Ann Allergy Asthma Immunol 113 (2014) 187e192
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Asthma and toxocariasis Lingling Li, MD *; Wei Gao, MD *; Xiao Yang, MD y; Dandan Wu, MD z; Hui Bi, MD z; Sini Zhang, MD z; Mao Huang, PhD z; and Xin Yao, PhD z * Department
of Respiratory Medicine, The First Affiliated Hospital of Nanjing Medical University, Nanjing, China Department of Urology, The First Affiliated Hospital of Nanjing Medical University, Nanjing, China z Department of Respiratory Medicine, The First Affiliated Hospital of Nanjing Medical University, Nanjing, China y
A R T I C L E
I N F O
Article history: Received for publication February 19, 2014. Received in revised form May 14, 2014. Accepted for publication May 18, 2014.
A B S T R A C T
Background: Asthma is the most common chronic disease in children and a major public health problem in adults. A relation between asthma and toxocariasis has been hypothesized. Objective: To evaluate the strength of association between asthma and Toxocara species seropositivity by conducting a systematic review and a meta-analysis of available data. Methods: PubMed, EMBASE, Web of Science, and the reference lists of all relevant articles and books were screened until October 2013. Two authors independently assessed eligibility and study quality and extracted data. A common odds ratio was estimated using a random-effects meta-analysis model of aggregated published data. Results: Ten studies met the inclusion criteria, for a total of 1,530 participants (723 cases and 807 controls). This meta-analysis found a significantly higher prevalence of Toxocara canis infection in patients with asthma than in controls (odds ratio 3.36, 95% confidence interval 1.76e6.42, P < .001). Similar results were found when meta-analysis was restricted to the studies considering only children or adults. Conclusion: The results support the existence of a positive association between asthma and Toxocara species seropositivity. Ó 2014 American College of Allergy, Asthma & Immunology. Published by Elsevier Inc. All rights reserved.
Introduction Asthma is the most common chronic disease in children; it is also a major public health problem in adults. During the past 30 years, its prevalence has increased up to 10% in adults, and it affects nearly 1 in 8 children worldwide, particularly in the United States.1e3 Therefore, identification of risk factors in the occurrence and development of asthma is important. Human toxocariasis is a cosmopolite helminthic zoonosis caused by Toxocara canis and Toxocara cati, which are common roundworms of dogs and cats, respectively. These helminths have a cosmopolitan distribution, and seroprevalence studies have indicated that Toxocara species are the most common cause of helminthic infections in humans worldwide.4 Helminthic infection
Drs Li, Gao, and Yang contributed equally to this work. Reprints: Xin Yao, PhD, Department of Respiratory Medicine, The First Affiliated Hospital of Nanjing Medical University, 300 Guangzhou Road, Nanjing, China; E-mail: [email protected]. Disclosure: Authors have nothing to disclose. Funding: This work was supported by the National Major Scientific and Technological Special Project for Significant New Drug Development (grant 2011ZX09302003-02), the Jiangsu Province Major Scientific and Technological Special Project (grant BM2011017), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (grant JX10231801).
often occurs when humans accidentally ingest the microscopic, oval, and thick-shelled, embryonic eggs (shed in dog and cat feces) that contain Toxocara larvae.5 The reported prevalence of soil contamination with Toxocara species eggs varies among studies, from 6.6% to 87.1%.6e13 The clinical manifestations in toxocariasis vary widely from asymptomatic cases (most common) to severe organ injury,14 which are determined by the parasite load, the sites of larval migration, and the host’s inflammatory reaction. There are 2 “classic” clinical syndromes resulting from infection: visceral larva migrans and ocular larva migrans.15,16 Two less severe syndromes also have been described: covert toxocariasis, seen mainly in children and characterized by fever, headache, and behavioral and sleep disturbances, and common toxocariasis, seen predominantly in adults with weakness, pruritus, rash, and abdominal pain.17 Despite being the most prevalent human helminthic infection in some industrialized countries,5 toxocariasis remains relatively unknown to the public.18 In addition, the true magnitude of the global burden of Toxocara-associated human disease has yet to be evaluated.19 Epidemiologic studies20e22 and experimental models23 have suggested that infection with Toxocara worms contributes to the development of allergic diseases, including asthma.24 Moreover, Walsh and Haseeb25 identified a significant independent association between Toxocara infection and lung function. Toxocariasis is a
http://dx.doi.org/10.1016/j.anai.2014.05.016 1081-1206/Ó 2014 American College of Allergy, Asthma & Immunology. Published by Elsevier Inc. All rights reserved.
