In the European Community Respiratory Health Survey I (ECRHS I) conducted in the years 1990-1991 the IgE sensitisation to D. pteronyssinus, measured by Pharmacia CAP System,1 was 9% (IgE ≥0.35 kU/l) in a random sample of young adults in Reykjavik and suburbs.2 By skin prick test (SPT) 6.1% were positive to D. pteronyssinus, using a cut off limit of 3 mm³. These findings in Reykjavik are comparable to the results in Uppsala Sweden, where corresponding figures were 7.9% and 7.4%, respectively.3

As a part of the ECRHS II indoors protocol (www.ecrhs.org) 197 randomly selected adults from ECRHS I were visited in their homes between March 2001 and January 2002. Dust samples were collected from their mattresses, according to the protocol, for measurement of house dust mite (HDM) allergen concentration and furthermore for the determination of the number and type of HDM.4 Only two mites, both D. pteronyssinus, were found in two separate dust samples. Mite allergen analysis identified a minimal amount of Der f 1 in two dust samples taken from other homes. Additionally, samples were collected in November 2002 from mattresses and bedroom floors in homes of 10 HDM allergic patients, not included in ECRHS II. The samples were examined for mites only but no mites were found.4 These results indicate that exposure to HDM in the Reykjavik area is extremely rare.

According to Korsgaard the risk of being sensitised to HDM in a cool temperate climate is assumed to be a consequence of indoor exposure.5 Assuming this is right the question rises how people in Reykjavik with positive SPT or specific IgE to D. pteronyssinus have become sensitised and about its clinical relevance. Therefore we decided to investigate what characterized those individuals who had specific IgE to D. pteronyssinus in ECRHS I in comparison with individuals, from the same cohort, positive to grass pollen antigens, but with no antibodies to D. pteronyssinus.

Of 601 participants in ECRHS I who provided blood for specific IgE, 529 (88.0%) participated in ECRHS II with a new blood sample. The participation rate in this cohort was substantially higher than average for all centres in ECRHS.6 The median follow up time was 8.38 years. The mean age at ECRHS II was 41.7 years, women constituted 54% of the samples.7 Of 58 with detectible D. pteronyssinus specific IgE in ECRHS I, 49 participated in ECRHS II and constitute our study population, of whom 25 were still positive to D. pteronyssinus, but 24 had become negative at that time point and 4 who were negative at ECRHS I had become positive in ECRHS II. They are not included in the study. A control group included participants with specific IgE to timothy grass in ECRHS II, but no IgE to D. pteronyssinus.

The study population was divided into three groups (Table 1):

1) Individuals with D. pteronyssinus specific IgE, both in ECRHS I and II (group +/+).

2) Individuals with D. pteronyssinus specific IgE in ECRHS I, but not in ECRHS II (group +/-).

3) Individuals with timothy grass specific IgE at ECRHS II but negative on specific IgE to D. pteronyssinus at ECRHS I and II (group -/-).

All participants were contacted in 2004 and asked detailed questions about childhood environment, travelling abroad, farming, hobbies and allergic symptoms to shellfish and mosquito bites. Data from ECRHS II were also used in subsequent analyses, namely questionnaire data on respiratory symptoms, asthma and allergic symptoms in addition to spirometry results8 and test results for bronchial responsiveness based on a methacholine challenge.9

Total IgE was measured as a part of the ECRHS II protocol, specific IgE value to D. pteronyssinus and grass was measured both in ECRHS I and II, but frozen venous blood samples from ECRHS II were used to measure serum IgE antibodies to the following available antigens reported to cross-react to HDM; the storage mite Lepidoglyphus destructor (L. destructor), shrimp (P. borealis), cockroach (B. germanica), mosquito (A. communis), tropomyosin (tropomyosin Pen a 1) and blood worm (C. thummi) by an enzymatic immunoassay (CAP-FEIA) performed according to the manufacturer's instructions (Pharmacia & Upjohn Diagnostics, Uppsala, Sweden). Results of ≥0.35 kU/l were considered positive. In ECRHS I L. destructor was included in the SPT panel.

The study design was approved by the local ethics committee.

Mean values with 95% confidence intervals were used for continuous variables. The two-sided Fischer's exact test and two-sided student's t-test were used for comparison of groups. A p value less than 0.05 was considered to indicate a statistically significant difference.

Of 89 individuals eligible in 2004, 79 (89%) could be reached and agreed to participate, 24 in group +/+, 20 in group +/-, and 35 in the control group (Table 1). Among the 44 subjects in groups +/+ and +/- with D. pteronyssinus specific IgE in ECRHS I, 20 (45%) had positive SPT to L. destructor at the same time, 12 in group +/+ and 8 in group +/-. One in the control group -/- had positive SPT to L. destructor in ECRHS I. In group +/+, 6 were exclusively positive to D. pteronyssinus in ECRHS II, when measured by specific IgE and only one of them had a positive SPT to L. destructor in ECRHS I.

