The authors have declared that no competing interests exist.
Conceived and designed the experiments: KCLC SR SL TK. Performed the experiments: KCLC SR TK. Analyzed the data: SR AR PM JMMJ KVS. Contributed reagents/materials/analysis tools: KHC AW BB MT GR SA IK AvB EE AH CK BS JS CMC AA CP OH SW UW UK. Wrote the paper: SR KCLC TK.
To examine the associations between pet keeping in early childhood and asthma and allergies in children aged 6–10 years.
Pooled analysis of individual participant data of 11 prospective European birth cohorts that recruited a total of over 22,000 children in the 1990s.
Ownership of only cats, dogs, birds, rodents, or cats/dogs combined during the first 2 years of life.
Current asthma (primary outcome), allergic asthma, allergic rhinitis and allergic sensitization during 6–10 years of age.
Three-step approach: (i) Common definition of outcome and exposure variables across cohorts; (ii) calculation of adjusted effect estimates for each cohort; (iii) pooling of effect estimates by using random effects meta-analysis models.
We found no association between furry and feathered pet keeping early in life and asthma in school age. For example, the odds ratio for asthma comparing cat ownership with “no pets” (10 studies, 11489 participants) was 1.00 (95% confidence interval 0.78 to 1.28) (I2 = 9%; p = 0.36). The odds ratio for asthma comparing dog ownership with “no pets” (9 studies, 11433 participants) was 0.77 (0.58 to 1.03) (I2 = 0%, p = 0.89). Owning both cat(s) and dog(s) compared to “no pets” resulted in an odds ratio of 1.04 (0.59 to 1.84) (I2 = 33%, p = 0.18). Similarly, for allergic asthma and for allergic rhinitis we did not find associations regarding any type of pet ownership early in life. However, we found some evidence for an association between ownership of furry pets during the first 2 years of life and reduced likelihood of becoming sensitized to aero-allergens.
Pet ownership in early life did not appear to either increase or reduce the risk of asthma or allergic rhinitis symptoms in children aged 6–10. Advice from health care practitioners to avoid or to specifically acquire pets for primary prevention of asthma or allergic rhinitis in children should not be given.
The causes of the worldwide asthma and allergy epidemic over recent decades remain uncertain. Environmental and lifestyle factors, possibly interacting with genetic variants, may play a role however clear evidence for a predominant risk factor is lacking. Pet exposure as a common indoor environmental exposure particularly in families with young children has been of increasing public health concern with regard to recommendations for primary prevention of respiratory and allergic disease. Considerable controversy exists as to whether particularly cat and dog exposure may be a risk or even a protective factor for developing asthma, allergic symptoms or allergic sensitization
Previous results have come predominantly from cross-sectional studies and may therefore be skewed due to recall bias with regards to pet keeping
Primary care practitioners are uncertain about respiratory health risks or benefits of furry pet ownership particularly in early childhood and what advice to give to parents. The objective of this study was to improve the evidence on the primary prevention of asthma and allergies in relation to pet keeping in early life, using data from a large data base of European birth cohort studies. The primary aim was to determine whether pet keeping in the first two years of life was associated with asthma in school-aged children (age 6 to 10 years). Secondarily, we aimed to assess whether pet-keeping was associated with other allergic diseases (allergic or non-allergic asthma, allergic sensitization or allergic rhinitis).
As part of the Global Allergy and Asthma European Network (GA2LEN,
For the present combined data analyses, three inclusion criteria were defined: (i) European population-based observational birth cohort studies focusing on allergy and asthma (with ethical approval from local review boards); (ii) recruitment of subjects in pregnancy, at birth or during the first year of life; (iii) at least 1 prospective assessment during 6–10 years of age (early school age); (iv) data on pet ownership assessed prospectively during the first 2 years. To avoid recall bias about early childhood exposures, cross-sectional studies of school-children were not considered. For each included study the raw individual level participant data was available for data analysis.