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common helminthiasis and is a potentially preventable disease; hence, estimation of the association between toxocariasis and asthma is necessary. Therefore, the authors conducted a systematic review and a meta-analysis to determine the relation between asthma and human toxocariasis and to identify the factors that potentially contribute to such a relation. Methods Search Strategy A computerized literature search was conducted in MEDLINE (PubMed, http://www.ncbi.nlm.nih.gov/pubmed/), EMBASE (http://www.embase.com/), and the Web of Science (http://apps. webofknowledge.com/) from their inception to October 1, 2013 by 2 independent investigators (L.L. and W.G.). The following keywords were used for asthma: asthma, bronchial spasm, bronchoconstriction, bronchial hyperreactivity, airway inflammation, wheeze, and wheezing. The following keywords were used for toxocariasis: toxocariasis, toxocarosis, toxocara, toxocara canis, and toxocara spp. Reviews and references of eligible studies were manually searched for additional relevant publications. No language restriction was implemented. Study Selection Criteria Considering asthma as the outcome and toxocariasis as the exposure, all studies that satisfied the following eligibility criteria were included: (1) caseecontrol, cohort, or cross-sectional studies; (2) original studies that had an independent study population; (3) studies that provided an adequate definition of asthma; (4) studies that provided an adequate definition and measurement of toxocariasis; and (5) studies that provided relevant and applicable quantitative information on the relation between asthma and toxocariasis. Data Extraction Based on the search strategy and selection criteria, 2 investigators (L.L. and W.G.) reviewed the titles, abstracts, and full articles to obtain the eligible studies. For each included study, the following data were collected: first author, year, and study population (number, age group, sex, and setting), study design, methods of Toxocara detection and diagnosis, types of specimen, and sample size. The numbers of Toxocara-positive and Toxocara-negative patients in the asthmatic group and control group of each study were recorded. Statistical Analysis Odds ratio (ORs) with 95% confidence intervals (CIs) are reported for toxocariasis seropositivity in patients with asthma compared with controls. Subgroup analyses were performed by age group and method of asthma definition to identify the association. The heterogeneity of the studies included in this meta-analysis was assessed using the Q statistic test and the I2 statistic test.26 A random-effects model was selected when the P value for heterogeneity was less than .1 or I2 was less than 50%; otherwise, a fixedeffects model was selected. Possible publication bias was evaluated by visual inspection of funnel plots and application of the Begg test.27 All statistical analyses were performed using STATA 10.0 (StataCorp, College Station, Texas). A P value less than .05 was identified as statistically significant. Results Literature Search Figure 1 shows the results of the systematic literature search. A total of 632 articles were reviewed, of which 365 were irrelevant
and 157 were duplicate studies. Thus, these articles were excluded from the study after screening the titles. Of the remaining 19 relevant studies, 3 were excluded for lack of available data, 2 were excluded for lack of sufficient data, and 4 were excluded for lack of a control group. Therefore, 10 articles were included in this study. These studies, which consisted of 7 caseecontrol studies, 2 cohort studies, and 1 cross-sectional study, were published from 1997 to 2012. Included Studies Ten studies were included,3,21,28e35 providing a total of 1,530 subjects, with a total T canis infection rate of 25.7% (393 of 1,530). The cumulative sample size of the asthmatic group was 723, of whom 251 were positive for T canis (34.7%). Of the 807 controls, 142 (17.6%) were positive for T canis. Three studies investigated the association between asthma and Toxocara in adults.3,29,32 The studies were conducted in 7 countries (United States, Argentina, Malaysia, Brazil, Turkey, Sri Lanka, and Romania). The general characteristics of the included studies are presented in Table 1. Asthma was defined as a physician-diagnosed condition in 6 articles.3,28,30e33 In 4 articles, the definition of asthma was based on asthma symptoms or use of asthma medication only.21,29,34,35 The presence of anti-Toxocara species antibodies in sera was assessed using antibodies and enzyme-linked immunosorbent assay. A questionnaire was administered to cases and controls in 3 studies to obtain demographic data and information concerning factors that are possibly associated with Toxocara species exposure and asthma.3,30,32 The laboratories that performed the analysis were blinded to the case and control status of sera samples in 2 studies.30,32,33 Meta-Analysis An assessment of heterogeneity of 10 studies included for the analysis indicated that the hypothesis of homogeneity could not be accepted (I2 ¼ 78.7%, P < .001), and the random-effects model was used to calculate the summary OR. Results are presented in Table 2 and Figure 2. A significant common OR of 3.36 (95% CI 1.76e6.42, P < .001) was estimated, suggesting a significant positive association between asthma and Toxocara species seropositivity. When analysis was restricted to the 3 studies considering only an adult population, the OR was 5.96 (95% CI 1.62e21.83, P < .001). The test for heterogeneity also was significant (P ¼ .089). The meta-analysis was restricted to the 7 studies considering Toxocara positivity only in childhood asthma (Fig 2), leading to an OR of 2.80 (95% CI 1.35e5.82, P < .001) and a significant test result for heterogeneity (P < .001). When restricting the analysis to asthma defined as a physician-diagnosed condition, the OR was 2.67 (95% CI 1.73e4.11) and the test result for heterogeneity was not significant (P ¼ .332). However, when restricting the analysis to asthma defined by the use of asthma medication or symptoms, the OR was 3.70 (95% CI 0.85e16.14) and a significant heterogeneity was found (P < .001). The result is shown in Figure 3. Evaluation of Publication Bias Publication bias was evaluated using the Begg funnel plot, and visual asymmetry was not found in the funnel plot analysis (P ¼ .441; Fig 4). Discussion The authors analyzed previous results on the potential association between T canis infection and asthma. The present study showed that Toxocara-positive results were significantly higher in those with asthma than in those without asthma based on the meta-analysis of 10 studies (OR 3.36, 95% CI 1.76e6.42, P < .001).
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Figure 1. References searched and selection of studies in the meta-analysis.
To the best of the authors’ knowledge, this study is the first metaanalysis of the association between toxocariasis and asthma. Among the 10 studies included, 7 indicated a significant association between Toxocara exposure and asthma. Sharghi et al30 indicated no association between Toxocara infections and asthma; however, they found a significant association between asthma and risk factors and between Toxocara infection and risk factors. In another study, no significant difference in anti-Toxocara IgG positivity was found between the asthmatic and control groups.32 However, a significant difference was found between the atopic asthmatic and control groups. Fernando et al33 identified Toxocara seropositivity as a significant risk factor in the development of asthma. However, this indication was true only when a model with
a single explanatory variable was used to analyze the data. When a model with multiple explanatory variables was used, no significant difference was found in the Toxocara seroprevalence for the group of children with asthma compared with age-, sex-, and ethnic group-matched controls. An association between toxocariasis and other risk factors of asthma has been suggested,33 rather than a direct association between toxocariasis and asthma. Furthermore, results from some studies support a more specific association between Toxocara infection and asthma in low- and middle-income countries.3,21,30 In the present study, a significant association was found between Toxocara infection and asthma. The possible role of Toxocara infection in the development of asthma might be related to
Table 1 Selected design characteristics of studies included in an analysis of the relation between asthma and toxocariasis, 1997 to October 2012 Study
Study design
Country
Sample size Population age (y)
Hakim et al,28 1997 Minvielle et al,29 1999 Sharghi et al,30 2001 Chan et al,21 2001 Figueiredo et al,31 2005 Kuk et al,3 2006 Kustimur et al,32 2007 Fernando et al,33 2009 Lopez et al,34 2010 Cobzaru et al,35 2012
caseecontrol caseecontrol caseecontrol caseecontrol crossesectional cohort caseecontrol cohort caseecontrol caseecontrol
Malaysia Argentina USA Malaysia Brazil Turkey Turkey Sri Lanka Argentina Rumania
45 82 124 324 208 103 184 196 100 164
Abbreviation: ELISA, enzyme-linked immunosorbent assay.