There were significantly more men in the +/+ group than in the control group or 18 (75%) vs. 11 (31%) (p < 0.01). The mean age was comparable between the groups.

No difference was observed between the groups in terms of history of wheezing, asthma or history of eczema (Table 2). In group +/- 50% reported symptoms of nasal allergies, including hay fever, compared with 83% in the control group (p < 0.05). The grass sensitive control group reported significantly more allergic symptoms when exposed to pollens than the HDM groups (p < 0.001). Exposure to dust was not associated with more symptoms in the HDM sensitised subjects compared with the control group. Bronchial hyperresponsiveness (BHR) was measured by a methacholine challenge test using PD20 = 1.0 mg methacholine as a cut off level. There was a non-significant tendency for the control group to show a positive BHR more often, or 19% compared with 14% in group +/+ and 12% in group +/-. The mean forced expiratory volume in 1 second (FEV1), as a percentage (%) of predictive value, was 100, 103 and 101 for groups +/+, +/- and -/-, respectively.

The re-evaluation in 2004 revealed that significantly more subjects in group +/+ had lived on a farm during summer holidays as children or adolescents than in the control group (p < 0.05) (Table 3). Among those 6 in the +/+ group, solely positive to D. pteronyssinus, 5 had been on farms during the summer vacation as a children or adolescents. Living or travelling abroad was not different for the groups, but being an owner or caretaker of aquarium fishes before the age of 12 was more often associated with present HDM allergy (p < 0.05). Horse riding, clinical reactions from eating shellfish and reactions to mosquito bites were more commonly reported in the HDM allergic group but did not reach statistical significance (Table 3).

Total serum IgE (geometric mean) was significantly higher in group +/+ than in the control group, 111 vs. 43 kU/l (p < 0.01). Specific IgE (geometric mean) to D. pteronyssinus in ECRHS I was significantly higher in group +/+ (2.1 kU/l) than in group +/- (0.78 kU/l) (p < 0.01).

Eighteen subjects (75%) in group +/+ and 4 subjects (20%) in group +/- displayed an IgE antibody level of ≥0.35 kU/l to one or more of the six allergens measured, but none in the control group (p < 0.0001, p < 0.05, respectively) (Table 4). Among group +/+, L. destructor was the most common allergen, with 16 (67%) positive subjects. Many of these were positive to more than one allergen; 13 were also positive to shrimp, 8 to cockroach, 4 to mosquito, 3 to tropomyosin 1 and 1 to blood worm. No correlation was found between the IgE value to D. pteronyssinus and the number of positive measurements to the other allergens.

In this study we examined what characterized the group of individuals in the ECRHS I, positive to specific IgE to D. pteronyssinus in an area with practically no HDM, and compared them with a group positive to specific IgE to timothy grass. Among the 89 selected for this follow-up study 28 had allowed home visits to search for HDMs without a single mite being found. Seasonal variation is unlikely to explain these results as dust samples were collected over a ten-month period and mite allergens found were negligible. In the study men comprised 75% of group +/+ but only 31% of the control group. In contrast similar prevalence of positive SPT to all allergens has been reported previously for both genders.3

Many cross-sectional studies in adult populations have reported decreasing prevalence of allergic sensitization with increasing age,10 which may reflect the natural course of allergic sensitization or a cohort effect, i. e. increase in prevalence of allergic sensitization among younger birth cohorts. In our study 24 out of 49 that were positive to HDM in ECRHS I had become negative to HDM 8.4 years later while 4 individuals had become positive. Those who became negative had lower IgE values at ECRHS I (mean IgE 0.78 kU/l) compared with those who did not (mean IgE 2.1 kU/l). When all centres in ECRHS II are taken together no significant overall changes were found in the prevalence of IgE sensitization to HDM between ECRHS I and II.6 In that sense Reykjavik differs from the average centre in the ECRHS II.

Specific IgE to D. pteronyssinus does not seem to reflect clinical symptoms in a dusty environment in contradiction to grass sensitisation in pollen-loaded areas. This lack of a clinically relevant association underlines the importance of caution when interpreting positive HDM results, at least in Iceland.

When seeking explanation for why so many are sensitive to HDM in HDM free environments we have three possibilities in mind: sensitisation may have occurred when travelling or living abroad, sensitisation may have occurred when travelling or living in other parts of the country, and the third possibility is that specific IgE to D. pteronyssinus may express cross-sensitisation to other allergens.

No relationship was found between sensitisation to HDM and staying outside Iceland. Group +/+ had dwelled significantly more often on farms during the summer vacation. It is a possibility that exposure to HDM on farms has led to sensitisation, as information about HDM in farmhouses is still not available.

