Toxocara infection in pregnant women: seroprevalence and risk factors

Abstract

Background

Human toxocariasis caused by T. canis is a worldwide and typically neglected zoonotic parasitic disease. Certain behaviors such as a geophagia habit, poor personal hygiene, close contact with young dogs, and ingestion of raw meat, as well as age, and socioeconomic status, affect the prevalence of the disease. In this study, we aimed at investigating the frequency of Toxocara infection among pregnant women and the epidemiological factors associated with disease.

Methods

Toxocariasis-specific IgG antibodies were measured using a commercial ELISA technique in 200 pregnant women between December 2021 and May 2022. A questionnaire filled by participants included options for demographic information (age, gender, residency), gestational age, number of previous pregnancies, history of abortion, drug use, comorbidities, history of parasitic disease, and keeping pets (dogs or cats).

Results

In total, 15 (7.5%) of the 200 participants (mean age 29.7 ± 18.7 years) had anti-T. canis antibodies. High-titer antibodies were most prevalent among the subjects aged 25–29 years. Logistic regression analysis showed that the pregnant women who had a history of abortions (P = 0.029), and keeping pets and contact with dogs and cats (P = 0.031) had an increased risk of acquiring toxocariasis. The study showed that demographic characteristics such as age group, pregnancy trimester, and underlying conditions were not significantly associated with toxocariasis (P > 0.05).

Conclusion

Considering the significant prevalence of toxocariasis in pregnant women as well as its health risks, preventive health measures against the toxocariasis risk factors seem more necessary.

Introduction

Human toxocariasis is a zoonotic parasitic disease caused by the migration of the nematode helminths Toxocara species (T. canis in dogs or T. cati in cats) larvae to the organs and tissues of animals and birds [1]. Human infection occurs when Toxocara larval eggs are accidentally ingested or through the consumption of raw or undercooked meat and giblets [2]. Subsequently, the larvae are released into the small intestine, penetrate the intestinal wall, and are transported through the bloodstream to tissues and organs, but do not develop into adult forms [3]. The larvae preferentially migrate to the liver, brain, and eye, causing the syndromes of visceral larval migration, ocular larval migration, neural toxocariasis, and covert Toxocara infection [4].

The diagnosis of human toxocariasis is mainly based on clinical, epidemiological, and laboratory data, which include imagining exams, blood exams, eosinophilia, total IgE levels, and serological tests [5]. Enzyme-linked immunosorbent assay is one of the most important tests that are available for the immunodiagnostic of toxocariasis using Toxocara excretory-secretory antigens. Due to the greater sensitivity and specificity of the Toxocara-ELISA, investigations demonstrate that this method is a better technique for detecting patients having visceral or ocular involvement consistent with toxocariasis [5,6,7,8,9].

In many developing countries, toxocariasis is a neglected zoonotic disease in humans, and this parasite is classified among the top six parasitic infections of public health priority by the World Health Organization and the Centers for Disease Control [11]. Several studies worldwide have described the extent of the high prevalence of toxocariasis. Globally, Toxocara infection is found in many countries, and prevalence rates can reach as high as 40% or more [12]. The serological studies on the prevalence of Toxocara in pregnant women in Iran are very limited. In a relatively similar study, the rate of seropositivity of anti-Toxocara antibodies by indirect ELISA in pregnant women in Ilam City western Iran was reported to be 21.2% [13].

It has been shown that people with chronic helminthic infection are more susceptible to intracellular pathogens, and these infections can subsequently cause congenital malformation [14]. However, limited reports of congenital infections caused by Toxocara have been published, which is largely due to the lack of attention to the investigation of the infection, despite its universality, in pregnant women. The only record of congenital Toxocara infection in humans occurred in a premature infant with retinopathy with blood eosinophilia whose mother was seropositive for Toxocara species [15]. In developing countries such as Iran, the serological investigation of this parasite has received less attention, and few studies have been conducted to determine the seroprevalence of toxocariasis in high-risk groups such as pregnant women.

