients and in the control groups
      First episodeDepressedInpatientHealthy
      (n = 40)(n = 46)(n = 58)(n = 48)
      IgG-positive24 (60%)15 (32.6%)32 (55.2%)14 (29.2%)
      IgM-positive2 (5%)2 (4.3%)2 (3.4%)2 (4.2%)
      The weight of evidence approach using the Bradford Hill criteria revealed a 92% probability of a causal association (Table 2).
      Table 2Weight of evidence approach using the Bradford Hill criteria for the association between toxoplasmosis and schizophrenia
      Hill's criterionEvidenceProbability (%)Probability × weight for category 1Probability × weight for category 2A
      Constant−14.7799−10.08346
      StrengthStrong OR detected804.97841.5384
      ConsistencyMany studies have revealed the association953.857951.71285
      SpecificityOther neurological disorders reported60−1.6722−2.3262
      TemporalityMaternal exposure to toxoplasmosis and exposure to cat in childhood are the risk factors for schizophrenia in adults1007.6578.281
      Dose–responseEpidemiologically there are many similarities between toxoplasmosis and schizophrenia60−2.1168−2.1204
      PlausibilityMice and rats with T. gondii have cognitive dysfunction and behavioral changes6013.81513.0134
      CoherenceT. gondii affects neurons and glial cells800.076968−0.2672
      Experimental evidenceAnti-schizophrenia drugs can inhibit growth of T. gondii400.3372−0.2672
      AnalogySome infective agents may contribute to schizophrenia80−1.0352−0.8088
      Sum11.1184188.67599
      Final probabilitye11.118418/(e11.118418 + e8.67599) = 92%
      The prevalence of anti-T. gondii IgG antibodies in the first-episode schizophrenia group was not significantly different to that of the inpatient group (p > 0.05). With regard to the control groups, there was also no statistically significant difference between the IgG levels of those with a depressive disorder and healthy controls (p > 0.05). However, there was a significant increase in IgG levels in both first-episode and inpatient schizophrenia individuals compared with depressed and healthy controls, respectively (p < 0.05).
      IgM antibodies were detected in 4.1% of cases with schizophrenia (5% in the first-episode group and 3.4% in the inpatient group) and 4.3% of controls (4.3% in depressed and 4.2% in healthy controls). There were no statistically significant differences between IgM levels in the examined groups (p > 0.05).
      No association was found between international standard unit values of IgG or IgM antibodies to T. gondii for all seropositive and seronegative sera in all groups and the risk of schizophrenia (Figure 1, Figure 2).
      Figure thumbnail gr1
      Figure 1Mean and standard deviation of international standard units (units/ml) for anti-Toxoplasma gondii IgG antibodies in all examined groups.
      Figure thumbnail gr2
      Figure 2Mean and standard deviation of international standard units (units/ml) for anti-Toxoplasma gondii IgM antibodies in all examined groups.

