BioMed Research International
Volume 2018 (2018), Article ID 4723739, 9 pages
https://doi.org/10.1155/2018/4723739

Research Article

Toxoplasma gondii Infection in Diabetes Mellitus Patients in China: Seroprevalence, Risk Factors, and Case-Control Studies

Yong-Xin Li,¹ Hai Xin,¹ Xiang-Yan Zhang,² Cui-Ying Wei,³ Yu-He Duan,⁴ Hao-Fu Wang,¹ and Hai-Tao Niu⁵

¹Department of Vascular Surgery, The Affiliated Hospital of Qingdao University, Qingdao, China
²Department of Pathology, The Affiliated Hospital of Qingdao University, Qingdao, China
³Department of Pneumology, The Affiliated Hospital of Qingdao University, Qingdao, China
⁴Department of Pediatric Surgery, The Affiliated Hospital of Qingdao University, Qingdao, China
⁵Department of Urinary Surgery, The Affiliated Hospital of Qingdao University, Qingdao, China

Correspondence should be addressed to Hao-Fu Wang; wanghaof2018@163.com and Hai-Tao Niu; niuhaitao@qduhospital.cn

Received 11 April 2018; Revised 5 August 2018; Accepted 6 November 2018; Published 18 December 2018

Academic Editor: Eric N. Villegas

Copyright © 2018 Yong-Xin Li et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The association between Toxoplasma gondii (T. gondii) infection and diabetes mellitus remains controversial. With the improvement of living standards, the prevalence rate of diabetes is steadily increasing in China. Thus, it is necessary to explore the possible association between toxoplasmosis and diabetes mellitus in China. Hence, case-control studies were conducted to explore the T. gondii seroprevalence and identify the risk factors and possible transmission routes of T. gondii infection in different types of diabetes, including type 1 diabetes (T1DM), type 2 diabetes (T2DM), and gestational diabetes (GDM) patients in China. Four hundred serum samples for each type of diabetes mellitus, matched with 400 control subjects for each group, were collected and examined for anti-T. gondii IgG and IgM antibodies using commercially available enzyme immunoassay kits. The total T. gondii seroprevalence in T1DM, T2DM, and GDM patients was 16.50%, 23.50%, and 21.25%, respectively. Each type of diabetes mellitus patients had a significantly higher T. gondii seroprevalence than the control subjects. Multivariate regression identified three variables as risk factors for T. gondii infection in diabetes patients, including keeping cats at home and consumption of raw oysters for T1DM patients and consumption of raw/undercooked meat and raw oysters for T2DM patients, which may help to guide future research and control policies in diabetes mellitus patients.

1. Introduction

Toxoplasma gondii, an obligate intracellular opportunistic parasite, can infect nearly all types of warm-blood animals, including humans [1]. Notably, nearly one-third of the human population worldwide have been estimated to be infected with this parasite, and over 7% of Chinese have chronic T. gondii infection [1–3]. In general, most T. gondii infections do not cause significant clinical symptoms [4]. But in some cases, infected persons may present clinical symptoms of toxoplasmosis such as lymphangoncus, cerebral, and eye diseases [4–6]. In some extreme cases, T. gondii infection can be reactivated and lead to a life-threatening disease with involvement of the central nervous system in immunocompromised patients [4, 5]. T. gondii can reach many organs of the host after infection [7], including the pancreas [8].

Diabetes mellitus is a common chronic metabolic disease and more than 300 million persons worldwide are projected to be affected by this disease in 2030 [9]. With the improvement of living standards, the prevalence rate of diabetes has steadily increased in China. The sensitivity and susceptibility to various infections can be higher in diabetes mellitus patients [10]. In some cases, the Apicomplexan parasite, T. gondii, has been proposed as a possible cause of diabetes, and current information is nearly predicated on this issue [11–13]. Meanwhile, chronic toxoplasmosis has been considered as a potential risk factor for type 2 diabetes (T2DM) identified by a meta-analysis of studies on the association between chronic toxoplasmosis and diabetes mellitus [13]. However, type 1 diabetes (T1DM) patients in Colombia were found to have significantly lower T. gondii seroprevalence [14]. In USA, no association was found between T. gondii infection and diabetes mellitus in a prospective cohort of elderly Latinos [15]. Since data of previous studies on the association between T. gondii infection and diabetes mellitus remain controversial, we conducted matched case-control studies to determine whether T. gondii seropositivity is associated with different clinical types of diabetes mellitus, including T1DM, T2DM, and gestational diabetes (GDM), and explore the risk factors for T. gondii infection in diabetes patients for the first time in China.

