House dust endotoxin exposure mitigates asthma severity in a tropical environment

Abstract

Rationale Studies have shown exposure to bacterial endotoxin to be associated with asthma severity. Asthma prevalence and morbidity has been increasing disproportionately in tropical, developing countries, where sources of domestic endotoxin, especially humidity, animals in close proximity to humans, and high allergen levels are extreme. To date, no studies have characterized domestic endotoxin exposure in the tropics nor examined the impact of this exposure on asthma. Methods To investigate the relationship between exposure to house dust endotoxin (HDE) and asthma severity in an Afro-Caribbean asthmatic population from Barbados, we measured HDE concentrations in the homes of 87 asthmatic sib-pairs. Asthma severity was assessed using a validated questionnaire that yields a quasi-quantitative Asthma Severity Score. Household characteristics considered to reflect overall household hygiene were assessed and tested for association with HDE concentrations. Results We observed HDE concentrations to be high relative to reports in urban settings in non-tropical environments (mean living room composite=21,061 EU/m 2, range=2649-185,468 EU/m 2). Several household characteristics were positively associated with HDE levels, including carpeting (p=0.01), decreased frequency of vacuuming (p=0.003), and cats in the home (p=0.001). Increasing endotoxin content was associated with decreasing asthma severity scores among skin test negative individuals only (p=0.01). Conclusions HDE levels were high in this tropical environment, but comparable to levels observed in farming populations. Despite early and significant exposure to endotoxin, asthma prevalence is high in this population. The inverse relationship between HDE levels and asthma severity supports the idea that microbial load may be protective and highlights the complex role of HDE in disease expression.