Association between latent toxoplasmosis and bipolar disorder: a case-control seroprevalence study

Abstract

Background

Toxoplasma gondii (T. gondii) is the most successful obligate protozoan that can infect warm-blooded vertebrate hosts. Some researchers suggest that the presence of Toxoplasma cysts in the brain can lead to mental disorders. Bipolar disorder (BD) is one of the serious neuropsychiatric disorders. Several studies have shown a high seroprevalence of T. gondii in bipolar patients. Therefore, this study aims to determine the prevalence of this infection in patients with BD.

Methods

In this case-control study, anti-Toxoplasma immunoglobulin (Ig) G and IgM antibodies were measured in serum samples from 115 patients with BD and 115 subjects without this disorder from the general population using commercially available enzyme-linked immunosorbent assay. Demographic characteristics of the patient and control groups, information about T. gondii infection and BD, and their potential risk factors were analyzed. We utilized the Mann-Whitney U test for continuous variables, the chi-square test for categorical data, and multivariate logistic regression to assess T. gondii infection and BD, with significance set at P < 0.05.

Results

Twenty-eight (24.34%) of 115 patients with BD and 10 (8.7%) of 115 controls had anti-T. gondii IgG antibodies. IgM antibodies against T. gondii were not reported to be positive in any participants. Furthermore, there was a statistically significant difference in the results [odds ratio (OR) = 2.89: 95% confidence interval (CI) = 1.08–7.73. P = 0.03]. Within the study population, various factors were identified as significant risk factors for BD: sex (OR 8.10, 95% CI 3.16–20.75), age 20–50 (OR 5.11, 95% CI 1.81–14.45), age over 50 (OR 19.54, 95% CI 4.02–94.89), education level (OR 0.24, 95% CI 0.09–0.60), working status (non-employment, OR 4.12, 95% CI 1.65–10.30), and income (middle, OR 0.29, 95% CI 0.10–0.89; high, OR 0.12, 95% CI 0.01–0.77), all with P-values less than 0.05. In addition, in the group of patients, there was no statistically significant relationship between T. gondii infection with the type of bipolar disease (P = 0.93), the severity of the disease (P = 0.61), and the history of suicide attempts (P = 0.63).

Conclusion

This study showed that toxoplasmosis is a risk factor for BD and increases the chance of developing BD. However, more studies with a larger sample size are recommended to clarify the development pathways of this disorder and provide new strategies for the prevention and treatment of this disease.

Clinical trial

Not applicable.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12879-024-10405-0.

Introduction

Toxoplasma gondii (T. gondii) is a ubiquitous parasitic protozoan that can infect intermediate hosts including humans, ruminants, rodents, and birds [1–6]. This parasite has been reported even in cold-blooded animals [7]. It is estimated that approximately 30–50% of the world’s people are infected with this parasite, and the highest prevalence is in tropical countries. This infection is mostly obtained by consuming fruits and vegetables contaminated with oocysts or eating cysts in raw or undercooked meat [8]. T. gondii has a complex life cycle including the sexual cycle in the definitive host and the asexual cycle in the intermediate hosts [9]. There are three pathogenic forms of the parasite: sporozoites (in oocysts), tachyzoites (in tissue sections or body fluid smears), and bradyzoites (in tissue cysts). Tachyzoites are responsible for the acute phase and clinical manifestations, and bradyzoites are responsible for the chronic phase of the disease [10]. Clinical symptoms such as encephalitis, chorioretinitis, and pneumonitis in immunocompromised individuals and abortion in pregnant women are among the consequences of T. gondii infection [11]. Congenital toxoplasmosis in neonates can result in serious neurological birth defects, mental retardation, and blindness [12]. Toxoplasmosis can cause changes in people’s behavior and personality [13]. This parasite has a specific tropism for brain tissue. Tachyzoites invade astrocytes, microglia, and neurons in the brain and create cysts in these cells. By producing large amounts of neurotransmitters such as dopamine, Toxoplasma increases the production of bradyzoites and destroys cyst walls, which may be responsible for behavioral changes [14, 15]. Various factors such as the number and location of parasite cysts in the brain in the chronic stage of infection, the genetic susceptibility of the person, the severity of the initial acute infection, ingestion of different stages of the parasite, and time of the person’s life at when the infection occurs [16, 17].

