Methods: A screening questionnaire survey was performed in 7164 children from urban Guangzhou and 6087 from rural Conghua. In the second stage, subsamples of 854 children (419 from Guangzhou, 435 from Conghua) were recruited for a case-control study including detailed questionnaire enquiring family history, early life environmental exposures, dietary habits, and testings including histamine airway provocation, skin prick test, and serum antibody analysis. House dust samples from 76 Guangzhou and 80 Conghua families were obtained to analyze levels of endotoxin, house dust mite and cockroach allergens.
Results: The prevalence of doctor-diagnosed-asthma was lower in children from Conghua (3.4%) than Guangzhou (6.9%, p<0.001) in the screening survey. A lower percentage of asthma was found in rural compared to urban subjects (2.8% vs. 29.4%, p<0.001) in case-control study. Atopy (odds ratio 1.91, 95% confidence interval 1.58-2.29), parental allergic diseases (2.49, 1.55-4.01), hospitalization before age 3 (2.54, 1.37-4.70), high milk product consumption (1.68, 1.03-2.73) and dust Dermatophagoides farinae 1 level (1.71, 1.34-2.19) were positively, while crop farming before age 1 (0.15, 0.08-0.32) and dust endotoxin level (0.69, 0.50-0.95) were negatively associated with asthma.
Conclusions: A variety of environmental factors were found to be associated with asthma. Parental allergic diseases, atopy, diet and early life exposures might explain the lower prevalence of asthma in the rural environment in southeast China.