Levels of Environmental Endotoxin and Prevalence of Atopic Disease
- Jose E. Gereda, MD;
- Donald Y. M. Leung, MD, PhD;
- Andrew H. Liu, MD
- KEYWORDS:
- asthma
- atopic hypersensitivity
- endotoxins
- environmental exposure
- hypersensitivity, immediate
- rural health
To the Editor: While the prevalence of asthma and allergic disease is increasing worldwide,1 both diseases appear to be less common in rural settings in developing countries and farming communities in industrialized countries.2 Indeed, some locales seem to be almost free of asthma.3 To explain the low prevalence of asthma in these settings, the "hygiene hypothesis" suggests that early childhood exposure to high levels of bacterial and viral pathogens leads to a lower risk of asthma and atopy.4-5 The immune response to these infections would presumably inhibit helper T cell type 2 (TH2)–type allergic responses.
We recently reported on the potential of environmental endotoxin—a cell-wall component from gram-negative bacteria that is ubiquitous in the environment—to provide an atopy-protective effect.6 Infants sensitized to common allergens had significantly lower levels of house-dust endotoxin in their homes. Furthermore, higher house-dust endotoxin levels correlated with increased proportions of interferon-γ–producing CD4 TH1 lymphocytes in peripheral blood, suggesting that household endotoxin exposure may have an atopy-protective effect by augmenting THl-type immune development. Therefore, we sought to determine if people in locales with a lower prevalence of asthma and atopy have greater exposure to environmental endotoxin.
Methods
We measured levels of house-dust endotoxin obtained from homes in 3 different settings: urban homes in Denver; farm homes and associated barns (with animals such as cows, horses, goats, chickens, cats, and dogs) located in Colorado except for 1 pair each in New Hamphire, Wyoming, and Missouri, respectively; and homes in rural India and Peru that are inhabited by similar animals. Methods for house dust collection, preparation, and endotoxin measurement were described previously.5
Results
Urban homes had significantly lower house-dust endotoxin levels than farm homes and rural homes in developing countries (P<.001, Wilcoxon rank sum test) (Figure 1). Farm barns had significantly higher endotoxin levels compared with both farm homes (P<.001, t test) and rural homes in developing countries (P = .03, t test). Farm home and associated barn endotoxin levels were significantly correlated (Spearman r = 0.67, P = .02).
Figure. House-Dust Endotoxin Concentrations From Homes in Denver, US Farms, and Rural Areas of Peru and India
House-dust endotoxin levels are reported in endotoxin units (EU)/mL, using reference standard endotoxin provided by the US Food and Drug Administration. Log transformation of endotoxin values normalized the distribution of the data. Data points with error bars and associated numbers indicate the geometric mean values and SEM for each location subgroup. Wilcoxon rank sum test, P<.001. Farm barns outnumber farm homes because some homes had more than 1 associated barn and also because some barns were sampled with permission from workers when farm homeowners were not home.
Comment
Greater levels of exposure to environmental endotoxin from early childhood, especially in rural areas of developing countries and in farming communities, may help explain the low prevalence of asthma and allergies observed in children raised in these environments. If environmental endotoxin exposure in early life has an atopy-protective effect, then TH1-type activity may be induced separately from exposure to serious infections, thus suggesting a possible strategy for allergy and asthma prevention.
