© 2002 American Thoracic Society
Ascaris lumbricoides Infection Is Associated with Increased Risk of Childhood Asthma and Atopy in Rural ChinaChanning Laboratory, Department of Medicine, Brigham and Women's Hospital; Division of Pulmonary and Critical Care Medicine, Beth Israel Deaconess Medical Center; Harvard Medical School, Program of Population Genetics, Harvard School of Public Health, Boston, Massachusetts; and Anhui Medical University Center for Ecogenetics and Disease Control, Hefei, Anhui, China Correspondence and requests for reprints should be addressed to Dr. Lyle Palmer, Channing Laboratory, Brigham and Women's Hospital and Harvard Medical School, 181 Longwood Avenue, Boston, MA 02115. E-mail: lyle.palmer{at}channing.harvard.edu
There is growing international interest in the possible relationships between helminthic infection and allergic disease, although the nature of the relationships remains uncertain and controversial. The interrelationships of current and past infection with Ascaris lumbricoides and asthma and atopy were investigated in a cross-sectional sample of 2,164 children between the ages of 8 and 18 years from Anqing Province, China. The children were sampled from a larger family-based study of the genetics of asthma. The prevalence of either a history of or a positive stool examination for Ascaris was 24.5%. Asthma was defined for analytic purposes using previously validated, stringent criteria including airways responsiveness to methacholine. Independently of the other factors assessed, infection with A. lumbricoides was associated with increased risk of asthma (p < 0.001), an increased number of skin tests positive to aeroallergens (p < 0.001), and an increased dose–response slope to methacholine (p = 0.003). The association of sensitization to common aeroallergens with increased asthma risk was enhanced in those children infected with Ascaris, and such infection was associated with an increased risk of asthma independent of sensitization to aeroallergens in this selected population. These data suggest a complex relationship between ascariasis and susceptibility to childhood asthma among predisposed children that may involve an interaction with the immune response to inhaled aeroallergens.
Key Words: asthma • atopy • helminthic infection • child • China
Asthma, the most common chronic disease among children in developed nations (1), carries a substantial direct and indirect economic cost worldwide (2). Whereas most asthma originates in childhood, the natural history of asthma is poorly understood. It has been widely observed that the prevalence of asthma and other allergic diseases has risen over the past two decades in industrialized nations (3). However, the baseline prevalence of childhood asthma is substantially lower in developing countries (1). Little is known about the etiology of asthma in mainland China. In developing tropical countries, endemic helminthic infection is a major public health problem (4). Ascariasis is among the most common helminthic infections worldwide; the global infection burden was recently estimated to be approximately 1.5 billion people, representing an overall prevalence of 36% (4). A recent nationwide survey in China suggested that Ascaris lumbricoides infection was the most common helminthiasis, with an overall prevalence of 47% and an estimated 531 million infections, and was most prevalent in children between 5 and 19 years old (5). The epidemiologic observation that asthma is very common and helminthic infection relatively uncommon in developed countries and that the converse is true in many developing countries has led to speculation that the two phenomena may be inversely associated (6–8). However, the relationship between asthma and helminthic infection remains uncertain. Previous studies in endemically parasitized non-Chinese populations have variously argued that helminthiasis causes asthma (9–14), inhibits asthma (15–17), or is unrelated to asthma (18, 19). The aim of this study was to investigate the association of infection with A. lumbricoides (round worm) and asthma and atopy in a rural Chinese population of children from families ascertained on the basis of asthma. A related aim was to explore possible associations between the pathogenic mechanisms underlying atopic disease and immune responses to helminthic infection in the Chinese population.
The study was conducted in families of asthmatic subjects from the eight rural counties of Anqing Province, China, and has been previously described (20–22). The current study population comprised all children from these families who were aged between 8 and 18 years and had parasite data available.
