Background
The incidence of allergic diseases is rising in industrialized countries and urbanized areas worldwide [
1,
2]. While genetic factors are important in determining the risk of development of asthma, environmental factors are still likely to be the primary determinants of expression of atopic diseases [
3]. Atopic disorders comprise a range of allergic diseases including allergic asthma, anaphylaxis, allergic rhinitis and atopic eczema. Atopic eczema affects approximately 5-20% of children worldwide and mostly children living in urban areas [
4,
5]. The highest prevalence rates of atopic eczema (more than 15%) are observed in urban Africa, the Baltics, Australia, New Zealand and Northern and Western Europe. The lowest prevalence rates (less than 5%) are observed in China, Eastern Europe, and Central Asia [
4]. In several studies a lower prevalence rate of allergic diseases in rural compared to urban children has been observed [
5‐
8]. This difference has been attributed to differences in exposure to aeroallergens and irritant environmental triggers as well as infection with pathogens that can act as allergic disease risk modifiers [
9,
10].
The so called ‘hygiene hypothesis’ proposes that diminished microbial exposure during early childhood increases susceptibility to allergic diseases [
11]. The implementation of hygiene measures as well as the use of antibiotics and vaccines can explain the decreased exposure to different pathogens, particularly in Western countries. Originally, it was proposed that an increased exposure to bacteria and viruses would lead to a strong Th1 response limiting the development of a Th2 immune response, which is typically associated with allergic diseases. However, helminth infections are known to induce a Th2 response and an inverse association between infection with different helminth species and allergic conditions has been reported [
12]. It has recently become clear that the interaction between helminth infection and allergic diseases involves regulatory T cells, that dampen both Th1 and Th2 effector responses [
13,
14].
Although several studies have demonstrated a negative association between intestinal helminth infections and allergic diseases, this association is not consistently observed [
2]. In a study including Ethiopian children, there was no significant association between helminth infections and atopic diseases [
9], while in pediatric studies from Brazil both positive as well as negative associations between
Ascaris lumbricoides infection and atopic diseases were observed [
15‐
17]. A systematic review and meta-analysis of epidemiologic studies on the association between intestinal parasite infection and the presence of atopy showed a consistent protective effect of infection with
Ascaris lumbricoides,
Trichuris trichiura, hookworm and
Schistosoma sp. [
18].
There are only a few reports assessing the association of protozoan infections with allergic diseases [
17,
19‐
21].
Giardia lamblia was associated with allergic diseases in children from an urban area of Venezuela [
19] while no significant relationship between detection of
G. lamblia and allergies was observed in urban areas of Brazil [
21]. Both studies included children older than 2 years of age. To our knowledge, the relationship between protozoan infections and allergic symptoms has not been investigated in children under 2 years of age.
In rural areas of Venezuela, parasitic infections are commonly observed in children under 16 years of age [
22]. Children aged between 12 and 24 months are at a critical stage of growth and development which increases their vulnerability to the detrimental effects of helminth infections. Recent evidence suggests that if interventions to reduce helminth infections and associated malnutrition are not implemented before 24 months of age, future growth and development will be affected [
23,
24]. Helminthiasis and giardiasis in school-age Venezuelan children were associated with acute and chronic nutritional status respectively [
25]. The vicious cycle of malnutrion and parasitic infections is associated with immune modulation. A disturbed immune balance may play a role in the development of immune-mediated diseases like atopy.
We performed a cross-sectional survey to study the association of helminth infections, protozoan infections and malnutrition with atopic eczema and recurrent wheezing in Warao Amerindian children aged 0 to 2 years.
Results
From August to November 2012, 229 children 0 to 24 months of age were included. All parents/guardians were willing to answer questions and cooperate with physical examination of the child. Fecal samples were collected from 100 children (44%). Children from whom stool samples were obtained were more likely to suffer from stunting than children from whom stool samples were not obtained (56% vs. 40%, p = 0.013). Characteristics of the study population are shown in Table
3.