Previous studies showed that a significant difference was observed between the prevalence of Toxocara seropositivity in pregnant women and some demographic and clinical parameters [16]. Therefore, the study aimed to estimate the prevalence of anti-Toxocara serum antibodies in pregnant women referred to the women’s hospital, which is a hospital focused on the demanding specialist medical care of women and newborns. Considering the knowledge of the serological status of pregnant women as one of the high-risk groups for Toxocara infection, they can provide useful information for this group and the health care of the community.

Materials and methods

Study participants

A cross-sectional study was conducted between December 2021 and May 2022 including pregnant women followed at Kamali Comprehensive Specialized Hospital, Vice-Chancellor for Education at the Alborz University of Medical Sciences, Iran. A standardized questionnaire was designed and applied during face-to-face interviews. The questionnaire included data about participants’ demographics such as age, pregnancy trimester, abortion history, the number of past pregnancies, history of drug use, potential risk factors for Toxocara infection (pet keeping and exposure to domestic animals), and detailed information on clinical characteristics and conditions such as diabetes, heart disease, central nervous system disorders, liver, and kidney diseases that may be considered “underlying diseases”. The human research ethical approval for the study was granted by the Ethical Committee at the University of Medical Sciences in Alborz (IR.ABZUMS.REC.1401.142), and informed consent was obtained from the participants before data collection. A written informed consent form was received from each study participant.

Serum samples collection

Under sterile conditions, five milliliters of venous blood were drawn from each subject in a clean blood collection tube by trained medical laboratory technicians. Serum samples were separated by centrifuging at 2,500 rpm for five minutes. The sera were collected in two mL tubes and stored at -20 °C until being used for the detection of anti-Toxocara IgG antibodies by serological assay.

Laboratory test

Anti-Toxocara antibodies were detected by a commercial ELISA kit (T. canis IgG-ELISA, NovaTec Company, Dietzenbach, Germany) as described by the manufacturer in the test instructions. The results were calculated as follows if the absorbance values (OD at 450 nm) were less than 9 NTU: negative; 10 NTU: cut-off point; between 9 and 11 NTU: grey zone which needs to be repeated; and more than 11 NTU: positive, as recommended by the manufacturer.

Statistical analysis

Positive result percentages by subgroup were estimated with a 95% confidence interval. Outcome data were initially evaluated by univariate analysis (Pearson’s chi-squared test), and variables with a statistical significance of < 0.20 in the univariate model were included in logistical regression analyses to assess risk factors associated with pregnant women’s seropositivity. From the regression coefficients for each predictor variable, odds ratio (OR) values were estimated per point together with the 95% CI. Results with P-values of less than 0.05 were considered to be statistically significant.

Results

Out of 200 pregnant women who participated in the study, 15 (7.5%) had positive anti-Toxocara antibodies. Table 1 lists demographic data on the studied people. The results of the present study showed that toxocariasis was prevalent in pregnant women in the Karaj district; therefore, evaluating the association between toxocariasis and several potential predictor variables to determine risk factors for the infection was performed. In the multivariate logistic regression analysis, abortion history (AOR = 5.40; 95% CI: 4.04–7.02; P = 0.017), and pet keeping (AOR = 3.75; 95% CI: 4.48–7.18; P = 0.036) had a statistically significant association with infection among the pregnant women. It should be mentioned that the statistical significance between the prevalence and the history of abortion does not mean the protective effect of Toxocara-related antibodies and this finding is probably due to the low volume of the studied samples. The analysis suggests that Toxocara infection is not significantly associated with other considered socio-demographics using the multivariate logistic regression analysis.

Table 1 Results of the univariate analysis of the studied variables

The history of drug medication in the last months before completing the questionnaire was recorded in 106 women: 18 (17.0%) were in the first, 31 (29.2%) were in the second and 57 (53.8%) were in the third trimester of pregnancy. The drugs that were taken included: Metronidazole, Folic acid, Multivitamins, hypo-hyperthyroidism drugs, Cefixime (antibiotic), Estradiol and Progesterone, Insulin, and Ranitidine. Of those who had a history of taking medicines, 10 (66.7%) had anti-Toxocara antibodies.