2. Materials and Methods

2.1. Study Sites

The study was conducted in The Affiliated Hospital of Qingdao University, Qingdao (35°35′–37°09′N, 119°30′–121°00′E), Shandong province, eastern China. The Affiliated Hospital of Qingdao University is a large comprehensive hospital in Shandong province that occupies an important position in the national medical system. The patients at this hospital mainly come from five provinces (Shandong, Jiangsu, Liaoning, Jilin, and Heilongjiang).

2.2. Study Design and Sample Collection

Through case-control studies, we studied T. gondii seroprevalence and identified the risk factors and possible transmission routes of T. gondii infection in diabetes mellitus patients and control subjects in China between September 2014 and January 2017. A total of 1200 diabetes mellitus patients who visited the Affiliated Hospital of Qingdao University were included in the study. Three types of diabetes mellitus patients (T1DM, T2DM, and GDM) were invited to participate in this study. The number of patients of each type was 400 (Tables 1–3).

Table 1: Seroprevalence of T. gondii infection by sociodemographic factors in type 1 diabetes (T1DM) patients and controls in eastern China.

Characteristic
T1DM (N=400)
Controls (N=400)
T1DM vs.

Prevalence of T. gondii infection
Prevalence of T. gondii infection
Controls

No. tested
No. positive
%
P-value
No. tested
No. positive
%
P-value
P-value

Age

    ≤ 30
62
15
24.19
0.187
62
10
16.12
0.591
0.263

    31-40
70
12
17.14

85
12
14.12

0.604

    41-50
100
13
13.00

90
8
8.89

0.367

    51-60
92
18
19.57

86
8
9.30

0.053

    >60
76
8
10.53

67
8
11.94

0.789

Region

    Shandong
170
34
20.00
0.237
170
23
13.53
0.493
0.110

    Jilin
141
21
14.89

141
13
9.22

0.144

    Heilongjiang
89
11
12.36

89
10
11.24

0.816

Gender

    Male
207
31
14.98
0.395
221
23
10.41
0.447
0.155

    Female
193
35
18.13

179
23
12.85

0.160

Area of residence

    Urban
202
30
14.85
0.370
211
16
7.85
0.031
0.019

    Rural
198
36
18.18

189
27
14.29

0.299

Employment

    Unemployed
156
28
17.95
0.261
137
21
15.33
0.147
0.549

    Employed part-time
139
26
18.71

161
13
8.07

0.006

    Employed full-time
105
12
11.43

102
12
11.76

0.940

Obesity and overweight

    Yes
156
21
13.46
0.191
194
24
12.37
0.596
0.762

    No
244
45
18.44

206
22
10.68

0.021

Keeping cats at home

    Yes
50
16
32.00
0.002
58
10
17.24
0.138
0.074

    No
350
50
14.29

342
36
10.53

0.134

Consumption of raw/undercooked meat

    Yes
26
8
30.77
0.043
60
10
16.67
0.174
0.140

    No
374
58
15.51

340
36
10.59

0.052

Consumption of oyster

    Raw
94
32
34.04
<0.001
134
16
11.94
0.845
<0.001

    Boiled
306
34
11.11

266
30
11.28

0.950

Consumption of fish

    Raw
147
34
23.13
0.007
167
19
11.38
0.948
0.006

    Boiled
253
32
12.65

233
27
11.59

0.721

Consumption of raw vegetables and fruits

    Yes
204
37
18.14
0.368
228
27
11.84
0.805
0.066

    No
196
29
14.80

172
19
11.05

0.287

Exposure to soil

    Yes
232
40
17.24
0.639
247
32
12.96
0.246
0.190

    No
168
26
15.48

153
14
9.15

0.182

Gardening or agricultural activities

    Yes
257
42
16.34
0.909
257
33
12.84
0.260
0.261

    No
143
24
16.78

143
13
9.09

0.053

Washing hands before meals

    Yes
262
44
16.79
0.827
266
28
10.53
0.390
0.036

    No
138
22
15.94

134
18
13.43

0.559

Source of drinking water

    Spring/well
85
19
22.35
0.101
68
13
19.12
0.031
0.625

    Tap
315
47
14.92

332
33
9.94

0.054

Total
400
66
16.50

400
46
11.50

0.042

Table 2: Seroprevalence of T. gondii infection by sociodemographic factors in type 2 diabetes (T2DM) patients and controls in eastern China.