Table 3
Characteristics of the study population (n = 229)
Age (months), mean (SD), | 13 (6) |
Male | 115 (50) |
Female | 114 (50) |
Community, n% | |
Araguabisi | 16 (7) |
Araguaimujo | 27 (12) |
Arature | 21 (9) |
Bonoina | 24 (11) |
Guayaboroina | 9 (4) |
Guayo | 26 (11) |
Ibaruma | 20 (9) |
Jobure de Curiapo | 31 (14) |
Merejina | 14 (6) |
Nabasanuka | 26 (11) |
Winikina | 15 (7) |
Malnourished: stunting (HAZ < -2 SD), n% | 107 (47) |
Malnourished: underweight (WAZ < -2 SD), n% | 41 (18) |
Malnourished: wasting (WHZ < -2 SD), n% | 15 (7) |
Atopic eczema, n% | 43 (19) |
Recurrent wheezing in children ≥ 12 months of age, n% | 28 (23) |
Helminth infection, n% | 26 (26) |
Ascaris lumbricoides
| 15 (15) |
Trichuris trichiura
| 4 (4) |
Strongyloides stercoralis
| 11 (11) |
Necator americanus
| 2 (2) |
Ancylostoma duodenale
| 1 (1) |
Protozoan infection, n% | 59 (59) |
Giardia lamblia
| 47 (47) |
Dientamoebe fragilis
| 4 (4) |
Cryptosporidium parvum
| 15 (15) |
Blastocystis hominis
| 5 (5) |
Iodamoeba sp
| 1 (1) |
Entamoeba dispar
| 1 (1) |
Chilomastix mesnili
| 1 (1) |
The prevalence of atopic eczema in the study population was 19% (n = 43) and recurrent wheezing was observed in 23% (n = 28). A borderline significant trend towards more atopic eczema in children suffering from recurrent wheezing was observed (32% vs. 16%, p = 0.055). Infection with an intestinal parasite was present in 70% (n = 70) of the children that handed in fecal samples. In 26 children (26%), a helminth infection was detected while 59 children (59%) suffered from a protozoan infection.
G. lamblia was the most frequently detected pathogen, infecting 47 of the included children (47%). Characteristics of included children and the association between parasitic infections and atopic eczema and recurrent wheezing are presented in Tables
4 and
5. There were no significant differences regarding age or sex between the children with or without atopic eczema. The prevalence of atopic eczema was significantly higher in stunted compared with non-stunted children in multivariable analysis (OR 4.3, 95% CI 1.3 – 13.6, p = 0.015).
Table 4
Characteristics of Warao Amerindian children 0 to 2 years of age, malnutrition and parasitic infections compared with atopic eczema*
Age (months), mean (SD) | 14 (6) | 13 (6) | 0.38 | 1.0 (0.96-1.1) | # |
Sex, n (%) | | | 0.62 | | |
Male | 13 (57) | 39 (51) | | 1 | # |
Female | 10 (43) | 38 (49) | | 0.79 (0.31-2.0) | |
Stunting vs. non-stunting (HAZ < -2 SD), n (%) | 18 (78) | 38 (49) | 0.018 | 3.7 (1.2-11.0) | 4.3 (1.3-13.6) |
Underweight vs. not underweight (WAZ < -2 SD), n (%) | 5 (22) | 17 (22) | 0.97 | 0.98 (0.32-3.0) | 0.52 (0.15-1.8) |
Wasting vs. non-wasting (WHZ < -2 SD), n (%) | 0 (0) | 6 (8) | # | # | # |
Parasitic infections, n (%) | | | | | |
Helminth infection present vs. absent | 6 (26) | 20 (26) | 0.99 | 1.0 (0.35-2.9) | 0.98 (0.32-3.0) |
Protozoan infection present vs. absent | 11 (48) | 48 (62) | 0.22 | 0.55 (0.22-1.4) | 0.69 (0.26-1.8) |
Table 5
Characteristics of Warao Amerindian children 0 to 2 years of age, malnutrition and parasitic infections compared with recurrent wheezing*
Age (months), mean (SD) | 18 (3) | 17 (4) | 0.57 | 1.0 (0.90-1.2) | # |
Sex, n (%) | | | 0.10 | | |
Male | 13 (81) | 26 (58) | | 1 | 1 |
Female | 3 (19) | 19 (42) | | 0.32 (0.079-1.3) | 0.28 (0.061-1.3) |
Stunting vs. non-stunting (HAZ < -2 SD), n (%) | 12 (75) | 25 (56) | 0.18 | 2.4 (0.67-8.6) | 4.5 (0.97-21.2) |
Underweight vs. not underweight (WAZ < -2 SD), n (%) | 4 (25) | 12 (27) | 0.90 | 0.92 (0.25-3.4) | 0.78 (0.15-4.1) |
Wasting vs. non-wasting (WHZ < -2 SD), n (%) | 0 (0) | 4 (9) | # | # | # |
Parasitic infections, n (%) | | | | | |
Helminth infection present vs. absent | 10 (63) | 27 (60) | 0.86 | 0.90 (0.28-2.9) | 0.77 (0.20-3.0) |
Protozoan infection present vs. absent | 13 (81) | 21 (47) | 0.024 | 5.0 (1.2-19.8) | 6.7 (1.5-30.5) |