Asthma definition
children (1e10) physician diagnosis adults (18e65) use of asthma medication children (2e15) physician diagnosis children (mean age 8.1 3.1) use of asthma medication children (1e14) physician diagnosis adults (18) physician diagnosis adults (18) physician diagnosis children (5e12) physician diagnosis children (3e13) use of asthma medication children (5e16) use of asthma medication
Exposure Specimen assessment ELISA and asthma symptoms ELISA ELISA and asthma symptoms ELISA ELISA ELISA ELISA ELISA and asthma symptoms ELISA and asthma symptoms ELISA
serum serum serum serum serum serum serum serum serum serum
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Table 2 Results of included studies on the association between asthma and toxocariasis, 1997 to October 2013 Study
With Without Seropositivity asthma, asthma, in n n participants with asthma, n (%)
18 Hakim et al28 Minvielle et al29 38 30 95 Sharghi et al Chan et al21 66 Figueiredo et al31 106 53 Kuk et al3 Kustimur et al32 124 33 Fernando et al 100 47 Lopez et al34 35 Cobzaru et al 76
27 44 229 58 102 50 60 96 53 88
10 26 14 9 65 7 12 29 27 52
(57.8) (68.4) (14.7) (21.2) (61.3) (13.2) (9.7) (29.0) (57.4) (68.42)
Seropositivity OR (95% CI) in participants without asthma, n (%) 4 6 5 24 49 1 3 10 28 12
(15.4) (13.6) (2.2) (8.6) (48.0) (2.0) (5.0) (10.4) (52.8) (13.6)
7.19 13.7 0.89 2.85 1.71 7.46 2.03 3.51 1.21 13.7
(1.75e29.5) (4.57e41.2) (0.40e2.00) (0.96e8.49) (0.99e2.98) (0.88e63.0) (0.55e7.50) (1.51e8.32) (0.55e2.66) (6.31e29.9)
Abbreviations: CI, confidence interval; OR, odds ratio.
immune response. Several studies that used murine models for toxocariasis have shown that infection with T canis leads to persistent pulmonary inflammation, eosinophilia, IgE production, airway hyperreactivity, and production of T-helper cell type 2
cytokines.36e38 Induction of T-helper cell type 2emediated immune responses, which include the production of high levels of IgE, inflammation of the airways, and accumulation of eosinophils, are common features in allergic asthma and toxocariasis.39 In addition, infections with this helminth share common clinical features, such as wheezing, coughs, mucus hypersecretion, and bronchial hyperreactivity, with allergic asthma. Investigation of immunologic and molecular mechanisms that can explain the observed association is indispensable to establish and understand the relation between Toxocara infection and allergic asthma. Given that the studies included in this meta-analysis involved different age groups, participants were divided into adults (17e65 years old) and children (1e17 years old). Altogether, 7 studies on childhood asthma and 3 studies on adult asthma were identified as relevant and included in the analyses. Random-effects models showed significant association between asthma and toxocariasis for the children (OR 2.80, 95% CI 1.35e5.82) and adults (OR 5.96, 95% CI 1.62e21.8). The results suggest that Toxocara infection had a stronger association with asthma in adults than in children. However, Toxocara infection, particularly that caused by T canis, occurs mainly in children who have close contact with infected household pets or who have been exposed to eggs in the environment. This finding can have several explanations. First, Toxocara seropositivity
Figure 2. Odds ratio (OR) and 95% confidence interval (CI) estimates for toxocariasis seropositivity in patients with asthma compared with controls. Meta-analysis randomeffects estimates were used. The sizes of the squares reflect the weighting of the included studies. Bars represent 95% confidence intervals. The center of the diamond represents the summary effect; left and right points of the diamond represent the 95% confidence interval.
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Figure 3. Subgroup analysis of association between asthma and toxocariasis according to definition of asthma. Odds ratios (ORs) from each study and common ORs were estimated using a random-effects model. CI, confidence interval.
could represent past rather than recent infection. Second, adults may have had toxocariasis during their childhood and exhibit cryptic infection. Third, the effect of T canis may be less important in adult-onset asthma, because the risk factors may be much more heterogeneous (environmental exposures, such as tobacco and occupation) than in childhood asthma. Considering that the diagnosis of asthma was different in various studies, the authors conducted a subgroup analysis according to the definition of asthma. The result suggested a significant association between asthma and toxocariasis in the physician-diagnosed group (OR 2.67, 95% CI 1.73e4.11). However, in 3 studies, patients using asthma medication or experiencing asthma symptoms were considered to have asthma (OR 3.70, 95% CI 0.85e16.14), indicating no association between asthma and toxocariasis. This finding may have 2 explanations: (1) a more specific definition of asthma was associated with a higher effect estimate and (2) the sample sizes of the 3 studies were too small and high heterogeneity existed. Significant heterogeneity was observed among estimates across all 10 studies and among the subgroups when stratified by age and definition of asthma. The sources of heterogeneity contained the following 3 aspects. (1) The study designs were different. Although 8 studies were caseecontrol studies, 1 was a cohort study and 1 was
a cross-sectional study. (2) The definition of asthma varied among studies. Heterogeneity was remarkably increased in the group using asthma medication or asthma symptoms to diagnose asthma (I2 ¼ 91.6%), indicating that the definition of asthma seemed to make significant contributions to heterogeneity. (3) The sample size may have contributed to the heterogeneity. Increased risk of asthma after Toxocara infection was observed in studies conducted in different countries with different designs and measurements of asthma, thereby conferring a level of robustness to this result. Various important points should be considered when interpreting these data. This meta-analysis was based on summary statistics from observational studies. These studies do not confirm causal relations, which require randomized control trials, and they do not allow adjustments for confounding factors. In the studies evaluated, participants with and without asthma were generally enrolled from a hospital setting, and their source population often was not clearly defined. Hence, a possible recruitment bias may have occurred, particularly in rural settings, where people who receive care are not representative of the general population. The increased risk of Toxocara for asthma could be attributed to other factors, such as, race, residence, and playground. Therefore, adjusting for various confounders is needed.
