Pet-keeping in childhood and adult asthma and hay fever: European community respiratory health survey☆☆☆

Article Outline

• Abstract
• Methods
  • Data collection
  • Statistical analysis
• Results
• Discussion
  • Methodologic considerations
  • Cats: Symptoms, sensitization, and tolerance
  • Dogs: Antiallergic but pro-inflammatory?
  • Birds: Nonspecific pro-inflammatory?
• Conclusion
• Acknowledgements
• Appendix
  • Principal participants
• References
• Copyright

Abstract 

Background: Whether pet-keeping early in life protects against or promotes allergy remains unclear. Objective: Our aim was to examine the effects of childhood pet-keeping on adult allergic disease in a large international population-based study, including information on sensitization, adult pet-keeping, and pet prevalence in the populations. Methods: We used information from structured interviews (n = 18,530) and specific IgE to common aeroallergens in blood samples (n = 13,932) from participants in the European Community Respiratory Health Survey (ECRHS) to analyze the associations between keeping pets and adult asthma and hay fever. Results: Keeping cats in childhood was associated with asthma only among atopic subjects, an association that varied between centers (P = .002) and was stronger where cats where less common (< 40% cats: odds ratio_wheeze [OR_wheeze] = 1.84, 95% CI = 1.31-2.57; 40%-60% cats: OR_wheeze = 1.33, 95% CI = 1.10-1.61; ≥60% cats: OR_wheeze = 0.98, 95% CI = 0.73-1.33). Dogs owned in childhood or adulthood were associated with asthma among nonatopic subjects (childhood: OR_wheeze = 1.28, 95% CI = 1.13-1.46; adulthood: OR_wheeze = 1.31, 95% CI = 1.14-1.51; both: OR_wheeze = 1.69, 95% CI = 1.40-2.04). In atopic subjects, those who had owned dogs in childhood had less hay fever (OR = 0.85; 95% CI = 0.73-0.98) and no increased risk of asthma (OR_wheeze = 1.01, 95% CI = 0.87-1.17). Respiratory symptoms were more common in subjects who had owned birds during childhood (OR_wheeze = 1.12; 95% CI = 1.02-1.23) independent of sensitization. Conclusions: The effects of pet-keeping in childhood varied according to the type of pet, the allergic sensitization of the individual, and the wider environmental exposure to allergen. Cats owned in childhood were associated with more asthma in sensitized adults who grew up in areas with a low community prevalence of cats. Dogs owned in childhood seemed to protect against adult allergic disease but promote nonallergic asthma. (J Allergy Clin Immunol 2003;112:289-300.)

Keywords:  Cats, dogs, birds, asthma, hay fever, sensitization, tolerance, hygiene hypothesis, ECRHS

Abbreviations:  ECRHS , European Community Respiratory Health Survey, OR , Odds ratio

Whether pet-keeping early in life protects against or promotes allergic disease remains unclear. Traditionally, pets have been considered a major promotor of allergic disease, and several studies show how pets increase the risk for asthma (Table I).¹, ² However, an increasing bulk of evidence indicates protective effects of early exposure to pets³, ⁴, ⁵, ⁶, ⁷, ⁸, ⁹, ¹⁰, ¹¹, ¹², ¹³ (Table I) or animals in a farm environment.¹⁴, ¹⁵ The literature is complex and contradictory and subject to a bias toward publication of more “interesting” results. Little information exists on whether the effects of exposure to pets in early life on asthma and allergy persist into adulthood.², ³, ⁴, ⁵

Table I. Literature overview of childhood pet-keeping and (subsequent) atopic sensitization and disease

(+), Positive association; (-) , negative association, (0) , no association.

To date, we know of 3 possible mechanisms through which contact with animals might influence immunologic priming early in life: allergen stimulation causing sensitization, allergen stimulation inducing tolerance, and microbial stimulation enhancing immune deviation toward a nonallergic T_H1 profile. The common assumption that exposure to allergens is the main determinant of allergy has lately been vigorously debated.¹⁶, ¹⁷, ¹⁸ Since the hygiene hypothesis was introduced in 1989,¹⁹ there has been increasing focus on how altered microbial stimulation during immunologic priming early in life could contribute to the recent increase in allergy in the Western world.²⁰, ²¹, ²² The concept of tolerance is applied in immunotherapy. It has recently been suggested that high-dose cat exposure might induce tolerance toward cat sensitization.²³, ²⁴ This implies that a change from high to moderate exposure to cat allergen could, paradoxically, increase cat sensitization in a community. Finally, early life exposure to animals might possibly influence the development of the airways and the lung parenchyma through nonallergic mechanisms involving endotoxin or other pro-inflammatory substances²⁵ or transmission of microbes causing manifest lower airways infections.²⁶, ²⁷ The individual response to each of these mechanisms is likely to depend on host factors.¹⁸, ²⁸

