Key results
In this three-generation study, the prevalence of self-reported asthma increased across generations. Overall, growing up on a farm was associated with a reduced risk of developing asthma in participants born from 1945-1999. However, this association was mainly driven by the parent generation born from 1945 to 1973. When stratified by birthyear into ten-year-intervals, a more pronounced inverse association between growing up on a farm and the risk of developing asthma was seen among the older generation born in the 1940s-1960s, while the association appeared more variable in the younger generations born during the 1970s and thereafter. No formal interaction was found between either place of upbringing and generation, or place of upbringing and birthyear upon the effect on asthma.
The association between farm upbringing and ever asthma stratified by generation showed a relatively large shift between crude and adjusted estimates among offspring. This difference was not observed among parents. A complete case analysis on offspring with complete information (n=5,838) showed a crude estimate (OR=1.01; 95%CI 0.69-1.47), which did not differ markedly from the adjusted estimate (OR=1.03; CI95% 0.71-1.51). This suggest that little residual confounding was removed when adjusting for sex, parental smoking in offspring childhood, parental asthma and study centre. Instead, the difference in crude and adjusted may potentially be attributed to the differences in subjects with missing information on covariates and subjects with a complete set of data.
Theassociation between farm upbringing and asthma stratified by birthyear into ten-year-intervals appeared variable and with wide confidence intervals among subjects born after 1970. Especially in the 1980s the confidence interval appeared very wide. This was assessed not to be related to the number of subjects as this did not differ markedly (n=2240) compared to the number of subjects in the other strata with narrower confidence intervals. Considering the crude and adjusted estimates of each stratum it appeared that the estimates changed substantially more in the 1980s than in any of the other strata. A complete case analysis on subjects born in the 1980s resulted in a crude estimate (OR=1.58; 95%CI 0.89-2.81), not substantially different from the adjusted estimate (OR=1.68; 95%CI 0.93-3.02), which suggests that the difference in crude and adjusted estimates also here may potentially be attributable to differences in the subjects with missing information on covariates and subjects included in the complete case analysis.
The analysis stratified by study centre showed overall homogeneity among all study centers except Albacete, for which the risk of asthma appeared higher among subjects with farm upbringing compared with subjects growing up in the city. The subgroup of subjects from Albacete, who were born on a farm consisted of 29 subjects, of whom only 8 reported asthma (data not shown). Generally, stratifying by center caused a reduced number of subjects in each stratum, and thus, centre estimates should be interpreted with the limited power in mind. Center analysis stratified by birthyear into ten-year-intervals was not possible due to power issues.
Interpretation
In line with the findings of this study, a register-based study by Bråbäck et al.[17] involving male subjects born between 1952-1981 found an increasing prevalence of asthma from 2.0% in cohorts born 1952-1956 to 7.2% in cohorts born 1977-1981. A cohort study by Timm et al.[19] involved subjects born between 1945 and 1971. Both of these studies found the protective effect of farm upbringing on the risk of developing inflammatory diseases to be a rather modern phenomenon. However, this three-generation study including subjects born between 1945-1999 showed an overall consistent association between farm upbringing and asthma, except for subjects born during the 1980s.
One study by Ober et al. found that the prevalence of asthma was substantially lower in Amish children growing up in families using traditional farming methods compared to Hutterite children, with families practicing a modern and industrialized farming method. Otherwise, Hutterite and Amish children were similar with respect to ancestry and lifestyle.[20] However, these observations were not consistent with the results of our study as the association between asthma prevalence and farm upbringing did not differ substantially between 1940s and 1990s despite considerable changes in the farming methods during these years.
Strengths and limitations
An important strength of this study is the large three-generation study design conducted in Europe and Australia. To the best of our knowledge, this is also the first three-generation study to investigate the association between farm upbringing and asthma at different times in history.
The response rates and drop-outs of the cohort studies have been considerable, leading to a risk of selection bias. At baseline 86% of the invited RHINE cohort answered the questionnaire. After 20 years of follow-up, the response rate was 53%.[23] Only one third of the invited RHINESSA cohort answered the questionnaire (personal communication, Signe Timm). A study on long-term participation and follow-up found the baseline prevalence of asthma similar for long-term follow-up participants compared to non-participants. In RHINE, only minor differences in more exposure-outcome pairs were found between long-term follow-up participants and baseline participants.[23] Thus in the parental generation we do not believe the selection to be related to outcome and therefore we do not suspect differential selection bias. For the offspring and grandparental generations, we cannot rule out selection bias in either directions.