Bipolar disorder (BD) is a mood disorder with intermittent periods of depression and mania or hypomania [13]. Numerous factors including genetic, neurobiological, personality, and environmental can play a role in causing this disease [18, 19]. BD affects approximately 2.4% of the world’s population [20]. There are hypotheses that T. gondii can cause BD by causing cysts in the central nervous system (CNS) or by affecting neurotransmitters [13]. Enhanced metabolism of dopamine due to parasitic infection has been particularly noted in the limbic region, which is known to be affected in BD [21]. The genome of the parasite contains two genes that encode enzymes with tyrosine hydroxylase activity; notably, one of these genes is activated during the formation of T. gondii bradyzoites. Tyrosine hydroxylase serves as the rate-limiting enzyme for dopamine synthesis in the brain, facilitating the conversion of L-tyrosine to L-dihydroxyphenylalanine [22]. In vitro studies indicate that dopamine itself may influence various aspects of T. gondii, including its production, proliferation, chemoattraction, infection efficiency, and stage conversion within the brain. Research involving both mammalian dopaminergic cells infected with T. gondii and brain sections from infected mice demonstrated a significant uptick in dopamine metabolism in neural cells, revealing a direct correlation between the number of infected cells and the amount of dopamine released [21]. The release of dopamine induced by T. gondii has been proposed as a significant neurochemical mechanism underlying the psycho-behavioral abnormalities observed in Toxoplasma-infected rodents, and potentially in humans as well [21, 23–25]. Also, indirect evidence such as the reduction of behavioral changes associated with T. gondii by haloperidol (an antipsychotic prescribed in the manic phase of the disease) shows the role of this parasite in causing BD [26]. Valproic acid (a first-line stabilizing drug) and haloperidol can inhibit the proliferation of T. gondii in fibroblast cells [27]. In addition, some studies reported a positive association between toxoplasmosis and BD [28, 29]. However, other reports have rejected such an association [30, 31]. Due to the inconsistency in the results of the available articles, this study was designed to help further clarify the association between T. gondii infection and BD.

Material and method

Patients and controls

In this case-control study, participants with BD (n = 115) were selected from psychiatric clinics affiliated with Birjand University of Medical Sciences and diagnosed by experienced psychiatrists. Specifically, the patients who participated in this study were hospitalized individuals with a diagnosis of BD, ensuring a clinical population of individuals experiencing significant mental health challenges. For the control group (n = 115), healthy volunteers with no personal or family history of psychiatric disorders were recruited. These individuals were referred to local health centers while undergoing routine pre-marriage evaluations, providing a sample of the general population that was not affected by psychiatric conditions. The sample size was determined based on the study of Hamdani et al. [29]. The participants were asked to complete the relevant questionnaire including demographic information (gender, age, education level, marital status, and underlying disease), information related to possible factors affecting the development of BD (place of residence, job, and income) to compare and eliminate possible confounding factors, and information related to ways of transmission of T. gondii infection (blood injection, cupping, contact with animals, drinking water, how to wash vegetables, contact with garden soil, eating habits and job characteristics of people).

Serological analysis

Blood samples (5 ml) were collected from all patients and control groups. Then the blood samples were centrifuged at 1000 rpm and the serums were separated and kept at -20 °C until analysis. Sera of all participants were analyzed for anti- T. gondii IgG and IgM antibodies with a commercially available enzyme-linked immunosorbent assay kit (Pishtaz Teb Diagnostics kits, Iran). The IgG and IgM antibody titers were read at an optical density of 490 nm using an automatic ELISA reader (Epoch, USA). The cut-off point of ELISA was 10IU/ml. The results below that considered as negative and higher than that considered as positive, according to the manufacturer’s guidelines.