Characteristic
T2DM (N=400)
Controls (N=400)
T2DM vs.

Prevalence of T. gondii infection
Prevalence of T. gondii infection
Controls

No. tested
No. positive
%
P-value
No. tested
No. positive
%
P-value
P-value

Age

    ≤ 30
37
9
24.32
0.956
37
4
10.81
0.793
0.127

    31-40
83
19
22.89

87
11
12.64

0.080

    41-50
96
23
23.96

99
13
13.13

0.052

    51-60
93
24
25.81

97
12
12.37

0.018

    >60
91
19
20.88

90
7
7.78

0.012

Region

    Shandong
156
45
28.85
0.074
156
25
16.03
0.025
0.007

    Jilin
142
32
22.54

142
17
11.97

0.019

    Heilongjiang
102
17
16.67

102
5
4.90

0.007

Gender

    Male
202
50
24.75
0.551
211
27
12.80
0.492
0.002

    Female
198
44
22.22

189
20
10.58

0.002

Area of residence

    Urban
200
50
25.00
0.479
229
18
7.86
0.005
<0.001

    Rural
200
44
22.00

171
29
16.96

0.223

Employment

    Unemployed
163
43
26.38
0.118
169
24
14.20
0.326
0.006

    Employed part-time
133
23
17.29

148
13
8.78

0.033

    Employed full-time
104
28
26.92

83
10
12.05

0.012

Obesity and overweight

    Yes
152
30
19.74
0.152
190
20
10.53
0.470
0.017

    No
248
64
25.81

210
27
12.86

<0.001

Keeping cats at home

    Yes
52
19
36.54
0.017
56
9
16.07
0.279
0.015

    No
348
75
21.55

344
38
11.05

<0.001

Consumption of raw/undercooked meat

    Yes
28
14
50.00
<0.001
161
5
3.11
<0.001
<0.001

    No
372
80
21.51

339
42
12.39

0.001

Consumption of oyster

    Raw
91
41
45.05
<0.001
143
15
10.49
0.559
<0.001

    Boiled
309
53
17.15

257
32
12.45

0.119

Consumption of fish

    Raw
150
50
33.33
<0.001
167
22
13.17
0.454
<0.001

    Boiled
250
44
17.60

233
25
10.73

0.031

Consumption of raw vegetables and fruits

    Yes
253
66
26.09
0.109
286
32
11.19
0.581
<0.001

    No
147
28
19.05

114
15
13.16

0.203

Exposure to soil

    Yes
228
54
23.68
0.920
248
34
13.71
0.120
0.005

    No
172
40
23.26

152
13
8.55

<0.001

Gardening or agricultural activities

    Yes
248
56
22.58
0.580
254
35
13.78
0.096
0.011

    No
152
38
25.00

146
12
8.22

<0.001

Washing hands before meals

    Yes
270
63
23.33
0.910
285
30
10.53
0.232
<0.001

    No
130
31
23.85

115
17
14.78

0.075

Source of drinking water

    Spring/well
90
20
22.22
0.745
64
13
20.31
0.020
0.776

    Tap
310
74
23.87

336
34
10.12

<0.001

Total
400
94
23.50

400
47
11.75

<0.001

Table 3: Seroprevalence of T. gondii infection by sociodemographic factors in gestational diabetes (GDM) patients and controls in eastern China.

Characteristic
GDM (N=400)
Controls (N=400)
GDM
vs.