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Figure 4. Publication bias in studies of the association between asthma and toxocariasis assessed by the Begg funnel plot. No significant funnel asymmetry that could indicate publication bias was observed. The horizontal line in the funnel plot indicates the random-effects summary estimate, and the sloping lines indicate the expected 95% confidence interval for a given standard error (s.e.), assuming no heterogeneity between studies. logor, natural logarithm of odds ratio.
Considering the diagnosis of toxocariasis, most of these studies used enzyme-linked immunosorbent assay based on the Toxocara spp excretory-secretory antigen to measure antibodies against this parasite. Different kits (commercial or in house) with different sera dilutions are used in enzyme-linked immunosorbent assays. In 2 studies, the mean anti-Toxocara antibody titer was significantly higher in patients with bronchial asthma than in children without asthma.21,28 Evaluations and reports on the sensitivity and specificity of the used assays were not performed in the included studies, which would have been helpful. Furthermore, a single seropositivity has limited pathologic significance and might represent past rather than recent infection. A potential weakness of the present study is the use of different asthma definitions in the included articles, which can lead to some misclassifications and subsequent bias toward the null hypothesis. Another weakness is the limited number of studies and their small samples. These 2 factors could affect the results, which would show no statistically significant association between toxocariasis and asthma. In conclusion, a positive association between Toxocara species seropositivity and asthma has been shown found, although it does not confirm the causal relation. It indicates a need to enforce methods of control and preventive measures against Toxocara infection. More studies that include larger populations, standardized serologic assays, and statistical analyses are required to clarify the association between Toxocara species and asthma. Further investigations on the immunologic and molecular mechanisms that can explain the observed association are needed to understand the relation between Toxocara infection and asthma. Investigation of the possible factors that contribute to the observed association between these 2 disorders also is necessary. References [1] Akinbami L. The state of childhood asthma, United States, 1980e2005. Adv Data. 2006;381:1e24. [2] Lai CK, Beasley R, Crane J, et al. Global variation in the prevalence and severity of asthma symptoms: phase three of the International Study of Asthma and Allergies in Childhood (ISAAC). Thorax. 2009;64:476e483. [3] Kuk S, Ozel E, Oguzturk H, et al. Seroprevalence of Toxocara antibodies in patients with adult asthma. South Med J. 2006;99:719e722. [4] Rubinsky-Elefant G, Hirata CE, Yamamoto JH, et al. Human toxocariasis: diagnosis, worldwide seroprevalences and clinical expression of the systemic and ocular forms. Ann Trop Med Parasitol. 2010;104:3e23.
[5] Hotez PJ, Wilkins PP. Toxocariasis: America’s most common neglected infection of poverty and a helminthiasis of global importance? PLoS Negl Trop Dis. 2009;3:e400. [6] Dada BJ, Lindquist WD. Studies on flotation techniques for the recovery of helminth eggs from soil and the prevalence of eggs of Toxocara spp in some Kansas public places. J Am Vet Med Assoc. 1979;174:1208e1210. [7] Emehelu CO, Fakae BB. Prevalence of Toxocara canis ova on playgrounds of nursery schools in Nsukka, Nigeria. Int J Zoonoses. 1986;13:158e161. [8] Shimizu T. Prevalence of Toxocara eggs in sandpits in Tokushima city and its outskirts. J Vet Med Sci. 1993;55:807e811. [9] Oteifa NM, Moustafa MA. The potential risk of contracting toxocariasis in Heliopolis district, Cairo, Egypt. J Egypt Soc Parasitol. 1997;27:197e203. [10] Chorazy ML, Richardson DJ. A survey of environmental contamination with ascarid ova, Wallingford, Connecticut. Vector Borne Zoonotic Dis. 2005;5: 33e39. [11] Guimaraes AM, Alves EG, de Rezende GF, et al. Toxocara sp. eggs and Ancylostoma sp. larva in public parks. Brazil. Rev Saude Publica. 2005;39:293e295. [12] Rokicki J, Kucharska AP, Dzido J, et al. Contamination of playgrounds in Gdansk city with parasite eggs. Wiad Parazytol. 2007;53:227e230. [13] Tiyo R, Guedes TA, Falavigna DL, et al. Seasonal contamination of public squares and lawns by parasites with zoonotic potential in southern Brazil. J Helminthol. 2008;82:1e6. [14] Taylor MRH, Keane CT, O’Conner P, Mulvihill E, Holland C. The expanded spectrum of toxocaral disease. Lancet. 1988;1:692e695. [15] Despommier D. Toxocariasis: clinical aspects, epidemiology, medical ecology, and molecular aspects. Clin Microbiol Rev. 2003;16:265e272. [16] Sharghi N, Schantz P, Hotez PJ. Toxocariasis: an occult cause of childhood neuropsychological deficits and asthma? Semin Pediatr Infect Dis. 2000;11: 257e260. [17] Pawlowski Z. Toxocariasis in humans: clinical expression and treatment dilemma. J Helminthol. 2001;75:299e305. [18] Wells DL. Public understanding of toxocariasis. Public Health. 2007;121: 187e188. [19] Smith H, Holland C, Taylor M, et al. How common is human toxocariasis? Towards standardizing our knowledge. Trends Parasitol. 2009;25:182e188. [20] Buijs J, Borsboom G, Renting M, et al. Relationship between allergic manifestations and Toxocara seropositivity: a cross-sectional study among elementary school children. Eur Respir J. 1997;10:1467e1475. [21] Chan PW, Anuar AK, Fong MY, et al. Toxocara seroprevalence and childhood asthma among Malaysian children. Pediatr Int. 2001;43:350e353. [22] Desowitz RS, Rudoy R, Barnwell JW. Antibodies to canine helminth parasites in asthmatic and nonasthmatic children. Int Arch Allergy Appl Immunol. 1981; 65:361e366. [23] Pinelli E, Brandes S, Dormans J, et al. Infection with the roundworm Toxocara canis leads to exacerbation of experimental allergic airway inflammation. Clin Exp Allergy. 