This meta-analysis was conducted according to the principles stated in the Declaration of Helsinki. All included birth cohort studies were approved by their local Institutional Review Boards and all participants' parents provided written informed consent. The Institutional Review Boards were for MAS: Ethical Review Board Charité – Universitätsmedizin Berlin, Berlin (Germany); BAMSE: Regional Ethical Review Board, Karolinska Institutet, Stockholm (Sweden); ECA: The regional committee for medical and health profession research ethics, South-East, (Norway); PIAMA-NHS: Ethical Review Boards Utrecht CCMO P04.0071C, Rotterdam MEC 2004-152, Groningen M 4.019912 (The Netherlands); LISA: Ethics committees of the Bavarian General Medical Council, the University of Leipzig, and the Medical Council of North-Rhine-Westphalia (Germany); GINI-B: Ethics committees of the Bavarian General Medical Council, the University of Leipzig, and the Medical Council of North-Rhine-Westphalia (Germany); ARC: The Regional Scientific Ethical Committee for Southern Denmark (Denmark); AMICS-Barcelona: Clinical Research Ethical Committee of the Parc de Salut Mar, IMIM, Barcelona (Spain); AMICS-Menorca: Comite etic d'investigacio clinica de les Illes Balears (Spain); Leicester: Leicestershire, Northamptonshire and Rutland Research Ethics Committees 1 and 2 (UK); Isle of Wight: lsle of Wight, Portsmouth & SE Hants HA Local Research Ethics Committee (UK).
Since current “wheeze” is not very specific for asthma
“Allergic asthma” was defined as the presence of the primary outcome “asthma” and a positive serum specific immunoglobulin E (s-IgE)>0.35 kU/l to (i) any aero- and/or food allergen. Further definitions of allergic asthma were specified as asthma with a positive s-IgE to: (ii) any aero-allergen (in- or outdoor); (iii) cat allergen; (iv) dog allergen. “Non-allergic asthma” was defined as the presence of “asthma” without sensitization to any tested aero-/food allergen (s-IgE≤0.35 kU/l). The reference groups were non-asthmatic children without allergic sensitization.
“Allergic sensitization” regardless of symptoms was defined as a positive s-IgE test >0.35 kU/l for the following categories: cat, dog, any indoor, any outdoor, any aero-, and any aero-/food allergen.
“Allergic rhinitis” included parent-reported symptoms during the last 12 months (ISAAC core questions: sneezing, runny or blocked nose without a cold or flu) plus s-IgE>0.35 kU/l against at least 1 aero-allergen.
Based on parent-completed questionnaires or interviews between the children's birth (or during pregnancy) and second birthday, we defined 6 pet ownership categories: (i) cat(s) only; (ii) dog(s) only; (iii) cat(s)
Our primary aim was to examine the effect of pet ownership at any time between birth and the 2nd birthday. In addition, to evaluate whether the timing of pet ownership is relevant, we examined different exposure periods: at time of birth; between birth and 1st birthday, and between 1st and 2nd birthdays.
Eleven variables, if available, collected by parental questionnaires or interviews, were considered as possible confounders in the adjusted analyses of the individual birth cohorts: 1. family history (parents and siblings) of asthma and/or allergic rhinitis (yes versus no); 2. family history of pet allergy (yes versus no); 3. maternal smoking during pregnancy (yes versus no); 4. postnatal maternal smoking from after birth to last follow-up between 6 to 10 years of age (‘regular smoker’ and ‘irregular smoker’ versus ‘never smoke’ as reference category); 5. educational level of parents at birth of child (by tertile according to school years as proxy for socio-economic status); 6. one or more older siblings (yes versus no); 7. home/apartment with convenient ground access (ground or 1st floor versus 2nd floor or higher); 8. crowding at home (number of persons per square meter or room; in quintiles, with the lowest quintile as reference category); 9. gender (boys versus girls); 10. breast feeding duration (in months); 11. doctor's diagnosed eczema any time between birth and 2 years (yes versus no).