Prevalence of T. gondii infection
Prevalence of T. gondii infection
Controls

No. tested
No. positive
%
P-value
No. tested
No. positive
%
P-value
P-value

Age

    ≤ 30
185
35
18.92
0.043
201
26
12.94
0.017
0.107

    31-40
184
38
20.65

71
19
26.76

0.294

    >40
31
12
38.71

28
3
10.71

0.014

Region

    Shandong
239
59
24.69
0.079
239
32
13.39
0.451
0.002

    Jilin
65
8
12.31

65
8
12.31

1.00

    Heilongjiang
95
18
18.95

95
8
8.42

0.035

Area of residence

    Urban
202
45
22.28
0.612
245
25
10.20
0.165
0.001

    Rural
198
40
20.20

155
23
14.84

0.192

Employment

    Unemployed
126
24
19.05
0.081
98
15
15.31
0.470
0.464

    Employed part-time
143
39
27.27

197
20
10.15

<0.001

    Employed full-time
131
22
16.79

105
13
12.38

0.343

Obesity and overweight

    Yes
79
15
18.99
0.582
117
9
7.69
0.086
0.018

    No
321
70
21.81

283
39
13.78

0.011

Keeping cats at home

    Yes
40
12
30.00
0.135
47
4
8.51
0.433
0.010

    No
360
73
20.28

353
44
12.46

0.005

Consumption of raw/undercooked meat

    Yes
33
12
36.36
0.027
77
5
6.49
0.098
<0.001

    No
367
73
19.89

323
43
13.31

0.021

Consumption of oyster

    Raw
84
30
35.71
<0.001
153
18
11.76
0.909
<0.001

    Boiled
316
55
17.41

247
30
12.15

0.084

Consumption of fish

    Raw
137
42
30.66
<0.001
158
17
10.76
0.086
<0.001

    Boiled
263
43
16.35

242
41
16.94

0.858

Consumption of raw vegetables and fruits

    Yes
282
64
22.70
0.275
299
37
12.37
0.692
0.001

    No
118
21
17.80

101
11
10.89

0.149

Exposure to soil

    Yes
233
47
20.17
0.534
254
34
13.39
0.261
0.045

    No
167
38
22.75

146
14
9.59

0.002

Gardening or agricultural activities

    Yes
257
52
20.23
0.505
263
35
13.31
0.265
0.034

    No
143
33
23.08

137
13
9.49

0.002

Washing hands before meals

    Yes
266
54
20.30
0.513
302
38
12.58
0.529
0.013

    No
134
31
23.13

98
10
10.20

0.011

Source of drinking water

    Spring/well
78
14
17.95
0.840
66
15
22.73
0.003
0.476

    Tap
322
61
18.94

334
33
9.88

<0.001

Total
400
85
21.25

400
48
12.00

<0.001

A total of 1200 control subjects, matched with diabetes mellitus patients by age, gender, and residence, were included in the study. Serum samples were obtained from persons who participated in health screening at the Affiliated Hospital of Qingdao University.

Approximately 5 mL of venous blood samples was drawn from participants who gave their consent to participate in this study. Blood samples were left overnight at room temperature to allow clotting and centrifuged at 3000 rpm for 10 minutes. The sera were collected in Eppendorf tubes and stored at 4°C for 24-72 hours and then kept at -20°C until testing.

2.3. Sociodemographic, Clinical, and Behavioral Data Collection

Sociodemographic data including age, gender, area of residence, and employment were obtained from all participants. Clinical data including the type of diabetes and behavioral data including obesity and overweight, keeping cats at home, consumption of raw/undercooked meat, fish, oysters, raw vegetables and fruits, gardening or agricultural activities, exposure to soil, source of drinking water, and washing hands before meals were collected from the participants. These variables were selected based on published literature [15, 16]. Data was obtained from the patients/guardians, medical examination records, and informants. Patients were invited to provide veridical information and were informed that their data were used in a confidential manner.

2.4. Serological Assay

Sera were analyzed for the presence of IgG and IgM antibodies against T. gondii using commercially available enzyme immunoassay kits (Demeditec Diagnostics GmbH, Germany) according to the manufacturer’s instructions. Clinical specificity and sensitivity of IgG kit were 99% and 98%, respectively. Clinical specificity and sensitivity of IgM kit were 99% and 100%, respectively [16, 17]. Positive and negative serum controls were included in every plate. To avoid bias of results, the serology test was performed double-blinded. Samples from diabetes mellitus patients and control group were randomly mixed, and the person performing the test did not know the source of samples in advance [16, 17].