Statistical analysis

All statistical analyses were performed using Prism 5 (GraphPad Software Inc., San Diego, CA, USA). Mann-Whitney U test was used to compare the differences in continuous variables between groups. The chi-square test was used for categorical variables. Multivariate logistic regression was used to determine the association between T. gondii infection and BD by adjusting the effect of confounding variables. Odds ratios (OR) and 95% confidence intervals (CIs) were calculated. The level of significance was set at P < 0.05.

Results

This study was performed on 230 people including 115 people with BD and 115 healthy people who were randomly selected. Comparison of sociodemographic data, possible risk factors for T. gondii infection, T. gondii seropositivity status and clinical characteristics in the patient and control groups are shown in Table 1. None of the subjects was in an acute phase of T. gondii infection and was not positive for IgM T. gondii antibody. Anti-T. gondii IgG positivity was found in 28 of the 115 patients with BD and 10 of the 115 control subjects (Table 1).

Table 1.

Demographic characteristics of the patient and control groups

Variables	Patients’ group	Control group	P- value
T. gondii IgG			0.001*
Positive	28 (24.3%)	10 (8.7%)
Negative	87 (75.7%)	105 (91.3%)
T. gondii IgM			--
Positive	0 (0%)	0 (0%)
Negative	115 (100%)	115 (100%)
Sex			0.002*
Female	45 (39.1%)	68 (59.10%)
Male	70 (60.9%)	47 (40.9%)
Residence			0.39
Urban	96 (83.5%)	91 (79.1%)
Rural	19 (16.5%)	24 (20.9%)
Marital status			< 0.001*
Single	33 (28.7%)	58 (50.40%)
Married	44 (38.30%)	57 (49.6%)
Divorced/widowed	38 (33%)	0 (0%)
Level of education			< 0.001*
Up to 12 years/high school	103 (89.6%)	73 (63.50%)
University degree	12 (10.40%)	42 (36.5%)
Any other disease			0.003*
No	90 (78.3%)	106 (92.2%)
Yes	25 (21.7%)	9 (7.8%)
Age (years)			< 0.001*
10–20	9 (7.80%)	33 (28.7%)
20–50	81 (70.4%)	78 (67.8%)
> 50	25 (21.7%)	4 (3.5%)
Working status			0.002*
Self-employment	30 (26.10%)	43 (37.40%)
Employee	7 (6.10%)	19 (16.50%)
other	78 (67.8%)	53 (46.10%)
Income			0.001*
Low	76 (74.8%)	63 (54.8%)
Middle	21 (18.30%)	27 (23.50%)
High	8 (7%)	25 (21.7%)

Values are expressed as n (%)

According to the results, there is a statistically significant relationship between the results of IgG antibodies in the case and control groups. In other words, the difference in IgG titer between seropositive patients and the seropositive control group was significant (P < 0.05). The patient and the control groups have significant differences in terms of sex, marital status, level of education, any other disease, age, working status, and income (P < 0.05) (Table 1).

In the present study, the examination of the frequency distribution of blood transfusion, cupping, contact with cats, contact with animals (cow, goat/sheep, and dog), drinking water (tap water, well/spring water, and mineral/boiling/purified water), meat-related jobs, jobs related to milk, washing vegetables (water, chlorine/dishwashing liquid, and salt), sausage, and garden soil in people with and without T. gondii infection in both case and control groups did not show any significant statistical difference (P > 0.05) (Table 2).

Table 2.