Discussion

The Toxocara seropositivity in the current study (7.5%) suggested that toxocariasis is present among pregnant women. The findings obtained in the current study were consistent with a previous study result from Brasília, Brazil (7.4%) [17]. The current seroprevalence was much lower than the findings in Nigeria (382/413, 92.5%) [18]. In a study by Raissi and colleagues in Ilam, the prevalence of anti-Toxocara antibodies in pregnant women was reported as 21.2% [13].

Keeping pets such as dogs and cats is a known risk factor for human infection due to the potential for soil contamination in public places with parasite eggs. In our study, a significant relationship was found between toxocariasis and contact with dogs and cats (P = 0.031). One reason for this could be pet contact with stray dogs and cats, which are more likely to come into contact with contaminated soil, thus having the potential for transmission. On the other hand, according to some researchers, the possibility of transmission of this disease by animals through direct physical contact with humans is high, because the ovum excreted in the feces of dogs and cats takes 2 weeks to reach the infectious stage. Therefore, soil is necessary as a reservoir of Toxocara for the spread of the disease [19].

In the current study, there was a significant relationship between a history of abortion and seropositivity of anti-Toxocara IgG antibodies (P = 0.029). Regarding the seropositivity of Toxocara and abortion, there are controversial results. In the study of Santos et al. [20] among pregnant women with a positive Toxocara serological assay, 55.6% had reproductive disorders (abortion, difficulty conceiving, premature birth). Pereira and colleagues showed that in pregnant women with IgG positive for Toxocara, 21.7% had at least one miscarriage, but they did not consider this variable as a significant risk factor [17]. In contrast, Taylor et al. found a statistically significant relationship between serum positivity against Toxocara IgG antibodies and previous abortions [21]. They found that in 35.0% of seropositive pregnant women, there is a history of miscarriage compared to 8.6% of serologically negative cases. Some researchers discuss the possibility of larval migration through host tissues and causing tissue damage that is associated with abortion [22]. Despite the only recorded case of congenital infection in human toxocariasis, the CDC does not consider this parasite to be a placental transmissible pathogen [23, 24]. However, there is evidence of vertical transmission by Toxocara species in definitive hosts. The larvae of Toxocara in the paratenic host show the capacity and tendency to migrate during pregnancy [25]. On the other hand, cases of Toxocara mammary transmission due to breastfeeding to children have been reported, all of which reinforce the importance of conducting studies in this field in humans [24].

We examined the level of IgG antibodies in different months of pregnancy; our results indicated the peak rise of anti-Toxocara IgG in the seventh month of pregnancy. The results of studies showed that the maternal IgG antibodies could migrate through the placenta and it is suggested that this starts from the 16th week of pregnancy and then rises continuously from the 22nd week until the fetus shows a level of IgG antibodies similar to an adult [23].

Our data demonstrated that there is no significant association between the prevalence of Toxocara infection and underlying conditions in pregnant women (P = 0.200). Although chronic medical conditions, what many may consider “underlying conditions”, include diabetes, heart disease, obesity, cancer, and kidney disease. Underlying diseases make people more vulnerable to the infection [26].

Finally, the findings of the current study showed that Toxocara seropositivity was not associated with some pregnancy risk factors (age group, pregnancy trimester, number of past pregnancies, and history of drug use) in the studied subjects that were analyzed separately (P > 0.05).

Limitations

The present study faced limitations including resource constraints and lack of access to confirmatory serological tests such as western blotting. A molecular test with high sensitivity/specificity can be performed to overcome some limitations such as cross-reactivity. Another limitation is the age distribution of the studied subjects; the number of pregnant women in different age groups was heterogeneous. On the other hand, in the current study some risk factors in contracting toxocariasis such as consumption of raw and undercooked meat or sitting vegetables, as well as the level of education and occupation of the participants, were not investigated, which should be investigated in future studies.

Conclusion

This study was conducted due to the lack of information about toxocariasis from the studied subjects, as far as we know, this is the first study to investigate Toxocara infection in pregnant women in our region. The findings of the present study show that toxocariasis is common in the studied individuals and there is a significant relationship between Toxocara infection in pregnant women and a history of abortion as well as keeping pets. It is recommended that more samples from pregnant women be screened and analyzed using serological techniques such as WB or molecular-based methods as confirmatory tests.