On the basis of today's knowledge, it is not possible to give confident advice concerning pet-keeping and allergy. In this article, we analyze data from the multicultural and population-based European Community Respiratory Health Survey (ECRHS) to study how keeping cats, dogs, and birds in childhood is related to adult asthma and hay fever. We have taken into account atopic sensitization, current adult pet-keeping, and more general exposure to cats measured as the population prevalence of cat ownership. The associations of pets and atopic sensitization in the ECRHS are specifically dealt with previously. Svanes et al³ revealed that subjects who had kept a dog in childhood were less often sensitized toward common aeroallergens (cat, grass, house dust mite); for childhood cat exposure a protective effect was restricted to sensitization toward grass and cat in those with parental allergy. Roost et al⁵ showed more sensitization toward cat in adults who currently kept a cat, but less cat sensitization in predisposed subjects who had kept a cat in childhood.

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Methods 

Data collection 

The methodology for the ECRHS has been fully described elsewhere.²⁹ Participating centers selected an area defined by pre-existing administrative boundaries with a population of at least 150,000. A random population sample of at least 1500 men and 1500 women aged 20 to 44 years was selected. In stage I, subjects were sent the ECRHS self-completed screening questionnaire. In stage II, a smaller random sample of subjects who had completed the screening questionnaire was invited to attend for a more detailed interviewer-led questionnaire, lung function testing, and blood tests. Specific IgE to common aeroallergens was measured in serum samples and was considered to be positive if >0.35 kU/L. Details of the IgE measurements are described elsewhere.³⁰ The informed consent of all participants was obtained, and the study was approved by all the involved ethics committees.

This analysis includes data collected from 1991 to 1993 for 18,530 subjects from 36 centers representing 16 countries, of which 13,932 subjects (75%) provided blood tests for measurement of specific IgE. Information on pet-keeping, symptoms, and potential confounding variables was obtained from the interviewer-led questionnaire. The wording of the questions on pet-keeping is given in the footnote to Table II. Atopy was defined as having specific IgE to cat, timothy grass, house dust mite, and/or Cladosporium .

Table II. Prevalence of pet-keeping in subjects aged 20 to 44 years by country

Statistical analysis 

Logistic regression models were used to assess the independent effects of pet-keeping in childhood and adulthood on adult symptoms of asthma and hay fever. Adjustments were made for other childhood factors, sex, age, smoking habits (never, ex-smoker, and current smoking), occupation (European Economic Community Status Groups-14),³¹ and study center. Potential heterogeneity between centers was studied by meta-analyses.³² Stratified analyses and analyses of interaction did not show differences in association between keeping pets in childhood and adult symptoms according to sex or smoking status. All analyses were carried out using the statistical software program Stata.³³

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Results 

Pet-keeping was common in all centers (Table II), and 65% of the subjects had kept pets in childhood, whereas 41% currently kept pets. People often had more than 1 pet and tended to keep the same pet in childhood and adulthood (Table III). Asthma and allergy in parents and siblings did not seem to influence systematically the decision to keep pets. Smokers more often kept pets, and this was true among our study subjects and their parents. Subjects from large families more often had cats or dogs in childhood but less often had cats and dogs when adults. Women more often kept cats in childhood and adulthood. Childhood cats were more common in the earlier birth cohorts, whereas children in the later birth cohorts were more likely to keep birds.

Table III. Factors related to childhood and adult pet-keeping (mutually adjusted and adjusted for center) in 17,546 men and women with complete information on all included variables

OR, Odds ratio.

Boldfaced odds ratios refer to associations with P < .05.

Respiratory symptoms were more common in subjects who had kept dogs or birds in childhood and in subjects who currently kept a dog (Table IV).

Table IV. Childhood and adult pet-keeping as related to adult asthma and hay fever

Boldfaced odds ratios refer to associations with P < .05.

OR, Odds ratio.