Association of toxoplasmosis with its potential risk factors

Variables	Patients’ group	T. gondi positive	Control group	T. gondii positive	P- value
Blood transfusion
No	99 (86.10%)	23 (82.10%)	114 (99.10%)	9 (90.00%)	0.53
Yes	16 (13.90%)	5 (17.90%)	1 (0.90%)	1 (10.00%)
Cupping
No	98 (85.20%)	24 (85.70%)	97 (84.30)	7 (70.00)
Yes	17 (14.80%)	4 (14.30%)	18 (15.70)	3 (30.00)	0.99
Contact with cats
No	78 (67.80%)	18 (64.30%)	100 (87.00%)	9 (90.00%)	0.64
Yes	37 (32.20%)	10 (35.70%)	15 (13.00%)	1 (10.00%)
Contact with animals
Cow	8 (34.80%)	3 (42.86%)	1(3.10%)	0 (0%)
Goat/sheep	10 (43.50%)	2 (28.57%)	29 (90.60%)	5 (100%)	0.6
Dog	5 (21.70%)	2 (28.57%)	2 (6.30%)	00 (0%)
Drinking water
Tap water	105(91.30%)	24 (85.70%)	49 (42.60%)	3 (30.00%)
Well/spring water	0 (0%)	0 (0%)	11 (9.60%)	0 (0%)	0.25
Mineral/boiling/purified water	10 (8.70%)	4 (14.30%)	55 (47.80%)	7 (70.00%)
Meat-related jobs
No	106 (92.20%)	27 (96.40%)	104 (90.40%)	10 (100%)	0.45
Yes	9 (7.80%)	1 (3.60%)	11 (9.60%)	0 (0%)
Jobs related to milk
No	108(93.90%)	27 (96.40%)	104 (90.40%)	10 (100%)	0.99
Yes	7 (6.10%)	1 (3.60%)	11 (9.60%)	0 (0%)
Washing vegetables
Water	33 (28.70%)	7 (25.00%)	56 (48.70%)	6 (60.00%)
Chlorine/dishwashing liquid	56 (48.70%)	15 (53.60%)	32 (27.80%)	2 (20.00%)	0.84
Salt	26 (22.60%)	6 (21.40%)	27 (23.50%)	2 (20.00%)
Sausage
No	19 (16.50%)	4 (14.30%)	61 (53.00%)	7 (70.00%)	0.99
Yes	96 (83.50%)	24 (85.70%)	54 (47.00%)	3 (30.00%)
Garden soil
No	81 (70.40%)	19 (67.90%)	89 (77.40%)	6 (60.00%)	0.81
Yes	34 (29.60%)	9 (32.10%)	26 (22.60%)	4 (40.00%)

Logistic regression with toxoplasmosis showed a positive association between BD and toxoplasmosis (OR = 2.89, CI 95%: 1.08–7.73, P = 0.03). To test whether toxoplasmosis or toxoplasmosis-specific risk factors are responsible for this association, a more complex analysis model including toxoplasmosis, sex, age (20–50 and > 50), marital status, level of education, working status (employee and other), income (middle and high), and any other disease was done.

Risk factors such as sex, age (20–50 and > 50), level of education, working status (non-employee job), and income (middle and high) were associated with BD (P < 0.05) (Table 3).

Table 3.

Association of T. gondii infection with BD and potential risk factors for BD

	P	OR	-95%CL	+ 95%CL
Toxoplasma gondii	0.03*	2.89	1.08	7.73
Sex	< 0.001*	8.10	3.16	20.75
Age (20–50)	0.002* 0.001*	5.11	1.81	14.45
Age (> 50)	< 0.001*	19.54	4.02	94.89
Marital status	0.38	1.40	0.65	3.00
Level of education	0.003*	0.24	0.09	0.60
Working status (Employee)	0.77	1.30	0.22	7.64
Working status (other)	0.002*	4.12	1.65	10.30
Income (Middle)	0.03*	0.29	0.10	0.89
Income (High)	0.02*	0.12	0.01	0.77
Any other disease	0.23	2.01	0.63	6.34

Associations were measured with one multivariate Logistic regression with all listed factors as independent variables. Significant Odds Ratio (OR) is denoted with asterisk

In addition, according to the results, in the group of patients, there was no statistically significant relationship between T. gondii infection with the type of bipolar disease, the severity of the disease, and the history of suicide attempts (P > 0.05) (Table 4).

Table 4.