For each cohort, a multivariable logistic regression analysis was used to calculate the adjusted odds ratio (OR) and 95% confidence intervals (CI) to estimate the effect of pet exposure in the first 2 years on the primary (current asthma) and secondary outcomes at age 6 to 10 years. Adjustment was performed for 7 potential confounders that were available for all studies (these were factors 1, 4–6, and 9–11 as listed above) and in addition, for all factors available for the respective cohort. Furthermore, we performed sensitivity analyses using (i) only the 7 potential confounders available for all studies, and (ii) using a propensity score approach for adjustment
The combining of results from all cohorts was done by random-effect meta-analyses with the inverse-variance method, based on the assumption that the associations in the different cohorts are not identical, estimating the average of the associations
In subgroup analyses, we assessed the associations for the following groups: (i) parents with and (ii) without asthma or allergic rhinitis ever; (iii) parents with and (iv) without pet allergies ever; (v) parents with asthma and/or allergic rhinitis, but without pet allergies; and (vi) parents without any allergies. Furthermore, we analyzed studies with high and those with low prevalence of pet ownership separately, and compared cohorts from major climatic regions in Europe (Nordic, Maritime, Central, and South).
In all analyses, a level of 0.05 was considered as statistically significant, without adjustment for multiple testing. Heterogeneity among the studies was tested using chi-squared Q-statistic and I2. We performed meta-analyses with Review Manager version 5.0 (German Cochrane Centre, Freiburg, Germany) and all other analyses with SAS version 9.1 (SAS Institute, Cary, NC, USA).
11 European studies, including the largest and oldest birth cohorts that were specifically designed to examine asthma and allergies, expressed interest and were included in the combined analyses. The recruitment of newborns and their families took place from 1989 (Isle of Wight, UK) to 1998 (DARC, Denmark and Leicester, UK) (
Birth cohort acronym | Study setting/first year of recruitment | Children initially recruited, N | Pet ownership at age 0–2 y | |||||
% | ||||||||
(n/N) | ||||||||
Cat(s) only |
Dog(s) only |
Cat(s) and dog(s) only |
Bird(s) only |
Rodent(s) only |
No furry or feathered pets | |||
ECA | Oslo | 3754 | 7.9 | 8.8 | 0.9 | 4.3 | 1.5 | 76.6 |
Norway, 1992 | (1877) |
(222/2810) | (247/2810) | (24/2810) | (122/2810) | (42/2810) | (2153/2810) | |
BAMSE | Stockholm | 4089 | 9.6 | 4.4 | 1.5 | 2.4 | 1.6 | 80.6 |
Sweden, 1994 | (355/3719) | (163/3719) | (54/3719) | (89/3719) | (60/3719) | (2998/3719) | ||
DARC | Odense | 562 | 16.2 | 11.0 | 4.0 | 3.6 | 2.3 | 62.9 |
Denmark, 1998 | (72/445) | (49/445) | (18/445) | (16/445) | (10/445) | (280/445) | ||
Leicester 1998 | Leicester | 566 |
16.4 | 16.4 | 4.4 | 1.5 | 6.5 | 54.8 |
UK, 1998 | (86/524) | (86/524) | (23/524) | (8/524) | (34/524) | (287/524) | ||
Isle of Wight | Isle of Wight | 1456 | 25.5 | 18.5 | 11.5 | 3.5 | 2.5 | 38.5 |
UK, 1989 | (274/1074) | (199/1074) | (123/1074) | (38/1074) | (27/1074) | (413/1074) | ||
PIAMA-NHS | Multicenter | 3291 | 27.8 | 8.7 | 5.7 | 6.1 | 6.2 | 45.5 |
The Netherlands, 1996 | (729/2620) | (228/2620) | (148/2620) | (160/2620) | (163/2620) | (1192/2620) | ||
MAS | Multicenter | 1314 | 11.6 | 6.1 | 1.6 | 8.5 | 3.4 | 68.8 |
Germany, 1990 | (108/933) | (57/933) | (15/933) | (79/933) | (32/933) | (642/933) | ||
LISA | Multicenter | 3097 | 9.6 | 6.1 | 1.5 | 4.7 | 5.4 | 72.7 |
Germany, 1997 | (232/2406) | (147/2406) | (37/2406) | (112/2406) | (129/2406) | (1749/2406) | ||
GINI-B | Multicenter | 3739 | 6.8 | 7.2 | 1.9 | 4.8 | 4.3 | 75.0 |
Germany, 1996 | (162/2377) | (172/2377) | (44/2377) | (113/2377) | (103/2377) | (1783/2377) | ||
AMICS-Barcelona | Barcelona | 487 | 6.2 | 9.8 | 1.0 | 21.8 | 4.2 | 57.0 |
Spain, 1996 | (12/193) | (19/193) | (2/193) | (42/193) | (8/193) | (110/193) | ||
AMICS-Menorca | Menorca | 485 | 4.3 | 18.3 | 6.5 | 18.1 | 2.0 | 50.8 |
Spain, 1997 | (17/398) | (73/398) | (26/398) | (72/398) | (8/398) | (202/398) | ||
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ECA: Environment and Childhood Asthma Study. BAMSE: Children, Allergy, Milieu, Stockholm, Epidemiological Survey. DARC: Danish Allergy Research Council Study. PIAMA-NHS: The Prevention and Incidence of Asthma and Mite Allergy – Natural History Study. MAS: Multi-center Allergy Study. LISA: Lifestyle-related Factors on the Immune System and Development of Allergies in Childhood Study. GINI-B: German Infant Nutritional Intervention Study (observational part). AMICS: Asthma Multicentre Infant Cohort Study.