Helminthic Infection
Spirometry and Airways Responsiveness
Total Serum IgE Levels and Allergy Skin Testing
Definition of Asthma
Statistical Methods The total serum IgE levels and the DRS were skewed with a long right-hand tail and were loge transformed before analysis. A constant was added to each DRS measurement to allow loge transformation when the measure was less than or equal to zero. Cigarette smoking history in parents and children was categorized as: 0 = never smoked; 1 = past smoker; 2 = current smoker. Asthma, sex, and positive skin test reactivity to each aeroallergen were analyzed as binary variables. All other variables were analyzed as continuous. Analysis and data management were performed using STATA v5 (Stata Corporation, College Station, TX) and SPlus v4.5 (Mathsoft Inc., Cambridge, MA).
Subject Characteristics The study originally included 3,372 subjects 18 years of age or younger in 1,617 families of asthmatic subjects in Anqing, China. The present analyses were limited to the 2,164 children (in 1,152 families) aged between 8 and 18 years for whom there was information on helminthic infection. The characteristics of participating children are summarized in Table 1. The average number of children per family was 1.9; this was less than the minimum sibship size of 2 enrolled in the parent study due to the exclusion of children not matching the entry criteria for the current analysis. A positive history or stool examination of A. lumbricoides infection was present in 15.9% (n = 345) and 12.2% (n = 264) of subjects, respectively. The concordance between a positive history of infection and a positive examination result for Ascaris was poor (Cohen's Kappa was 0.02). Therefore, infection with Ascaris within each individual was also analyzed by use of a variable that was coded as positive if either the history or the examination was positive. Serum total IgE levels ranged from 2.0 to 10,137.5 IU/ml. The number of positive skin tests ranged from 0 to 9; 40% of subjects were sensitized to at least one aeroallergen (Table 1). Cigarette smoking had a low prevalence among the children, was very common among fathers, and was almost absent in the mothers (Table 1).
Bivariate GEE analysis suggested that higher BMI (odds ratio [OR] = 0.82/kg/m2, 95% confidence interval [CI] = 0.77–0.88, p < 0.001), increasing height (OR = 0.05/m, 95% CI = 0.02–0.14, p < 0.001), and increasing age (OR = 0.87/yr, 95% CI = 0.82–0.92, p < 0.001) were closely associated with decreased risk of asthma. Neither male sex (OR = 1.05, 95% CI = 0.80–1.40, p = 0.71) nor cigarette smoking history (OR = 1.40, 95% CI = 0.94–2.08, p = 0.10) was significantly associated with asthma risk. Paternal (OR = 1.08, 95% CI = 0.89–1.31, p = 0.44) or maternal (OR = 0.73, 95% CI = 0.43–1.23, p = 0.24) smoking history was not significantly associated with asthma risk in offspring. As has been previously shown in this population (22), bivariate analysis indicated that loge total serum IgE levels (OR = 1.23/loge IU/ml, 95% CI = 1.07–1.40, p = 0.003) and the number of skin tests positive to aeroallergens (OR = 1.10/positive skin test, 95% CI = 1.01–1.19, p = 0.03) were positively associated with asthma risk.
Association of Helminthic Infection with Asthma Multivariate GEE analyses confirmed these associations. Independently of other potential predictors, a positive history or stool examination for A. lumbricoides was the best overall predictor of increased asthma risk (Table 2, Model 1). This relationship was also independent of the association of asthma with sensitization to aeroallergens (Table 2, Model 2). Further multivariate analysis suggested an important interaction between the number of skin tests positive to aeroallergens and a positive history or stool examination for A. lumbricoides (OR = 1.25, 95% CI = 1.13–1.37, p < 0.001). The results suggested that the association of sensitization to aeroallergens with asthma risk was enhanced in those subjects who had a positive history or examination for A. lumbricoides infection (Table 3).