The prevalence of recurrent wheezing in the last 12 months was 21% (n=48) and for children aged 12 months and above, it was 23% (n=28). There were no significant differences regarding age or sex between the children with or without recurrent wheezing. Stunting was more often observed in children with recurrent wheezing compared with children without recurrent wheezing, but this difference was not statistically significant in multivariable analysis (OR 4.5, 95% CI 0.97 - 21.2, p = 0.055). Children with recurrent wheezing were significantly more often infected with a protozoan infection compared with children that did not suffer from recurrent wheezing in multivariable analysis (OR 6.7, 95% CI 1.5 – 30.5).
Discussion
This cross-sectional survey showed that Warao Amerindian children in Venezuela aged 0 to 2 years suffer from a high prevalence of atopic eczema and recurrent wheezing. Recurrent wheezing prevalence rates were significantly positively associated with protozoan infections while rates of atopic eczema were significantly higher in stunted children. Helminth infections were not significantly associated with either atopic eczema or recurrent wheezing.
The prevalence of recurrent wheezing in our study population was 23%. Similar prevalence rates of recurrent wheezing in children aged 0 to 2 years were observed in urban areas of Brazil and Norway [
34,
35], although study definitions regarding the minimal number of wheezing episodes (respectively three in the first year of life and two in the first two years of life) were not identical to our study. In a rural area of Colombia, the prevalence of recurrent wheezing (≥3 episodes) in children aged 24 months was 14% [
36]. Definitions of recurrent wheezing in young children used in different studies vary widely. Our definition of recurrent wheezing has been used previously in preschool children [
37,
38]. In urban areas with hospitals keeping patient records, questionnaires for recurrent wheezing in infants can be validated by comparing the parental answers to questionnaires with diagnoses made by pediatric respiratory specialists and this has led to the use of such questionnaires in urban health centres in Latin America [
39,
40]. The diagnostic value of these questionnaires in rural indigenous populations remains to be seen.
Almost one fifth of the children included in this survey were diagnosed with atopic eczema (19%). In Shanghai in children 3 to 6 years of age an overall atopic eczema prevalence of 8% was observed, ranging from 5% in rural areas to 13% in urban areas [
5]. In Europe, the overall prevalence of atopic dermatitis in 1133 children living in five European countries in farming and non-farming rural areas was 16% (range 8%-30%) [
41].
We observed a significantly higher prevalence of atopic eczema in children suffering from stunting compared with children who were not stunted and a trend towards more stunting in children with recurrent wheezing compared to children without recurrent wheezing. Stunting indicates chronic malnutrition. In children with malnutrition, cell-mediated (T-cell) immunity, IgA levels in secretions, complement levels, and phagocytosis are all diminished [
42]. Whether stunted children with an altered immune response have a higher risk of atopic reactions than non-stunted children warrants further investigation. In the group of children with wasting, indicating acute malnutrition, we did not record cases of atopic eczema and only two cases of recurrent wheezing. As this acute presentation of malnutrition develops in a short period of time, it is possible that malnutrition-related alterations in the immune response that might be associated with the development of atopic eczema have not yet developed in children with an acute form of malnutrition.