2.5. Statistical Analysis

Results were analyzed with SPSS 18.0 software package. For the univariate analysis, Chi-square test was used to compare the categorical variables. The Mantel-Haenszel test was used to probe any differences between the patient and control groups. Multivariate regression models were used to adjust for potential confounders. Variables were included in the multivariate analysis if they had a p value ≤0.25 in the univariate analysis [17, 18]. Odds ratios (ORs) and the corresponding 95% confidence interval (CI) were calculated, in order to identify the independent risk factors for T. gondii infection. Results with a p value <0.05 were considered as statistically significant.

2.6. Ethics Approval and Consent to Participate

The study protocol was reviewed and approved by the Ethics Committee of the Affiliated Hospital of Qingdao University. The aim of the study was explained to the patients and they provided written consent for their participation in the study. Control sera were collected from volunteers.

3. Results

3.1. Epidemiology of T1DM Patients with T. gondii Infection

T1DM patients (16.50%) had a significantly higher T. gondii seroprevalence than the control subjects (11.50%) (p=0.042). Of these, 53 T1DM patients (13.25%) were found to be positive for T. gondii IgG antibodies, as compared to 40 controls (10.00%), and the difference was not statistically significant (p=0.152). T. gondii IgM antibodies were detected in 15 of the 400 T1DM patients and in seven of the 400 controls (3.75% versus 1.75%, respectively, p=0.084). The details of T1DM patients and control subjects, including age distribution, gender, employment, and area of residence, are shown in Table 1. The highest seroprevalence of T. gondii infection was detected in T1DM patients in the age range of ≤30 years (24.19%). T1DM patients living in Shandong province (20.00%) had a higher T. gondii seroprevalence than those living in Jilin (14.89%) and Heilongjiang (12.36%) provinces, but the difference was not significant (p=0.237). There were no significant differences between female (18.13%) and male (14.98%) T1DM patients (p = 0.395). The seroprevalence of T. gondii infection among the T1DM patients who lived in rural areas (18.18%) was slightly higher than those who lived in urban areas (14.85%), but the difference was not statistically significant (p=0.370). Moreover, T. gondii infection seroprevalence was not significantly different among T1DM patients with different types of employment (p=0.261).

3.2. Epidemiology of T2DM Patients with T. gondii Infection

T2DM patients (23.50%) had a significantly higher T. gondii seroprevalence than the control subjects (11.75%) (p<0.001). A total of 77 T2DM patients (19.25%) were found to be positive for T. gondii IgG antibodies, as compared to 37 controls (9.25%), and the difference was statistically significant (p<0.001). T. gondii IgM antibodies were detected in 19 of the 400 T2DM patients and in 11 of the 400 controls (4.75% versus 2.75%, respectively, p=0.137). The details of T2DM patients and control subjects, including age distribution, gender, employment, and area of residence, are shown in Table 2. The highest seroprevalence of T. gondii infection was detected in T1DM patients in the age range of 51-60 years (25.81%). T2DM patients living in Shandong province (28.85%) had a higher T. gondii seroprevalence than those living in Jilin (22.54%) and Heilongjiang (16.67%) provinces, but the difference was not significant (p=0.074). There were no significant differences between male (24.75%) and female (22.22%) T2DM patients (p = 0.551). The seroprevalence of T. gondii infection among the T2DM patients who lived in urban areas (25.00%) was slightly higher than those who lived in rural areas (22.00%), but the difference was not statistically significant (p=0.479). Moreover, T. gondii infection seroprevalence was not significantly different among T2DM patients with different types of employment (p=0.118).