Keeping a dog during childhood was related to less adult hay fever. These associations differed significantly according to atopic status (Table V). Data for wheeze are presented, but the results were similar for the other respiratory symptoms and asthma medication. Among atopic subjects, keeping a cat during childhood was related to more adult respiratory symptoms, whereas keeping a dog in childhood seemed to protect against adult hay fever. Among those who were not atopic, dogs kept at any time of life were related to more adult respiratory symptoms. The association between birds kept during childhood and adult respiratory symptoms was independent of atopic status.

Table V. Childhood and current adult pet-keeping as related to wheeze and hay fever at age 20-44 years, in 4514 subjects with atopic sensitization (specific IgE to house dust mite, cat, grass, and/or mold) and 9407 subjects without

Boldfaced odds ratios refer to associations with P < .05.

OR, Odds ratio.

In atopic subjects, the association between keeping a cat during childhood and wheeze (and other respiratory symptoms) varied greatly between centers (P _{heterogeneity} = .002) (Fig 1, A ).

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• Fig. 1. 

  A, Odds ratios for the association of childhood cats and adult wheeze in sensitized subjects by center. Adjustment within center for other childhood pets, adult pets, parental and sibling allergy, own and parental smoking, number of siblings, age, sex, and occupation. For each center, horizontal lines indicate 95% CI. For combined odds ratio, diamond indicates 95% CI from model, with center as random effect. The size of each square is proportional to the sample size. B, Log odds ratio for association of childhood cats and adult wheeze in sensitized subjects and prevalence of childhood cats by center. r = -0.488.

The magnitude of the associations was inversely related to the prevalence of keeping cats during childhood in each center (r = -0.488) (Fig 1B , ). In centers in which more than 60% reported having a cat during childhood, there was no increase in risk for wheeze related to cats kept during childhood (OR 0.98, 95% CI = 0.73-1.33). In centers with 40% to 60% childhood cats, the OR was 1.33 (95% CI = 1.10-1.61), and in centers with <40% childhood cats the OR was 1.84 (95% CI = 1.31-2.57). This interaction by prevalence of cats kept during childhood was significant (<40% versus ≥60% childhood cats: P _interaction = .008), whereas there was no association with prevalence of cat ownership in the center at the time of the survey.

The increased asthma risk in nonatopic subjects who had kept a dog in childhood was consistent across centers (P _{heterogeneity} = .3) (Fig 2), whereas the lack of association between dogs kept during childhood and wheeze among atopic subjects showed some heterogeneity (P _{heterogeneity} = .046).

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• Fig. 2. 

  Odds ratios for the association of childhood dogs and adult wheeze in nonsensitized subjects by center. Adjustment within center for other childhood pets, adult pets, parental and sibling allergy, own and parental smoking, number of siblings, age, sex, and occupation. For each center, horizontal lines indicate 95% CI. For combined odds ratio, diamond indicates 95% CI from model, with center as random effect. The size of each square is proportional to the sample size.

Differences in the prevalence of keeping dogs during childhood could not explain this heterogeneity. The protective effect of dogs kept during childhood on hay fever among the atopic subjects was homogeneous across centers (P _{heterogeneity} = .5) (Fig 3), as was the association of birds kept during childhood and adult asthma (P _{heterogeneity} = .2).

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• Fig. 3. 

  Odds ratios for the association of childhood dogs and adult hay fever in nonsensitized subjects by center. Adjustment within center for other childhood pets, adult pets, parental and siblings allergy, own and parental smoking, number of siblings, age, sex, and occupation. For each center, horizontal lines indicate 95% CI. For combined odds ratio, diamond indicates 95% CI from model, with center as random effect. The size of each square is proportional to the sample size.

The association of cats kept during childhood and asthma among atopic adults was significant in those who kept a cat only in childhood but not currently as adults (Table VI). The strongest associations between respiratory symptoms and dog ownership among the nonatopic were among those who had kept dogs both in childhood and adulthood. The protective effect of keeping a dog on hay fever among atopic subjects was restricted to those who had kept a dog in childhood.

Table VI. Age of pet-keeping as related to adult respiratory symptoms, asthma medication, and hay fever in 9088 nonatopic subjecs and 4374 atopic subjects with complete information

Boldfaced odds ratios refer to associations with P < .05.

OR, Odds ratio.

