Longitudinal evidence that indoor allergen exposure causes morbidity in sensitized individuals with asthma is scarce. We evaluated the association of allergen sensitization and home exposure to short- and long-term morbidity in 140 women with asthma and to asthma prevalence in 458 women from metropolitan Boston. Cockroach (Blattella germanica), dust mite (Dermatophagoides farinae), and cat (Felis domesticus) allergens in home dust samples, and specific immunoglobulin E antibodies were measured at outset, and doctor-diagnosed asthma and markers of asthma morbidity were ascertained by questionnaire during a 4-year follow-up. Cat- and cockroach-sensitive (immunoglobulin E immunocap [Cap] class ⩾ 1) women with asthma reported greater morbidity in the past year at the start, and during follow-up, if high levels of the relevant allergen were found. Women with asthma sensitized to cat allergen and with concentrations at 8 μ g/g or greater were more likely to have used steroid (adjusted odds ratio [95% confidence interval] 2.7 [1.2–6.2]) and wheezed without a cold (odds ratio 6.8 [3.3– 14.0]) during follow-up. Those sensitized and exposed to cockroach (Bla g 1 or 2 ⩾ 2 U/g) were at least three times more likely to have used steroid and to have attended a hospital emergency room; the size of the effect upon steroid use was maintained, but the precision was reduced and the 95% confidence interval included one (p = 0.07), with adjustment for race and poverty. We conclude that cockroach and cat allergens may contribute to asthma morbidity in sensitized women.
Keywords: home allergens; cockroach; cat; sensitization; asthma morbidity
Longitudinal epidemiologic evidence for indoor allergen exposure as a cause of morbidity in sensitized adults with asthma is surprisingly sparse, despite reproducible laboratory evidence that bronchial provocation with allergens (e.g., cat [Felis domesticus] [1, 2], mite [Dermatophagoides farinae] [3], or cockroach [Blattella germanica] [4]) in sensitized patients can produce symptoms or a decline in lung function. For dust mite, avoidance of allergen in sensitized subjects with asthma has resulted in short-term improvement in asthma morbidity or amount of immunoglobulin E (IgE) (5-7). Clinical trial data evaluating the success of cat or cockroach allergen avoidance in improving asthma morbidity are lacking (8). The National Cooperative Inner City Asthma Study recently demonstrated that cockroach-allergic children with asthma from U.S. inner city communities are at risk for greater asthma morbidity if elevated amounts of cockroach allergen are present in their homes (9). In this study, we prospectively evaluate the role of indoor allergen sensitization and exposure as a cause of morbidity in women with asthma from a range of socioeconomic backgrounds from metropolitan Boston. Among women with and without asthma, we also examine the cross-sectional relationship of cat, cockroach, and dust mite allergen levels in the home to sensitization, the level of specific IgE, and asthma prevalence.
Our study population comprised the mothers who participated in the Epidemiology of Home Allergens and Asthma Study, a longitudinal birth and family cohort study, described elsewhere (10-12). The family was eligible for inclusion only if either parent had a doctor-diagnosed specific sensitization or allergic disease; the mothers therefore comprised a population of women enriched for allergic disease.
At screening interview, each woman was asked detailed questions regarding her current and past history of respiratory disease (questions detailed in the online data supplement), and virtually identical questions were asked at 12-month telephone interviews for the following 4 years. Doctor-diagnosed asthma was defined as a positive response to the question: “Has a doctor ever said that you have asthma?” In those with asthma, subsidiary questions were used to derive indicators of current and long-term morbidity. A bout of asthma during the past 12 months of 1 week or more duration was defined as “prolonged illness,” treatment in a hospital emergency room (ER) for asthma in the past 12 months was defined as “use of a hospital ER,” and use of an oral or inhaled steroid during the worst bout of illness was defined as “use of steroid.” Women whose chest had ever sounded wheezy or whistling in the absence of a cold (upper respiratory viral infection) in the past 12 months were defined as having “wheeze without a cold.” A positive response to any one of these indicators was defined as “overall asthma morbidity.” The ER question changed at 4 years; it was analyzed to 3 years only.
