Results
A total of 408 participants, including 200 patients with asthma as a
case group with a mean ± SD age of 8.19 ± 3.2 years and 208 individuals
without asthma as a control group with a mean ± SD age of 7.79 ± 3.4
years, were included in the study. According to the statistical test,
both patient and control groups did not differ significantly in age,
location, and sex distribution. (P ≥ 0.05) (Table 1) .
The prevalence of Toxocara infection in the study population (patients
and control individuals) was 6.9% (95% CI, 3–5.62%; 28/408). Sixteen
out of the 200 children with asthma (8%; 95%CI, 2.11–6.49%) and 12
out of the 208 children without asthma (5.8%; 95%CI 2.71–5.9%) were
seropositive for anti-Toxocara IgG antibodies. There were no
statistically significant differences in the prevalence of toxocariasis
between children with asthma and children without asthma (OR, 1.420;
95% CI, 0.654-3.083; P value = 0. 373) (Table 2).
In subgroup analyses based on sociodemographics, Toxocara infection
seropositivity was significantly associated with asthma in children
whose parents were farmers (OR, 5.647; 95%CI, 1.168-27.299; P-value =
0.017), and those with frequent contact with soil (OR, 3.225; 95%CI,
1.044-9.961; P-value = 0.033). More details and significance for other
variables are shown in Table 3.
The multivariate analysis showed that a history of atopic asthma in the
family (OR, 3.463; 95% CI, 2.005- 5.982; P-value < 0.001),
soil contact (OR, 2.658; 95% CI, 1.675- 4.218; P-value <
0.001), and eating unwashed (raw) vegetables (OR, 2.575; 95% CI, 1.454-
4.558; P-value < 0.001) were independent factors associated
with asthma (Table 3). Additionally, no significant association was
found between seropositivity to Toxocara spp. and asthma.
Discussion
The prevalence of asthma in children has been reported from 2% to 37%
in different parts of the world (10). The prevalence rate of childhood
asthma has been reported to be 10.9% in Iran (11). Identifying the risk
factors that can affect allergy and asthma is crucial. Some previous
experimental and epidemiological studies have suggested that
geo-helminth infection, such as toxocariasis, might play an etiological
role in developing asthma and other allergic disorders(4, 12) However,
according to the hygiene hypothesis, this relationship is still
controversial and awaits to be explored (13). This investigation
discovered that children with asthma had a greater seroprevalence ofToxocara infection than age- and gender-matched controls, showing
that Toxocara seropositivity was not an independent risk factor
for developing asthma in this pediatric population in Boyer-Ahmad
Province.
Regarding the seroprevalence of toxocariasis in asthmatic and healthy
children, the present study was in agreement with some previous
epidemiologic studies where no significant difference in Toxocara seropositivity was observed between the two groups (14, 15). In contrast
to our study, several studies have reported a statistically significant
difference between the seroprevalence of Toxocara infection among
asthmatic and healthy children(16-19). In a study by Cobzaru et al., the
seroprevalence of toxocariasis in asthmatic children (68.42%) was
significantly higher than in the control subjects (13.63%) (16).
Moreover, two published meta-analysis studies in 2014 and 2018 indicated
that Toxocara infection could be a potential risk factor for
developing asthma (20, 21). An explanation for these different results
could be attributed to the age of the study population, geographic and
environmental conditions in each region, the rate of soil contamination
with Toxocara spp. eggs in the studied areas, parasite load,
personal hygiene, study design, and different sensitivity and
specificity methods used to diagnose toxocariasis (commercial ELISA or
Western blot). Although there was no significant difference in the
prevalence of toxocariasis between children with and without asthma in
this study, asthmatic children who had frequent contact with soil (one
of the most important known risk factors for toxocariasis) and those
whose fathers were farmers had significantly higher Toxocara seropositivity than healthy children. These findings indicate that this
region of Iran is contaminated with Toxocara eggs, and the risk
of acquiring infection is high, particularly for children who frequently
play in similar conditions in the southwest of Iran. Our study also
found that children with a history of atopic asthma in the family, soil
contact, and eating unwashed raw vegetables were more susceptible to
having asthma. Some studies confirm a positive association between these
underlying factors with asthma (22-24). In the present study, the
prevalence of toxocariasis in the total study population (asthmatic and
non-asthmatic children) was 6.9%. Several case-control studies had a
higher overall prevalence than our results (12, 16, 25). For example, in
two studies by Momen et al. (in Iran) and Cobzaru et al. (in Romania),
the seroprevalence of toxocariasis in the study population was reported
to be 33.35.% and 41%, respectively (12, 16). The fact that most of
the children in our study lived in the city and their lifestyle was such
that the majority had no history of exposure to dogs and cats may
explain the lower overall prevalence of toxocariasis in our study
compared to most other studies.
On the contrary, some studies report a lower prevalence of toxocariasis
than the current study. In this regard, Darvish et al. and Sadri et al.
reported the prevalence of toxocariasis at 2.48% and 1.09%,
respectively, which may be due to the use of different serological
diagnosis methods and antigenic sources(17, 26). A relatively high
prevalence of HT is seen in children (6.9%) in current study.
Therefore, control and preventive measures against toxocariasis must be
considered part of the study region’s healthcare system policies. Some
suggested preventive strategies are public education of parents about
ways of transmitting the infection, proper fencing around farms and
children’s playgrounds, and deworming of dogs and cats. The most
critical limitations were the small sample size and the absence of
confirming methods for definitive toxocariasis diagnosis (such as
Western blotting). Despite the mentioned limitations, using native
secretory-excretory antigen (ES) with acceptable sensitivity and
specificity significantly reduces cross-reactions (Barr et al. 2014). In
a study done by Zibaei et al. to diagnose toxocariasis, the sensitivity
and specificity of ELISA using T. cati ES antigens were 97.0%
and 96.7% respectively (27). Accurate diagnosis of asthmatic children
by an experienced pediatrician in allergy and immunology based on the
ISAAC questionnaire and good matching of groups in terms of age, gender,
socioeconomic status, and place of residence were among the strengths of
our study.