Investigating the relationship between T. gondii infection with the type and severity of BD and the history of suicide attempts in the patients group

Variables	Patients group	T. gondii positive	T. gondii negative	P
Bipolar disorder type
Type 1	91 (100%)	22 (24.20%)	69 (75.80%)	0.93
Type 2	24 (100%)	6 (25.00%)	18 (75.00%)
Severity of the disease
0–1 hospitalization	54 (100%)	12 (22.20%)	42 (77.80%)	0.61
Hospitalized more than once	61 (100%)	16 (26.20%)	45 (73.80%)
Suicide attempt history
No	86 (74.78%)	20 (71.40%)	66 (75.90%)	0.63
Yes	29 (25.22%)	8 (28.60%)	21 (24.10%)

Discussion

In recent years, the possible association between T. gondii infection and mental disorders has been investigated in several systematic reviews and meta-analyses. The results of these studies showed that chronic T. gondii infection might be one of the related factors with schizophrenia [32, 33], obsessive-compulsive disorder [34], Alzheimer’s disease [35, 36], addiction [32], epilepsy [37, 38], suicide attempts [39], autism [40], and headache [41] but not with major depression disorder [32, 42], attention-deficit hyperactivity disorder [43], and Parkinson’s disease [36]. This parasite can disrupt the level of dopamine in the brain and cause changes in the CNS [21, 44]. BD is a chronic and recurrent psychiatric disorder and one of the main causes of disability and mortality in the world, which can be affected by many factors, including infectious agents [25]. T. gondii is one of these infectious agents whose relationship with BD has been evaluated in several systematic reviews. In a systematic review, the association of several psychiatric disorders including schizophrenia (OR = 1.81), bipolar disorder (OR = 1.52) obsessive-compulsive disorder (OR = 3.4), and addiction (OR = 1.91) with toxoplasmosis was reported [32]. One study has indicated that T. gondii infection is associated with BD (OR = 1.26). In this study, eight related articles were analyzed and the main limitation of the study was the very small number of included articles [13]. Also, a meta-analysis found increased odds of being seropositive for toxoplasmosis in BD patients compared to healthy controls (OR = 1.69). Of note, this review pooled data from 23 independent studies with a total of 12,690 participants (4,021 with BD and 8,669 controls) [45]. Nevertheless, due to the small number of studies that were analyzed and also due to the contradiction in the results of the original studies regarding the relationship between toxoplasmosis and BD, the need for more studies in this field seems necessary. Therefore, this study was targeted to investigate the relationship between T. gondii infection and BD.

Results of the present study suggested that latent infection with the parasite T. gondii could be a risk factor for acquiring BD. People with T. gondii infection are about 2.89 times more likely to report having BD than people without T. gondii (P < 0.05). The present results are in agreement with the results of case-control and cross-sectional studies [29, 46, 47]. A case-control study in Ethiopia showed that the seropositivity rate for T. gondii infection was significantly higher in bipolar patients compared to healthy controls [28]. Another study in France investigated the potential association between exposure to T. gondii and BD in a sample of 110 patients with bipolar disorder and 106 healthy controls. The results of this study showed that the seropositive group for T. gondii IgG antibody compared to the seronegative group had 2.7 times the chance of developing BD [29]. In a cross-sectional survey conducted in China, Liu et al.. reported that T. gondii seropositivity in psychiatric patients, including patients with bipolar disorder, is maintained at a higher level than in the general population (OR = 2.327) [47].

In contrast to our study, Khademvatan et al.. did not report a significant difference between seropositivity and BD [48]. Similarly, two studies in Germany [49, 50] and Mexico [51] with small sample sizes reported no significant association between T. gondii seroprevalence and BD.