The methods used in collecting home dust samples and the processing and assay of each allergen have been detailed elsewhere (10, 12). Each sample was analyzed for allergens from a dust mite (Der f 1) and cat (Fel d 1), recorded in units of micrograms per gram of dust, and cockroach (Bla g 1, and if sufficient dust was available, Bla g 2), recorded in units per gram of dust. Bedroom concentrations were available for women who shared a bedroom with their baby, derived as the maximum of the concentrations for the parent's bed, child's bed, and bedroom floor.
A serum sample drawn at screening was analyzed for total IgE and for specific IgE (cockroach, mite, and cat). We determined sensitivity as specific IgE at immunocap (Cap) Class level 1 or above (> 0.35 U/ml) and high specific IgE as Cap Class 3 or above (> 3.5 U/ml). Individuals with sensitivity to any one of these allergens or to ryegrass, ragweed, Alternaria, or Aspergillus or with total IgE greater than or equal to 200 U/ml, were defined as being “allergic.”
Race/ethnicity and area poverty (based on the proportion living below the poverty level) were determined and categorized for analysis as described before (11).
Statistical analyses were performed using SAS statistical software (SAS Institute, Inc., Cary, NC). We analyzed the relation of allergen level (high/low) and sensitization to the risk of asthma and to measures of asthma morbidity, at screening and during follow-up in women with diagnosed asthma, using multiple logistic regression. Evidence of interaction between sensitization and allergen level was assessed using the likelihood ratio test. To the unadjusted model (Model 1), we added age, race, and poverty to assess confounding (Model 2). We accounted for correlations between repeated observations on the same individual using generalized estimating equations (13). See online data supplement.
Four hundred and ninety-nine mothers (and families) entered the longitudinal study and received a home visit 2–3 months after the screening interview; 485 (97%), 481 (96%), 464 (93%) and, to date, 452 (91%) were followed up by phone at 1, 2, 3, and 4 years, respectively. Serum was not available in 41 cases, and a complete set of specific IgE was available for 446 women. The 458 women with assessment of IgE to at least one allergen were between 18 and 46 years of age at screening, the majority were white (79%), college educated (78%), and on a household income of $50,000 or more (Table 1). At the home visit, few were current smokers (5%). In this group enriched for allergic disease, 30.6% reported doctor-diagnosed asthma.
Characteristics | Whole Sample (n = 458) | Subset with Diagnosed Asthma (n = 140) | ||||||
---|---|---|---|---|---|---|---|---|
Number | Percent | Number | Percent | |||||
Age group | ||||||||
18–30 | 112 | 24.5 | 52 | 37.1 | ||||
30–33 | 116 | 25.3 | 36 | 25.7 | ||||
33–36 | 113 | 24.7 | 23 | 16.4 | ||||
36–46 | 117 | 25.5 | 29 | 20.7 | ||||
Race | ||||||||
White | 363 | 79.3 | 94 | 67.1 | ||||
Black | 51 | 11.1 | 24 | 17.1 | ||||
Hispanic | 24 | 5.2 | 13 | 9.3 | ||||
Asian | 16 | 3.5 | 6 | 4.3 | ||||
Other | 4 | 0.9 | 3 | 2.1 | ||||
Area poverty, | ||||||||
% below poverty line | ||||||||
< 5 | 180 | 39.3 | 45 | 32.1 | ||||
5–10 | 130 | 28.4 | 39 | 27.9 | ||||
10–20 | 105 | 22.9 | 36 | 25.7 | ||||
> 20 | 43 | 9.4 | 20 | 14.3 | ||||
Household income, $ | ||||||||
> 50,000 | 319 | 69.7 | 80 | 57.1 | ||||
30,000–50,000 | 83 | 18.1 | 35 | 25.0 | ||||
< 30,000 | 43 | 9.4 | 21 | 15.0 | ||||
Unknown | 13 | 2.8 | 4 | 2.9 | ||||
Maternal education | ||||||||
College education or higher | 357 | 77.9 | 92 | 65.7 | ||||
No college education | 101 | 22.1 | 48 | 34.3 |
Cockroach. Eleven percent of the study population was sensitized to cockroach. The maximal home level of cockroach allergen, which occurred almost invariably (86%) in the kitchen, was equal to or above 2 U/g for 13% (61/458) of the women. There was substantial overlap between levels of Bla g 1 and Bla g 2.