and no other furry or feathered pet.
3754 initially recruited and followed-up until 2 years; for ECA: 1877 invited for longer follow-up.
More children are part of the Leicester 1998 cohort, but for the present meta-analyses on early pet ownership we included only children who were recruited during the 1st year of life.
Pet ownership ranged from around 60% (Isle of Wight, UK) to around 20% (BAMSE, Stockholm, Sweden), only cat ownership from 28% (Dutch PIAMA-NHS) to 4% (Menorca, Spain), and only dog ownership from 18% (the 2 islands Menorca and Isle of Wight) to 4% (BAMSE) (
The prevalence of current asthma at 6–10 years ranged from 2.9%–18.7% (
Meta-analyses of the adjusted odds ratios of
Birth cohort, country (sorted from north to south) | Age of children at follow-up (years) | Follow-up rate | Asthma | Allergic asthma | Allergic rhinitis | Allergic sensitisation |
% | % | % | % | |||
(n/N) | (n/N) | (n/N) | (n/N) | |||
ECA | 10 | 84% | 11.9 | 7.6 | 13.9 | 33.2 |
Norway | (120/1010) | (73/963) | (135/972) | (325/979) | ||
BAMSE | 8–9 | 84% | 9.3 | 5.2 | 8.0 | 26.0 |
Sweden | (308/3330) | (165/3187) | (255/3202) | (637/2451) | ||
DARC | 6 | 81% | 7.7 | 4.3 |
4.1 |
35.9 |
Denmark | (35/457) | (19/441) | (18/441) | (168/468) | ||
Leicester 1998 | 6 | 57% | 18.7 | n.a. | n.a. | n.a. |
UK | (66/353) | |||||
Isle of Wight | 10 | 94% | 16.3 | 8.8 | 11.9 | 33.0 |
UK | (223/1370) | (110/1257) | (142/1196) | (314/952) | ||
PIAMA-NHS | 8 | 83% | 7.1 | 2.7 | 5.5 | 29.7 |
The Netherlands | (194/2720) | (70/2596) | (131/2374) | (383/1289) | ||
MAS | 10 | 58% | 11.2 | 9.1 | 20.0 | 48.1 |
Germany | (68/606) | (54/592) | (147/735) | (343/713) | ||
LISA | 6 | 71% | 3.0 | 1.2 | 5.5 | 26.7 |
Germany | (66/2185) | (25/2144) | (109/1988) | (318/1193) | ||
GINI-B | 6 | 59% | 2.9 | 1.3 | 4.5 | 27.2 |
Germany | (64/2179) | (28/2143) | (90/2020) | (257/945) | ||
AMICS-Barcelona | 6 | 64% | 12.5 | 3.9 |
5.8 |
18.9 |
Spain | (39/312) | (11/284) | (15/259) | (54/286) | ||
AMICS-Menorca | 6 | 94% | 7.9 | 3.0 |
0.5 |
12.3 |
Spain | (36/458) | (13/435) | (2/438) | (43/349) | ||
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n.a. = not assessed.
in DARC, AMICS-Barcelona and AMICS-Menorca, sensitization data were only available for the age of 4 years.