2008;38:649e658. [24] Asher MI. Recent perspectives on global epidemiology of asthma in childhood. Allergol Immunopathol (Madr). 2010;38:83e87. [25] Walsh MG, Haseeb MA. Toxocariasis and lung function: relevance of a neglected infection in an urban landscape. Acta Parasitol. 2014;59:126e131. [26] Higgins JP, Thompson SG. Quantifying heterogeneity in a meta-analysis. Stat Med. 2002;21:1539e1558. [27] Begg CB, Mazumdar M. Operating characteristics of a rank correlation test for publication bias. Biometrics. 1994;50:1088e1101. [28] Hakim SL, Thadasavanth M, Shamilah RH, et al. Prevalence of Toxocara canis antibody among children with bronchial asthma in Klang Hospital, Malaysia. Trans R Soc Trop Med Hyg. 1997;91:528. [29] Minvielle MC, Niedfeld G, Ciarmela ML, et al. Asthma and covert toxocariasis. Medicina (B Aires). 1999;59:243e248. [30] Sharghi N, Schantz PM, Caramico L, et al. Environmental exposure to Toxocara as a possible risk factor for asthma: a clinic-based caseecontrol study. Clin Infect Dis. 2001;32:E111eE116. [31] Figueiredo SD, Taddei JA, Menezes JJ, et al. Clinicaleepidemiological study of toxocariasis in a pediatric population. J Pediatr (Rio J). 2005;81:126e132. [32] Kustimur S, Dogruman AF, Oguzulgen K, et al. Toxocara seroprevalence in adults with bronchial asthma. Trans R Soc Trop Med Hyg. 2007;101:270e274. [33] Fernando D, Wickramasinghe P, Kapilananda G, et al. Toxocara seropositivity in Sri Lankan children with asthma. Pediatr Int. 2009;51:241e245. [34] Lopez ML, Bojanich MV, Jacobacci JM, et al. Toxocara canis and bronchial asthma. Medicina (B Aires). 2010;70:75e78. [35] Cobzaru RG, Ripa C, Leon MM, et al. Correlation between asthma and Toxocara canis infection. Rev Med Chir Soc Med Nat Iasi. 2012;116:727e730. [36] Buijs J, Egbers MW, Lokhorst WH, et al. Toxocara-induced eosinophilic inflammation. Airway function and effect of antieIL-5. Am J Respir Crit Care Med. 1995;151:873e878. [37] Kayes SG. Nonspecific allergic granulomatosis in the lungs of mice infected with large but not small inocula of the canine ascarid, Toxocara canis. Clin Immunol Immunopathol. 1986;41:55e65. [38] Pinelli E, Withagen C, Fonville M, et al. Persistent airway hyperresponsiveness and inflammation in Toxocara caniseinfected BALB/c mice. Clin Exp Allergy. 2005;35:826e832. [39] Pinelli E, Dormans J, Van Die I. Toxocara and asthma. In: Holland C, Smith H, eds. Toxocara: The Enigmatic Parasite. Oxfordshire, UK: CABI Publishers; 2006: 42e57.
Contents lists available at ScienceDirect
Asthma and toxocariasis Lingling Li, MD *; Wei Gao, MD *; Xiao Yang, MD y; Dandan Wu, MD z; Hui Bi, MD z; Sini Zhang, MD z; Mao Huang, PhD z; and Xin Yao, PhD z * Department
of Respiratory Medicine, The First Affiliated Hospital of Nanjing Medical University, Nanjing, China Department of Urology, The First Affiliated Hospital of Nanjing Medical University, Nanjing, China z Department of Respiratory Medicine, The First Affiliated Hospital of Nanjing Medical University, Nanjing, China y
A R T I C L E
I N F O
Article history: Received for publication February 19, 2014. Received in revised form May 14, 2014. Accepted for publication May 18, 2014.
A B S T R A C T
Background: Asthma is the most common chronic disease in children and a major public health problem in adults. A relation between asthma and toxocariasis has been hypothesized. Objective: To evaluate the strength of association between asthma and Toxocara species seropositivity by conducting a systematic review and a meta-analysis of available data. Methods: PubMed, EMBASE, Web of Science, and the reference lists of all relevant articles and books were screened until October 2013. Two authors independently assessed eligibility and study quality and extracted data. A common odds ratio was estimated using a random-effects meta-analysis model of aggregated published data. Results: Ten studies met the inclusion criteria, for a total of 1,530 participants (723 cases and 807 controls). This meta-analysis found a significantly higher prevalence of Toxocara canis infection in patients with asthma than in controls (odds ratio 3.36, 95% confidence interval 1.76e6.42, P < .001). Similar results were found when meta-analysis was restricted to the studies considering only children or adults. Conclusion: The results support the existence of a positive association between asthma and Toxocara species seropositivity. Ó 2014 American College of Allergy, Asthma & Immunology. Published by Elsevier Inc. All rights reserved.
Introduction Asthma is the most common chronic disease in children; it is also a major public health problem in adults. During the past 30 years, its prevalence has increased up to 10% in adults, and it affects nearly 1 in 8 children worldwide, particularly in the United States.1e3 Therefore, identification of risk factors in the occurrence and development of asthma is important. Human toxocariasis is a cosmopolite helminthic zoonosis caused by Toxocara canis and Toxocara cati, which are common roundworms of dogs and cats, respectively. These helminths have a cosmopolitan distribution, and seroprevalence studies have indicated that Toxocara species are the most common cause of helminthic infections in humans worldwide.4 Helminthic infection
Drs Li, Gao, and Yang contributed equally to this work. Reprints: Xin Yao, PhD, Department of Respiratory Medicine, The First Affiliated Hospital of Nanjing Medical University, 300 Guangzhou Road, Nanjing, China; E-mail: [email protected]. Disclosure: Authors have nothing to disclose. Funding: This work was supported by the National Major Scientific and Technological Special Project for Significant New Drug Development (grant 2011ZX09302003-02), the Jiangsu Province Major Scientific and Technological Special Project (grant BM2011017), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (grant JX10231801).