Association of Helminthic Infection with Atopy and Airways Responsiveness Bivariate GEE analysis indicated that a positive stool examination (OR = 1.43, 95% CI = 1.11–1.86, p = 0.006) but not a positive history (OR = 1.04, 95% CI = 0.81–1.32, p = 0.77) for A. lumbricoides infection was associated with increased total serum IgE levels in the subsample of children with IgE levels available. The geometric mean total IgE in those with a positive stool examination was 206.0 IU/ml and in those without a positive stool examination was 147.0 IU/ml. Multivariate GEE analysis indicated that a positive stool examination or positive history for Ascaris infection was associated with increased number of positive skin reactivity tests (Regression coefficient [ß] = 0.32, SD = 0.08, p < 0.001; model adjusted for sex and age) and increased loge dose–response slope to me- thacholine challenge (ß = 0.10, SD = 0.03, p = 0.003; model adjusted for sex and height). Multivariate analysis of individual allergens indicated that a positive history or stool examination for A. lumbricoides was associated (p < 0.001) with sensitization to all of the aeroallergens tested, with the exception of D. pteronyssinus, D. farinae, mold, and cockroach (data not shown).
Our study was designed to investigate the interrelationships between asthma, atopy, and helminthic infection in rural China in a large sample of children from asthmatic families and has shown that infection with A. lumbricoides was closely associated with increased risk of childhood asthma, increased airways responsiveness to methacholine, and sensitization to common aeroallergens. The major limitations of our study are its cross-sectional design and potential information biases relating to the collection of questionnaire data. The cross-sectional nature of the data meant that a temporal relation between ascariasis and the development of asthma could not be demonstrated. However, our sample size was large, standardized protocols were employed, and Ascaris stool examination data, physiological indices of airways responsiveness and atopy, and anthropometric data were measured objectively. Further, we used a stringent algorithm for defining ‘asthma’ for our analysis that had been previously validated in the Anqing population (28). There was only a limited and sporadic history of anti-Ascaris treatment, and medication use for asthma was virtually absent (no use of corticosteroids) (20) in the rural Anqing population at the time of this study, making it unlikely that our results were strongly confounded by treatment. Stools were also examined for schistosomiasis, ancylosyomiasis, and enterobiasis; however, the prevalence of these parasites was too low (< 3%) to be analyzed further in this study. Finally, the familial aggregation of asthma, atopy, and helminthic infection and the potential interrelationships between the genetic and environmental components of variance of these outcomes could not be investigated in the current study population due to the strong ascertainment scheme and the lack of quantitative worm burden data. Although our findings may not be generalizable to the general population of Anqing due to the selected nature of the population, they do apply to children at high risk of developing asthma (relatives of asthmatic subjects). The prevalence of asthma among children in the Anqing general population is less than 5% (21); a prohibitively large sample size would therefore be needed to study the relation between ascariasis and asthma in the Chinese general population. This cross-sectional, family-based cohort study is, to our knowledge, the first to investigate the interrelationships of childhood asthma, asthma-associated physiological traits, and helminthic infection in a Chinese population. The inconsistent results of previous studies investigating the association between asthma and helminthiasis (7, 8) may relate to their relatively small sample sizes, to the lack of objective physiological measures related to asthma, and to the lack of a stringent definition of asthma. Our results are consistent with previous studies in non-Chinese populations that have suggested an association between helminthiasis and an increased risk of asthma (9–14, 30, 31) and suggest that ascariasis may act to potentiate the TH2 immune response characteristic of childhood asthma (8). Lifestyle changes and improved public health in industrialized nations such as the United States have resulted in a relatively low prevalence of parasitic infection but have also been associated with a marked increase in the incidence and severity of asthma and other atopic disorders (8). Our study raises the possibility that immune responses to parasitization and the predisposition to atopic disease may share common mechanisms. The close association of ascariasis with increased total serum IgE levels, sensitization to common aeroallergens, and increased airways responsiveness to methacholine were consistent with this finding and provide evidence of two potential mechanisms for the observed association of Ascaris infection with childhood asthma. Both experimental studies (32) and studies in humans (33) have demonstrated that infection with