In the middle of the last century, when the majority of the participants in the study were growing up, exposure to hay dust was very common among the urban population because of summer vacations on farms among youngsters, in particular boys. Exposure to hay can also be related to horse riding and horse feeding, which is a very popular hobby in Iceland. In ECRHS I, were participants came from Reykjavik and suburbs, 69% of all men and 31% of all women had been on farms during their summer holidays and 69% of participating men and 41% of participating women had handled hay at some time or been exposed to hay dust.11 Of those who had been exposed to hay dust 21.5% reported some symptoms when exposed and 16.7% were positive to L. destructor (SPT ≥1 mm) compared with 7.8% among those not exposed to hay dust.11 Among farmers in Iceland and their families storages mites, in particular L. destructor, are the main cause of atopy and atopic diseases.12

Previous studies have reported allergic cross-sensitivity between HDM and other arthropods, among them L. destructor.13 L. destructor accounted for 25% of all storage mites found in Icelandic hay 1981.14 Cross-sensitisation may also exist between HDM (Der p 10, Der f 10) and muscle protein, tropomyosin.15 Similar proteins occur in a wide range of arthropods, including shrimps and lobsters.15 Tropomyosins are also found in other invertebrates such as cockroaches, chironomids and mosquitoes.15, 16 RAST inhibition studies have shown that cross-sensitivity exists between D. pteronyssinus and both cockroaches and chironomids.17, 18 Bloodworms are chironomid larvae that are used as aquarium fish food, and allergy against them has been reported in persons handling them as fish food.18 Cross-sensitisation has also been found between HDM, crustaceans and mosquitoes.16, 19 In our study, group +/+ was more exposed to hay dust environments than the control group, and kept aquarium fishes more often than the control group. Group +/+ had also more often symptoms after eating shellfish or after mosquito bites, although these differences were not significant. Our results, as demonstrated in Table 4, indicate that serum-specific IgE to HDM may be an expression of sensitisation to other allergens, cross-reacting to HDM. This is supported by negative findings for the same cross-reactive allergens in the control group of grass-sensitised individuals. The storage mite L. destructor is responsible for two thirds of these responses and there seems to have been some cross-sensitisation between all the allergens measured. The clinical relevance of these positive measurements is however questionable. In group +/+, 58% were shrimp positive; only 13% of them reported clinical reactions when eating shellfish compared with 9% in the control group. As shellfish is a common ingredient in the Icelandic diet, these results do not appear to express clinical allergy to shrimps. All but one of the 14 shrimp positive individuals were also positive to L. destructor.

Altogether, 17% in group +/+ had a specific IgE to the shrimp allergen tropomyocin, thus indicating that tropomyosin can only be responsible for a small part of the cross-sensitisation between D. pteronyssinus and shrimp. This is in contrast to previously reported cross-reactivity between HDM and crustaceans where tropomyosin has been reported to be the major cross-reactive allergen.15, 20

Compared with none in the control group, 33% in group +/+ had specific IgE to cockroach and 17% to mosquito, even though there are neither mosquitoes nor cockroaches found in Iceland. These people might have been sensitised when living or travelling abroad, but this may as well be explained by cross-sensitisation. The fact that none in the control group were positive to these allergens makes the latter alternative likely.

There is good evidence for a dose-response association between HDM exposure, HDM sensitisation and asthma.21, 22 Sensitisation to HDM has been considered to be a major independent risk factor for asthma in all areas where the climate supports mite growth.23, 24 In our study there was no tendency to bronchial hyperresponsiveness or asthma in group +/+ compared with the control group. This shows that allergy to HDM has little clinical relevance as such in Reykjavik. HDM sensitisation might, however, be a surrogate marker of a sensitisation to other allergens cross-reacting to HDM where the clinical relevance is not fully revealed.

In summary people in Reykjavik with specific IgE to D. pteronyssinus are more often men who spent time on farms in childhood and to day have a high prevalence of IgE antibodies cross-reactive to D. pteronyssinus, which indicates that IgE antibodies to HDM may be an expression of sensitisation to other allergens cross-reacting to HDM, supported by negative findings for cross-reacting allergens in the control group. It is also possible that exposure to HDM on farms has led to sensitisation, as information about HDM in farmhouses is still not available. Specific IgE to D. pteronyssinus does not seem to reflect clinical symptoms in dusty environments and we believe that a positive HDM test should be interpreted with precaution. Therefore, traditional recommendation to those with a positive specific IgE to HDM should only be put forward when based on the presence of clinical symptoms as well.

ACKNOWLEDGEMENTS

The study was funded by the National University Hospital Science Fund, whom the authors thank cordially for the support. Pharmacia Diagnostic, Copenhagen, Denmark supported the study by free reagents for specific IgE measurement to L. destructor, shrimps, cockroach, mosquito, tropomyosin and blood worm, which we are grateful for. We also thank Thorkil Hallas, Copenhagen Denmark for critical revision of the manuscript and much good advice.

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