often occurs when humans accidentally ingest the microscopic, oval, and thick-shelled, embryonic eggs (shed in dog and cat feces) that contain Toxocara larvae.5 The reported prevalence of soil contamination with Toxocara species eggs varies among studies, from 6.6% to 87.1%.6e13 The clinical manifestations in toxocariasis vary widely from asymptomatic cases (most common) to severe organ injury,14 which are determined by the parasite load, the sites of larval migration, and the host’s inflammatory reaction. There are 2 “classic” clinical syndromes resulting from infection: visceral larva migrans and ocular larva migrans.15,16 Two less severe syndromes also have been described: covert toxocariasis, seen mainly in children and characterized by fever, headache, and behavioral and sleep disturbances, and common toxocariasis, seen predominantly in adults with weakness, pruritus, rash, and abdominal pain.17 Despite being the most prevalent human helminthic infection in some industrialized countries,5 toxocariasis remains relatively unknown to the public.18 In addition, the true magnitude of the global burden of Toxocara-associated human disease has yet to be evaluated.19 Epidemiologic studies20e22 and experimental models23 have suggested that infection with Toxocara worms contributes to the development of allergic diseases, including asthma.24 Moreover, Walsh and Haseeb25 identified a significant independent association between Toxocara infection and lung function. Toxocariasis is a
http://dx.doi.org/10.1016/j.anai.2014.05.016 1081-1206/Ó 2014 American College of Allergy, Asthma & Immunology. Published by Elsevier Inc. All rights reserved.
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common helminthiasis and is a potentially preventable disease; hence, estimation of the association between toxocariasis and asthma is necessary. Therefore, the authors conducted a systematic review and a meta-analysis to determine the relation between asthma and human toxocariasis and to identify the factors that potentially contribute to such a relation. Methods Search Strategy A computerized literature search was conducted in MEDLINE (PubMed, http://www.ncbi.nlm.nih.gov/pubmed/), EMBASE (http://www.embase.com/), and the Web of Science (http://apps. webofknowledge.com/) from their inception to October 1, 2013 by 2 independent investigators (L.L. and W.G.). The following keywords were used for asthma: asthma, bronchial spasm, bronchoconstriction, bronchial hyperreactivity, airway inflammation, wheeze, and wheezing. The following keywords were used for toxocariasis: toxocariasis, toxocarosis, toxocara, toxocara canis, and toxocara spp. Reviews and references of eligible studies were manually searched for additional relevant publications. No language restriction was implemented. Study Selection Criteria Considering asthma as the outcome and toxocariasis as the exposure, all studies that satisfied the following eligibility criteria were included: (1) caseecontrol, cohort, or cross-sectional studies; (2) original studies that had an independent study population; (3) studies that provided an adequate definition of asthma; (4) studies that provided an adequate definition and measurement of toxocariasis; and (5) studies that provided relevant and applicable quantitative information on the relation between asthma and toxocariasis. Data Extraction Based on the search strategy and selection criteria, 2 investigators (L.L. and W.G.) reviewed the titles, abstracts, and full articles to obtain the eligible studies. For each included study, the following data were collected: first author, year, and study population (number, age group, sex, and setting), study design, methods of Toxocara detection and diagnosis, types of specimen, and sample size. The numbers of Toxocara-positive and Toxocara-negative patients in the asthmatic group and control group of each study were recorded. Statistical Analysis Odds ratio (ORs) with 95% confidence intervals (CIs) are reported for toxocariasis seropositivity in patients with asthma compared with controls. Subgroup analyses were performed by age group and method of asthma definition to identify the association. The heterogeneity of the studies included in this meta-analysis was assessed using the Q statistic test and the I2 statistic test.26 A random-effects model was selected when the P value for heterogeneity was less than .1 or I2 was less than 50%; otherwise, a fixedeffects model was selected. Possible publication bias was evaluated by visual inspection of funnel plots and application of the Begg test.27 All statistical analyses were performed using STATA 10.0 (StataCorp, College Station, Texas). A P value less than .05 was identified as statistically significant. Results Literature Search Figure 1 shows the results of the systematic literature search. A total of 632 articles were reviewed, of which 365 were irrelevant
and 157 were duplicate studies. Thus, these articles were excluded from the study after screening the titles. Of the remaining 19 relevant studies, 3 were excluded for lack of available data, 2 were excluded for lack of sufficient data, and 4 were excluded for lack of a control group. Therefore, 10 articles were included in this study. These studies, which consisted of 7 caseecontrol studies, 2 cohort studies, and 1 cross-sectional study, were published from 1997 to 2012. Included Studies Ten studies were included,3,21,28e35 providing a total of 1,530 subjects, with a total T canis infection rate of 25.7% (393 of 1,530). The cumulative sample size of the asthmatic group was 723, of whom 251 were positive for T canis (34.7%). Of the 807 controls, 142 (17.6%) were positive for T canis. Three studies investigated the association between asthma and Toxocara in adults.3,29,32 The studies were conducted in 7 countries (United States, Argentina, Malaysia, Brazil, Turkey, Sri Lanka, and Romania). The general characteristics of the included studies are presented in Table 1. Asthma was defined as a physician-diagnosed condition in 6 articles.3,28,30e33 In 4 articles, the definition of asthma was based on asthma symptoms or use of asthma medication only.21,29,34,35 The presence of anti-Toxocara species antibodies in sera was assessed using antibodies and enzyme-linked immunosorbent assay. A questionnaire was administered to cases and controls in 3 studies to obtain demographic data and information concerning factors that are possibly associated with Toxocara species exposure and asthma.3,30,32 The laboratories that performed the analysis were blinded to the case and control status of sera samples in 2 studies.30,32,33 Meta-Analysis An assessment of heterogeneity of 10 studies included for the analysis indicated that the hypothesis of homogeneity could not be accepted (I2 ¼ 78.7%, P < .001), and the random-effects model was used to calculate the summary OR. Results are presented in Table 2 and Figure 2. A significant common OR of 3.36 (95% CI 1.76e6.42, P < .001) was estimated, suggesting a significant positive association between asthma and Toxocara species seropositivity. When analysis was restricted to the 3 studies considering only an adult population, the OR was 5.96 (95% CI 1.62e21.83, P < .001). The test for heterogeneity also was significant (P ¼ .089). The meta-analysis was restricted to the 7 studies considering Toxocara positivity only in childhood asthma (Fig 2), leading to an OR of 2.80 (95% CI 1.35e5.82, P < .001) and a significant test result for heterogeneity (P < .001). When restricting the analysis to asthma defined as a physician-diagnosed condition, the OR was 2.67 (95% CI 1.73e4.11) and the test result for heterogeneity was not significant (P ¼ .332). However, when restricting the analysis to asthma defined by the use of asthma medication or symptoms, the OR was 3.70 (95% CI 0.85e16.14) and a significant heterogeneity was found (P < .001). The result is shown in Figure 3. Evaluation of Publication Bias Publication bias was evaluated using the Begg funnel plot, and visual asymmetry was not found in the funnel plot analysis (P ¼ .441; Fig 4). Discussion The authors analyzed previous results on the potential association between T canis infection and asthma. The present study showed that Toxocara-positive results were significantly higher in those with asthma than in those without asthma based on the meta-analysis of 10 studies (OR 3.36, 95% CI 1.76e6.42, P < .001).