In this study the questionnaire data were cross-sectional, which involved a risk of recall error. The potential misclassification caused by recall error was unlikely to be differential and is more likely to lead to a potential underestimation of these associations. Furthermore, time to follow up varied between subjects as questionnaires were answered in 2010 (ECRHS) and 2013 (RHINESSA) giving information on subjects born between 1945 and 1999. Another study found that growing up on a livestock farm had a stronger protective effect on late-onset asthma diagnosed after the age of ten.[21] The youngest subjects were 14 years at time of follow up, meaning that some of these subjects could be misclassified as “not yet identified cases”. This misclassification was most likely unrelated to exposure among the younger subjects, thus nondifferential causing the association to be underestimated. Additionally, the variable age of the subjects when participating in the study was problematic, as some had a long time at risk since exposure, while others had a shorter time at risk.
Another limitation of this study was that outcome was assessed as self-reported ever asthma. This means that some of the older cases reporting “asthma” could instead be suffering from COPD (Chronic Obstructive Pulmonary Disease). This misclassification was assessed not to be related to place of upbringing, but as COPD is typically caused by long-term exposure to irritants such as cigarette smoke, the misclassification would be related to the age of the subjects. The potential misclassification would therefore be non-differential causing an underestimation of the association between place of upbringing and ever asthma, especially among the older generations of the study population. The same applies to subjects with early life wheeze, who could also be misclassified as subjects with ”ever asthma”. Another consideration regarding self-reported ever asthma is that it also relied on the subjects to see a doctor. Subjects with farm upbringing could be less prone to see a doctor, however this will be influenced by local factors in each study centre. The diagnosis of asthma might be more used by younger generations, which could contribute to the increase in asthma prevalence across generations.
Subjects were giving information on behalf of their parents and grandparents regarding place of upbringing, asthma status and smoking. A study on the agreement between offspring’s and parent’s reporting of parental place of upbringing showed that offspring misclassification was highly dependent on their own place of upbringing, as they tended to report the same for their parents as for themselves.[24] The potential misclassification was assessed to be unrelated to parental asthma status and therefore non-differential. Furthermore, Timm et al. performed a quantitative bias analysis presenting similar results when using direct or secondhand information in parental place of upbringing.[22] We have no information on misclassification when offspring were reporting on behalf of their grandparents but believe that this involved a comparable risk of non-differential misclassification.
As seen in Table 1 subjects with missing information regarding parental asthma among offspring were more pronounced among rural and farm children compared to children raised in the city. In adjusted analysis, subjects with missing information on covariates such as parental asthma were omitted. Among subjects with missing information, we also found a skewed distribution with regard to asthma status, meaning that this drop out was both related to exposure and outcome hence a differential dropout. The subgroup of offspring growing up on a farm consisted of 368 subjects of whom only 60 had asthma (data not shown). This meant that even small proportions of subjects could have a strong influence on the distribution.
Place of upbringing was considered as a crude measure for microbial exposure, which was a strong assumption, as not all farms were equal regarding microbial substances.[10, 25] Of all subjects growing up on a farm, 484 reported a farm without livestock. 2,017 reported a farm with livestock, of these 1,769 were parents and 248 were offspring. Information on grandparents did not differentiate farms with or without livestock. Ege et. al found that only 24% of agricultural farms did not also raise livestock and 37% of the livestock farms also performed agriculture. Both agriculture and frequent stays in animal sheds were found to have a protective effect on asthma.[10] This indicated that farms often were combining field and livestock farming, which means that distinguishing these as two separate exposures can be problematic. On basis of this, farms with or without livestock were merged under the category “farm”.
Analysis on data from the Nordic part of the ECRHS cohort, named RHINE (Respiratory Health in Northern Europe) supported the assumption that subjects growing up on a farm were more likely to keep pet animals than subjects growing up in the city (data not shown). Also, this factor did not differ over time (data not shown). One study investigating the effect of pet keeping on asthma and allergies showed no association between keeping dogs, cats, birds and rodents in early life and asthma among school children.[26] The same applies to a German study on early-life predictors of asthma, which also found no association between asthma and pet ownership[27].