Sensitization to cockroach allergen was substantially more common in those for whom the level of Bla g 1 or Bla g 2 was equal to or above 2 U/g in the kitchen, and there was a trend for sensitization prevalence to rise as kitchen cockroach levels rose (Table 2). Adjustment for race and area poverty attenuated the size of this effect (Table 2) but did not completely explain the association. Few individuals had high levels of IgE (⩾ Cap 3) to cockroach, but these were almost exclusively in homes with high levels of cockroach allergen. The association between allergen level and specific sensitization was similar in the allergic subgroup (data not shown).
IgE Level | IgE ⩾ Cap 1 Adjusted OR (95% CI) | |||||||
---|---|---|---|---|---|---|---|---|
Allergen | No. | ⩾ Cap 1 (No. [%]) | ⩾ Cap 3 (No. [%]) | |||||
Bla g 1 or Bla g 2, U/g | ||||||||
Kitchen | ||||||||
< 0.05 | 224 | 14 (6.2) | 4 (1.8) | 1 | ||||
0.05–2 | 166 | 15 (9.0) | 2 (1.2) | 1.14 (0.51–2.57) | ||||
⩾ 2 | 51 | 21 (41.2) | 10 (19.6) | 3.97 (1.57–10.0) | ||||
Der f 1, μg/g | ||||||||
Living room | ||||||||
< 2 | 238 | 84 (35.3) | 55 (23.1) | 1 | ||||
2–10 | 97 | 36 (37.1) | 25 (25.8) | 1.18 (0.71–1.96) | ||||
⩾ 10 | 112 | 40 (35.7) | 24 (21.4) | 1.13 (0.69–1.84) | ||||
Bedroom (if parents and child share) | ||||||||
< 2 | 119 | 44 (37.0) | 28 (23.5) | 1 | ||||
2–10 | 60 | 20 (33.3) | 13 (21.7) | 1.04 (0.52–2.11) | ||||
⩾ 10 | 72 | 32 (44.4) | 25 (34.7) | 1.68 (0.87–3.22) | ||||
Fel d 1, μg/g | ||||||||
Living room | ||||||||
< 1 | 168 | 53 (31.6) | 19 (11.3) | 1 | ||||
1–8 | 166 | 47 (28.3) | 14 (8.4) | 0.89 (0.69–2.02) | ||||
⩾ 8 | 103 | 35 (34.0) | 23 (22.3) | 1.18 (0.69–1.46) |
Mite and cat. Levels of Der f 1 tended to be higher in bedrooms and living rooms, of cat in living rooms. Sensitization to dust mite occurred more frequently among women in bedrooms with high (⩾ 10 μg/g) levels of Der f 1, but limited data were available on parental bedroom dust mite levels, and the association between dust mite levels and allergy was weak (p = 0.2) (Table 2). The level of Fel d 1 in the home was not related to the level of maternal cat-specific IgE (Table 2). In the allergic subgroup, sensitization to mite was more common in those with high (⩾ 10 μg/g) than those with low (< 2 μg/g) levels of mite in the bedroom (odds ratio, 2.5; 95% confidence interval, 1.0–6.1), but the association between exposure and sensitization to cat remained weak.
Cross-sectionally at screening, doctor-diagnosed asthma and overall asthma morbidity were both significantly more common in those with home levels of Bla g 1 or Bla g 2 at above 2 U/g. Using this cutoff as a marker of high exposure, there were relatively few with high exposure to either of the cockroach allergens, and few with low exposure but sensitization to cockroach. Nevertheless, the prevalence of doctor-diagnosed asthma and of overall asthma morbidity were both more common in those sensitized to cockroach and increased further in those who were both sensitized and had high levels of cockroach in the kitchen (doctor-diagnosed asthma, 86%; overall asthma morbidity, 52%) (Figure 1). For diagnosed asthma, this interaction remained of borderline significance after controlling for age, race/ethnicity, and area poverty (p = 0.06). In those who were not cockroach sensitive, there was no difference in asthma prevalence between those with low and high exposures.

