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Background: The aim of the present study was to determine the distribution of mite allergens in pig-farming environments in comparison to urban homes and the relationship between exposure to mite allergens and sensitization to the respective allergens in 100 pig farmers with work-related respiratory symptoms.
Methods: The concentration of storage mite (Lep d 2) and house-dust-mite (Der p 1, and Der f 1, Der 2) allergens in dust collected from five different sampling sites (floor, wall, grain mill, transit floor, and farmers' mattresses) was studied in relationship to the respective sensitization rates. Allergen concentrations in the mattresses were compared to those determined in mattresses from 22 urban dwellers.
Results: Median concentrations of Der p 1 and Der 2 in the mattresses of the farmers were significantly higher than in the urban dwellers' samples (53.4 μg/g dust vs 1.05 μg/g dust, P=0.001; 19.6 μg/g dust vs 2.2 μg/g dust, P<0.0001, respectively). Allergen concentrations in the transit areas were strongly related to bedroom exposure. In a multiple logistic regression model, a weak but significant relationship between Der p 1 exposure and sensitization to Der p 1 was found. Despite these findings, the prevalence of sensitization to mite allergens in the farmers (18%) was comparable to the prevalence in the general population.
Conclusions: Allergen exposure at the work place is strongly related to the concentration of allergens in farmers' beds. Exposure to domestic mite allergens should be taken into account when assessing occupational exposure to allergens and the respiratory health of farmers.
Farmers who work in barns or with animals frequently develop obstructive airway diseases ( 1, 2). In randomly selected farming populations, the prevalence of sensitization to storage mites is from about 5% ( 3) to 6% ( 4), with considerably higher rates in asthmatic farmers ( 3). Studies involving farmers from Sweden ( 5), Finland ( 6), and Denmark ( 3) have found Lepidoglyphus destructor to be the most important species inducing storage mite allergy, and comparable results have been reported in the UK ( 7, 8). High numbers of storage mites are present in crops ( 3, 9), but storage mites have also been found in the farmers' mattresses, with a median number of 60 mites (Acarus siro, Tyrophagus putrescentiae, L. destructor, or any storage mite) per gram dust ( 3).
Besides being rather time-consuming, counting mites as an index of allergen exposure may not optimally reflect the biologically relevant allergen concentrations at the workplace. The study of Campbell et al. ( 10) was the first to investigate the prevalence of a number of potential allergens in barns. The major findings were that L. destructor allergens were abundantly present, and that in some barns allergens reactive with antisera to Dermatophagoides sp. could be demonstrated. No such data, however, are available regarding the distribution of these allergens in animal confinement houses where pigs or poultry are kept.
The main L. destructor allergen, Lep d 2, has been purified and characterized ( 11, 12), and it has become possible to measure Lep d 2, as well as group 1 and group 2 allergens of Dermatophagoides ( 13–17), by ELISA.
to assess the concentrations of L. destructor (Lep d 2) and Dermatophagoides (Der p 1, Der f 1, and Der 2) allergens in farming environments
to study the relationship between allergen concentration and sensitization to the respective allergens in pig farmers with work-related respiratory symptoms.
Additionally, we determined the antigen concentrations in the mattresses of the farmers and of 22 urban dwellers.
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- Material and methods
The four main findings of this study were as follows:
The median concentrations of Der p 1 were 50-fold higher in farmers' beds than in the beds of the group of urban dwellers (53.40 μg/g vs 1.05 μg/g, P=0.001), and higher than concentrations reported from many other places around the world ( 20–24).
Mite concentrations in the transit areas were shown to be related to dust and storage mite concentrations in farmers' beds.
In a multivariate model, a dose-response relationship was seen between Der p 1 concentrations in farmers' beds and sensitization to D. pteronyssinus.
Despite high Der p 1 concentrations in dust samples taken from farming and home environments, farmers did not show a higher prevalence of sensitization than the general population in Germany (18%).
L. destructor was found to be abundantly distributed. Since storage mites feed on molds, they can flourish on moldy grain. Our findings are consistent with this observation, since we found the highest concentrations of Lep d 2 (median 0.52 μg/g) in the grain mill. Echechipia et al. ( 19) detected Lep d 2 in 23% of house-dust samples in an urban population, with low levels (0.05–0.20 μg/g) in 10%, moderate levels (0.21–2.00 μg/g) in 6%, high levels (2.01–10.00 μg/g) in 3%, and very high levels (>10 μg/g) in 4%. While Lep d 2 in our study was detected in 41% of the urban beds, none of the samples were above 2 μg/g. Forty-one percent were found to have low concentrations, and 9% had moderate concentrations of L. destructor. In contrast, 16% of farmers' beds had concentrations of Lep d 2 of 0.2–2 μg/g, but only one farmer's mattress had high concentrations (>10 μg/g).