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Figure 1. References searched and selection of studies in the meta-analysis.
To the best of the authors’ knowledge, this study is the first metaanalysis of the association between toxocariasis and asthma. Among the 10 studies included, 7 indicated a significant association between Toxocara exposure and asthma. Sharghi et al30 indicated no association between Toxocara infections and asthma; however, they found a significant association between asthma and risk factors and between Toxocara infection and risk factors. In another study, no significant difference in anti-Toxocara IgG positivity was found between the asthmatic and control groups.32 However, a significant difference was found between the atopic asthmatic and control groups. Fernando et al33 identified Toxocara seropositivity as a significant risk factor in the development of asthma. However, this indication was true only when a model with
a single explanatory variable was used to analyze the data. When a model with multiple explanatory variables was used, no significant difference was found in the Toxocara seroprevalence for the group of children with asthma compared with age-, sex-, and ethnic group-matched controls. An association between toxocariasis and other risk factors of asthma has been suggested,33 rather than a direct association between toxocariasis and asthma. Furthermore, results from some studies support a more specific association between Toxocara infection and asthma in low- and middle-income countries.3,21,30 In the present study, a significant association was found between Toxocara infection and asthma. The possible role of Toxocara infection in the development of asthma might be related to
Table 1 Selected design characteristics of studies included in an analysis of the relation between asthma and toxocariasis, 1997 to October 2012 Study
Study design
Country
Sample size Population age (y)
Hakim et al,28 1997 Minvielle et al,29 1999 Sharghi et al,30 2001 Chan et al,21 2001 Figueiredo et al,31 2005 Kuk et al,3 2006 Kustimur et al,32 2007 Fernando et al,33 2009 Lopez et al,34 2010 Cobzaru et al,35 2012
caseecontrol caseecontrol caseecontrol caseecontrol crossesectional cohort caseecontrol cohort caseecontrol caseecontrol
Malaysia Argentina USA Malaysia Brazil Turkey Turkey Sri Lanka Argentina Rumania
45 82 124 324 208 103 184 196 100 164
Abbreviation: ELISA, enzyme-linked immunosorbent assay.
Asthma definition
children (1e10) physician diagnosis adults (18e65) use of asthma medication children (2e15) physician diagnosis children (mean age 8.1 3.1) use of asthma medication children (1e14) physician diagnosis adults (18) physician diagnosis adults (18) physician diagnosis children (5e12) physician diagnosis children (3e13) use of asthma medication children (5e16) use of asthma medication
Exposure Specimen assessment ELISA and asthma symptoms ELISA ELISA and asthma symptoms ELISA ELISA ELISA ELISA ELISA and asthma symptoms ELISA and asthma symptoms ELISA
serum serum serum serum serum serum serum serum serum serum
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Table 2 Results of included studies on the association between asthma and toxocariasis, 1997 to October 2013 Study
With Without Seropositivity asthma, asthma, in n n participants with asthma, n (%)
18 Hakim et al28 Minvielle et al29 38 30 95 Sharghi et al Chan et al21 66 Figueiredo et al31 106 53 Kuk et al3 Kustimur et al32 124 33 Fernando et al 100 47 Lopez et al34 35 Cobzaru et al 76
27 44 229 58 102 50 60 96 53 88
10 26 14 9 65 7 12 29 27 52
(57.8) (68.4) (14.7) (21.2) (61.3) (13.2) (9.7) (29.0) (57.4) (68.42)
Seropositivity OR (95% CI) in participants without asthma, n (%) 4 6 5 24 49 1 3 10 28 12
(15.4) (13.6) (2.2) (8.6) (48.0) (2.0) (5.0) (10.4) (52.8) (13.6)
7.19 13.7 0.89 2.85 1.71 7.46 2.03 3.51 1.21 13.7
(1.75e29.5) (4.57e41.2) (0.40e2.00) (0.96e8.49) (0.99e2.98) (0.88e63.0) (0.55e7.50) (1.51e8.32) (0.55e2.66) (6.31e29.9)
Abbreviations: CI, confidence interval; OR, odds ratio.