Fig. 1. Shaded bars represent diagnosed asthma and solid bars represent overall asthma morbidity.
[More] [Minimize]Doctor-diagnosed asthma was also more common in those with mite sensitization and in those with cat sensitization, but levels of allergen did not modify the relationship between sensitization and asthma (data not shown).
Cockroach. Only 18 women with diagnosed asthma were sensitized and exposed (kitchen) to Bla g 1 or Bla g 2 at 2 U/g or greater. However, of these individuals, 33% used a steroid, 35% attended a hospital ER, and 22% had prolonged illness in the past 12 months at screening, compared with just 12, 10, and 11% in the other diagnosed subjects with asthma (p = 0.02, 0.006, and 0.17, respectively) (Table 3). These differences were greater than those seen in relation to living room exposure to cockroach, but there were insufficient exposed individuals with asthma (n = 4) with exposure measurements in the bedroom to compare the effects of kitchen and bedroom exposures. During the four (or three) years of follow-up, the sensitized and exposed individuals continued to report greater use of steroids and substantially greater use of hospital ER facilities. They also tended to have more wheeze in the absence of a cold (55% versus 39%) and more bouts of prolonged illness (19% versus 12%) than those in the remaining three groups, though not significantly so. The 18 individuals were characterized, but not exclusively so, by residence in an area of high poverty (12), and the majority were of black (9) or Hispanic (6) race. The increase in steroid use was sustained after adjusting for race or ethnicity (adjusted odds ratio 5.2 [1.6– 16.3]). The size of the effect estimate was maintained after additional control for area poverty, although the precision of the estimate decreased and the confidence interval included 1 (adjusted odds ratio 4.3 [0.9–21.3], p = 0.07) (Table 4). The effect upon use of hospital ER facilities was also no longer significant at the 5% level after adjustment for area poverty (p = 0.1). Similar relations were seen longitudinally in those with asthma morbidity in the past year at screening (data not shown).
Exposure/ Sensitization Group | Steroid | ER Visit* | Wheeze No Cold | Prolonged Illness | Overall Morbidity | |||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
OR (95% CI) Model 1 | OR (95% CI) Model 2 | OR (95% CI) Model 1 | OR (95% CI) Model 2 | OR (95% CI) Model 1 | OR (95% CI) Model 2 | OR (95% CI) Model 1 | OR (95% CI) Model 2 | OR (95% CI) Model 1 | OR (95% CI) Model 2 | |||||||||||
Cockroach | ||||||||||||||||||||
Low exposure or | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | ||||||||||
not sensitized | ||||||||||||||||||||
High exposure | 3.1 | 4.3 | 6.8 | 4.0 | 1.7 | 1.5 | 1.6 | 1.4 | 1.7 | 1.7 | ||||||||||
and sensitized | (1.3–7.4) | (0.9–21.3) | (2.0–23.0) | (0.6–25.0) | (0.7–4.1) | (0.5–4.5) | (0.6–4.3) | (0.3–6.8) | (0.7–4.3) | (0.5–5.6) | ||||||||||
Mite | ||||||||||||||||||||
Low exposure or | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | ||||||||||||
not sensitized | ||||||||||||||||||||
High exposure | 0.7 | 0.8 | – | – | 1.1 | 1.2 | 0.5 | 0.6 | 1.3 | 1.4 | ||||||||||
and sensitized | (0.3–1.5) | (0.3–1.8) | (0.5–2.8) | (0.5–3.1) | (0.2–1.5) | (0.2–2.0) | (0.6–2.9) | (0.6–3.2) | ||||||||||||
Cat | ||||||||||||||||||||
Low exposure or | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | ||||||||||
not sensitized | ||||||||||||||||||||
High exposure | 2.8 | 2.7 | 1.6 | 1.7 | 5.6 | 6.8 | 1.0 | 0.9 | 5.4 | 6.2 | ||||||||||
and sensitized | (1.2–6.4) | (1.2–6.2) | (0.3–8.4) | (0.3–9.8) | (2.7–11.6) | (3.3–14.0) | (0.5–2.4) | (0.3–2.1) | (2.4–12.1) | (2.7–14.1) |