The results for Der p 1 agree with the investigations of Iversen et al. ( 3), who also found very high concentrations of house-dust mites in the mattresses of farmers (median count was 1480 living mites/g dust, D. pteronyssinus, D. farinae, any other house-dust mite). Exposure to more than 2 μg/g of group 1 mite allergen or 100 mites per gram dust is considered to increase the risk of sensitization and symptoms; exposure to more than 10 μg/g or 500 mites per gram dust is thought to increase the risk of acute asthma attacks ( 18, 25). Wahn et al. ( 26) proposed even lower thresholds. They suggest a low allergen exposure below concentrations of 0.4 μg Der p 1/g dust and a significant allergen exposure in the range of 0.4–2 μg Der p 1/g dust. In our study, only 13% of the mattresses were below the level of 2 μg/g. Antigen levels equal to and above 2 μg/g but below 10 μg/g were measured in 11% of the mattresses. A possible explanation could be the way farmers were recruited for the study. All subject farmers complained of work-related respiratory symptoms. Thus, one might speculate that the less “hygienic” conditions causing a higher concentration of mites are overrepresented in the sample. Furthermore, it is possible that D. pteronyssinus finds better growth conditions in a rural environment than in urban homes. Indoor temperature and humidity have the greatest influence on the growth of house-dust mites, and reports suggest that 7 g/kg is the level of absolute humidity above which excess mite growth will occur ( 27). Homes with a higher number of occupants, those located on the first floor, and private buildings tend to have a higher concentration of Der p 1 and Der 2 antigens ( 28–30). Older mattresses, use of a cover or under blanket, a higher weight of sampled dust, and a higher ratio of inhabitants per m2 were significantly associated with increased concentrations of Der f 1. On the other hand, lower Der f 1 concentrations were found when interior spring mattresses were used ( 31). However, Der f 1 was the only allergen which tended to be higher in urban mattresses than in farmers' mattresses. Which of these factors were responsible for our results will be elucidated in future studies.
We could show a significant relationship between mite concentrations in transit areas and farmers' beds. Thus, among farmers, mite concentrations at occupational locations are related to bedroom exposure. Allergen avoidance in the workplaces of symptomatic farmers alone may be insufficient.
Only two serum samples contained IgE antibodies to L. destructor alone. The majority of samples (10%) contained IgE antibodies to all three mites tested. This may be due to cross-reactivity between these two families of Acaridae mites, as shown by Luczynska et al. ( 32) and Griffin et al. ( 33), who performed RAST inhibition studies. However, van Hage-Hamsten et al. ( 5) suggested that each of the storage mites and D. pteronyssinus possess unique allergens. Ventas et al. ( 11) also found that the major allergen of Lep d 2 is not responsible for allergenic cross-reactivity between L. destructor and D. pteronyssinus. One explanation is that most individuals were exposed to both storage mites and house-dust mites and became sensitized to both. Our findings demonstrate the complexity of the immunologic responses to the different mite species. Nevertheless, there was a tendency for higher Lep d 2 concentrations in the swine confinement houses in farmers sensitized to L.destructor that was significant for the floor samples. In contrast, concentrations of Der p 1, Der f 1, and Der 2 tended to be higher mostly in the mattresses of sensitized farmers. Due to the different habitat of the mite species, it might be that the sites of sensitization to house-dust mites and storage mites are different. However, when the logistic regression model was repeated, using sensitization to L. destructor as dependent variable and mite concentrations in the confinement houses as predictor variables, no dose-response relationship was seen (data not shown). One reason might be the low number of sensitized farmers in the present study.
A relationship between exposure and sensitization to Der p 1 has been shown in many other investigations ( 25, 26, 34) and was confirmed in a multiple logistic regression model in our study. Because of the high Der p 1 and Der 2 levels in farmers' beds, it might have been expected that these farmers would have a higher sensitization rate than the urban population; however, the prevalence of sensitization to house-dust mites among the farmers was not significantly higher than that of a randomly selected urban population ( 35) in the same region of Germany (farmers vs urban dwellers aged 20–44: 24%vs 19%). In general population samples in the European Health Survey, Burney et al. ( 36) reported that 7–35% of adults aged 20–44 were sensitized to house-dust mites, whereas 24% of the farmers in this age group were sensitized to house-dust mites.
Our results are compatible with observations on sensitization rates among children growing up on farms. Children living on a farm had a significantly lower sensitization rate than children living in a rural environment but not on a farm ( 37, 38). Therefore, another possible explanation for the lack of an association between sensitization and exposure in our study might be a protective effect of dietary or environmental factors which seem to be relevant to farming environments, such as endotoxin. It may well be that “lifestyle” factors responsible for an increase in sensitization to common allergens in urban dwellers, such as spending most of the time in well-insulated buildings, may not be true of farmers and their children growing up on farms.
In conclusion, in farmers sensitized to any mite allergen, avoidance of workplace exposure may not be sufficient to control symptoms due to domestic exposure because of house-dust mites in mattresses. Further studies are needed on indoor factors associated with high Der p 1 and Der 2 antigen concentrations in farmers' mattresses, and the reason for similar sensitization rates in farmers exposed to higher allergen levels than those of the total population.