A. lumbricoides can potentiate sensitization to allergens. Our study has shown the association of increased risk of childhood asthma with an interaction between enhanced sensitivity to common inhaled allergens and Ascaris infection. This suggests that the association of Ascaris infection with asthma, increased skin test reactivity, and increased airways responsiveness could reflect a pathway to airway inflammation related to potentiation of immune responses to aeroallergens (32). This effect could be due to an immediate influence of Ascaris infection or to a predisposition to the clinical expression of allergies in those subjects predisposed to more persistent Ascaris infection. Our study suggests that immune responses to parasitization and the predisposition to asthma may share common causal pathways. There is consistent evidence from family studies and animal models that both asthma and resistance to helminthic infection are under substantial genetic control (7, 8, 34, 35). As other studies have suggested, such pathways may well involve shared genetic determinants (8, 36). Previous epidemiological studies have hypothesized that the discordant relationship between the high prevalence of atopy and the low prevalence of asthma and other allergic disease in the Chinese population is due to the action of other adjuvant factors in determining the clinical expression of asthma (37). Our finding that Ascaris infection was associated with asthma independently of aeroallergen sensitization (Table 2, Model 2) and with airways responsiveness suggests that helminthic parasitization may represent one such adjuvant factor. This second potential pathway involving Ascaris infection may influence airways responsiveness independently of atopy and, in addition to potentially shared genetic determinants, could in part be due to acute pulmonary hypersensitivity responses caused by the tissue-migratory juvenile lung stage of the parasite (31). A model of the possible influence of helminthic infection on the allergic reactivity of tropical populations has been proposed by Lynch (38). With mild helminthiasis, reactivity to specific allergens is elevated by the nonspecific potentiation of IgE synthesis against environmental antigens and possibly also by direct reactivity against the parasite. This increase in allergic reactivity reaches a peak, after which further stimulation results in suppressive effects on reactivity to specific allergens that may be due to saturation of mast cell Fc receptors and/or an inhibition of specific IgE synthesis by the polyclonal response (15, 39–43). The overall prevalence of Ascaris infection and the mean total serum IgE levels were suggestive of light-to-moderate infection intensities in the majority of the children studied. Further, several previous studies and nationwide surveys of China found that the intensity of Ascaris infection was generally light (5, 44–46). Together with the positive relationship between Ascaris infection and increased skin test reactivity in our study, these observations are consistent with the hypothesis that potentiation of the specific immune response and concomitant increased susceptibility to allergic disease may be associated with moderate levels of helminthic infection, and heavy infection may produce the opposite effect (38). Alternatively, our finding may mean that the ecologic relationship between lower asthma prevalence and increased parasitism observed in many developing countries (6) reflects confounding factors such as diet, exposure to bacterial infections, and antibiotic use (8). Asthma and other allergic diseases are the result of complex interactions between largely unknown genetic and environmental mechanisms. Ascaris infection, although associated with increased asthma risk, likely interacts with many other exposures and individual genetic predispositions and therefore may have only a relatively small etiologic fraction at the population level. Asthma is increasing in prevalence, and hence in clinical and economic importance, in China (47). Whereas asthma is relatively uncommon compared with industrialized countries, the absolute numbers of asthmatic children are very significant given the enormous population base of China. The prevalence of infection with A. lumbricoides in China is highest in children, suggesting that public health measures should be targeted at this age group (5). Our study suggests that there may be some value in an evaluation of the effects of targeted antihelminthic treatment on the expression of asthma in children living in endemically parasitized areas of China.
The authors thank the study participants, the staff of Anhui Medical University, and the Anqing Health Bureau. They also thank Professor Neil Lynch for helpful comments on this manuscript. This study was supported in part by NIH Grant-HL/AI56371 and by Millennium Research. Dr. Celedón is supported by a Charles A. King Trust Fellowship Award.
This article has an online data supplement, which is accessible from this issue's table of contents online at www.atsjournals.org Received in original form July 5, 2001; accepted in final form January 15, 2002
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