immune response. Several studies that used murine models for toxocariasis have shown that infection with T canis leads to persistent pulmonary inflammation, eosinophilia, IgE production, airway hyperreactivity, and production of T-helper cell type 2
cytokines.36e38 Induction of T-helper cell type 2emediated immune responses, which include the production of high levels of IgE, inflammation of the airways, and accumulation of eosinophils, are common features in allergic asthma and toxocariasis.39 In addition, infections with this helminth share common clinical features, such as wheezing, coughs, mucus hypersecretion, and bronchial hyperreactivity, with allergic asthma. Investigation of immunologic and molecular mechanisms that can explain the observed association is indispensable to establish and understand the relation between Toxocara infection and allergic asthma. Given that the studies included in this meta-analysis involved different age groups, participants were divided into adults (17e65 years old) and children (1e17 years old). Altogether, 7 studies on childhood asthma and 3 studies on adult asthma were identified as relevant and included in the analyses. Random-effects models showed significant association between asthma and toxocariasis for the children (OR 2.80, 95% CI 1.35e5.82) and adults (OR 5.96, 95% CI 1.62e21.8). The results suggest that Toxocara infection had a stronger association with asthma in adults than in children. However, Toxocara infection, particularly that caused by T canis, occurs mainly in children who have close contact with infected household pets or who have been exposed to eggs in the environment. This finding can have several explanations. First, Toxocara seropositivity
Figure 2. Odds ratio (OR) and 95% confidence interval (CI) estimates for toxocariasis seropositivity in patients with asthma compared with controls. Meta-analysis randomeffects estimates were used. The sizes of the squares reflect the weighting of the included studies. Bars represent 95% confidence intervals. The center of the diamond represents the summary effect; left and right points of the diamond represent the 95% confidence interval.
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Figure 3. Subgroup analysis of association between asthma and toxocariasis according to definition of asthma. Odds ratios (ORs) from each study and common ORs were estimated using a random-effects model. CI, confidence interval.
could represent past rather than recent infection. Second, adults may have had toxocariasis during their childhood and exhibit cryptic infection. Third, the effect of T canis may be less important in adult-onset asthma, because the risk factors may be much more heterogeneous (environmental exposures, such as tobacco and occupation) than in childhood asthma. Considering that the diagnosis of asthma was different in various studies, the authors conducted a subgroup analysis according to the definition of asthma. The result suggested a significant association between asthma and toxocariasis in the physician-diagnosed group (OR 2.67, 95% CI 1.73e4.11). However, in 3 studies, patients using asthma medication or experiencing asthma symptoms were considered to have asthma (OR 3.70, 95% CI 0.85e16.14), indicating no association between asthma and toxocariasis. This finding may have 2 explanations: (1) a more specific definition of asthma was associated with a higher effect estimate and (2) the sample sizes of the 3 studies were too small and high heterogeneity existed. Significant heterogeneity was observed among estimates across all 10 studies and among the subgroups when stratified by age and definition of asthma. The sources of heterogeneity contained the following 3 aspects. (1) The study designs were different. Although 8 studies were caseecontrol studies, 1 was a cohort study and 1 was
a cross-sectional study. (2) The definition of asthma varied among studies. Heterogeneity was remarkably increased in the group using asthma medication or asthma symptoms to diagnose asthma (I2 ¼ 91.6%), indicating that the definition of asthma seemed to make significant contributions to heterogeneity. (3) The sample size may have contributed to the heterogeneity. Increased risk of asthma after Toxocara infection was observed in studies conducted in different countries with different designs and measurements of asthma, thereby conferring a level of robustness to this result. Various important points should be considered when interpreting these data. This meta-analysis was based on summary statistics from observational studies. These studies do not confirm causal relations, which require randomized control trials, and they do not allow adjustments for confounding factors. In the studies evaluated, participants with and without asthma were generally enrolled from a hospital setting, and their source population often was not clearly defined. Hence, a possible recruitment bias may have occurred, particularly in rural settings, where people who receive care are not representative of the general population. The increased risk of Toxocara for asthma could be attributed to other factors, such as, race, residence, and playground. Therefore, adjusting for various confounders is needed.
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Figure 4. Publication bias in studies of the association between asthma and toxocariasis assessed by the Begg funnel plot. No significant funnel asymmetry that could indicate publication bias was observed. The horizontal line in the funnel plot indicates the random-effects summary estimate, and the sloping lines indicate the expected 95% confidence interval for a given standard error (s.e.), assuming no heterogeneity between studies. logor, natural logarithm of odds ratio.
Considering the diagnosis of toxocariasis, most of these studies used enzyme-linked immunosorbent assay based on the Toxocara spp excretory-secretory antigen to measure antibodies against this parasite. Different kits (commercial or in house) with different sera dilutions are used in enzyme-linked immunosorbent assays. In 2 studies, the mean anti-Toxocara antibody titer was significantly higher in patients with bronchial asthma than in children without asthma.21,28 Evaluations and reports on the sensitivity and specificity of the used assays were not performed in the included studies, which would have been helpful. Furthermore, a single seropositivity has limited pathologic significance and might represent past rather than recent infection. A potential weakness of the present study is the use of different asthma definitions in the included articles, which can lead to some misclassifications and subsequent bias toward the null hypothesis. Another weakness is the limited number of studies and their small samples. These 2 factors could affect the results, which would show no statistically significant association between toxocariasis and asthma. In conclusion, a positive association between Toxocara species seropositivity and asthma has been shown found, although it does not confirm the causal relation. It indicates a need to enforce methods of control and preventive measures against Toxocara infection. More studies that include larger populations, standardized serologic assays, and statistical analyses are required to clarify the association between Toxocara species and asthma. Further investigations on the immunologic and molecular mechanisms that can explain the observed association are needed to understand the relation between Toxocara infection and asthma. Investigation of the possible factors that contribute to the observed association between these 2 disorders also is necessary. References [1] Akinbami L. The state of childhood asthma, United States, 1980e2005. Adv Data. 2006;381:1e24. [2] Lai CK, Beasley R, Crane J, et al. Global variation in the prevalence and severity of asthma symptoms: phase three of the International Study of Asthma and Allergies in Childhood (ISAAC). Thorax. 2009;64:476e483. [3] Kuk S, Ozel E, Oguzturk H, et al. Seroprevalence of Toxocara antibodies in patients with adult asthma. South Med J. 2006;99:719e722. [4] Rubinsky-Elefant G, Hirata CE, Yamamoto JH, et al. Human toxocariasis: diagnosis, worldwide seroprevalences and clinical expression of the systemic and ocular forms. Ann Trop Med Parasitol. 2010;104:3e23.
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