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Discussion 

Keeping a cat during childhood was associated with asthma but only among atopic subjects. This association was, however, highly heterogeneous between centers, and the magnitude of the association was inversely related to the prevalence of cats kept during childhood. Thus, in subjects who grew up in an area with relatively low public exposure to cats, having their own cat was associated with an increased risk for adult asthma. In subjects from areas with a very high prevalence of cats, however, a cat at home was not associated with increased asthma risk. Dog keeping in childhood or adulthood was related to more adult asthma among nonatopic subjects. Among the atopic, dog keeping in childhood was related to reduced risk for adult hay fever. These associations were consistent across the ECRHS, including centers with great differences in prevalence of symptoms, habits of pet-keeping, and sociocultural conditions. The associations of cat and dog ownership with adult asthma and hay fever differed substantially according to allergic disposition, whereas subjects who had birds in childhood had more adult respiratory symptoms independent of atopic status. Each pet showed different associations with asthma and hay fever. This is plausible, because the animal species have different allergenic and microbial properties and interact with children at different ages. These findings add to the evidence that early life contact with animals does influence the development of the immune system and the airways, with the potential to prevent or promote subsequent disease, even in adulthood.

Methodologic considerations 

The major problem in the interpretation of analyses of pets and allergy, is that the study subjects cannot be randomly assigned with regard to pet-keeping. People choose whether and what kind of pet to keep on the basis of a range of motives. Doctors in many countries expect allergy to be a main determinant in this choice, whereas the literature gives contradictory information.⁵, ³⁴, ³⁵ If families with allergy tend to avoid keeping pets, this should have been reflected in apparent “protective” effects of cats and dogs on both asthma and hay fever among predisposed subjects. We did not find such evidence for selective avoidance of pets during childhood, possibly because there was little focus on allergen avoidance during the period when our subjects were children (born 1945-1972). Furthermore, subjects who had gotten rid of a childhood pet because of symptoms would still report having had the pet. In agreement with other studies,¹⁰, ¹³ allergic disease in siblings or parents was not an important determinant for pet-keeping in childhood compared with the strong effects of sociocultural factors (Table III).

With regard to adult pet-keeping, however, the results must be interpreted with caution, because there were indications of complex selection phenomena (Table VI). Cats were more strongly related to adult asthma in sensitized subjects who had kept cats only in childhood, possibly indicating that those who got symptoms from their childhood cat avoided keeping cats when adults. (Alternately, subjects who stopped keeping a cat might have lost their tolerance and developed more symptoms related to community exposure.) Adult cat keeping was not associated with wheeze, not even among the cat sensitized, possibly because of selective avoidance. Childhood dog owners seemed to keep dogs when adults despite respiratory symptoms, compatible with investigations showing that people are reluctant to get rid of their dog even when it causes symptoms.³⁵ Less respiratory symptoms in those who kept a bird only as adults might indicate that adult subjects with asthma avoided getting birds unless they had kept them in childhood (a protective effect from keeping birds during adult life seems unlikely).

Recall bias is another problem, but a pet is an important feature of childhood that people are likely to remember accurately. The cross-sectional design of the study is a weakness, because our measure of exposure is very crude, whereas the biologic influence of pet contact is expected to be specific with regard to time and dose of exposure. Our analyses can therefore not reveal very distinct patterns and might underestimate true associations.

The inheritable susceptibility to asthma and allergy is well known, but direct genetic markers are not available. In this study, the presence of atopic sensitization was used as a measure of allergic predisposition, thereby characterizing the subject's own immunologic constitution based on confirmed production of specific IgE. This measure is not influenced by information bias and is feasible in clinical consulting if further research supports this approach.