The overall gastrointestinal parasitic infection prevalence in children 0 to 2 years of age in our study was 70%. The prevalence of helminth infections was 26%. Similar helminth infection prevalence rates in children under 2 years of age were observed in rural areas in other South American countries [
24,
43,
44]. The prevalence of gastrointestinal protozoan infections in our study population was 59% with
G. lamblia as the most prevalent protozoan infection, infecting 47% of the children. The prevalence of
G. lamblia infections we observed is higher than prevalence rates in an earlier study performed in the Orinoco Delta between 2010 and 2011. This study included 141 children aged 1 to 15 years and recorded a protozoan infection prevalence of 65% and a
G. lamblia infection prevalence of 37% [
22]. In pre-school children, a higher prevalence of
G. lamblia infections in the rainy season compared with the dry season has been observed [
45]. Our study was performed in the rainy season while the earlier study was performed during a whole year, which can explain the higher prevalence of
G. lamblia in our study compared to the earlier study
. In other South American countries the prevalence of
G. lamblia in children aged 0 to 2 years is approximately 20% [
43,
44]. However, these studies detected
G. lamblia infections only microscopically. We detected
G. lamblia by both microscopy and real time PCR, a technique with a very high sensitivity for protozoan infections [
28].
The presence of protozoan infections was significantly associated with a high prevalence of recurrent wheezing in our study. Two earlier studies performed in Venezuela in 1993 and 1998 showed similar positive associations between
G. lamblia and allergic diseases [
19,
20]. A recent Brazilian study showed no association between
G. lamblia and symptoms of asthma or skin allergy [
21]. However, all of these studies included children older than 2 years of age. Helminth infections were not associated with either recurrent wheezing or atopic eczema in our study while an inverse association between infection with different helminth species and allergic conditions has been reported in other studies [
16‐
18] However, the children included in these studies were also older than 2 years. The immune system of infants and young children shows great plasticity and is more amenable to modulation than the immune system of older children and adults [
46]. Age is therefore likely to be a major determinant of the interaction between intestinal parasitic infections and the host immune respons and more studies including children younger than 2 years of age are warranted.
The interpretation of a diagnosis of recurrent wheezing in children younger than 2 years of age is questionable. Wheezy lower respiratory tract illnesses are common in early childhood and are often associated with viral infections, particularly with respiratory syncytial virus (RSV) [
47,
48]. Recurrent wheezing in our study children could thus be the mere result of repeated viral infections rather than a marker for bronchial allergy. However, even if recurrent wheezing was caused by viral respiratory tract infections, wheezing episodes could still be related to an allergic predisposition of the child. Prospective studies show that infants with repeated episodes of viral bronchiolitis often develop allergic asthma during childhood [
49‐
51] and it has been suggested that a predisposition for bronchial obstructive disease underlies both recurrent wheezing in young children and asthma development during childhood [
52]. In a recently published large prospective birth cohort study, two-third of the children with recurrent wheezing episodes by 2 years of age was diagnosed with asthma or bronchial hyper responsiveness or used asthma medication at 16 years of age [
35]. Atopy as measured by skin prick tests and atopic dermatitis in infancy has also been identified as a predictor for the subsequent development of asthma [
51,
53]. Another study among schoolchildren living in a poor rural region of tropical Latin America, showed a predominance of non-atopic compared with atopic wheeze. However the subjects of this study were between 6 and 16 years old and thus not comparable to our study population [
54]. The positive association between atopic eczema and recurrent wheezing that we observed indicates that recurrent wheezing in our studied children could be a marker for susceptibility for atopic asthma. However, follow-up studies in this population at an older age are required to establish a possible association of recurrent wheezing with subsequent asthma development. Furthermore, it is recommendable to include other measures of atopy such as skin prick tests in future follow-up studies.