Cat. There were 19 doctor-diagnosed individuals with asthma who were both sensitized and exposed to high levels of cat allergen in the living room. At screening, these individuals reported significantly more overall asthma morbidity in the past year (79% versus 47%), and specifically more wheeze without a cold (74% versus 41%) (Table 3). During follow-up, this group continued to experience more years with some morbidity from the condition and especially more wheeze without a cold and greater use of steroid (Table 3). This group was generally white (16) and on a high income (13). Adjustment for race/ethnicity and markers of socioeconomic status did not alter the size or significance of the effect upon steroid use or wheeze without a cold (Table 4). There were weaker trends in the same direction with exposure in the bedroom (data not shown). Sensitization to cat or cockroach was associated with wheeze with cold; exposure did not modify the relationship (data not shown).
Mite. There was no evidence of an increase in morbidity with high Der f 1 levels either in the living room (Table 3) or bedroom of mite-sensitized individuals.
The size of the effects of cockroach and cat sensitization plus exposure on overall asthma morbidity was not altered by adjusting each for the other (odds ratio for cat 5.7 [2.5–13.3] and for cockroach 2.0 [0.7–5.3]). There was no increase in relation to the combination of mite sensitization and exposure even after allowing for the effects of the other allergens (odds ratio for overall asthma morbidity 1.0 [0.5–2.1]) and before and after adjusting for race/ethnicity and area poverty.
In this prospective study of women from a range of socioeconomic backgrounds, for both cockroach and cat allergens, the highest levels of asthma morbidity were associated with the presence of both sensitization to the allergen and the presence of elevated concentrations of the allergen in the home. This association was found for several measures of asthma morbidity, including wheeze apart from cold, ER visits for asthma, and steroid use.
This is the first prospective epidemiologic study to demonstrate a relationship between cat allergen concentrations in the home and asthma morbidity among specifically sensitized women. For cat allergen, the relationship persisted after we controlled for socioeconomic factors such as race or ethnicity and area poverty. Separation of the effects of these aspects of socioeconomic disadvantage from the effects of sensitization and high exposure to cockroach allergen was more problematic because of the close correlation between the two. Poverty and race are two fairly crude estimates of socioeconomic status, and it is possible that the effect of cockroach is subject to residual confounding by other factors associated with deprivation. Nevertheless, the sizes of the effect estimates of cockroach exposure were retained in our fully adjusted models, even though the precision of the estimate decreased, suggesting that confounding does not explain the relation that we have observed. In fact, the effects of poverty and race upon morbidity, which varied but was strongest for ER attendance, were reduced and lost statistical significance in the multivariate model after allowing for the effect of cockroach. This suggests that it is the effect of cockroach that contributes to the socioeconomic divide in asthma morbidity rather than vice versa.
Allergen concentrations are inevitably a crude assessment of exposure, and further errors would have accrued from the fact that we used allergen measurements made near the start of follow-up; 43% of women moved home at some stage before the end of follow-up (usually not in the first year, in accordance with the cohort entry criteria). Misclassification of exposure may have tended to weaken our findings with respect to cockroach. We also had insufficient numbers to look at effects on morbidity of cockroach exposure in the bedroom, which in the Inner City Asthma Study was found to have the stronger effect. It is therefore possible that the effect of cockroach exposure upon morbidity is greater than we have estimated.