In the present study, we found anti-T. gondii IgG positivity rates as 24.3% and 8.7% in the patient group and control group. In addition, in this study, out of 230 blood samples of case and control individuals, no positive samples of IgM antibody were reported. Therefore, none of the participants was in the acute phase of T. gondii infection. In a similar study, IgM antibody was not reported positive in the blood samples of any of the case and control groups [29]. However, in another study, the serum level of IgM T. gondii antibody was significantly higher in the group with BD than in the control group [46]. Reasons for the contradictory results of the studies can be explained by differences in the characteristics of the patients studied, including differences in the genetic background of the participants, the severity and duration of the illness, the presence of concurrent psychiatric illness, the duration of the acute or chronic infection, methodological differences in the serological diagnosis, and differences in the prevalence of T. gondii infection [52, 53]. Various studies examining the association between T. gondii infection and BD employ different sampling techniques, diagnostic criteria for BD, and serological methods for detecting T. gondii antibodies. For instance, while our study utilized a case-control design with serological analysis via ELISA, another study may have employed a cross-sectional design or different serological techniques. These variations can lead to differing prevalence rates and associations. For example, a study by Hamdani et al.. (2013) [29] found a significant association using a similar case-control design, while Khademvatan et al.. (2013) [48] reported no significant difference with a smaller sample size. Also, genetic susceptibility can play a role in how individuals respond to T. gondii infection and their risk for developing BD. Variations in genes related to immune response, such as those encoding cytokines or neurotransmitter receptors, may influence both susceptibility to infection and vulnerability to psychiatric disorders. Environmental factors, including socioeconomic status, urban versus rural living conditions, and exposure to cats (the definitive host for T. gondii), also vary widely across populations. These factors can affect the prevalence of T. gondii infection and the expression of BD. For instance, a study by Liu et al.. (2022) [47] highlights how urbanization and lifestyle factors may contribute to differing rates of T. gondii exposure. Additionally, the availability of healthcare resources and public health measures can affect both the detection and management of infections and mental health conditions.

According to the results of the current study, there was no statistically significant relationship between T. gondii infection with the type of bipolar disease, the severity of the disease, and the history of suicide attempts (P > 0.05). It should be noted that the severity of the disease in this study was defined based on the number of hospitalizations due to the recurrence of BD. So the number of hospitalizations of 2 or more was considered as a severe type of BD [54]. To date, no study has been conducted on the impact of T. gondii infection and increased risk of hospitalization for BD recurrence. Therefore, it seems that additional studies are needed to determine the role of this factor in increasing the risk of recurrence and hospitalization in BD.

Our study has limitations, such as the small sample size: the study included 115 patients with BD and 115 controls. Although this sample size is not uncommon in preliminary studies, it may limit the statistical power of the findings. The study design did not include long-term follow-up of participants, which limits the ability to establish causality between T. gondii infection and the development or exacerbation of bipolar disorder symptoms. Also, given that the study was a case-control study, the control and case groups were not matched for some variables such as gender, marital status, and education level. This mismatch can introduce selection bias, as these variables may themselves influence the risk of developing BD and potentially confound the association with T. gondii infection.

Conclusion

The findings of the present study confirm the hypothesis that latent T. gondii infection is associated with bipolar disorder. It was shown that T. gondii infection might increase the risk of BD by 2.89 times. These results pave the way for future research to explore the underlying mechanisms by which T. gondii may influence neuropsychiatric conditions. Understanding these mechanisms could lead to the development of targeted interventions that address both the infection and its potential neuropsychiatric consequences. Furthermore, this study highlights the need for larger, longitudinal studies to better assess the causal relationships between T. gondii infection and BD. Such research could also evaluate potential preventive measures, such as T. gondii screening among individuals at higher risk for BD, particularly in areas with high infection prevalence.

Electronic supplementary material

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Author contributions

M. Z. and AS. P conceptualized and supervised the study. AA. E. and AS. T. carried out the methodology and analysis of the study. F. R., E. N., and T. N. write initial draft of the manuscript and critically reviewed and revised the manuscript. All authors read and approved the final manuscript.

Funding

This article is the outcome of the research project number 456136, conducted with the financial support of the Vice Chancellor for Research at Birjand University of Medical Sciences.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Declarations

Ethics approval and consent to participate

The study protocol received approval from Birjand University of Medical Sciences, under the ethical code number IR.BUMS.REC.1399.385. All participants in our study provided informed consent prior to their inclusion. We adhered to all relevant guidelines and regulations throughout the study.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.