Unfortunately most studies in the farming environment did not report the prevalence of parental history nor did they report the effect size of parental risk in the farming population. This is, however, a critical issue as the so called healthy worker effect (HEW) may be a rather trivial explanation of the results.
Specifically, it is a sampling bias: the kind of subjects that voluntarily enroll in a clinical trial and actually follow the experimental regimen are not representative of the general population. They can be expected, on average, to be healthier as they are concerned for their health [or as ill people already dropped out]
At least Braun-Fahrländer 1999 reported that allergic parents were seen much less at farms: allergic rhinitis 12.7% versus 29.4% (P=0.001). A history of allergy at farms is no more a risk factor as it is otherwise in the general population. How that?
Are there any studies in adults? I know of three studies (and one review Le Moual N 2008).
Leynaert 2001 showed only a slightly reduced prevalence of “allergy” (39.1% vs 41.5%, NS) while her table 4 is most interesting. The association started only after year 1960 which points towards misclassification as far as the analysis is not stratified by year of birth.
Remes 2002 showed a dose dependent effect decline between farming (36.2%) and controls (31.6%, P=0.075),
Perkin 2006 also found some significant lower prevalence in farmers 47.3% versus 57.7%, P<0.001.
A HWE is therefore likely.
There are six studies (Thelin 1994, Braback 2006, Chenard 2007, Thaon 2011, Elholm 2013 and Spierenburg 2015) that examined in detail a possible relationship of HWE, allergy and farming. Unfortunately the examination period in five of these studies is too short for any conclusion while Braback 2006 seems to be the only reliable study.
Also from this study, we can safely conclude, that there is a significant HWE.
Addendum 22 Nov 2019
It seems that I missed some papers on HWE and farming. For asthma it is important to discriminate atopic and non-atopic asthma.
Timm 2019: a hard to understand 3 generation study of unclear asthma type. Point estimates of parental asthma on farm upbringing are not really a measure of HWE – shuffling exposure and outcome distorts temporality. Neither protection by farm nor HWE found but also selection bias (approx. 50% dropout) and non-differential bias for residency.
Eduard 2015: 1,964 farming students. Current asthma in farmers was 3.0% compared to 6.3% in farmers who had retired early. No HWE found at entry level but during farm work.
Farmers who had changed the type of production had an increased risk of current asthma (OR 9.8), compared with those who had not changed production, indicating a strong selection effect.
Vogelzang 1999: 400 pig farmers, X-sectional point estimates, not a real HWE study, although HWE offered as explanation.
Health-based selection of nonasthmatics for pig farming, which tends to mask a work-related hazard for asthma, is offered as an explanation for these results.
Taken together – more data but not more knowledge.
Addendum 21 Feb 2021
A new farming paper shows parental history basically same vein as Braun-Fahrländer 1999. If we could get also paternal history, this would explain the whole farming effect.