Prolonged illness and wheeze apart from colds tended to be increased with cockroach sensitization independently from exposure, whereas for outcomes that may be stronger indicators of more severe asthma, such as steroid use and ER visits, both exposure and sensitization were needed for increased morbidity. This is consistent with previous data suggesting that cockroach allergen may be a cause of more severe asthma than other allergens, either because of particular characteristics of the allergen or because of host vulnerability in disadvantaged communities (14). Emergency room visit rates were much higher for mothers with sensitization and exposure to cockroach than for the combination of sensitization and exposure to either cat or mite. In children from the inner cities of the U.S., the combination of cockroach sensitization and exposure has also been found to be associated with increased use of the health care system and clinical symptoms (9). Case–control studies have demonstrated that children presenting with acute asthma exacerbations are more likely than nonasthmatic control children to have both sensitization and exposure to indoor allergens (cockroach, dust mite, or cat) (15, 16). However, in these studies, sensitization to allergen had the stronger effect, and the contribution of allergen exposure in the home was not clear. Our findings and the findings in the Inner City Asthma Study suggest that in communities where home cockroach infestation is common, ER visits for asthma may, in part, be attributable to cockroach exposure in sensitized patients with asthma.
The strongest previous data supporting the association between exposure and acute exacerbation of asthma come from the controlled laboratory exposure studies in which sensitized patients with asthma have been exposed to allergen. In the case of dust mite and cockroach, these experiments have been conducted through inhalation of nebulized allergen (3, 4), which could potentially result in nonspecific as well as specific bronchial irritation and reactivity. However, increased eosinophilia as well as lung function decline in response to bronchial provocation suggest that the response is allergen-specific (17). These experiments have the advantage of control over exposure in well-characterized patients; our epidemiologic findings have the advantage of demonstrating effects of exposure and sensitization in women with asthma in their home environment.
For cockroach allergen, our cross-sectional data at entry to the study also suggested a relation of home allergen concentration with sensitization and a relation of the combination of sensitization and allergen concentration with asthma prevalence. Other cross-sectional studies have found an association between allergen concentration and specific sensitization to cockroach among subjects with asthma (18, 19). We found this association in a broader population of women with and without asthma. Our findings support the hypothesis that cockroach allergen exposure may contribute to sensitization and asthma development. However, no cross-sectional study can establish the temporal relationship between exposure and these outcomes. Moreover, current exposure does not necessarily reflect the exposure that preceded and possibly contributed to asthma development. Our study is further limited by the fact that our exposure data were collected 2–3 months after collection of our baseline sensitization and asthma prevalence data, rather than at the same point in time.
Our cross-sectional data on home cat allergen concentrations suggested no association between allergen exposure and sensitization. Cross-sectional studies of children have suggested associations between a report of home exposure to cat in the first 6 months of life and sensitization to cat (20, 21), but these studies may be subject to recall bias. Because cat allergen exposure can take place not only at home, but also in public places, the relative importance of home exposure in allergy or asthma development is unknown (8). We found a relation between sensitization and asthma prevalence, but this may simply reflect the fact that the prevalence of allergy in asthmatic persons is higher than the prevalence of allergy in nonasthmatic persons. A similar argument may be applied to the association of dust mite sensitization with asthma in our study.
There is strong laboratory evidence that dust mite exposure can worsen asthma (22) and that dust mite avoidance may decrease IgE levels to mite and improve asthma symptoms (23, 24), at least on a short-term basis. We found no prospective evidence that dust mite concentrations were related to asthma morbidity in sensitized asthmatic women, though our estimate for exposure was imperfect, because we did not have measurements of dust mite concentrations in many of the parents' bedrooms.
In summary, our findings provide prospective epidemiologic evidence that both cockroach allergen and cat allergen may contribute to asthma morbidity in sensitized women. This study provides evidence for the importance of allergy testing in patients with asthma to tailor recommendations regarding exposure mitigation to the allergies of the individual patient. However, the feasibility of various methods of home allergen reduction needs further investigation, as does the level of allergen reduction needed to significantly improve asthma morbidity in allergic adults and children with asthma.
Supported by National Institutes of Health grants RO1 A1/EHS35786 and A1 20565 and by UK Medical Research Council (S.A.L.).
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