Main results were similar when analyzing shorter pet exposure time periods (e.g. around birth or during first 12 months) or in sensitivity analyses using a propensity score to control for potential confounding. Also, meta-analyses in subgroups showed no significant association of pet ownership and asthma among parents with or among those without asthma/allergies, in cohorts with only high or those with only low pet prevalence, or in subgroups of cohorts from 4 major climatic regions in Europe.
No significant associations that would suggest effect modification were found when we analyzed two-way interactions between pet exposure and parental allergies, maternal prenatal smoking, or postnatal maternal smoking.
The overall prevalence of allergic asthma (defined as current asthma and sensitization to ≥1 aero-allergen) in early school age was 4.0%, ranging from 1.2% at six years to 9.1% at 10 years (
Meta-analyses of the adjusted odds of
Asthma without sensitization to any aero- or food allergen (“non-allergic asthma”) was not significantly associated with cat (OR 0.99, 0.51–1.94), dog (OR 1.35, 0.71–2.57), bird (OR 1.80, 0.80–4.04) or rodent ownership (OR 1.70, 0.53–5.44) in the meta-analyses of all cohorts.
The prevalence of sensitization to ≥1 aero-allergen ranged from 26%–33% during 6–10 years (
Meta-analyses of the adjusted odds of
Meta-analyses of the adjusted odds of
Insufficient data for the definition of the primary endpoint ranged from 5% (AMICS-Menorca) to 54% (MAS) of the participants; from 10% (AMICS-Menorca) to 55% (MAS) for defining allergic asthma; 10% (AMICS-Menorca) to 48% (ECA) for allergic rhinitis, and 17% (DARC) to 75% (GINI-B) for the definition of allergic sensitization.
Our meta-analyses showed that ownership of single types of furry pets or birds in the first 2 years of life neither increased nor decreased the risk of asthma, non-allergic asthma (not sensitized to any aero- or food-allergen), allergic asthma or allergic rhinitis (both included sensitization to at least 1 aero-allergen) in school-aged children. However, living with furry pets in the first 2 years appeared to reduce the likelihood of becoming sensitized to aero-allergens in early school-age regardless of respiratory symptoms.
An older meta-analysis, mainly with cross-sectional studies from the 1990s, showed a slightly increased risk of asthma or wheezing for children >6 years in relation to any pet exposure, but did not analyze different types of pets
The reduced sensitization to aero-allergens related to furry pet ownership is consistent with similar findings in several previous studies, particularly for dogs
Our approach was different than a previous meta-analysis on this topic because we were able to collect, harmonize, and combine the individual participant data from 11 birth cohorts instead of using published risk estimates based on heterogeneous outcome and exposure definitions and age groups
The large sample allowed the definition of mutually exclusive pet exposure categories: ownership of “only cat(s)”, “only dog(s)”, “only rodent(s)”, and “only bird(s)”. This is another unique feature of our collaborative study compared to previous studies, which did not separate potential effects of “clean” pet exposure categories. Furthermore, analyzing the time at birth, the first and the second year of life separately, the results were very similar compared with the whole period of the first 2 years. This suggests that our results are robust, and do not point towards a narrow post natal period with increased susceptibility to pet exposure in the home.
For the outcome definition, previous studies used single variables such as parent-reported wheezing or doctor's-diagnosed asthma, which may have over- or underestimated the real prevalence of asthma. To avoid a potential over-estimation of asthma prevalence we used a more stringent definition for the primary outcome asthma based on at least 2 out of the 3 conditions parent-reported wheezing, doctor's-diagnosed asthma and asthma medication
A limitation of previous studies may have been the lack of sufficient adjustment for potential confounding. The size of our sample had enough statistical power to take into account potential confounders including family, social and domestic factors in most cohorts
Although the birth cohorts come from different climatic European regions, include children born in different years (between 1989–1998), have urban and rural/island study settings, different prevalences of allergies and patterns of pets, the statistical tests for heterogeneity were rarely significant, which strengthens the findings and generalizability of our analyses.