In this survey a stool sample was collected in 100 of the 229 subjects (44%). The prevalence of stunting was higher in children from whom stool samples were obtained than in children without a stool sample. A possible explanation for this finding is that parents of malnourished children were more likely to collect stool samples because of concerns about the state of health of their children.
The diagnosis of atopic eczema was based on the Nottingham- and SCORAD-protocol [
30,
31]. Both instruments are designed for diagnosing clinical atopic dermatitis. Additional information about the skin condition and familiar predisposition was obtained in order to distinguish between atopic dermatitis and other skin conditions such as scabies and viral or antibiotic rash. However, we cannot exclude the possibility that the presence of other skin conditions has led to false-positive observations and overreporting of the prevalence rate of atopic eczema. We observed that atopic eczema in children was often not identified as a problem by their parents. Dermatitis and rash were attributed to sweating and often no treatment was given, even if a topical treatment was provided by a local nurse. Severe cases of dermatitis were recognized as aberrant and a reason to consult a doctor. Scratching, which can easily lead to secondary infections in this setting, was generally not considered to be a problem. As atopic eczema in young children can lead to significant health problems [
55], qualitative studies are needed to determine whether parents have disease awareness of atopic eczema. Possibly, there is a need for health and educational services on this topic.
In addition to the already mentioned limitations, eg the interpretation of the diagnosis of recurrent wheezing and the lack of blood samples or skin-prick tests, there are several other limitations to our study, some of which are related to the challenging logistics of conducting an epidemiological study in an area with a poor infrastructure characterized by low literacy and poor access to health care. Although we performed a multivariable analysis taking into account possible confounders such as age and sex, unmeasured factors may have caused residual confounding. Male gender, low birth weight (<2,500 g), low gestational age (<37 weeks), breastfeeding for less than 6 months, congenital heart disease, family history of atopy, asthma, smoking exposure and stove warming, have been identified as significant risk factors for recurrent wheezing while the presence of pets at home seems to be a protective factor [
56]. It is difficult to obtain information on birth weight and gestational age in the Warao population, because documentation on new-borns is not collected consistently. Most women deliver their children at home and do not visit a hospital. There are no records available on gestational age because women generally do not know their exact pregnancy duration. As virtually all Warao children are breastfed until at least 12 months of age, it is not likely that the lack of information on duration of breastfeeding has influenced our study results. The prevalence of congenital heart disease is unknown. Also, the lack of information on smoking exposure and the presence of pets in the household may have caused residual confounding.
Additionally, although included villages were chosen to be geographically spread throughout the Antionio Díaz municipality, no randomization procedure was performed for the selection of villages. If reported wheezing episodes were related to viral infections, substantial variation between communities in the prevalence of recurrent wheezing could be associated with the rapid spread of viral infections within communities. Because most Warao people live with several families together in one house, viral infections spread rapidly, leading to a rise in overall prevalence when a few community members become infected. Finally, the use of a questionnaire may have caused recall bias. However, a questionnaire is the only available instrument in the Warao population because hospital data on atopic diseases are scarce due to the lack of personal medical records.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
MO participated in the design of the study, the collection of data, the statistical analysis, the interpretation of data and drafting the manuscript. LV participated in the design of the study, the collection of data, the statistical analysis, the interpretation of data, drafting the manuscript and revising it critically. PH participated in the design of the study and revised the manuscript critically for important intellectual content. BN advised on patient recruitment and revised the manuscript critically for important intellectual content. AM participated in the collection of data and helped to draft the manuscript. NA participated in the collection of data and helped to draft the manuscript. RM carried out fecal microscopical analysis. JR participated in the data analysis and helped with the PCR of feces. EP advised on data analysis and revised the manuscript critically for important intellectual content. JW participated in the design of the study, coordinated the field work, and revised the manuscript critically for important intellectual content. All authors read and approved the final manuscript.