3.3. Epidemiology of GDM Patients with T. gondii Infection

GDM patients (21.25%) had a significantly higher T. gondii seroprevalence than the control subjects (12.00%) (p=0.042). A total of 70 GDM patients (17.50%) were found to be positive for T. gondii IgG antibodies, as compared to 37 controls (9.25%), and the difference was statistically significant (p<0.001). T. gondii IgM antibodies were detected in 18 of the 400 GDM patients and in 11 of the 400 controls (4.50% versus 2.75%, respectively, p=0.186). The details of GDM patients and control subjects, including age distribution, gender, employment, and area of residence, are shown in Table 3. The highest seroprevalence of T. gondii infection was detected in GDM patients in the age range of >40 years (38.71%), and significant difference was found among different age groups (p=0.043). GDM patients living in Shandong province (24.69%) had a higher T. gondii seroprevalence than those living in Jilin (12.31%) and Heilongjiang (18.95%) provinces, but the difference was not significant (p=0.079). The seroprevalence of T. gondii infection among the GDM patients who lived in rural areas (20.20%) was slightly lower than those who lived in urban areas (22.28%), but the difference was not statistically significant (p=0.612). Moreover, T. gondii seroprevalence was not significantly different among GDM patients with different types of employment (p=0.081).

3.4. Risk Factors Associated with T. gondii Infection

Univariate analysis showed that some lifestyle variables of T1DM patients had a p value ≤0.25, including obesity and overweight, keeping cats at home, consumption of raw/undercooked meat, oysters, and fish, and source of drinking water. Six lifestyle variables of T2DM patients had a p value ≤0.25 through univariate analysis. They are obesity and overweight, keeping cats at home, consumption of raw/undercooked meat, fish, oysters, and raw vegetables and fruits. In GDM patients, keeping cats at home and consumption of raw/undercooked meat, fish, and oysters had a p value ≤0.25 by univariate analysis. In the multivariate analysis, keeping cats at home (OR=2.885; 95% CI: 1.37-6.07; p = 0.005) and consumption of oysters (OR=13.19; 95% CI: 2.91-59.82; p = 0.001) were associated with significantly increased odds of T. gondii infection in T1DM patients (Table 4). Consumption of raw/undercooked meat (OR=2.663; 95% CI: 1.08-6.56; p = 0.033) and oysters (OR=4.785; 95% CI: 1.98-11.45; p<0.001) was associated with significantly increased odds of T. gondii infection in T2DM patients (Table 4). There was no evidence of a significant association between T. gondii status and the selected variables in GDM patients (Table 4).

Table 4: Multivariate analysis of selected characteristics of diabetic patients and their association with Toxoplasma gondii infection.

Type of diabetic patients
Characteristi
Adjusted Odds rati
95% Confidence interval
P value

T1DM
Obesity and overweight
Keeping cats at home
0.584
2.885
0.32-1.08
1.37-6.07
0.086
0.005

Consumption of raw/undercooked meat Consumption of oyster
2.177
13.19
0.81-5.88
2.91-59.82
0.125
0.001

Consumption of fish
0.295
0.06-1.36
0.117

Source of drinking water
1.713
0.78-3.77
0.181

T2DM
Obesity and overweight
Keeping cats at home
Consumption of raw/undercooked meat
0.673
1.584
2.663
0.39-1.15
0.79-3.17
1.08-6.56
0.147
0.193
0.033

Consumption of oyster
4.758
1.98-11.45
<0.001

Consumption of fish
0.747
0.32-1.72
0.493

Consumption of raw vegetables and fruits
1.296
0.72-2.33
0.385

GDM
Keeping cats at home
Consumption of raw/undercooked meat
1.376
1.906
0.63-2.99
0.83-4.36
0.420
0.127

Consumption of oyster
1.764
0.78-3.99
0.173

Consumption of fish
1.516
0.72-3.18
0.271

variables with a p <0.25 in the univariate analysis were included.

by age and the other characteristics included in this table.

4. Discussion

The association between T. gondii infection and diabetes mellitus remains controversial, with few studies reporting conflicting results [19, 20]. Thus, the present study was conducted to determine whether T. gondii infection is associated with different types of diabetes mellitus in eastern China. The results showed that diabetes mellitus patients had higher frequencies of antibodies against T. gondii as compared to control subjects. Thus, our findings based on serological methods supported an association between diabetes mellitus and T. gondii infection.

Type 1 diabetes mellitus (T1DM) is an autoimmune disease with complex interactions between genetic and environmental factors [14]. The enteroviruses and other infectious agents were found to be associated with T1DM [21]. In the present study, T1DM patients had a significantly higher T. gondii seroprevalence than the controls (p=0.042), suggesting that T1DM patients are more likely to be infected with T. gondii. However, further targeted studies should be conducted to explore and confirm the association between T1DM and T. gondii infection.