Cats: Symptoms, sensitization, and tolerance 

Several studies indicate that keeping a cat during childhood might confer protection against becoming sensitized to cats⁵, ⁶, ⁹, ³⁶, ³⁷ and to other common aeroallergens,³ particularly among predisposed subjects.³, ⁵, ³⁶ High-dose cat exposure might induce tolerance in some individuals²³, ²⁴ through production of IgG4 rather than IgE, stimulated by the T_H2 cytokine IL-4.²⁴ We found that in subjects who did not become sensitized (ie, who developed tolerance), keeping cats during childhood did not make any difference with regard to later respiratory symptoms. When sensitization did occur in those who kept cats during childhood, the risk of adult asthma was increased. A stronger effect among the sensitized was also found in a study of adults³⁸ relating current cat allergen level with respiratory symptoms. In 1 study of children, exposure to cats increased the risk of wheeze in the children of asthmatic mothers but reduced the risk in the children of mothers without asthma.¹¹ However, a net protective effect was found in the total cohort,¹² and a protective effect of cats was also found in another cohort of high-risk children.⁹ The discrepancy between these studies of children and our findings in adults could be related to the difference in age span but also to differences in community level of allergen exposure. Increased asthma risk in sensitized subjects who had kept cats in childhood was restricted to subjects who grew up in areas in which the public exposure to cat was limited. In areas in which >60% of the study subjects kept cats, there was no increase in asthma risk among those who had kept a cat during childhood, and there was even an indication of a protective effect. Our study included only a small number of areas with very high community prevalence of pets, thus we could not determine with certainty whether the OR of childhood cats and adult asthma with further increasing community prevalence of childhood cats tended toward 1 or 0. An OR of 1 would mean that pet-keeping at home did not add to the risk related to high exposure in the community, whereas an association tending toward 0 (as indicated by Fig 1 B , ) would mean that high exposure at home offered relative tolerance toward high public exposure.

Dogs: Antiallergic but pro-inflammatory? 

Dogs kept in childhood, previously shown to protect against allergic sensitization,³ also seemed to protect against adult hay fever in the sensitized population. Protective effects of dogs with regard to atopic sensitization are similarly reported in several studies of children,⁷, ¹², ¹³, ³⁹ particularly in high-risk children.⁷, ¹² However, among the nonatopic subjects of our study, dog ownership was associated with a substantial increase in risk for adult respiratory symptoms. There was evidence of a cumulative effect with a 70% increase in wheeze related to dog keeping in both childhood and adulthood. These findings were highly significant, consistent across 36 different populations, and robust to different methods of analysis. The association was similar in never-smokers, and was thus not caused by uncontrolled confounding by smoking. There are no comparable studies of adults. Nafstad et al⁷ showed more bronchial hyperresponsiveness related to dog keeping in infants, but other studies of children show protective effects of dogs on wheeze also in subjects without allergic predisposition.¹³, ³⁹

Microbial challenge could possibly explain the protective effects of dogs on allergic sensitization and on allergic symptoms in sensitized subjects, as well as the demonstrated adverse effects of dogs on adult asthma. Dogs often have close physical contact with people, especially with very small children, nuzzling them in the face and licking their hands. The dog's fur might carry microbes from soil. Dogs, as opposed to cats, might thus provide a wide microbial exposure early in life that might possibly promote a nonallergic immunologic maturation during this sensitive age window. However, in nonsensitized subjects who do not need this additional microbial stimulation because they already have a competent nonallergic immune system, the harmful consequences of microbial challenge might be dominant. Both harmful and protective effects of dogs could be mediated through the bacterial product endotoxin, which is found in increased levels in homes with dogs.⁴⁰ The pro-inflammatory effects of these substances and the related increase in respiratory symptoms are well known,²⁵ whereas exposure during the early stages of immune priming seem to confer protection against allergy.⁴¹, ⁴² A recent study showed less atopic but more nonatopic asthma related to increasing levels of endotoxin.⁴¹ However, in another study, the protective effect of exposure to dogs was independent of endotoxins,¹² suggesting a role for other mediators. The microbial closeness of the dog with human beings²⁶, ²⁷ could also increase the possibility for lower airways infections predisposing to asthma.⁴³, ⁴⁴ Harmful effects of pro-inflammatory substances and infections from dogs are likely to increase with age. An age-dependent balance of beneficial and harmful effects of dogs could explain the cumulative effect of dog exposure in this study and the different results in children and adults. This view is supported by a meta-analysis showing protective effects of pets in studies of younger children but harmful effects in studies of older children.¹

Birds: Nonspecific pro-inflammatory? 

Bird keeping in childhood was found to increase the risk for adult respiratory symptoms independent of allergic disposition. Adult occupational exposure to poultry is known to increase respiratory symptoms.⁴⁵, ⁴⁶ Rees et al⁴⁶ showed that this was unrelated to sensitization, which is in accordance with our study of childhood exposure to pet birds. Melsom et al¹⁴ showed increased asthma in children living in households with poultry kept indoors. A direct toxic or airways irritant effect from ammonia or other substances in bird manure is a possible explanation. Childhood exposure could cause adult symptoms by an impact during critical stages of lung development.