Avoidance behavior in families with allergies could be an explanation for the “protective effect” of pet keeping seen in some previous studies (reverse causation)
Another possible limitation of our analyses is that for some birth cohorts the outcome was only available for 6 years of follow-up (at this age asthma may not have been fully developed in some subjects), whereas for others we could include the 8 and 10 year follow-up data. Comparing the effect estimates of the individual birth cohorts, we did not find that cohorts with a 6 year follow-up differed from the older cohorts; instead we found rather homogeneous results across the cohorts.
Some cohorts assessed more potential confounding variables than others. However, when we repeated our analyses with only those confounders that were assessed in all studies our results did not change considerably. Since most cohorts did not ask for the number of pets at home, we could not examine the possibility of a dose-response relationship of pet keeping.
A risk of participation bias in each included study could be present and it could be different for each study (e.g. due to regional differences in disease awareness or in recruitment strategies). However, while this might influence the observed prevalences for allergic diseases, this should less influence any association between pet ownership and allergic disease. On the other hand, each cohort had different numbers of observations available to define the primary and secondary outcomes, and some kind of selection bias cannot be excluded.
Assessing total exposure to pet allergens in early life was outside the aim and scope of the present study. Furthermore, total allergen exposure, which is virtually impossible to measure, would not influence the scientific evidence for giving advice on pet keeping or not.
When interpreting the results, the reliance upon parents' questionnaire data should be kept in mind. It should be noted, however, that standardized ISAAC questions were used to assess allergic symptoms and diseases. In addition, the quality of data might not be equal across the included studies due to data collection timing and methods.
We examined various secondary endpoints while performing over a hundred additional explorative analyses of the whole dataset and of subgroups. Some of these subgroups included only 2 or 3 birth cohorts if these were the only ones with sufficient exposure and/or outcome data. Almost all analyses showed no associations between exposure and secondary outcomes with a few exceptions, e.g. a positive association between cat ownership and asthma in combination with sensitization to dog or cat allergens, however dog ownership was not associated with asthma in combination with dog or cat allergens. Another positive association was found between ownership of both cat(s) and dog(s) and non-allergic asthma; however cat and dog ownership alone was not associated with non-allergic asthma. Since we did not correct for multiple testing, the statistically significant results in some of the subgroup analyses should be interpreted cautiously and as results of explorative analyses keeping in mind the possibility of false positive findings.
This pooled analysis of individual participant data from 11 European birth cohorts found no association between ownership of single types of furry and feathered pets in the first 2 years of life and asthma or allergic rhinitis in school children aged 6–10. For primary prevention of asthma and allergy, we found no evidence for health care practitioners to give parents specific advice on avoiding or acquiring pets in early childhood. To evaluate the effect of pet keeping in early childhood on e.g. developing eczema, further pooled birth cohort data analyses are needed rather than single birth cohort analyses.
We thank all birth cohort study investigators and participants.
Members of the GA2LEN WP 1.5 ‘Birth Cohorts’ working group include also the following researchers: Magnus Wickman (Sachs' Children's Hospital), Eva Hallner, Johan Alm (Institute of Environmental Medicine), Catarina Almqvist (Dept. of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden); Göran Wennergren, Bernt Alm (Dept. of Pediatrics, Queen Silvia Children's Hospital, University of Gothenburg, Sweden); Joachim Heinrich (Institute of Epidemiology, Helmholtz-Zentrum München, German Research Centre for Environmetal Health, Munich, Germany); Henriette A. Smit (Center for Prevention and Health Services Research, National Institute for Public Health and the Environment, Bilthoven, The Netherlands); Carel Thijs, Monique Mommers (School of Public Health and Primary Care, Maastricht University, Maastricht, The Netherlands); Carsten Bindslev-Jensen (Dept. of Dermatology), Susanne Halken, (Dept. of Pediatrics, Odense University Hospital, Denmark); Maria Pia Fantini, Francesca Bravi (Dept. of Public Health, Alma Mater Studiorum, University of Bologna, Italy); Daniela Porta, Francesco Forastiere (Dipartimento di Epidemiologia ASL Rm E, Rome, Italy); Adnan Custovic (Wythenshawe Hospital, University of Manchester, UK); Ruta Dubakiene (Allergy Center, Vilnius University, Lithuania); Jestinah Mahachie (Dept. of Oto-Rhino-Laryngology, University of Ghent, Belgium).