Type 2 diabetes mellitus (T2DM), a major global health problem, is a complex metabolic disease [11]. The incidence of T2DM has notably increased in recent years, in both developed and developing countries [22]. Various infections, including T. gondii, may easily appear in T2DM patients because they are immunocompromised [23]. In the present study, T2DM patients had a significantly higher T. gondii seroprevalence than the control subjects (p<0.001). These evidences indicated a potential association between T. gondii infection and T2DM implying that T. gondii infection may increase susceptibility to T2DM, while T2DM patients are more vulnerable to opportunistic infections such as T. gondii. Importantly, the clinician should pay more attention to T. gondii infection when they diagnose and treat T2DM patients given the high prevalence of T. gondii infection in T2DM patients, and T. gondii infection may aggravate the T2DM status.

T. gondii infection during pregnancy may cause serious consequences such as miscarriage, microcephaly, hydrocephalus, and severe neurological disorders in the fetus [5]. In addition, in immunodeficient individuals, released bradyzoites from tissue cysts switching back into rapidly multiplying tachyzoites could cause reactivation of latent infection and dissemination throughout the body [24]. The immune system in diabetes mellitus patients is affected, and GDM patients are more susceptible to T. gondii infection. In the present study, GDM patients had a significantly higher T. gondii seroprevalence than the control subjects (p<0.001). Thus, serological screening of GDM patients is needed, followed by proper treatment of the T. gondii infection [25]. Moreover, information about toxoplasmosis and its transmission routes should be given to GDM patients as part of prenatal care.

The first epidemiological investigation on T. gondii infection in humans in China was conducted in Guangxi Zhuang Autonomous Region in 1978 [26]. Now, toxoplasmosis has become a notifiable disease in China. However, there are no national guidelines for the prevention of toxoplasmosis in China. Humans acquire the infection through three major routes: consumption of undercooked meat containing T. gondii tissue cysts, ingesting oocysts-contaminated water, soil, vegetables, and fruits, and transmission from mother to fetus during pregnancy [1, 27]. As expected, we found that keeping cats at home and consumption of raw/undercooked meat were associated with significantly increased odds of T. gondii infection in diabetic patients. These two risk factors have been identified in many studies in China [16, 28–30]. Interestingly, fresh oyster consumption was also a potential risk factor for T. gondii infection in T1DM and T2DM patients, which was similar to a study reported from the United States [31]. T. gondii oocysts can be washed into the sea via rainwash and runoff [32, 33], and shellfish including oysters, clams, and mussels can ingest the oocysts directly from seawater [1, 32–37]. In China, T. gondii oocysts have been detected in oysters [38] and consumption of fresh oysters is common in recent years, which may explain the higher T. gondii seroprevalence in the diabetes mellitus patients who consume raw oysters than those who do not consume raw oysters. Thus, knowledge of these risk factors will help in prevention efforts.

Some limitations of the present study should be kept in mind. First, our study participants might not represent the general clinically healthy individuals, pregnant women, and diabetes mellitus population due to the potential limitation of enrollment methods. Therefore, potential selection bias should be considered when interpreting our results. Second, serology could not clearly indicate the infection status as current infection or past infection; potential bias caused by such misclassification could not be eliminated. Moreover, molecular identification, taxonomy, genetic variation, and diagnosis of T. gondii should be considered in further studies. Third, more effective statistical analysis methods should be used to confirm the association between diabetes mellitus and T. gondii infection. Therefore, our results need to be proved in further studies.

5. Conclusion

This study provided serological evidence of an association between T. gondii infection and three types of diabetes mellitus (T1DM, T2DM, and GDM). Moreover, keeping cats at home and consumption of raw/undercooked meat and raw oysters were risk factors for T. gondii positivity using multivariate regression, which may help to guide future research and control policies. Further studies should be conducted to elucidate the role of T. gondii in diabetes mellitus.

Data Availability

The clinical and behavioral data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Authors’ Contributions

Yong-Xin Li and Hai Xin are equal contributors.

Acknowledgments

Project support was provided by the Natural Science Foundation of Shandong Province (ZR2016HQ18, ZR2014HM088) and National Natural Science Foundation of China (81472411, 81772713, and 81372752).

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