      4. Discussion

      Our findings replicate the results of recent studies that have shown higher anti-T. gondii IgG antibodies in individuals with schizophrenia than in controls.
      • Yolken R.H.
      • Dickerson F.B.
      • Fuller Torrey E.
      Toxoplasma and schizophrenia.
      • Cetinkaya Z.
      • Yazar S.
      • Gecici O.
      • Namli M.N.
      Anti-Toxoplasma gondii antibodies in patients with schizophrenia—preliminary findings in a Turkish sample.
      Using the Bradford Hill criteria we established that the association between toxoplasmosis and schizophrenia is causative. We also found that both first-episode (those who have not had a previous hospitalization) and inpatient schizophrenia patients had significantly elevated levels of IgG antibodies to T. gondii compared with depressed and healthy controls, respectively, but we did not find any statistical difference in the levels of IgG antibodies between first-episode and inpatient schizophrenia cases. It is possible that the sample size in this study was not sufficient to obtain significant results or that hospitalization of the patients did not have any effect on the contamination of inpatients with T. gondii. Also, we found no significant difference between individuals with depressive disorders as a confounding factor and the healthy control group in the relationship between T. gondii infection and schizophrenia.
      As shown in other studies,
      • Leweke F.M.
      • Gerth C.W.
      • Koethe D.
      • Klosterkotter J.
      • Ruslanova I.
      • Krivogorsky B.
      • et al.
      Antibodies to infectious agents in individuals with recent onset schizophrenia.
      • Tamer G.S.
      • Dundar D.
      The schizophrenia and Toxoplasma gondii connection: infectious, immune or both?.
      IgM antibodies were not significantly different between the groups. This is due to the fact that IgM is an indicator of recent infection and becomes negative within 4–12 weeks,
      • Fayer R.
      Toxoplasmosis update and public health implications.
      so presumably is not associated with the increased risk of schizophrenia.
      As yet, the mechanism of T. gondii infection on the manifestation of schizophrenia is not clear. T. gondii infects the central nervous system (CNS) and forms latent tissue cysts in neurons and other permanent cells.
      • Tenter A.M.
      • Heckeroth A.R.
      • Weiss L.M.
      Toxoplasma gondii: from animals to humans.
      Research has found that mice and rats infected with T. gondii experience cognitive dysfunction and behavioral changes and that infected humans show changes in psychomotor skills and personality,
      • Witting P.A.
      Learning capacity and memory of normal and Toxoplasma-infected laboratory rats and mice.
      • Webster J.P.
      • Brunton C.F.
      • Macdonald D.W.
      Effects of Toxoplasma gondii upon neophobia behavior in wild brown rats Rattus norvegicus.
      • Flegr J.
      Effects of Toxoplasma on human behavior.
      • Havlicek J.
      • Gasova Z.
      • Smith A.P.
      • Zvara K.
      • Flegr J.
      Decrease of psychomotor performance in subjects with latent asymptomatic toxoplasmosis.
      or suffer psychotic symptoms such as delusions and hallucinations.
      • Stibbs H.H.
      Changes in brain concentrations of catecholamines and indoleamines in Toxoplasma gondii infected mice.
      This parasite also manipulates the behavior of the intermediate host and causes neurological and psychiatric symptoms in some infected individuals.
      • Webster J.P.
      • Lamberton P.H.
      • Donnelly C.A.
      • Torrey E.F.
      Parasites as causative agents of human affective disorders? The impact of anti psychotic, mood-stabilizer and anti parasite medication on Toxoplasma gondii's ability to alter host behaviour.
      Different in neuropathological effects, T. gondii and schizophrenia affect neurons and glial cells. Schwarcz and Hunter speculated that prominent activation of resident glial cells, in particular astrocytes, by T. gondii infection in the CNS may play a role in the development of schizophrenia.
      • Schwarcz R.
      • Hunter C.A.
      Toxoplasma gondii and schizophrenia: linkage through astrocyte-derived kynurenic acid?.
      Astrocytes are the main cells responsible for the synthesis of kynurenic acid (KYNA), which inhibits N-methyl-d-aspartate (NMDA) and a7 nicotinic acetylcholine (a7nACh) receptors when present at slightly elevated concentration levels. These receptors are linked to cognitive processes. KYNA levels are also increased in the brains of schizophrenia patients.
      • Schwarcz R.
      • Rassoulpour A.
      • Wu H.Q.
      • Medoff D.
      • Tamminga C.A.
      • Roberts R.C.
      Increased cortical kynurenate content in schizophrenia.
      • Schwarcz R.
      • Pellicciari R.
      Manipulation of brain kynurenines: glial targets, neuronal effects and clinical opportunities.
      On the other hand, postmortem studies of brains from individuals who had schizophrenia have revealed glial abnormalities.
      • Cotter D.R.
      • Pariante C.M.
      • Everall I.P.
      Glial cell abnormalities in major psychiatric disorders: the evidence and implications.
      However, Conejero-Goldberg et al. did not detect T. gondii sequences in the postmortem brains by nested-polymerase chain reaction.
      • Conejero-Goldberg C.
      • Torrey E.F.
      • Yolken R.H.
      Herpesviruses and Toxoplasma gondii in orbital frontal cortex of psychiatric patients.
      Evidence points to the fact that T. gondii affects function and the levels of neurotransmitters including dopamine; likewise, individuals with schizophrenia have abnormal dopamine levels. Flegr et al. believe that the activation of interleukin-2 following infection by T. gondii may result in the subsequent increase in dopamine levels, and that dopamine is the missing link between schizophrenia and T. gondii infection.
      • Flegr J.
      • Preiss M.
      • Klose J.
      • Havlcek J.
      • Vitkov M.
      • Kodym P.
      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?.
      All these findings indicate that T. gondii may play an important role in the pathogenesis of schizophrenia.

      5. Conclusions

      We still do not know exactly what causes schizophrenia. Moreover, genetic and environmental factors contribute to susceptibility to this disorder. Many studies have demonstrated that the risk of schizophrenia runs in families.
      • Yolken R.H.
      • Dickerson F.B.
      • Fuller Torrey E.
      Toxoplasma and schizophrenia.
      People in urban areas have an increased risk of being diagnosed with schizophrenia in later life compared to those in rural areas.
      • Yolken R.H.
      • Dickerson F.B.
      • Fuller Torrey E.
      Toxoplasma and schizophrenia.
      It appears that a combination of genetic and acquired factors may be involved in schizophrenia.
      Our study supports the role of T. gondii in the etiology of schizophrenia and found no difference between first-episode and inpatient individuals with schizophrenia. This shows that chronic toxoplasmosis has notable and plausible epidemiologic and symptomatic similarities to schizophrenia. Furthermore, effective anti-schizophrenia and bipolar disorder drugs are able to inhibit the growth of T. gondii.
      • Jones-Brando L.
      • Fuller Torrey E.
      • Yolken R.
      Drugs used in the treatment of schizophrenia and bipolar disorder inhibit the replication of Toxoplasma gondii.
      However, in spite of the correlations hypothesized by many authors, further comprehensive studies and a greater accumulation of evidence are required to determine the precise association between these two disorders.

      Acknowledgements

      The authors wish to thank the vice-chancellor of research of Shahid Chamran University for a research grant for this project.
      Conflict of interest: No conflict of interest to declare.

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