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Conclusion 

This study showed complex associations of childhood pets and adult asthma and hay fever. Childhood exposure to cats, previously shown to protect against allergic sensitization, had no influence on adult asthma in those who did not become sensitized but was related to increased risk for adult asthma among the sensitized. However, when the community exposure to cats was high during childhood, keeping a cat at home represented no additional asthma risk. Whether in this case cat keeping could offer protection against adult asthma, indicating tolerance toward allergic symptoms in already sensitized subjects, needs further investigation. Dogs kept in childhood, previously shown to protect against allergic sensitization, seemed to also protect against hay fever symptoms in atopic subjects. However, for those who were not atopic a dog kept in childhood or during adult life was associated with a considerably increased risk of adult respiratory symptoms. Dogs might possibly stimulate a nonallergic immunologic maturation, but at the same time promote nonallergic asthma through alternative inflammatory mechanisms or transmission of infections. The choice of keeping dogs in families with children might thus have substantial long-term consequences for allergic disease, and further research is needed. There were clear differences related to atopic status, a feasible and potentially useful marker of immunologic constitution. These results should be a step toward individual clinical advice about pet-keeping on the basis of phenotype.

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Acknowledgements 

We thank the Norwegian Asthma and Allergy Foundation for their support and help with the study.

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Appendix 

Principal participants 

Coordinating Center (London) . P. Burney, S. Chinn, C. Luczynska, D. Jarvis, E. Lai.

Project Management Group. P. Burney (Project leader) S. Chinn, C. Luczynska, D. Jarvis, P. Vermeire (Antwerp); H. Kesteloot (Leuven); J. Bousquet (Montpellier); D. Nowak (Hamburg); the late J. Prichard (Dublin), R. de Marco (Verona); B. Rijcken (Groningen); J. M. Anto (Barcelona); J. Alves (Oporto); G. Boman (Uppsala); N. Nielsen (Copenhagen); P. Paoletti (Pisa).

Participating Centers. Austria : W. Popp (Vienna); Australia : M. Abramson, J. Raven, J. Rolland (Melbourne); Belgium : P. Vermeire, F. van Bastelaer (Antwerp South, Antwerp Central); Estonia : R. Jögi (Tartu); France : J. Bousquet J. Knani (Montpellier); F. Neukirch, R. Liard (Paris); I. Pin, C. Pison (Grenoble); A. Taytard (Bordeaux); Germany : H. Magnussen, D. Nowak (Hamburg); H. E. Wichmann, J. Heinrich (Erfurt); Greece : N. Papageorgiou, P. Avarlis, M. Gaga, C. Marossis (Athens); Iceland : T. Gislason, D. Gislason (Reykjavik); Ireland : J. Prichard, S. Allwright, D. MacLeod (Dublin); Italy : M. Bugiani, C. Bucca, C. Romano (Turin); R. de Marco Lo Cascio, C. Campello (Verona); A. Marinoni, I. Cerveri, L. Casali (Pavia); Netherlands : B. Rijcken, A. Kremer (Groningen, Bergen-op-Zoom, Geleen); New Zealand : J. Crane, S. Lewis (Wellington, Christchurch, Hawkes Bay); Norway : A. Gulsvik, E. Omenaas (Bergen); Portugal : J. A. Marques, J. Alves (Oporto); Spain : J. M. Antó, J. Sunyer, F. Burgos, J. Castellsagué, J. Roca, J. B. Soriano, A. Tobías (Barcelona); N. Muniozguren, J. Ramos González, A. Capelastegui (Galdakao); J. Castillo, J. Rodriguez Portal (Seville); J. Martinez-Moratalla, E. Almar (Albacete); J. Maldonado Pérez A. Pereira, J. Sánchez (Huelva); J. Quiros, I. Huerta, F. Pavo (Oviedo); Sweden : G. Boman, C. Janson, E. Björnsson (Uppsala); L. Rosenhall, E. Norrman, B. Lundbäck (Umeå); N. Lindholm, P. Plaschke (Göteborg); Switzerland : U. Ackermann-Liebrich, N. Künzli, A. Perruchoud (Basel); United Kingdom : M. Burr, J. Layzell (Caerphilly); R. Hall (Ipswich); B. Harrison (Norwich); J. Stark (Cambridge); United States : S. Buist, W. Vollmer, M. Osborne (Portland).

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